Design a hard sci-fi-warship (RAR).

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Re: Design a hard sci-fi-warship (RAR).

Postby LaCroix » 2017-02-07 07:50am

Flanking is a concept of relatively close combat.
If the engagement is done at distances that need light-seconds or minutes to describe properly, you need to change your position a LOT to get a minimal angular change to the line of attack. It's pretty easy to create a ship that can deas lit attacks coming from a 5 degree cone, but very hard to traverse that distances needed to get outside of that cone quick enough to make a difference.

So you need to deploy forces close to or already in the flanking position. Deploying in a spread out swarm will make it easy to target the one enemy you have a favorable position on.

But since the enemy will see that positioning ahead of time, he might change his swarm geometry to have a good position on you, from a different point of their swarm, or shift their attack vector to have som of your vessels out of range to increase their numerical odds.

We are pretty much back to the pre-battle maneuvering of set battles of ye olde days, where both sides shift forces around, and deploy and withdraw constantly until they one side finds a flaw in the enemies deployment and initiates the combat.
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-07 02:11pm

Exonerate wrote:I foresee problems with this on several levels.


I don't think you are describing problems so much as logical mechanics of such a set up.

1. It's difficult to maneuver without presenting a larger profile/unarmored part to your opponent. To do this, you'd need effective vector thrusting. It may be possible to do this outside effective weapons range, which brings us to #2...


Exactly, which is why I described it as a dance instead of a rush in guns blazing affair. Once you can't spin to use your main drive to maneuver because you are inside an enemies weapons range you are limited to whatever vectors your particular armor shadow will still cover your drive. This will lead to interesting choices in armor layout. Do I have a shield covering more degrees giving me more freedom to manuever or do I have a smaller one that confines me but gives me a better mass ratio? Or do I keep the same mass ratio but simply have a smaller but thicker shield?

2. It's difficult to outmaneuver an opponent with rough parity in terms of ships and acceleration/dV. If there's no terrain, it's pretty hard to hide what you're doing and nothing to stop an opponent from positioning themselves accordingly in response. I suppose this doesn't really hold true if you're fighting around a planet or some strategic objective, but since the OP specified deep space as the setting...


There is no terrain on a chess board other than the edges. Each player can see all the others pieces from the start, each player knows the exact capabilities or the other's piece (the only capability being how they move). Because of this, like chess, you have to be thinking many "turns" ahead in this sort of space combat setup. Since the stakes are life and death this will probably mean a lot of stalemates where because an opponent can know his position is suboptimal if he can withdraw they will.

But it should be noted that this isn't entirely positional warfare. There are still things you could have on your side that might make you accept combat at a degree of positional disadvantage such as differences in weapons and defense performance which the enemy may not know.

3. It's not entirely clear to me that "flanking" has significant advantages at all. The line 'em up and shoot Napoleonic tactics arose from the need to concentrate fire/forces to avoid getting defeated piecemeal by cavalry, primitive command and control, morale, etc, all of which apply much less here. Modern naval surface warfare worries about off-axis attacks because of a need to protect high value targets, kinematic engagement envelopes, potentially compressed detect-to-engage timelines, limited sensor resources, etc... if there's nothing analogous to a Carrier that forms the centerpiece of the fleet, a lot of requirements disappear and again, the "flat" terrain alleviates a lot of potential problems about getting blindsided.


So there is no getting blindsided. I doubt that fact will prevent all forms of space warfare.

Don't forget about inertia as well - one moment you're nicely positioned so that your entire fleet is fighting only a portion of theirs, the next you're pulling away out of range and need to burn in the other direction.


Also an interesting consequence of the proposed tactical setup. At a certain point once each side is within each others weapons envelopes its going to become similar to a joust, each side careening towards each other after accepting combat. Neither able to significantly alter their approach vectors due to being instakilled by a shut up the ass, each probably regretting unalterable decisions made previously. At this point its up to the skill of weapons officers and their ability to wield their lasers/particle cannons/mass drivers/missile batteries as well as the engineers who put together the ships armor scheme, fire controls, and mount stabilization systems.

I think about the gun vs armor problem this way: It takes energy to ablate/deform/penetrate armor. One can convert highly energetic but low mass stuff into weapons (Gunpowder, nuclear reactor into electrical power into rail/coilguns or big explosions, rocket fuel into very fast moving pieces of metal, etc), but we don't really have an effective way of converting that highly energetic stuff into effective passive protection. When you have to lug around lots of mass to defend against something that takes up less mass, all your requirements start ballooning real quick. That's not to say there won't be some level of protection vs micrometeorites, shrapnel, lasers, etc, but beyond a certain level, it stops making sense.


Which is why I believe IF armor does exists at all, its probably not going to be possible to be protect 100% of a ships surface area to a degree that is effective against main battery weaponry. Maybe not even 10%. Hence my description of starship combat being all about presenting the smallest cross section possible with the thickest armor possible, and the desire to circumvent this attempt by forcing an enemy to expose more than he wants too.


LaCroix wrote:Flanking is a concept of relatively close combat.


Thats an unfounded statement. Flanking is frequently used to describe manuevers or large formations from divisions to whole army groups. battle fleets crossing the "T" is essentially a flanking maneuver.

If the engagement is done at distances that need light-seconds or minutes to describe properly, you need to change your position a LOT to get a minimal angular change to the line of attack. It's pretty easy to create a ship that can deas lit attacks coming from a 5 degree cone, but very hard to traverse that distances needed to get outside of that cone quick enough to make a difference.


Very true. But sense the Jovian Confederation will see the exit burn of the Earth Combine fleet the second it happens, counter maneuvers can essentially happen starting exactly then with maybe months of dancing around depending on the power of the drives we are talking about.

So you need to deploy forces close to or already in the flanking position. Deploying in a spread out swarm will make it easy to target the one enemy you have a favorable position on.


Yep.

But since the enemy will see that positioning ahead of time, he might change his swarm geometry to have a good position on you, from a different point of their swarm, or shift their attack vector to have som of your vessels out of range to increase their numerical odds.


Yep. However, I will repeat that the only perfect information you have concerns positioning and whatever characteristics you know from observing their burns (mass, engine type, vector, etc) but not things like weapon types, effectiveness (laser spot probabilities at X range for example), etc. This is a big deal because if you don't know when you have to stop manuevering due to being inside the enemies effective weapons range, you might make erroneous maneuvering decisions, restricting yourself too early or too late.

We are pretty much back to the pre-battle maneuvering of set battles of ye olde days, where both sides shift forces around, and deploy and withdraw constantly until they one side finds a flaw in the enemies deployment and initiates the combat.


Pretty much. Its also a measure of risk tolerance and how that is calculated based on the balance between offensive and defensive systems. If any error means your class cannon is vaporized because you had a mere 1000km deficit in range vs your enemy you aren't going to commit except on the most perfect of circumstances as you define them. If its possible to have a slugging match where you can make two or three passes straight through each others weapons envlopes riding inertia like Exonerate said, you may take a chance on your weapons officer finding a seam in their armor before they do.

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Re: Design a hard sci-fi-warship (RAR).

Postby Sky Captain » 2017-02-07 04:47pm

Maybe a good design idea is to have two sets of vectorable engines one at the back, one at the front if engine technology allows that. It means ship don't necessarily have to expose vulnerable sides if it wants to thrust sideways. Also would make random maneuvering to throw off aim of ballistic weapons easy while still pointing frontal armor shield at the enemy ships.

Caiaphas wrote:I don't know if this helps or not, but the way that the current meta has balanced out on the Children of a Dead Earth forums, even with sharply limited engagement ranges, is in favor of relatively low-powered laser gunships fielding near-UV lasers in the tens to hundreds of megawatts. Those in turn are countered by swarms of several thousand micromissiles a few centimeters across and around a meter long, approaching at velocities in excess of 10 km/s, usually either coilgun-launched or fired from a larger, heavier missile bus shortly before terminal maneuvers.


That is quite possible. If you lack standoff warheads then only sure way how to overwhelm good point defense system is to swamp it with too many targets to handle.

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Re: Design a hard sci-fi-warship (RAR).

Postby The Romulan Republic » 2017-02-07 04:53pm

Or you can go the unsubtle route and simply carpet nuke the general area. Without shields, it won't matter if you don't get a direct hit.
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-07 11:09pm

That certainly defeated a limited armor scheme as you can just have your warheads detonate behind a limited shield. Sure the ship can spin to keep the shield facing the missle but what if there are two? Or like you said a general area bombardment.

The effectiveness of this sort of attack will depend on the balance of point defenses vs missiles. The radiation from a blast drops of quickly with distance, so if you can push out anti missile engagement far enough and with a good hit to kill percentage you may make missiles a losing bet due to the mass penalty the inflict on the launching ship.

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Re: Design a hard sci-fi-warship (RAR).

Postby LaCroix » 2017-02-08 05:16am

Especially since in space, you can safely use one nuclear missile to defend against a sizeable number of missiles. Thinking of it, that is a hard counter to missile swarms, so we might see warships with primarily energy weapons, and a secondary battery of nuclear missiles that exists primarily to counter missile and fighter swarms.

These missiles will be a lot smaller than offensive ones, for you only need to travel a few km out to intercept, so they make the mass penalty even worse.
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-11 07:54am

Simon_Jester wrote:As to the person who suggested spherical hulls and engines pointing every which way... This actually presents some very serious problems.

For one, a sphere has the minimum possible surface to volume ratio. You have lots of internal space, but a minimum of external space. That limits your ability to mount things on the hull (like airlocks and sensors and so on).

For another, you can never concentrate full firepower or sensor capabilities on any one target.

Another concern is armor protection. The good news is that if your ship could get shot at from any direction, a sphere minimizes the area of armor plate you need. The bad news is that this is only true if you assume that enemy fire can come from any direction equally easily... In which case you've basically already given up on being able to maneuver to position yourself during combat. It takes pretty unusual hard-SF scenarios to force that on someone.

Spherical designs are not remotely competitive with designs that concentrate all their armor in a single 'belt' or 'plate' designed to stop specific threats from hitting at specific angles. Plus, when the enemy hits your hull, the center of mass of the target they see is always perpendicular to their line of fire, so you don't get as much benefit from enemy shots 'grazing' your armor belt and slamming into disproportionate thicknesses of protective material- which is one of the few things that can make thin flank protection worthwhile in my opinion.


So I have been working on a drawing based around Shroom's amoeba ship, but more like the gooey candy with a hard chocolate shell and a cherry suspended in the middle. I wanted to reignite the spherical ship discussion starting with Simon's post.

1.) Oh surface area, this seems like a disadvantage only if you happen to have lots of things to mount. If you are porcupined with primary and secondaries like a WWII battleship this will be hard. But if you are relying on one or two primaries such as spinal weapons this may not matter. My big problem is radiator space.

2.) On alpha arcs, I don't see a lot of disadvantages here. On a sphere if you apply an equatorial mounting scheme you get alpha arcs fore and aft, which is a good scenario if you have a single thrusting vector. Everywhere else you have half firepower.

3.) On the armor, its a tradeoff of having the least surface to armor if 100% protection is required, but having no optimal cross section to present to a threat axis. The nature of the tactical situation will dictate how much of an advantage or disadvantage this is.

3.) On the armor front though the cross section disadvantage can be partially minimized by being able to readily and quickly spin the globe to present new armor faces as long as the center of gravity is maintained in the center of the sphere. Any ship can do this, but its more complicated for a long skinny ship to do so in directions other than rolling.

Specific to Shroom's idea.

1.) If we assume a rather large space for low density armor material (aerogels or foamed metals as discussed or something else), I have been working with one radius, of the profile presented to the enemy most of the area vulnerable to hits will just be armor protecting more armor. This seems like an inefficient use of space and mass to me.

2.) I wrestle with whether to have the outside armored shell be thrust bearing or to have an internal structure. I was thinking of flooding the porous armor medium with water or liquid hydrogen or other remass option to use hydraulic forces for thrust loads in the thrust bearing shell option. I am probably missing something horribly obvious with that idea. Of course to get the whipple effect of the low density porous armor medium some portion of that liquid will have to be drained.

3.) There are some center of mass issues if you want to keep it in the center of gravity in the center of the sphere as the engine is obviously not going to be there. So there has to be some off center balancing. In NY case this will move the none gooey core bits closer to the shell, reducing armor protection from certain directions.

4.) The same goes for the things mounted on the outside of the shell. Assuming you want the sensitive bits within the shell like magazines, laser generators, handling rooms, capacitors, etc. that will effectively reduce the armor thinness from the emplacement to the core unless what you are mounting is as hard as the armor.

4a.) If you don't have portions of the mounts inside the shell but instead outside the shell, they will have to be armored independently increasing armored surface area and thus overall mass. They will also greatly unbalance the sphere meaning complementary mounts to compensate on other portions of the shell or a complicated set of ballast tanks. You will probably need the tanks anyway if you are using weapons like missiles or rail guns that could eject significant levels off mass away from the ship which again could unbalance them. All of this is true of any ship, but a sphere in particular.

Just some thoughts. Please let me know what I am missing or got wrong.

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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-11 04:38pm

Patroklos wrote:
Simon_Jester wrote:As to the person who suggested spherical hulls and engines pointing every which way... This actually presents some very serious problems.

For one, a sphere has the minimum possible surface to volume ratio. You have lots of internal space, but a minimum of external space. That limits your ability to mount things on the hull (like airlocks and sensors and so on).

For another, you can never concentrate full firepower or sensor capabilities on any one target.

Another concern is armor protection. The good news is that if your ship could get shot at from any direction, a sphere minimizes the area of armor plate you need. The bad news is that this is only true if you assume that enemy fire can come from any direction equally easily... In which case you've basically already given up on being able to maneuver to position yourself during combat. It takes pretty unusual hard-SF scenarios to force that on someone.

Spherical designs are not remotely competitive with designs that concentrate all their armor in a single 'belt' or 'plate' designed to stop specific threats from hitting at specific angles. Plus, when the enemy hits your hull, the center of mass of the target they see is always perpendicular to their line of fire, so you don't get as much benefit from enemy shots 'grazing' your armor belt and slamming into disproportionate thicknesses of protective material- which is one of the few things that can make thin flank protection worthwhile in my opinion.
So I have been working on a drawing based around Shroom's amoeba ship, but more like the gooey candy with a hard chocolate shell and a cherry suspended in the middle. I wanted to reignite the spherical ship discussion starting with Simon's post.

1.) Oh surface area, this seems like a disadvantage only if you happen to have lots of things to mount. If you are porcupined with primary and secondaries like a WWII battleship this will be hard. But if you are relying on one or two primaries such as spinal weapons this may not matter. My big problem is radiator space.
Radiators are an issue. Sensors are an issue- remember, your ability to detect stuff coming towards you from a long way out, with good enough precision to have a fire control solution, is key to your ship's ability to defend itself. Small craft bays and airlocks are an issue.

2.) On alpha arcs, I don't see a lot of disadvantages here. On a sphere if you apply an equatorial mounting scheme you get alpha arcs fore and aft, which is a good scenario if you have a single thrusting vector.
Your alpha arcs are extremely narrow unless you mount your weapons in tiny little turrets on top of "spines" sticking up above the hull, which mandates that the weapons themselves be individually small and very numerous, which in turn runs you into the surface area constraints discussed. Basically, your only options for firing while engaging the enemy are to have your main engine pointed straight at them, or straight away from them. You have no more than a few degrees of leeway in terms of the angle between the threat axis and your thrust vectore.

This also means that your only option for 'sidestepping' enemy fire is whatever maneuvering thrusters you have mounted on the equator of the ship, which is a bad place to mount engines because it competes with things like weapons and sensors. And sidestepping is extremely useful for things like not getting pasted by ballistic railgun rounds.

3.) On the armor, its a tradeoff of having the least surface to armor if 100% protection is required, but having no optimal cross section to present to a threat axis. The nature of the tactical situation will dictate how much of an advantage or disadvantage this is.
If you can't predict the threat axis in advance in a hard-SF space battle, you've probably already lost, because you don't know the enemy is coming.

3.) On the armor front though the cross section disadvantage can be partially minimized by being able to readily and quickly spin the globe to present new armor faces as long as the center of gravity is maintained in the center of the sphere. Any ship can do this, but its more complicated for a long skinny ship to do so in directions other than rolling.
If you spin a globe, you lose the ability to control which direction your engines are thrusting in. And if you mounted your weapons equatorially, this automatically takes the enemy out of your alpha arc.

By contrast, a long skinny ship can roll to any desired angle without significantly interfering with its ability to accelerate along a desired axis.
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Re: Design a hard sci-fi-warship (RAR).

Postby Shroom Man 777 » 2017-02-12 04:58am

There should be a ship that's got components distributed on a mechanical framework that... rotates these components to provide maximum coverage. Like an orrery. A warship or a space station!
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-13 12:35pm

Simon_Jester wrote:Radiators are an issue. Sensors are an issue- remember, your ability to detect stuff coming towards you from a long way out, with good enough precision to have a fire control solution, is key to your ship's ability to defend itself. Small craft bays and airlocks are an issue.


I suppose it depends on the variety and number of things you want to mount. If you are for a VLSesque missile swarm generator with many doors for box launchers a sphere is probably not the best shape. Even in the carrier role how many bay doors do you actually need.

I see the point though. Another issue is that if you are trying to hide the delicate bits of your surface mounts inside the ship if they have particularly deep penetration towards the core things will start to get cramped depending on the size of the sphere due to them all penetrating at a different angle. This can be managed of course, but its not as nice as features on a flat surface lining up parallel to each other.

Your alpha arcs are extremely narrow unless you mount your weapons in tiny little turrets on top of "spines" sticking up above the hull, which mandates that the weapons themselves be individually small and very numerous, which in turn runs you into the surface area constraints discussed. Basically, your only options for firing while engaging the enemy are to have your main engine pointed straight at them, or straight away from them. You have no more than a few degrees of leeway in terms of the angle between the threat axis and your thrust vectore.


Again, this is a balancing issue based on the tactical environment. Sure you alpha are is limited, but you will never find yoruself in a negative arc either. Alpha arcs are great if weapons are devastating enough that there is little opportunity for reengagement due to one side or the other dieing quickly in a mutual charge (or a one sided charge if you catch someone out of position)

If the offensive to defensive balance is such that its more or a slugging match you may find yourself needing to maneuver and depending on your maneuvering characteristics that means maybe presenting more aspects of your ship than a carefully selected alpha arc. Now granted, if you are alpha arc encompasses more degrees in the first place you can maneuver longer while still maintaining it, but what happens when you can't?

This also means that your only option for 'sidestepping' enemy fire is whatever maneuvering thrusters you have mounted on the equator of the ship, which is a bad place to mount engines because it competes with things like weapons and sensors. And sidestepping is extremely useful for things like not getting pasted by ballistic railgun rounds.


Granted, but there are more maneuvering options than sidestepping. A sphere, assuming uniformish density, will perform pitch and yaw maneuvers much better than a long slender cylinder (also assuming uniform density for simplicity). That means it can use its main engine for maneuvering more readily.

If you can't predict the threat axis in advance in a hard-SF space battle, you've probably already lost, because you don't know the enemy is coming.


True, but some locals, like Mars for the Martian Defense Force, must be defended. And while knowledge of position and movement is great, there are other things involved. You might THINK your have positional superiority until you realize the Saturnian death pods are mounting longer range lasers than the last time you saw them.

If you spin a globe, you lose the ability to control which direction your engines are thrusting in. And if you mounted your weapons equatorially, this automatically takes the enemy out of your alpha arc.


It depends on where the fire is coming from. If you are taking it from the side you are spinning around your thrust access so it has no effect on your main engine thrusting, and if your weapons are equatorial you are unmasking other mounts as you mask others for a net of no change. Even if you are taking fire forward spinning on your thrust axis will greatly frustrate weapons like lasers as unless they are able to maintain a perfect spot directly on your nose you are increasing the spot surface by the circumference of whatever circle you are creating.



By contrast, a long skinny ship can roll to any desired angle without significantly interfering with its ability to accelerate along a desired axis.
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Rolling for either would not have the effect you are thinking. Rolling around the current thrust axis will with not effect the thrust vector of any ship. It also assumes you are burning through the combat phase, which you may not be doing if you already think you have a superior position and would rather shutter your engine so you can make these types of defensive spins without the effects you predict which would be true for spinning that is not around the thrust vector.

I should point out that the sphere has the advantage (again, uniform density) of having identical maneuvering characteristics for both rolling and pitch/yaw. A cylinder shaped ship will have superior roll characteristics, but inferior yaw/roll. There are advantages to both these situations but if you are trying to reorient your main drive for additional combat burns the sphere is superior. So If this is a slugging match where you both fly past each other the sphere will be more able to maintain an alpha arc on target as they pass (one on one encounter here).

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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-13 02:24pm

Patroklos wrote:Again, this is a balancing issue based on the tactical environment. Sure you alpha are is limited, but you will never find yoruself in a negative arc either. Alpha arcs are great if weapons are devastating enough that there is little opportunity for reengagement due to one side or the other dieing quickly in a mutual charge (or a one sided charge if you catch someone out of position)

If the offensive to defensive balance is such that its more or a slugging match you may find yourself needing to maneuver and depending on your maneuvering characteristics that means maybe presenting more aspects of your ship than a carefully selected alpha arc. Now granted, if you are alpha arc encompasses more degrees in the first place you can maneuver longer while still maintaining it, but what happens when you can't?
In the context of space warfare there is a huge gap of time between the point at which the enemy comes into range and the point at which you can no longer keep them in a chosen wide arc of fire.

The problem I'm trying to point out is that if your alpha arc constitutes an extremely tiny volume of space, your options for using it are so limited that it becomes nearly useless. You are effectively always limited to firing with whatever weapons happen to bear, because you have to be quite fortunate to get the enemy in your alpha arc.

That is an especially serious handicap in cases where (as you describe) pounding matches or extended exchanges of fire are required, which are precisely the conditions you expected to favor the sphere. Because under those conditions, not having an alpha arc means that you need to mount two weapons for every one you expect to be able to fire at the enemy, and so on. It's wasteful, and a more efficiently laid out ship* of similar mass can have major advantages.

*(Say, one with main weapons in turrets on the dorsal and ventral surfaces, permitting full broadside fire through a broad, roughly flat region surrounding the ship in most if not all directions)

This also means that your only option for 'sidestepping' enemy fire is whatever maneuvering thrusters you have mounted on the equator of the ship, which is a bad place to mount engines because it competes with things like weapons and sensors. And sidestepping is extremely useful for things like not getting pasted by ballistic railgun rounds.
Granted, but there are more maneuvering options than sidestepping. A sphere, assuming uniformish density, will perform pitch and yaw maneuvers much better than a long slender cylinder (also assuming uniform density for simplicity). That means it can use its main engine for maneuvering more readily.
Uh... actually no. A long slender cylinder (assuming it's got a rigid structure) can have its maneuvering thrusters at the ends of the cylinder, which means a lot more torque. This tends to cancel things out, or (in my opinion that I haven't yet done math on admittedly) more than cancel things out.

Thrusters to roll, yaw, or pitch a spherical ship are going to have to be mounted physically on the surface, too, firing roughly parallel to the surface. Which means that their "guts" cannot be buried deep within the hull. That is another factor taking up surface area, plus it means there are swathes of the outer hull that have rocket exhaust playing across them.

If you can't predict the threat axis in advance in a hard-SF space battle, you've probably already lost, because you don't know the enemy is coming.
True, but some locals, like Mars for the Martian Defense Force, must be defended. And while knowledge of position and movement is great, there are other things involved. You might THINK your have positional superiority until you realize the Saturnian death pods are mounting longer range lasers than the last time you saw them.


If you spin a globe, you lose the ability to control which direction your engines are thrusting in. And if you mounted your weapons equatorially, this automatically takes the enemy out of your alpha arc.
It depends on where the fire is coming from.
Yeah, but if your weapons are mounted equatorially, that means the enemy was already out of your alpha arc. They remain so, as noted. The point being that there is basically no realistic way, with a spherical ship, to maintain fire by all the ship's weapons, while dodging enemy fire using the main engines. Either you have to sacrifice the ability to bring all your weapons to bear, or you have to fly along a predictable trajectory where you have little or no freedom to be anywhere other than where the enemy expects you to be (and has presumably already fired a nuke at).

A ship with broadside turret armament can do that. Hell, World War II battleships do that, exactly that way! There's a reason people didn't just build big circular warships with guns bristling all along the equator. And hydrodynamics was far from the only reason.

[Incidentally this is also a problem with spinal armament, which is also something people never did with large warships in the Industrial Age for a good reason. It only really works if you expect to always be able to force the enemy to shoot at you from a very specific direction, and have such good lateral maneuverability that you can duck and weave in 3D rather than needing to turn to point your main engines]

By contrast, a long skinny ship can roll to any desired angle without significantly interfering with its ability to accelerate along a desired axis.
Rolling for either would not have the effect you are thinking. Rolling around the current thrust axis will with not effect the thrust vector of any ship. It also assumes you are burning through the combat phase, which you may not be doing if you already think you have a superior position and would rather shutter your engine so you can make these types of defensive spins without the effects you predict which would be true for spinning that is not around the thrust vector.
Frankly, yes I am assuming you're making main engine burns during combat.

There are extremely good reasons to do that, of the "not getting shot" variety. Again, think about wet-navy warships. They don't stop moving in order to shoot, and it would be extremely dangerous for them to do so in a threat environment dominated by ballistic weapons. The main reason naval guns had such terrible hit rates in the World War era was precisely because the time of flight was a minute or two, and ships could change its position, even its predicted future position, by large fractions (or even multiples) of their own length in that amount of time.

Remove that advantage, and the enemy's odds of hitting you go up by an order of magnitude. Or more (and I can do the math to prove this). Even though your spacecraft is still coasting ballistically, it's still a "sitting duck."
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Re: Design a hard sci-fi-warship (RAR).

Postby LaCroix » 2017-02-14 10:03am

Spinal weapons were not used in naval warships for a couple of reasons. Most important, line of sight. A spinal weapon can per definition, not be aimed in any way other than moving the ship. With no elevation, your range was far to limited.
(If you try and put a railway gun on a battleship to get elevation, it would no longer count as a true spinal weapon in my book, but even if we accept them, the use is limited.)

Such a weapon mount is highly problematic in open water where the ship is constantly moving, if only slightly. That alone might make it impossible to reliably hit a moving target.
With projectile fight times being quite long and the inherent inaccuracy, it made no sense to send a single projectile of multiple tons in one piece. Sending a dozen smaller ones of same total weight increased hit probablility a lot.

Detection range was pretty low, and gun ranges did not favor a bigger gun - 35 km ranges for 38cm guns vs 50 km for an 80cm gun. 2 shots/minute for 38cm vs 1 every 20-40 mins for 80cm. It just did not scale well.

It would mean that you could reliably one-shot disable any opponent you hit, but more agile opponents would swarm you, evade your fire, rendering your main weapon inefficient, or even damaging it if it was an exposed railroad gun type. The range vs speed difference is small enough that you could face that scenario. You have maybe 2 or 3 shots before they can fire back, but most likely, you need 50 to 100 to hit a target, going by historic data. 3 pr 4 shots later, they are right next to you.
You would need to add conventional turrets to fight them off and make them stay at resonable range, and at that point, the spinal gun is obsolete for obvious reasons.

A lot of factors that made them a stupid idea for ships do not apply for space. You have long ranges and (with lasers) virtual zero flight time, and no environmentally induced inacurracy, making hitting a sole problem of precision calculation and positioning. Spinal weapons are easier to install and maintain. They do make sense here.
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Re: Design a hard sci-fi-warship (RAR).

Postby Sky Captain » 2017-02-14 10:32am

If particle beam veapons are viable they would benefit greatly from spinal mounts, longer the accelerator, longer the effective range. Longest possible accelerator is one that runs the length of entire ship. In space detection ranges are much greater than engagement ranges making it highly unlikely to have a situation where ship with spinal mount cant turn in time to face the enemy.

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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-14 11:15am

Simon_Jester wrote:
Patroklos wrote:Again, this is a balancing issue based on the tactical environment. Sure you alpha are is limited, but you will never find yoruself in a negative arc either. Alpha arcs are great if weapons are devastating enough that there is little opportunity for reengagement due to one side or the other dieing quickly in a mutual charge (or a one sided charge if you catch someone out of position)


If the offensive to defensive balance is such that its more or a slugging match you may find yourself needing to maneuver and depending on your maneuvering characteristics that means maybe presenting more aspects of your ship than a carefully selected alpha arc. Now granted, if you are alpha arc encompasses more degrees in the first place you can maneuver longer while still maintaining it, but what happens when you can't?



In the context of space warfare there is a huge gap of time between the point at which the enemy comes into range and the point at which you can no longer keep them in a chosen wide arc of fire.



While that’s possible, perhaps even likely, there are a lot of reasons why the effective ranges will not be particularly large or that you won’t remain at the limit of effective range for long. Lasers have spotting issues for instance. And since it seems we often describe situations of ships hurtling at each other at great speed the range is closing fast.


The problem I'm trying to point out is that if your alpha arc constitutes an extremely tiny volume of space, your options for using it are so limited that it becomes nearly useless. You are effectively always limited to firing with whatever weapons happen to bear, because you have to be quite fortunate to get the enemy in your alpha arc.



You are exaggerating the smallness of the sphere’s alpha arc. It is indeed smaller but hardly “useless”


If we are talking extreme ranges as you suggest, there is no real way to avoid an opponent’s alpha arc if they have one. In that situation we are talking having to sidestep thousands of kilometers to move through one degree or your enemies arc (using your puny thrusters vice main drive no less) so the point is moot. If we are talking head onish approaches, if you have a forward alpha arc at all that’s all that matters. The advantage of the larger arc will only be apparent at close ranges, where the same maneuver energy allows you to transition through multiple degrees of arc relatively quickly. But you can compensate with your own maneuvering.


That is an especially serious handicap in cases where (as you describe) pounding matches or extended exchanges of fire are required, which are precisely the conditions you expected to favor the sphere. Because under those conditions, not having an alpha arc means that you need to mount two weapons for every one you expect to be able to fire at the enemy, and so on. It's wasteful, and a more efficiently laid out ship* of similar mass can have major advantages.



The sphere has an alpha arc. Two actually.


Your logic only follows if as the two pass each other they do not maneuver at all, either on their thrust axis or otherwise, or that each has the exact same maneuvering capabilities. In this case the ship with the larger alpha arc will indeed of course maintain it longer. I will point out though that eventually the cylinder will enter the spheres aft alpha arc and the cylinder is up a creek. Ships do maneuver, however, and different shaped ships maneuver differently.



*(Say, one with main weapons in turrets on the dorsal and ventral surfaces, permitting full broadside fire through a broad, roughly flat region surrounding the ship in most if not all directions)



I have been assuming this for the cylindrical ship. It should be noted spheres can have broadside optimized arcs too. Your setup can never have alpha arcs in all directions. Either you are superfiring forward or super firing aft, you can’t have both. And your ventral guns can never shoot “up”, nore your dorsal “down.”


I am using a cylinder btw because it’s going to make volume and mass math easier when the time comes.


I am not saying the cylinder does not have better possible alpha arcs. I am not saying large alpha arcs are not an advantage. I am simply pointing out that the sphere can also achieve alpha arcs and that beyond this there are other ways to compensate. It’s not a situation where whomever has the larger alpha arc wins automatically.


his also means that your only option for 'sidestepping' enemy fire is whatever maneuvering thrusters you have mounted on the equator of the ship, which is a bad place to mount engines because it competes with things like weapons and sensors.And sidestepping is extremely useful for things like not getting pasted by ballistic railgun rounds.



This is incorrect. If I am using ONE thruster you are right it has to be on the equator to sidestep. There is no reason to do so. No matter the shape a warship will be sprinkled liberally with maneuvering thrusters both for redundancy but also to achieve whatever delicate force balance is needed to maneuver as internal mass shifts (missiles are ejected, droplet radiator mass is lost while maneuvering, magazines cycle ordinance, propellant tanks empty and reballast, etc.). A smart move would to have some thrusters be gimballed to make them more useful.


If you have the luxury or using one thruster, hopefully a large one, thrusting through your center of mass that’s great. I imagine most will not rely on such a mechanism but will instead use multiple thrusters along the thrust axis for side step maneuvers.


Each ship can sidestep the same. The true disadvantage of a sphere in this context is that the sphere will always show the same cross section, while a cylinder can show an optimal one.


Uh... actually no. A long slender cylinder (assuming it's got a rigid structure) can have its maneuvering thrusters at the ends of the cylinder, which means a lot more torque. This tends to cancel things out, or (in my opinion that I haven't yet done math on admittedly) more than cancel things out.



So I guess we need to hash this out before continuing. I am using this as my starting reference for moment of inertia issues. http://www.projectrho.com/public_html/r ... efense.php





Let’s assume our two ships are a perfect sphere and a perfect cylinder. To avoid redoing half the math from Atomic Rockets we will use the characteristics of the cylinder used there which was 100 meters long, masses 1000 tons (1,000,000kg), and that mass is evenly distributed throughout the shape for a center of gravity 50 meters along its length. Let’s put the thrusters on the ends like you suggested and have one push the tail and the other the nose in opposite directions. This is all straight out of the source, I am just reproducing it here for convenience.


The formula for Moment of Inertia (I) of a rod rotating about its center = mL^2/12.


So here I = (1,000,000)(10,000)/12 or I = 833 million.


Assuming an angular acceleration of 1 radian/sec^2, that is after firing the thrusters for one second continuously this will give us an angular velocity of one rad/ecs (it will take 3.14 to make rotate 180 degrees), the torque needed to finish the maneuver is 833 million meter-Newtons. You are using two thrusters so each will need to apply 416 million meter-Newtons of torque. They are 50 meters from the center so the force each needs to apply is 416/50 or 8.33 million Newtons.


So that means with a mass of 1,000,000kg acceleration will be F/m or 8.3 m/s. This means each thruster will have to produce enough thrust to achieve .83g. That’s a ridiculously powerful thruster; we can probably expect realistic performance to be a small fraction of that and thus similar rotations to take many times 3.14 seconds.


So here is where my math begins, so check everything thrice. To create the equivalent sphere I am using 1 ton per m^3 for the density of both ships (easy math). This gives me a spherical ship with the dimensions:


r = 6.2035 m
V = 1000 m3
A = 483.597 m2
C = 38.9777 m


And for consistency the cylinder


r = 1.78412 m
h = 100 m
V = 1000 m3
lateral A = 1121 m2
top/bottom A = 9.99995 m2
A = 1141 m2


Here are the calculations for a sphere with the same mass and volume and uniform distribution of mass. To create the equivalent sphere I am using 1 ton per m^3 for the density of both ships:


I = (2mr^2)/(5) = ((2)(1,000,000)(6.2035^2))/(5) = 15.39 million


Thrusters x2 = 7.7 million meter-Newtons per thruster


Thruster placement at opposite ends of thrust axis means 7.7/6.2035 = 1.24 million Newtons


A= F/m = .77m/s


.77/9.81 = .078g


Now I just did math in public, which means I am standing by to be ruthlessly mauled for my mistakes. But IF the above was done correctly that means that a spherical can perform the same maneuver that the cylinder can for a fraction for the effort. There is a big difference between .83g and .08g.


The fact is that it is HARD to turn a spaceship. It’s much harder if you are long and skinny. This has a lot of implications to likely tactical developments. For instance, if you really want to be able to toss your nose around on a dime do you mount super powerful maneuvering thrusters? Is that worth it? If you don’t then you those fancy alpha arcs become less desirable over more all-around coverage because if you don’t kill your opponent in your first carefully choreographed optimal approach once he is in your baffles he might have weapons he can shoot up your ass, you’re probably minimally armored ass.


Thrusters to roll, yaw, or pitch a spherical ship are going to have to be mounted physically on the surface, too, firing roughly parallel to the surface. Which means that their "guts" cannot be buried deep within the hull. That is another factor taking up surface area, plus it means there are swathes of the outer hull that have rocket exhaust playing across them.



I am not following your logic. You do want the thrusters placed as far apart as possible yes, but in both cases, cylinder or sphere, you are going to want them under the armor. On the sphere you would want the thrusters burning tangentially to the circle you are trying to pitch around, and the hull will actually already curve away from the thruster. The same principle applies to the roll thrusters on a cylinder.


It’s actually the cylinder that has the problem you describe for pitch thrusters. Unless your ship has a pointy nose cone, any bluntness in features means there would be significant bulk between the thruster and space if you located it on the thrust axis. Unlike a sphere, the hull of the cylinder doesn’t naturally curve away.


]Yeah, but if your weapons are mounted equatorially, that means the enemy was already out of your alpha arc. They remain so, as noted. The point being that there is basically no realistic way, with a spherical ship, to maintain fire by all the ship's weapons, while dodging enemy fire using the main engines. Either you have to sacrifice the ability to bring all your weapons to bear, or you have to fly along a predictable trajectory where you have little or no freedom to be anywhere other than where the enemy expects you to be (and has presumably already fired a nuke at).



No matter your alpha arc, ships are going to get outside of it. That would be one of the number one imperatives of your enemy, to get outside of it while keeping you in his. If we are talking about multiple ship engagements and primarily forward firing arcs, it’s basically impossible to avoid this short of refusing battle.


As my maneuvering math above showed (assuming it holds up), the sphere is FAR more capable of keepings its alpha arcs on target. That mitigates somewhat is smaller cone shaped alpha arc. I get it, the bigger the alpha arc the better. But there are other arcs besides the alpha arc. The combination of better maneuverability and being able to stay in the fight even if it finds itself sub optimally oriented are significant advantages.


A ship with broadside turret armament can do that. Hell, World War II battleships do that, exactly that way! There's a reason people didn't just build big circular warships with guns bristling all along the equator. And hydrodynamics was far from the only reason.



The real reasons they didn’t build circular battleships is because they move through a dense fluid medium that requires them to be shaped the way they are, long and skinny. Those broadside arrangements are only optimal in light of the restrictions on the placement dictated by the shape of the ship for seagoing purposes. This has been the case for all of modern naval warfare, all of it is bent around that basic requirement.


[Incidentally this is also a problem with spinal armament, which is also something people never did with large warships in the Industrial Age for a good reason. It only really works if you expect to always be able to force the enemy to shoot at you from a very specific direction, and have such good lateral maneuverability that you can duck and weave in 3D rather than needing to turn to point your main engines]



I think the usual intent of spinal weaponry is to have a weapon with an effective range that makes you invulnerable and/or powerful enough to decide the engagement before the enemy can get squirrely.


Obviously physical weapon ranges are not so much a thing in space. But if you giant weapon also comes with giant spinal gyroscopes effective range might be.


Also remember that if you are planning to engage at extreme range alpha arcs are again not much or an issue. If something is even vaguely in front of your weapon it can be laid with the tiniest of maneuver. It’s the accuracy of your of maneuvers (and stabilizers and fire controls) that is the issue then, as that tiniest of maneuver again translates to millions of kilometers of lateral transition down range.


Frankly, yes I am assuming you're making main engine burns during combat.


There are extremely good reasons to do that, of the "not getting shot" variety. Again, think about wet-navy warships. They don't stop moving in order to shoot, and it would be extremely dangerous for them to do so in a threat environment dominated by ballistic weapons. The main reason naval guns had such terrible hit rates in the World War era was precisely because the time of flight was a minute or two, and ships could change its position, even its predicted future position, by large fractions (or even multiples) of their own length in that amount of time.



Ah, but the comparison falls flat because on a BB its main engine is its maneuvering engine. Well, more accurately the main engine provides all the maneuvering forces by having the rudder bleed energy from the hull through water resistance. On a space ship you don’t generally maneuver with your main engine, and your maneuver engines, unless they are in a direction that would provide a breaking force, won’t impact your current velocity.


Similar to the difference between an aircraft and a battleship, the problem with aircraft is that instead of coasting to a stop and bobbing harmlessly in the ocean when they run out of gas they fall out of the sky and crash violently. The difference between an aircraft and a spaceship is the spaceship DOESN’t stop when it runs out of propellant, it carries on smartly in the direction of its last burn for all of eternity. The crew just carry on in that direction until they starve or blow their brains out. If that vector was being traveled by a state of the art combat cruiser at combat speed who is going to catch you?


The husbanding of propellant is going to be a BIG deal in space combat. Especially for attackers who had to presumably burn much of theirs getting to the target even before battle starts. Even then you have to account for things like tanks being shot up. Are you going to have to vent some for heat radiation? Propellant conservation is a good reason to not go burning around everywhere all the time. I imagine a lot of pre combat decisions will revolve around making sure your closing vectors coincide with a return vector back to base. Of course your enemy will know this…


Some other reasons not to burn.


1.Do you actually want to close faster? If you are already registering good hits and your enemy not so much, you can't exactly point your exhaust a him and try to decelerate (you will bet shot up the ass) but you don’t need to help your enemy by increasing your closing speed either.


2.How much heat are you going to have to radiate or sump by burning? Is it more important to get a few more m/s or is it time to lay into them with your GW lasers with their heat disgorging inefficiencies?


3.Is there an enemy with a bead on your ass presently? Time to cut the engine and close the armored doors…


4.Do you want to change your rotation to the enemy but NOT change your current movement vector? Cot the engine, spin on an axis.


5.Do your engine burns destabilize your firing solutions, especially at long ranges where every vibration matters?




Remove that advantage, and the enemy's odds of hitting you go up by an order of magnitude. Or more (and I can do the math to prove this). Even though your spacecraft is still coasting ballistically, it's still a "sitting duck."



Again, this depends on the nature of the combat. If these are face to face charges at each other where you have to keep behind your armor shadow all burning does is bring you closer faster, making you bigger in the enemies scopes. In this case evasive maneuvers will be handled by thrusters.


If you are maintaining one of those precious alpha arcs you might not be able to face your main drive in a useful direction, in which all it could do would make you go a bit faster in a predictable vector. Sure, it technically opens up the possible area you could maneuver into, but FUCK THEY JUST SHOT AWAY #2 HYDROGEN TANK. So much for decelerating. I hear Alpha Centari is nice this time of 145 years from now.


I am going to throw another issue out there now which I tend to assume for space warships in general. Are there effective ways to armor or otherwise defend your exhaust from the gambit of space based weapons. I haven’t done the math on it, but It would make sense to me that a sufficiently powerful drive would be able to counteract kinetic war shots or even lasers. Thoughts?

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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-14 04:49pm

I think we need to address one specific, important point before we move on.

Patroklos wrote:
Frankly, yes I am assuming you're making main engine burns during combat.

There are extremely good reasons to do that, of the "not getting shot" variety. Again, think about wet-navy warships. They don't stop moving in order to shoot, and it would be extremely dangerous for them to do so in a threat environment dominated by ballistic weapons. The main reason naval guns had such terrible hit rates in the World War era was precisely because the time of flight was a minute or two, and ships could change its position, even its predicted future position, by large fractions (or even multiples) of their own length in that amount of time.
Ah, but the comparison falls flat because on a BB its main engine is its maneuvering engine. Well, more accurately the main engine provides all the maneuvering forces by having the rudder bleed energy from the hull through water resistance. On a space ship you don’t generally maneuver with your main engine, and your maneuver engines, unless they are in a direction that would provide a breaking force, won’t impact your current velocity.
It doesn't have to.

The advantage ships and planes have, of operating in a fluid medium where they can change heading without speeding up or slowing down much... Is offset by the advantage spacecraft have, of operating in a vacuum where you can tack on extra m/s of velocity in any direction you like with vector addition. Instead of having to keep the ship's bow and the ship's heading pointing in the same direction.

If my main engine can accelerate the ship at, say, 0.5 gravities, then all I have to do is point the ship in any desired direction and fire the engine. Suddenly, I am accelerating at (roughly) five meters per second squared. Regardless of where the enemy thought I was going to be before I started the engine burn...

Ten seconds into the engine burn, I have moved 250 meters away from my own predicted position. Twenty seconds in, I'm a kilometer away from where they thought I was going to be. Anything they aimed at me on the expectation that I would be over there is now going to miss, because I am over here instead. If we are, say, firing at each other with ballistic railgun shells that travel at 30 km/s from a range of six hundred kilometers, this makes it extremely unlikely that they will succeed in hitting me- or vice versa, if they can do the same thing I'm doing.

Conversely, if I'm making a main engine burn at 0.5 gravities and they 'aim ahead' accordingly... all I have to do is cut my engines, and within twenty seconds, I will be a kilometer behind where they expected me to be. Again, they miss.

Lower accelerations means it takes longer to generate such a large miss distance, of course. But essentially, any spacecraft including a Gemini capsule can dodge an unguided weapon reasonably well, if it has time and fuel to make a random engine burn between the moment the shot is fired and the moment the shot arrives. You don't have to know the shot is coming, any more than a WWII destroyer has to see the incoming shell to dodge it by zig-zagging.

Basically, if you are firing with unguided weapons, then once you get far enough away that the enemy has any realistic chance of sidestepping your shots, your hit probability drops off with the fourth power of the range. Because you're firing into a 2D "the target might be anywhere in this region" envelope, and the width and height of that envelope are both proportionate to the square of the amount of time you give the enemy to dodge.

With guided weapons this incentive is greatly diminished. But with guided weapons all discussion of 'alpha arc' becomes considerably less relevant, because there's no such thing as the 'alpha arc' of a box of VLS cells.

Similar to the difference between an aircraft and a battleship, the problem with aircraft is that instead of coasting to a stop and bobbing harmlessly in the ocean when they run out of gas they fall out of the sky and crash violently. The difference between an aircraft and a spaceship is the spaceship DOESN’t stop when it runs out of propellant, it carries on smartly in the direction of its last burn for all of eternity. The crew just carry on in that direction until they starve or blow their brains out. If that vector was being traveled by a state of the art combat cruiser at combat speed who is going to catch you?
Obviously it would be suicidal for a ship to expend all its delta-V on combat dodging and leave none to return. However, this is comparable to the situation with real life combat aircraft. A functional multirole combat aircraft has a "combat radius" defined by its ability to fly to the target, expend some fuel loitering or maneuvering around the target, and then come home.

A functional combat spaceship would have a delta-V budget to go to the battle, a delta-V budget for coming home, and a delta-V budget to expend on combat maneuvers. If there isn't enough surplus delta-V for combat maneuvers, the ship will not be able to fight effectively in any case,in which case you wasted mass on extra weapons and armor that should have been spent on extra fuel to increase the ship's mass fraction.

The husbanding of propellant is going to be a BIG deal in space combat. Especially for attackers who had to presumably burn much of theirs getting to the target even before battle starts. Even then you have to account for things like tanks being shot up. Are you going to have to vent some for heat radiation? Propellant conservation is a good reason to not go burning around everywhere all the time. I imagine a lot of pre combat decisions will revolve around making sure your closing vectors coincide with a return vector back to base. Of course your enemy will know this…
As noted, if your ship doesn't have a significant delta-V margin purely for combat maneuvers (which includes evasive action and delta-V lost to battle damage), then your ship is unfit for combat and will almost certainly lose to a more lightly built "combat ship" that does have such a reserve for maneuvers.

Some other reasons not to burn.

1.Do you actually want to close faster? If you are already registering good hits and your enemy not so much, you can't exactly point your exhaust a him and try to decelerate (you will bet shot up the ass) but you don’t need to help your enemy by increasing your closing speed either.
This is why the designs I'm advocating can burn their engines in directions besides "directly towards the enemy." Because sometimes, you want to be able to say "let's get 15 m/s of new velocity in the thefuckouttatheway direction" and not "prepare for RAMMING SPEED!"

2.How much heat are you going to have to radiate or sump by burning? Is it more important to get a few more m/s or is it time to lay into them with your GW lasers with their heat disgorging inefficiencies?
That's a legitimate calculation, I will admit.

3.Is there an enemy with a bead on your ass presently? Time to cut the engine and close the armored doors…
If "bead on your ass" just means "they're aiming at us successfully," that is exactly when you want to be dodging. Because a 10 m/s engine burn is going to hurt a lot less than taking a railgun to the face.

4.Do you want to change your rotation to the enemy but NOT change your current movement vector? Cot the engine, spin on an axis.
If your current movement vector is measured in kilometers per second, you have a lot of leeway to change your vector by tens of meters per second to dodge enemy fire. Especially since you can random-walk and avoid building up much net delta-V in any one direction.

5.Do your engine burns destabilize your firing solutions, especially at long ranges where every vibration matters?
THIS is a good point, to be fair- it can force you to choose between firing or maneuvering. On the other hand, this creates much stronger incentives to use weapons that are immune or highly resistant to the problem, because being able to get out of the way of the incoming fire is so goshdarn useful.

Remove that advantage, and the enemy's odds of hitting you go up by an order of magnitude. Or more (and I can do the math to prove this). Even though your spacecraft is still coasting ballistically, it's still a "sitting duck."
Again, this depends on the nature of the combat. If these are face to face charges at each other where you have to keep behind your armor shadow all burning does is bring you closer faster, making you bigger in the enemies scopes. In this case evasive maneuvers will be handled by thrusters.
Although in that case, you have every incentive to thicken your armor by narrowing your cross-section and packing as much ship as possible into said armor shadow.

If you are maintaining one of those precious alpha arcs you might not be able to face your main drive in a useful direction, in which all it could do would make you go a bit faster in a predictable vector. Sure, it technically opens up the possible area you could maneuver into, but FUCK THEY JUST SHOT AWAY #2 HYDROGEN TANK. So much for decelerating. I hear Alpha Centari is nice this time of 145 years from now.
That's the argument for having a large alpha arc and more flexibility to maneuver. In particular, to avoid getting hit in the first place.

Number Two hydrogen tank wouldn't have gotten shot to begin with, if the enemy's weapons fire was all going over there in that random patch of cosmic void, instead of over here in our living room.

Basically, you can't readily armor starships against plausible enemy weaponry without some degree of soft-SF technology. If armor is not a good defense, you really need to be able to dodge.

I am going to throw another issue out there now which I tend to assume for space warships in general. Are there effective ways to armor or otherwise defend your exhaust from the gambit of space based weapons. I haven’t done the math on it, but It would make sense to me that a sufficiently powerful drive would be able to counteract kinetic war shots or even lasers. Thoughts?
[/quote]A cloud of rocket exhaust would probably be a lot more effective at stopping lasers than kinetic weaponry, really- but honestly, high efficiency rocket drives would tend to have low exhaust mass and high exhaust velocity, both of which stop you from putting much 'cloud' between you and the lasers. Plus, your exhaust plume may not even be in a place you can use to block incoming fire, no matter what direction you point your ship's butt end towards, depending on the vectors involved and how fast you're moving relative to your opponent.
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Re: Design a hard sci-fi-warship (RAR).

Postby Sea Skimmer » 2017-02-14 07:39pm

LaCroix wrote:Especially since in space, you can safely use one nuclear missile to defend against a sizeable number of missiles. Thinking of it, that is a hard counter to missile swarms, so we might see warships with primarily energy weapons, and a secondary battery of nuclear missiles that exists primarily to counter missile and fighter swarms.

These missiles will be a lot smaller than offensive ones, for you only need to travel a few km out to intercept, so they make the mass penalty even worse.


This assessment is based I believe on the idea that a threat missile swarm will compose and behave something like a group of Harpoon missiles would, or similar. But realistically the cost and technology threshold for space warfare would be so high it'd be far different by the time we ever got their, a group of missiles is going to be much like a self escorting strike package of aircraft would be, just on on a one way flight. That's where that whole swarming drone warfare idea goes anyway.

The actual assured kill radius for nuke bursts in space against lightly shielded missiles could be kilometers across sure, its all about how much yield at that point, but conversely if a portion of the incoming missile swarm is itself armed with anti missiles it could destroy the nuclear interceptor at a distance of say, 50km without too much trouble either, and that's a very long way to kill things with nuclear radiation. The threat swarm could also use outlier missiles or a leading wave to fire nuclear casba howitzes or high power chemical lasers for overall defense suppression, targeting not just the threat ship but any known pickets it might already have deployed, and interceptors coming out to say hi. In fact none of the threat missiles might have a real anti ship warhead, as they could probably inflict lethal effects just by colliding at all, and put all warhead mass into further active offensive capabilities such as this. The enemy attacking ship might also be able to employ supporting blinding laser weapons at distances far beyond either sides ability to directly damage each other.

These missiles will be a lot smaller than offensive ones, for you only need to travel a few km out to intercept, so they make the mass penalty even worse.


This is true, a decently engineered space warship should have an incredibly high kill probability within a few kilometers of its own hull. Thing is any kind of enemy missile is already a threat at that point. It's space, no air to stop debris. So even if a conventional warhead shaped charge detonates 3km out, it could still easily hit you at full velocity. You'd have to target a swarm of hit to kill interceptors into charge mass, which would suddenly have accelerated to an additional 10km a second, covering that 3km is under a third of a second... if the threat missile was already going 10km/s relative speed this is now a 20km/s relative impact. It will only get worse from thence upward.
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Re: Design a hard sci-fi-warship (RAR).

Postby Shroom Man 777 » 2017-02-15 01:36pm

I wonder if it's possible - hard or not-hard sci-fi - to set up a SPACE KILL-WALL by having constantly flowing and bouncing-back projectiles, fragments that are passed around by two magnety emitters/reflectors on either ends. Or do this process with death lasers and super-mirrors. The emitters/reflectors would also move around in sync with each other. So there are like these moving grids that will chop up careless travelers. Of course, rudimentary sensors can detect these hazards and ships can maneuver, but that would mean they're gonna detour through pre-set "safe" paths that can then have delay-action, proximity-activated or trigger-activated laser-mines, bomb-pumped-lasers or encapsulated-torpedo-launchers.
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-18 10:09am

Simon_Jester wrote:It doesn't have to.

The advantage ships and planes have, of operating in a fluid medium where they can change heading without speeding up or slowing down much... Is offset by the advantage spacecraft have, of operating in a vacuum where you can tack on extra m/s of velocity in any direction you like with vector addition. Instead of having to keep the ship's bow and the ship's heading pointing in the same direction.


That’s true, with the caveat that unless you are tacking that m/s in the direction of your thrust axis it’s not going to be tacked on by the main engine. And it’s not any direction, in order to add m/s to any direction abaft of beam you are going to pay either some or all of your maneuvering force to breaking depending on how close to reversing direction you get. A plane can of course just reduce or shut off its engine and let the air resistance to that work for it, or keep burning and let banking forces turn the nose until its main engine is lined up for the desired maneuver.

The point is that in atmosphere your main and maneuvering engine is always the same, where in space its only the same for a very select set of circumstances. Basically you have to have maneuvering engines, and they have to do all the work themselves (no banking).

If my main engine can accelerate the ship at, say, 0.5 gravities, then all I have to do is point the ship in any desired direction and fire the engine. Suddenly, I am accelerating at (roughly) five meters per second squared. Regardless of where the enemy thought I was going to be before I started the engine burn...


Again, this holds true only for directions not abaft of beam if you were already under power, at which point you are technically accelerating as you say but part or all of that force is counteracting your original velocity along the old thrust vector.

And remember as I proved above pointing your main drive in a different direction is no easy task, especially if you are long and skinny. Suddenly is not how I would describe it, more like predictably.

Ten seconds into the engine burn, I have moved 250 meters away from my own predicted position. Twenty seconds in, I'm a kilometer away from where they thought I was going to be. Anything they aimed at me on the expectation that I would be over there is now going to miss, because I am over here instead. If we are, say, firing at each other with ballistic railgun shells that travel at 30 km/s from a range of six hundred kilometers, this makes it extremely unlikely that they will succeed in hitting me- or vice versa, if they can do the same thing I'm doing.

Conversely, if I'm making a main engine burn at 0.5 gravities and they 'aim ahead' accordingly... all I have to do is cut my engines, and within twenty seconds, I will be a kilometer behind where they expected me to be. Again, they miss.

Lower accelerations means it takes longer to generate such a large miss distance, of course. But essentially, any spacecraft including a Gemini capsule can dodge an unguided weapon reasonably well, if it has time and fuel to make a random engine burn between the moment the shot is fired and the moment the shot arrives. You don't have to know the shot is coming, any more than a WWII destroyer has to see the incoming shell to dodge it by zig-zagging.


Oh, you are absolutely correct, evasive maneuvers are certainly a thing (that spheres will always do better).

Whether it’s your main engine or a maneuvering thruster is irrelevant, they can each move you in a range or directions. The reason I mentioned not necessarily constantly burning your main engine is because it essentially only allows you to move up and down a straight line projecting forward of your current position the enemy already knows about, while they have no idea what direction your maneuvering thrusters will burn (especially if they are gimbled).

If we are talking about less about just a few seconds for enemy fire to reach you, and you are suffering from a particularly bad moment of intertia, you are probably only going to be able to pitch your nose a degree or two. This will of course provide some sort of cone for possible maneuver using that .5g, it will be long and skinny. So skinny that if we are using that 30km/s projectile it will pass through it so fast the maneuver profile is essentially 2D as far as the enemy is concerned assuming he is shooting at you in profile. If he shooting at your forward or aft face it makes not difference at all.

Your maneuvering thrusters, on the other hand, even though they can’t accelerate you in whatever direction as fast, can do so in any direction instantly, uniformly in any direction forward of the beam, and less so abaft of the beam but still somewhat. In fact, since they can offer a breaking force they greatly increase the range or possible positions up and down your original thrust vector. Turning off your main drive just varied how much m/s you could add, breaking increases that range to add and subtract.

But like I was saying if your pitch performance isn’t that great the cone from continued main engine burning is not going to add much, so you might start to think about if what you gain from not burning is worth more.

Basically, if you are firing with unguided weapons, then once you get far enough away that the enemy has any realistic chance of sidestepping your shots, your hit probability drops off with the fourth power of the range. Because you're firing into a 2D "the target might be anywhere in this region" envelope, and the width and height of that envelope are both proportionate to the square of the amount of time you give the enemy to dodge.


It’s only a 2D target if you are talking lasers or very fast kinetic rounds that can travel along a line through the volume faster than a crossection of the target could clear it that line if it was anywhere on it when the round entered it. Maneuvering will provide for a possible volume of locations, not just area.

But assuming your KE roun’ts aren’t that fast, this is why I don’t believe in all the hype about railgun/coilgun slugs. If you have a target that has any decent maneuvering capability at range you will need to fire off spreads of hundreds (conservatively) or slugs near simultaneously to register hits. Maybe if you are coordinating fleet fires on selected ships in the enemy formation this is possible, but I doubt a single ship at the tech levels we are talking about will have the number of mounts or mounts with the rapid fire capability to do it.

For this reason I would imagine for ranges where a rounds travel time is measured in multiple seconds rail guns will still shoot nuclear shells. If they don’t vaporize after hitting an enemy hull when dead reconing told them they should they auto detonate to see if they were at least close enough to get some radiation splashed on the target. We then get into the cost benefit analysis of how much faster you could shoot the shells if you didn’t have the warhead mass there, but that’s for the ship designers to do the math on.

With guided weapons this incentive is greatly diminished. But with guided weapons all discussion of 'alpha arc' becomes considerably less relevant, because there's no such thing as the 'alpha arc' of a box of VLS cells.


Sort of, it depends on how you launch the missiles. I am of the opinion that VLS won’t be the desired method because all of the deltaV has to come from the missile itself. I would suggest using launchers that are just giant railguns (tailored to be gentle on the more delicate missile than your bog standard slug thrower is) to get as much m/s as you can before using onboard stores. This means smaller missles (harder to detect, harder to hit) or missiles of the same size with more maneuvering/burn options.

The consequence of this is that your missiles have an alpha arc based on the launcher.

Another advantage of this method is there is no initial burn for the missile, which means the enemy may not know you shot a missile at them until it makes a burn at a later date for its terminal dash or other maneuver which could be half as close as otherwise. I can shoot missiles way out in front of a targets path to attack them from another aspect. Sure the enemy may never actually show up there, but maybe they do.

Obviously it would be suicidal for a ship to expend all its delta-V on combat dodging and leave none to return. However, this is comparable to the situation with real life combat aircraft. A functional multirole combat aircraft has a "combat radius" defined by its ability to fly to the target, expend some fuel loitering or maneuvering around the target, and then come home.


True, but those are estimates. Plenty of combat aircraft have found themselves in situations where they have to bail out because combat took to long despite all the best pre planning calculations. But when you bail out you either get stopped by air resistance and parachute to the ground which stops you, or the ground just stops you. Either way SOMETHING stops you whether you survive or not. Nothing stops you in space.

A functional combat spaceship would have a delta-V budget to go to the battle, a delta-V budget for coming home, and a delta-V budget to expend on combat maneuvers. If there isn't enough surplus delta-V for combat maneuvers, the ship will not be able to fight effectively in any case,in which case you wasted mass on extra weapons and armor that should have been spent on extra fuel to increase the ship's mass fraction.


True, but combat is unpredictable. The point is that there are situations, some predictable but many not, which may force a captain to think about not making that burn.

As noted, if your ship doesn't have a significant delta-V margin purely for combat maneuvers (which includes evasive action and delta-V lost to battle damage), then your ship is unfit for combat and will almost certainly lose to a more lightly built "combat ship" that does have such a reserve for maneuvers.


The fist German jet planes measured their combat availability times in minutes. Even jets today have a loiter time, let alone combat time, sometimes less than an hour after reasonable transit times. We still fight wars with them anyway.

This is why the designs I'm advocating can burn their engines in directions besides "directly towards the enemy." Because sometimes, you want to be able to say "let's get 15 m/s of new velocity in the thefuckouttatheway direction" and not "prepare for RAMMING SPEED!"


But as I proved above, that’s probably not going to be possible in lots of situations. You just can’t spin on your long axis that fast, or expose the ship in ways that will open up a useful main burn vector (if you don’t have 100% protection coverage).


If "bead on your ass" just means "they're aiming at us successfully," that is exactly when you want to be dodging. Because a 10 m/s engine burn is going to hurt a lot less than taking a railgun to the face.


I meant literally “is shooting up your pooper/ AKA engine bell”. I am doubtful of the ability to defend an actually burning main engine if the exhaust itself won’t do it. If it can’t, then you might be able to do it when it’s not burning. At least if you are are facing this situation it’s a good bet you were accelerating away from your enemy at the time and might open the distance outside his weapons effective range to start burning again before he can overtake your speed.

If your current movement vector is measured in kilometers per second, you have a lot of leeway to change your vector by tens of meters per second to dodge enemy fire. Especially since you can random-walk and avoid building up much net delta-V in any one direction.


Yeah, but you will do that with your maneuvering engine not your main engine. The point is if you can’t devend your main engine, or your armor protection only covers a certin arc, there are directions you can’t point your ship without exposing a critical weakness. This means your main engine (assuming its fixed, granted) simply can’t be used for certain maneuvers.

Even if it can, you don’t want to be burning it while you transition to the new burn position because it will be accelerating you in an arc of directions along the way. You may want that, and if so cool but you probably don’t.

THIS is a good point, to be fair- it can force you to choose between firing or maneuvering. On the other hand, this creates much stronger incentives to use weapons that are immune or highly resistant to the problem, because being able to get out of the way of the incoming fire is so goshdarn useful.


It also depends on if you can tank rounds or not. If you can, you may calculate your enemy need get three good shots on you, but you only need one more on him. So in that case you might make the trade knowing you will probably get hit, but you will still carry the day.

If we are talking glass cannons duking it out for sudden death elimination then yean, the tradeoff goes squarely to the maneuvering side. But even there since getting the first shot in as accurately as possible is so important stability may be more important than maneuverability.

Although in that case, you have every incentive to thicken your armor by narrowing your cross-section and packing as much ship as possible into said armor shadow.


Yes. Though there might be an incentive to increase your exposed armor area to allow for more weapons to see the target. I imagine the optimization math will keep engineers up at night.

That's the argument for having a large alpha arc and more flexibility to maneuver. In particular, to avoid getting hit in the first place.


The result of this exchange thus far is that increasing usable alpha arcs means being less maneuverable and visa versa just based on ship shape

I would point out that the logical result of expanding apha arcs is expanding the ships armor protection if armor is a thing which is also a determent to maneuvering due to mass penalties. There is little point I having portions of alpha arcs that you can’t use because you will be one shotted (this is a good argument against the utility of a sphere’s aft facing alpha arc as it can only be used by exposing the exhaust). Granted it might let you get one more shot off (maybe the pivotal one), but its use would be limited.

Number Two hydrogen tank wouldn't have gotten shot to begin with, if the enemy's weapons fire was all going over there in that random patch of cosmic void, instead of over here in our living room.


True, but neither did we design battleships with the assumption we could just design them to not get hit. Quite the opposite actually.

Basically, you can't readily armor starships against plausible enemy weaponry without some degree of soft-SF technology. If armor is not a good defense, you really need to be able to dodge.


Absolutely, the circumstances will dictate quite a bit. The defense to offense ratio will matter.

A cloud of rocket exhaust would probably be a lot more effective at stopping lasers than kinetic weaponry, really- but honestly, high efficiency rocket drives would tend to have low exhaust mass and high exhaust velocity, both of which stop you from putting much 'cloud' between you and the lasers. Plus, your exhaust plume may not even be in a place you can use to block incoming fire, no matter what direction you point your ship's butt end towards, depending on the vectors involved and how fast you're moving relative to your opponent.


This is my gut feeling as well unless we are talking torch ships. I have always assumed that the main engine, especially a burning one, is a great big weak spot.

Shroom Man 777 wrote:I wonder if it's possible - hard or not-hard sci-fi - to set up a SPACE KILL-WALL by having constantly flowing and bouncing-back projectiles, fragments that are passed around by two magnety emitters/reflectors on either ends. Or do this process with death lasers and super-mirrors. The emitters/reflectors would also move around in sync with each other. So there are like these moving grids that will chop up careless travelers. Of course, rudimentary sensors can detect these hazards and ships can maneuver, but that would mean they're gonna detour through pre-set "safe" paths that can then have delay-action, proximity-activated or trigger-activated laser-mines, bomb-pumped-lasers or encapsulated-torpedo-launchers.


I always figured this was a more plausible way to deal with those scifi prison doors and launch bay defenses that the go to force fields. The problem is that its actually pretty hard to reflect a laser with a mirror once its been focused into a beam of death. You can defocus it again with a mirror if you have the right one, but trying to bounce it it intact generally just gets you a burnt up mirror (which is why you can’t just strap mirrors on things to act as laser armor).

The other issue is that if where does the laser end up? If you are trying to confine it to a space you can’t just reflect it around infinetly. If you do have mirrors that can deflect a destructive mirror to bounce it around a volume unless they are 100% efficient eventually the mirrors will absorb all the lasers energy. Depending on the folume of space and mirros that might not mean much but if it is a small space like a prison door or launch bay door you are essentially just shooting the frame with a destructive laser with predictable results, PLUS your generater still has all the waste heat from its generation in the first place.

I guess you can have the laser escape out into space after its bounced around enough to proved the laser wall you want, but then you have a constantly running laser to keep the wall up. You also have a destructive laser constantly shooting out of your ship, so be carful where you point it!

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Re: Design a hard sci-fi-warship (RAR).

Postby Sea Skimmer » 2017-02-18 06:44pm

Shroom Man 777 wrote:I wonder if it's possible - hard or not-hard sci-fi - to set up a SPACE KILL-WALL by having constantly flowing and bouncing-back projectiles, fragments that are passed around by two magnety emitters/reflectors on either ends. Or do this process with death lasers and super-mirrors. The emitters/reflectors would also move around in sync with each other. So there are like these moving grids that will chop up careless travelers. Of course, rudimentary sensors can detect these hazards and ships can maneuver, but that would mean they're gonna detour through pre-set "safe" paths that can then have delay-action, proximity-activated or trigger-activated laser-mines, bomb-pumped-lasers or encapsulated-torpedo-launchers.


Physics wise that would work, but it would make no sense in an anti ship unless perhaps the enemy was spamming endless dumb weapons at you, anything else will just blowup a gap on approach. The magnetic launchers are drawing lots of power and thus bound to be hot and thus obvious targets (a normal space mine can be way cool in principle). You've basically invented space barbed wire in tactical use, but those magnetic launchers could just serve as aiming at the enemy cannon instead, and they'd draw waaaay less power long term because they aren't constantly active doing that. Since half your magnetic force is being used merely to catch your own projectiles your cutting your maximum peak firepower in half, and your still going to loose your ammo when it hits stuff.

This idea might make a lot more sense on the ground actually, all kinds of exotic land mines are plausible in the future as mine and drone warfare merge and take on all kinds of new roles. Also an idea like this could be used, up on poles, as a 'roof' to protect ground installations from indirect fire. That's not maybe the best thing to do, but it'd work. Basically it would be like generating your own slat armor ceiling over spans too wide for passive fortification to be viable at the same cost.

Another place this idea might make sense is actually in an active sweeping role. Say the enemy has cloaked ships. Ships that can bend light, so sweeps with laser weapons will not detect them, but they still physically-mass interact. So you have two formations of drone magnetic cannon sweeping ships start running grid patterns shooting up an area looking for said cloak, while your normal warships stand off in a circle waiting for sign of movement. This might take days and such, but real ASW searches do too.

Dragging around thin nets of carbon nanotube cables could also make sense thinking on that, if we could ever make the magically super several kilometer long threads required for space elevators some HUGE nets will be possible, the holes just have to be small enough that a whole enemy ship can't fit through, doesn't have to be dense.
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Re: Design a hard sci-fi-warship (RAR).

Postby Sea Skimmer » 2017-02-18 07:25pm

I would suggest that an azipod style engine which was also given some elevation control, more or less like a gun turret, would easily give you a main engine that can also provide a wide range of maneuvering control.

As far as ship going flying off into space goes, if you have a fleet train they can just come rescue you. A rescue ship can have a very slow thrust rate and it will still very quickly catch up to anything drifting. Combat rules would be no different then planes or warships, avoid combat if you've consumed over 1/3rd of your fuel. And just bring tankers, lots of tankers.

Patroklos wrote:I always figured this was a more plausible way to deal with those scifi prison doors and launch bay defenses that the go to force fields. The problem is that its actually pretty hard to reflect a laser with a mirror once its been focused into a beam of death. You can defocus it again with a mirror if you have the right one, but trying to bounce it it intact generally just gets you a burnt up mirror (which is why you can’t just strap mirrors on things to act as laser armor).


The SDI solution to this was a matrix of fine grains of sand in a transparent epoxy. Each one acting like a bad mirror reflecting the laser onto all the others around it, and they choose sand with the highest melting point they could, so it in effect becomes a 3D heat sink. A 1mm thick sample intended as body armor stoodup to laser pulses that would have been instantaneous kills on clothed humans. This idea would not stand up to unlimited sustained fire, but up to a certain threshold it would take no damage at all from laser hits.

A major warship would probably use something similar in concept but a lot different in scale because it has far more volume to play with. Ceramic ball armor for example is a thing already in tank armor and now literally in hoppers to reinforce walls, and if you could make reflective ones that also contained hydrogen then compartments filled with these could provide protection against all threats. Thermal, KE/shaped charge, fast neutrons and gamma rays. And since its just a mass of balls if you loose any in action you can physically go out and shove more into the compartments, and space weld patches over the holes, then get back into combat quickly.

Lasers can be pretty dangerous, but since the enemy ought to shoot back with his own lasers I think you'll fast find the most vulnerable thing around is the enemy laser optics. So a laser duel may quickly turn into both sides have no working lasers, but little other hull damage. Lasers can't be magically defended against with mirriors, but at ranges of hundreds and thousands of kilometers I don't think they'll actually work out to be too good of weapons.

As far as missiles go, it really really depends on how big and expensive the missiles are allowed to get. Unguided railgun shells are indeed difficult to hit anything with, but you could have several km/s of divert in a realistic guidance rocket in a 100kg projectile, without asking for any sci tech. Long range missiles could use all kinds of crazy nuclear power. Like two stage nuclear salt water rocket missile, only firing the second stage when its somewhere near the target.
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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-19 04:25pm

Sea Skimmer wrote:I would suggest that an azipod style engine which was also given some elevation control, more or less like a gun turret, would easily give you a main engine that can also provide a wide range of maneuvering control.
The big problem there is that you end up with an engine that's a significant fraction of the total size of your ship. Azipod engines are really small compared to the ships they push around, precisely because they have to mount on swivel mounts rather than being integrated into the hull.

The space-based equivalent would be a form of maneuvering thruster, an alternative to fixed-nozzle RCS blocks, not so much an alternative to the main engine assembly that gives you super-thrust gimballing.

As far as ship going flying off into space goes, if you have a fleet train they can just come rescue you. A rescue ship can have a very slow thrust rate and it will still very quickly catch up to anything drifting. Combat rules would be no different then planes or warships, avoid combat if you've consumed over 1/3rd of your fuel. And just bring tankers, lots of tankers.
This.


Patroklos wrote:
Simon_Jester wrote:It doesn't have to.

The advantage ships and planes have, of operating in a fluid medium where they can change heading without speeding up or slowing down much... Is offset by the advantage spacecraft have, of operating in a vacuum where you can tack on extra m/s of velocity in any direction you like with vector addition. Instead of having to keep the ship's bow and the ship's heading pointing in the same direction.
[snip well known physics]

The point is that in atmosphere your main and maneuvering engine is always the same, where in space its only the same for a very select set of circumstances. Basically you have to have maneuvering engines, and they have to do all the work themselves (no banking).
And as I have repeatedly said, my entire point is that it's desirable to configure the ship's main armament so that during combat it can use the main engine for combat maneuvers intended to defend against enemy fire. Otherwise, the main engine is effectively parasitic mass during combat; it doesn't contribute to survivability except insofar as you need it to get home.

If my main engine can accelerate the ship at, say, 0.5 gravities, then all I have to do is point the ship in any desired direction and fire the engine. Suddenly, I am accelerating at (roughly) five meters per second squared. Regardless of where the enemy thought I was going to be before I started the engine burn...
Again, this holds true only for directions not abaft of beam if you were already under power, at which point you are technically accelerating as you say but part or all of that force is counteracting your original velocity along the old thrust vector.

And remember as I proved above pointing your main drive in a different direction is no easy task, especially if you are long and skinny. Suddenly is not how I would describe it, more like predictably.
In a hard-SF context, we would normally assume that combat spacecraft approach each other ballistically, seeing one another well in advance. This provides plenty of time to rotate the ship so that the thrust axis points perpendicular to the line of flight- assuming you can do that, assuming your main weapons aren't fixed to point forward so that you're stuck with your thrust axis pointing directly at the enemy.

The enemy can presumably tell which way my nose is pointing and predict the direction any evasive main engine burns will take me. But he cannot predict when I will make those burns, or even if I will make them. Or, for that matter, if I do appear to be making a continuous burn, when I will abruptly decrease thrust, cut off one of my multiple main engines to decrease acceleration, or cut in an engine that was previously not running in order to abruptly change acceleration and impart new, unpredicted changes in velocity.

This specific part of the problem is very much similar to that of hitting a maneuvering target with World War era gunnery and torpedoes. Even given perfect knowledge of the target's location and heading, you can't stop them from zig-zagging around that base course. If you aim a weapon that cruises (ballistically or under power in water) at a fixed velocity towards the target's predicted location, and they maneuver, you miss the target. If you want meaningful hit rates, you either need a guided weapon, or you need to fire from close enough that the enemy can't get out of the way in time.

Oh, you are absolutely correct, evasive maneuvers are certainly a thing (that spheres will always do better).

Whether it’s your main engine or a maneuvering thruster is irrelevant, they can each move you in a range or directions. The reason I mentioned not necessarily constantly burning your main engine is because it essentially only allows you to move up and down a straight line projecting forward of your current position the enemy already knows about, while they have no idea what direction your maneuvering thrusters will burn (especially if they are gimbled).
What matters here isn't the direction in which you accelerate. It's the rate with which you accelerate.

If it takes N seconds the target to accelerate enough to 'sidestep' by 2-3 times its own length in a given direction, shots fired that take significantly more than N seconds to reach the target will nearly always miss. It'd be like trying to hit someone with a sniper rifle from fifty miles away- even if you can see them and aim at exactly where they're standing, they have to obligingly stand still (that is, coast ballistically) for a period of several minutes waiting for the bullet to arrive. They don't even have to see the bullet coming to foil an attack like that, they just have to walk leisurely around in unpredictable directions.

By contrast, doing the same thing at a range of one mile would be nearly impossible- because the bullet arrives within two to three seconds.

The trick is that unless your maneuvering thrusters are extremely beefy, you can't sidestep nearly as well using them as you can using the main engine. Therefore it takes longer for you to get out of the way of enemy fire when using maneuvering thrusters only. Therefore effective range against you is greater.

If your maneuvering thrusters ARE beefy, that imposes a considerable mass penalty because you need a large number of redundant engines all capable of providing significant acceleration to the ship, enough to move it several times its own length in a matter of seconds. The main engine is already capable of doing that- but you can't use it for that purpose if you're committed to pointing the thrust axis in a fixed direction because of your ship's geometry.

If we are talking about less about just a few seconds for enemy fire to reach you, and you are suffering from a particularly bad moment of intertia, you are probably only going to be able to pitch your nose a degree or two. This will of course provide some sort of cone for possible maneuver using that .5g, it will be long and skinny. So skinny that if we are using that 30km/s projectile it will pass through it so fast the maneuver profile is essentially 2D as far as the enemy is concerned assuming he is shooting at you in profile. If he shooting at your forward or aft face it makes not difference at all.
If we're talking about shell flight times of a few seconds with a 30 km/s railgun shell, we're talking about being at most a few hundred kilometers away from the enemy. You're not going to be able to dodge from such ranges, but the enemy will also have a very good chance of being able to avoid coming that close to you in the first place, or of crippling your ship with longer ranged weapons before you get there.

By the time you get within 300 kilometers and your railguns have a ten-second flight time to the enemy, they've had a LOT of chances to shoot at you with missiles, lasers, particle beams, and the like.

But like I was saying if your pitch performance isn’t that great the cone from continued main engine burning is not going to add much, so you might start to think about if what you gain from not burning is worth more.
Pitch performance doesn't need to be good, especially if you combine lateral acceleration from maneuvering thrusters and the main engine.

If my ship is 100 meters long and ten meters in diameter/width/whatever, and I have a 0.5g main engine plus 0.05g maneuvering thrusters...

In twenty seconds, the maneuvering thrusters can move me 25 meters in any direction I want. If I'm being fired at from abeam, that's enough to generate a miss if the shot was aimed at my center of mass, if I burn in the direction perpendicular to my line of flight.

In the same twenty seconds, the main engine can move me 250 meters in a single direction- or zero meters, or anywhere from 0 to 250. Again, enough to generate a miss.

Being able to fire both means that from the point of view of an enemy gunner, I am a 1000 square meter target that, in tewenty seconds' time, could be anywhere in a 50*250=12500 square meter area. Hit rates against me aren't going to be good even if they have perfect information on my speed and heading now.

The catch is that to get increased range of dodging options from the maneuvering thrusters requires a lot of beefing up of said maneuvering thrusters. With soft SF technology and super-maneuverable spacecraft this isn't a problem, but it's much more of an issue with hardish technology.

Basically, if you are firing with unguided weapons, then once you get far enough away that the enemy has any realistic chance of sidestepping your shots, your hit probability drops off with the fourth power of the range. Because you're firing into a 2D "the target might be anywhere in this region" envelope, and the width and height of that envelope are both proportionate to the square of the amount of time you give the enemy to dodge.


It’s only a 2D target if you are talking lasers or very fast kinetic rounds that can travel along a line through the volume faster than a crossection of the target could clear it that line if it was anywhere on it when the round entered it. Maneuvering will provide for a possible volume of locations, not just area.
It is a reasonable assumption, in the context of space warfare, that any unguided weapon is moving a lot faster than the amount of delta-V that the target can pile on in the time it takes the round to cross the target's maneuver envelope.

Otherwise the idea of hitting the target with unguided ANYTHING is a complete joke- it'd be like trying to shoot down WWII fighters with a projectile that travels at the speed of a WWII torpedo. Sure, in theory you could fire the projectile into the air at 50 miles an hour and hope the enemy plane is dumb enough to smack into it, but that's not the way to bet.

But assuming your KE roun’ts aren’t that fast, this is why I don’t believe in all the hype about railgun/coilgun slugs. If you have a target that has any decent maneuvering capability at range you will need to fire off spreads of hundreds (conservatively) or slugs near simultaneously to register hits. Maybe if you are coordinating fleet fires on selected ships in the enemy formation this is possible, but I doubt a single ship at the tech levels we are talking about will have the number of mounts or mounts with the rapid fire capability to do it.

For this reason I would imagine for ranges where a rounds travel time is measured in multiple seconds rail guns will still shoot nuclear shells. If they don’t vaporize after hitting an enemy hull when dead reconing told them they should they auto detonate to see if they were at least close enough to get some radiation splashed on the target. We then get into the cost benefit analysis of how much faster you could shoot the shells if you didn’t have the warhead mass there, but that’s for the ship designers to do the math on.
This is analogous to firing flak shells at enemy bombers because you just know your AA guns will literally never hit them with solid shot- it's a good idea. An alternate strategy if large numbers of nuclear warheads aren't a practical solution is to use actual flak shells, with a bog-standard bursting charge; this converts a single projectile into a large number of (still hellafast) bits of shrapnel expanding forward in a cone-shaped cloud from the point of detonation, in hopes of saturating the target's maneuver envelope. The individual bits of shrapnel aren't necessarily all that damaging, but you vastly increase hit probability, and against hard-SF spacecraft almost any damaging hit is likely to be a Big Deal even if the impactor in question 'only' carries a few megajoules of kinetic energy.

Either of these strategies will work, assuming you can solve the problem of designing a suitable fuze that can survive getting shot out of the railgun and/or engaged by the target's point defense lasers. And assuming the mass penalty of the heavier shell (and heavier gun to fire it) doesn't make the weapon system inviable. And assuming you don't just get straight-up targeted from ranges where even nuclear railguns cannot reply effectively, using weapons with shorter time of flight or better guidance packages.

Sort of, it depends on how you launch the missiles. I am of the opinion that VLS won’t be the desired method because all of the deltaV has to come from the missile itself. I would suggest using launchers that are just giant railguns (tailored to be gentle on the more delicate missile than your bog standard slug thrower is) to get as much m/s as you can before using onboard stores. This means smaller missles (harder to detect, harder to hit) or missiles of the same size with more maneuvering/burn options.

The consequence of this is that your missiles have an alpha arc based on the launcher.
On the other hand, it also means that the missile launch tube has to be really big. To impart, say, 1000 m/s of delta-V to the missile while launching, without exceeding, say, 1000 gravities of acceleration, the launch tube has to be about 500 meters long. Artillery shells are subject to insane accelerations precisely because they get boosted to Mach 2 or Mach 3 in a barrel no more than about ten meters long. Conversely, gun-launched missiles have to be fired at low muzzle velocities for the same reason.

You're going to be hard-pressed to give a gun-launched missile enough delta-V to do much good for its maneuver envelope and effective range, without either using an utterly huge launch tube OR designing the missile to withstand extreme artillery-esque accelerations.

Obviously it would be suicidal for a ship to expend all its delta-V on combat dodging and leave none to return. However, this is comparable to the situation with real life combat aircraft. A functional multirole combat aircraft has a "combat radius" defined by its ability to fly to the target, expend some fuel loitering or maneuvering around the target, and then come home.
True, but those are estimates. Plenty of combat aircraft have found themselves in situations where they have to bail out because combat took to long despite all the best pre planning calculations. But when you bail out you either get stopped by air resistance and parachute to the ground which stops you, or the ground just stops you. Either way SOMETHING stops you whether you survive or not. Nothing stops you in space.
Right- but the point is, you never see anyone refuse to maneuver their aircraft in combat for fear of running out of fuel. Nor do you see people design aircraft on the assumption that there's no point in making them able to maneuver.

A spacecraft designed for combat will necessarily have a large reserve of delta-V in hand for combat maneuvers. If it doesn't, then it shouldn't be going into combat in the first place, and if you don't have ships capable of meeting that performance requirement, you shouldn't be fighting a space war. Can something go wrong anyway? Sure, just like how in the Age of Sail, if your hull was riddled with gunfire it meant you were at greater risk of breaking up and sinking in a storm and drowning. But if you fight like you've got no delta-V budget for evasive maneuvers, then you're going to get shot dead from far outside your own effective weapons range, at which point all that delta-V you saved avails you very little.

What it comes down to is that in space, speed is more valuable, and armor is of limited practicality. Tricking the enemy into missing with decoys and ECM is best, but being able to dodge is critical because your trajectory is totally predictable if you aren't expending delta-V to maneuver. But under no circumstances should you court being hit yourself if you don't have to, because being hit by an enemy weapon is far more likely to kill you than having 50 m/s less delta-V than you want. Worst case, that still leaves you with the option of decelerating enough that a rescue ship can catch you after the battle.

As noted, if your ship doesn't have a significant delta-V margin purely for combat maneuvers (which includes evasive action and delta-V lost to battle damage), then your ship is unfit for combat and will almost certainly lose to a more lightly built "combat ship" that does have such a reserve for maneuvers.
The fist German jet planes measured their combat availability times in minutes. Even jets today have a loiter time, let alone combat time, sometimes less than an hour after reasonable transit times. We still fight wars with them anyway.
The first German jet planes also took devastating losses because they kept getting shot up on takeoff and landing due to their short combat availability times. To the point where they were of doubtful utility against enemy piston-engine fighters with greater endurance.

Running out of the ability to maneuver is death in space warfare, just as in air warfare. But for that exact reason, no weapon system will be very successful in the air OR in space if it lacks enough reserve maneuverability to make maneuvers while fighting. A combat aircraft whose options reduce entirely to "fly in straight line to target, drop bomb, return home" is going to get killed sooner or later. A combat spacecraft whose options reduce entirely to "accelerate to intercept enemy, coast ballistically, fire forward guns, pass the enemy, turn around, return home" is likewise going to get killed.

This is why the designs I'm advocating can burn their engines in directions besides "directly towards the enemy." Because sometimes, you want to be able to say "let's get 15 m/s of new velocity in the thefuckouttatheway direction" and not "prepare for RAMMING SPEED!"
But as I proved above, that’s probably not going to be possible in lots of situations. You just can’t spin on your long axis that fast, or expose the ship in ways that will open up a useful main burn vector (if you don’t have 100% protection coverage).
You don't have to pitch/yaw the ship quickly to get that new 15 m/s of velocity.

If "bead on your ass" just means "they're aiming at us successfully," that is exactly when you want to be dodging. Because a 10 m/s engine burn is going to hurt a lot less than taking a railgun to the face.
I meant literally “is shooting up your pooper/ AKA engine bell”. I am doubtful of the ability to defend an actually burning main engine if the exhaust itself won’t do it. If it can’t, then you might be able to do it when it’s not burning.
Unlikely unless you're dealing with comically feeble weaponry. Again, trying to armor against serious weaponry in space, given the energies involved and the amount of mass it takes to armor a ship all over, is a losing game. You can mount stuff like Whipple shields and anti-radiation protection that minimizes the effect of lightweight stuff going for a 'cheap and easy' kill. But there are limits.

Honestly, if you're worried about shots striking your engine bell, you should have started a turn to keep your engine bell away from the enemy in the first place. Before they got there, because they can't just magically teleport into a firing position on you. It's not World War Two and they can't swoop in from out of the sun and be hitting you from behind in seconds.

THIS is a good point, to be fair- it can force you to choose between firing or maneuvering. On the other hand, this creates much stronger incentives to use weapons that are immune or highly resistant to the problem, because being able to get out of the way of the incoming fire is so goshdarn useful.
It also depends on if you can tank rounds or not. If you can, you may calculate your enemy need get three good shots on you, but you only need one more on him. So in that case you might make the trade knowing you will probably get hit, but you will still carry the day.

If we are talking glass cannons duking it out for sudden death elimination then yean, the tradeoff goes squarely to the maneuvering side. But even there since getting the first shot in as accurately as possible is so important stability may be more important than maneuverability.
Stability is much more relevant to this discussion.

Remember your 6.2-meter radius thousand ton spherical ship? Try armoring it. You have a surface area of 482 square meters, which means that if your armor material masses one ton per square meter, you've increased the total mass of the ship by 50%, and reduced the mass fraction of the ship for purposes of delta-V calculations by a large factor.

Increased dry weight will not only eat into combat maneuverability (so that you get hit more often), but also your delta-V budget for deep space maneuvers. That's effectively reducing your cruising range and ability to get places, otherwise known as "so sorry, but we can't put this ship into Jupiter orbit in less than twelve months, so sorry about your pirate problem!"

To be fair, one ton of armor per square meter is pretty chunky- but it only translates as, say, four inches of steel plate. Yes, I know, you wouldn't actually use steel, the point is you have to use something, and since some of the threats (like radiation) are really only stopped by dense materials, you can't just use cotton candy or whatever. And while this level of armoring will stop some things (say, a one-gram fleck of shrapnel moving at 5 km/s), it won't stop just anything (say, a 100-gram fleck of shrapnel moving at 10 km/s). It's certainly not enough that you can be confident of laughing off a railgun to the face.

So honestly I'd rather use those 482 tons of extra mass to add more fuel tanks (and thrusters) and further increase maneuverability, in order to avoid getting hit in the first place.

"Glass cannon" is totally a good description of space warfare fought with technologies we can realistically predict being invented in the foreseeable future. Including exotic stuff like fusion rockets and X-ray laser guns.

Number Two hydrogen tank wouldn't have gotten shot to begin with, if the enemy's weapons fire was all going over there in that random patch of cosmic void, instead of over here in our living room.
True, but neither did we design battleships with the assumption we could just design them to not get hit. Quite the opposite actually.
That's because WWII battleships are less maneuverable than spacecraft- but still maneuverable enough to avoid the vast majority of enemy fire, despite being heavily armored!

Aircraft, by contrast, are almost totally unarmored, except for a handful of specific aircraft types designed to spend all their time loitering at or near ground level where any yutz with an AK can hit them. Because the mass penalty of armor plating against typical AA weapons (like a SAM the size of a telephone pole) is too great to offset the cost of extra fuel and munitions you could have carried.
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Re: Design a hard sci-fi-warship (RAR).

Postby Sea Skimmer » 2017-02-19 11:01pm

Simon_Jester wrote:The big problem there is that you end up with an engine that's a significant fraction of the total size of your ship. Azipod engines are really small compared to the ships they push around, precisely because they have to mount on swivel mounts rather than being integrated into the hull.


Its really not going to be that much bigger then what a fixed engine would have to be anyway, and its the only way your likely to get more then the smallest thrust deflection angles out of a realistic high power engine in this context.


The space-based equivalent would be a form of maneuvering thruster, an alternative to fixed-nozzle RCS blocks, not so much an alternative to the main engine assembly that gives you super-thrust gimballing.


It really depends heavily on what your engine actually is at that point. For some propulsion concepts simply having multiple nozzles wouldn't be a serious problem, for others the only option is complete replication of the powertrain.
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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-20 02:28pm

It's not so much that the engine pod is physically heavier than a fixed-mount engine. The problem is that the engine pod probably represents a large fraction of the ship's total mass. Plus, you have problems on a spacecraft if the axis of thrust doesn't point through the center of mass; you don't want your ship to go into a tumble.
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Re: Design a hard sci-fi-warship (RAR).

Postby Patroklos » 2017-02-20 02:32pm

Simon_Jester wrote:
Sea Skimmer wrote:I would suggest that an azipod style engine which was also given some elevation control, more or less like a gun turret, would easily give you a main engine that can also provide a wide range of maneuvering control.
The big problem there is that you end up with an engine that's a significant fraction of the total size of your ship. Azipod engines are really small compared to the ships they push around, precisely because they have to mount on swivel mounts rather than being integrated into the hull.


There are also issues with:

1.) Thrust loading. The more powerful you make the thruster the off access thrust forces will require more support structure. With a fixed RCS this is true too but if you are using a gimbaled or the mentioned azipod you are talking about a range of possible thrust loads.
2.) If you are using beefy engines, specifically nuclear engines, for your thrusters to provide anything approaching the useful maneuvering Simon describes (azipods generally have main engines in them in real life) you are talking about another shadow shield as well as making far more area around the ship no go radiation zones which will impact things like docking with other ships, scatter radiation from launching missiles into these volumes, cargo loading, etc.
3.) If you are using a type of engine with radioactive exhaust like say an open cycle gas core NTR, you are no spraying it everywhere. Essentially you are going to have to radiation shield the whole ship.

As far as ship going flying off into space goes, if you have a fleet train they can just come rescue you. A rescue ship can have a very slow thrust rate and it will still very quickly catch up to anything drifting. Combat rules would be no different then planes or warships, avoid combat if you've consumed over 1/3rd of your fuel. And just bring tankers, lots of tankers.


No. This is not how it works in space. If you are at a significant fraction of your ships max deltaV when your engine and/or propellant tanks are shot away, assuming your military ship is near the top end of the available capability scale, the chance of rescue is nil. The rescue ship, at best, will be able to match or maybe slightly overtake your velocity. If it is starting any distance away from your runaway start point or there is any significant time delay there is no catching up.

There is the possibility your terminal vector just happens to point in the direction of a rescue ship to allow interception vice overtaking (why I mentioned the good idea of having attack vectors coincide with RTB vectors), but that’s just throwing the dice.


Simon_Jester wrote: And as I have repeatedly said, my entire point is that it's desirable to configure the ship's main armament so that during combat it can use the main engine for combat maneuvers intended to defend against enemy fire. Otherwise, the main engine is effectively parasitic mass during combat; it doesn't contribute to survivability except insofar as you need it to get home.


I think you are beginning to understand…

Yes, the main engine for is going to be very useful for most desired combat maneuvers IF it is as fragile as I believe both of us think it is.

In a hard-SF context, we would normally assume that combat spacecraft approach each other ballistically, seeing one another well in advance. This provides plenty of time to rotate the ship so that the thrust axis points perpendicular to the line of flight- assuming you can do that, assuming your main weapons aren't fixed to point forward so that you're stuck with your thrust axis pointing directly at the enemy.


The main engine maneuvering restrictions regarding where you can point it only apply when combat is joined. When you are within your enemies known or predicted effective weapons range. If they can’t shoot you yet, who cares what weak spot point at them.

That said, your main engine still won’t be as useful as you think because it just takes so damn long to point it in most possible directions.

The enemy can presumably tell which way my nose is pointing and predict the direction any evasive main engine burns will take me. But he cannot predict when I will make those burns, or even if I will make them. Or, for that matter, if I do appear to be making a continuous burn, when I will abruptly decrease thrust, cut off one of my multiple main engines to decrease acceleration, or cut in an engine that was previously not running in order to abruptly change acceleration and impart new, unpredicted changes in velocity.


As I stated previously, all this cutting off or turning down engines you are describing is just your ability to move up and down your thrust vector. The enemy will know how far you can possible have moved forward along it and how far forward you must move along it (assuming they know your engine specs) technically this is an evasive maneuver, but not a particularly hard one to counter.

Now you can expand that line into a cone by using your main engine and your thrusters in combination. But because pitch performance is so poor, especially on the long cylinder you champion, that cone is not going to be very big. You can further increase that cone’s size by pitching AND accelerating in another direction from your main thrust axis with other thrusters, but its still not going to have the outsized effect you seem to be predicting.

This specific part of the problem is very much similar to that of hitting a maneuvering target with World War era gunnery and torpedoes. Even given perfect knowledge of the target's location and heading, you can't stop them from zig-zagging around that base course. If you aim a weapon that cruises (ballistically or under power in water) at a fixed velocity towards the target's predicted location, and they maneuver, you miss the target. If you want meaningful hit rates, you either need a guided weapon, or you need to fire from close enough that the enemy can't get out of the way in time.


Given the ships sunk and damaged at engagements like Jutland, there are many thousands of dead sailors who would take issue with your “meaningful hit rates” estimates.

It’s not pretty. It’s not efficient materially. It might take a long time. But you can get hits his way. Hits that win battles and kill.



What matters here isn't the direction in which you accelerate. It's the rate with which you accelerate.


No. Its both. If I can accelerate in one direction I can only be present on a line. If I can accelerate in two directions I can be present in an area. If I can accelerate in three directions I can be present in a volume. The more direction I add, the more volume that is. The benefits of this are obvious.

If it takes N seconds the target to accelerate enough to 'sidestep' by 2-3 times its own length in a given direction, shots fired that take significantly more than N seconds to reach the target will nearly always miss. It'd be like trying to hit someone with a sniper rifle from fifty miles away- even if you can see them and aim at exactly where they're standing, they have to obligingly stand still (that is, coast ballistically) for a period of several minutes waiting for the bullet to arrive. They don't even have to see the bullet coming to foil an attack like that, they just have to walk leisurely around in unpredictable directions.

By contrast, doing the same thing at a range of one mile would be nearly impossible- because the bullet arrives within two to three seconds.

[/quote]

This happens in real life. To real snipers. In this situation the snipers just get closer.

All you are doing is describing what would on Earth be the difference between max range and effective range. There is no max range in space, but you are absolutely right that the relationship between the speed of your projectiles and your targets maneuvering capabilites puts a hard cap on your kinetic weapons max effective range. Add to that your targeting ability and platform stability and that effective range is going to a whole lot less than the theoretical infinity.

So get closer. And yeah, if that interplay means kinetic weapons are only effective inside the effective weapons range of lasers or particle cannons or whatever other weapons are options at the time you may have just argued away the use of rail guns in your universe. It depends on the characteristics of all those weapons at their given tech level.


The trick is that unless your maneuvering thrusters are extremely beefy, you can't sidestep nearly as well using them as you can using the main engine. Therefore it takes longer for you to get out of the way of enemy fire when using maneuvering thrusters only. Therefore effective range against you is greater.


Exactly.

If your maneuvering thrusters ARE beefy, that imposes a considerable mass penalty because you need a large number of redundant engines all capable of providing significant acceleration to the ship, enough to move it several times its own length in a matter of seconds. The main engine is already capable of doing that- but you can't use it for that purpose if you're committed to pointing the thrust axis in a fixed direction because of your ship's geometry.


That’s my point Simon, that many things prevent you from using your engine the way you want to. It doesn’t matter how awesome it would be if you could use your main engine in any way you want. You can’t for the many reasons who have stated including:

1.) It’s a fragile weak spot you can’t point at the enemy.
2.) Moment of inertia means it’s physically unable to be pointed in most directions in a useful timeframe.
3.) If you are committed to maintaining the enemy in an alpha arc, unless that arc is 360x360 gun laying and unfettered main engine maneuvering are going to conflict in some way.

It would be supper awesome if you could just instantly point your main engine in any direction with zero regard for those three things. You would be the Brian Boitano of space maneuvering. But you do have to worry about those three things. This means the utility of your main engines for maneuvering are something <100%, thus other things may be of higher value than using them for maneuvering under certain circumstances.

If we're talking about shell flight times of a few seconds with a 30 km/s railgun shell, we're talking about being at most a few hundred kilometers away from the enemy. You're not going to be able to dodge from such ranges, but the enemy will also have a very good chance of being able to avoid coming that close to you in the first place, or of crippling your ship with longer ranged weapons before you get there.

By the time you get within 300 kilometers and your railguns have a ten-second flight time to the enemy, they've had a LOT of chances to shoot at you with missiles, lasers, particle beams, and the like.


Did they? Wouldn’t that depend on the universe in question? Do all ships have all those weapons on board.

This would be a case of weapons balancing, and it’s a who different discussion about when for instance particle beams become viable, when engine tech for missiles can be miniaturized to make them viable in place of rail guns, or when weapon mount stability makes maintaining a laser spot viable.

There is no reason to assume that because 30 km/s railguns exist particle beam weapons do too (also, there are a LOT of drawbacks to particle beam weapons which I would love to discuss).

Pitch performance doesn't need to be good, especially if you combine lateral acceleration from maneuvering thrusters and the main engine.


Yeah, but that performance is far and away inferior to the “spin and thrust with the main engine in any direction you want” you were talking about before.

If my ship is 100 meters long and ten meters in diameter/width/whatever, and I have a 0.5g main engine plus 0.05g maneuvering thrusters...

In twenty seconds, the maneuvering thrusters can move me 25 meters in any direction I want. If I'm being fired at from abeam, that's enough to generate a miss if the shot was aimed at my center of mass, if I burn in the direction perpendicular to my line of flight.

In the same twenty seconds, the main engine can move me 250 meters in a single direction- or zero meters, or anywhere from 0 to 250. Again, enough to generate a miss.


Again, this is not how it works unless you were at a relative dead stop to your enemy in the first place. This is probably not the case, you are instead hurtling on some direction and a significant fraction of your deltaV limits. This means that any maneuver abaft of beam will be in part of all a breaking force.

You are also a slave to your original velocity, which the enemy knows at the time of firing. They know you can't break significantly without flipping your main engine, which they know you can't do.

Being able to fire both means that from the point of view of an enemy gunner, I am a 1000 square meter target that, in tewenty seconds' time, could be anywhere in a 50*250=12500 square meter area. Hit rates against me aren't going to be good even if they have perfect information on my speed and heading now.


Again, you are not accounting for breaking.

The catch is that to get increased range of dodging options from the maneuvering thrusters requires a lot of beefing up of said maneuvering thrusters. With soft SF technology and super-maneuverable spacecraft this isn't a problem, but it's much more of an issue with hardish technology.


Hard scifi is the topic of this thread, no?

Basically, if you are firing with unguided weapons, then once you get far enough away that the enemy has any realistic chance of sidestepping your shots, your hit probability drops off with the fourth power of the range. Because you're firing into a 2D "the target might be anywhere in this region" envelope, and the width and height of that envelope are both proportionate to the square of the amount of time you give the enemy to dodge.


It is a reasonable assumption, in the context of space warfare, that any unguided weapon is moving a lot faster than the amount of delta-V that the target can pile on in the time it takes the round to cross the target's maneuver envelope.


Maybe, it depends on what power we give the engines, both main and maneuvering.

My point was that it’s a different game altogether if you are using a cylinder vice a sphere. A sphere has such superior pitch performance that it CAN use its main engine to create a give enough volume forward cone to make a 30/km kinetic round miss via volume considerations. I cylinder not so much.

And of course if you are using lasers volume doesn’t matter. If you are using volume effect weapons like nukes volume matters but no so much.

Otherwise the idea of hitting the target with unguided ANYTHING is a complete joke- it'd be like trying to shoot down WWII fighters with a projectile that travels at the speed of a WWII torpedo. Sure, in theory you could fire the projectile into the air at 50 miles an hour and hope the enemy plane is dumb enough to smack into it, but that's not the way to bet.


I am pretty sure most torpedoes went SLOWER than the surface ships they were targeting. Currently conventionally ones are the same speed or somewhat faster but not multiple times faster than their targets. Your example of a 30km/s kinetic round is probably 10x faster at least than a reasonable maneuvering thruster could accelerate a target ship.

But yeah, ranges are going to be closer than many anticipate.

This is analogous to firing flak shells at enemy bombers because you just know your AA guns will literally never hit them with solid shot- it's a good idea. An alternate strategy if large numbers of nuclear warheads aren't a practical solution is to use actual flak shells, with a bog-standard bursting charge; this converts a single projectile into a large number of (still hellafast) bits of shrapnel expanding forward in a cone-shaped cloud from the point of detonation, in hopes of saturating the target's maneuver envelope. The individual bits of shrapnel aren't necessarily all that damaging, but you vastly increase hit probability, and against hard-SF spacecraft almost any damaging hit is likely to be a Big Deal even if the impactor in question 'only' carries a few megajoules of kinetic energy.


The draw back of using bursting shells vice nukes is that those shells would produce single direction cones of effects versus omnidirectional radiation bursts.

Still not a bad idea, but in that case you should have the shells burst as soon as they enter a possible target maneuvering envelope because if you wait until after it would have hit with full mass the cone will expand primarily outside the envelope on the other end. Essentially you are giving up on a full kinetic impact.

The value of the nuke is that you can still try and hit the target with the full kinetic mass of the round, you only detonate the nuke when you are about to clear the target envelope and you want to “shoot” ie radiate backwards back into it.

On the other hand, it also means that the missile launch tube has to be really big. To impart, say, 1000 m/s of delta-V to the missile while launching, without exceeding, say, 1000 gravities of acceleration, the launch tube has to be about 500 meters long. Artillery shells are subject to insane accelerations precisely because they get boosted to Mach 2 or Mach 3 in a barrel no more than about ten meters long. Conversely, gun-launched missiles have to be fired at low muzzle velocities for the same reason.

You're going to be hard-pressed to give a gun-launched missile enough delta-V to do much good for its maneuver envelope and effective range, without either using an utterly huge launch tube OR designing the missile to withstand extreme artillery-esque accelerations.


True. But every little bit of deltaV counts.

Right- but the point is, you never see anyone refuse to maneuver their aircraft in combat for fear of running out of fuel. Nor do you see people design aircraft on the assumption that there's no point in making them able to maneuver.


We do see them chose between maneuvers, specifically ones that allow you to disengage vice continue.

A spacecraft designed for combat will necessarily have a large reserve of delta-V in hand for combat maneuvers. If it doesn't, then it shouldn't be going into combat in the first place, and if you don't have ships capable of meeting that performance requirement, you shouldn't be fighting a space war. Can something go wrong anyway? Sure, just like how in the Age of Sail, if your hull was riddled with gunfire it meant you were at greater risk of breaking up and sinking in a storm and drowning. But if you fight like you've got no delta-V budget for evasive maneuvers, then you're going to get shot dead from far outside your own effective weapons range, at which point all that delta-V you saved avails you very little.


Fuel is a concern. It always has been it always will be. This means you will make choices based on how much of it you have or how much of it you might still need. Its no different than armies and food, tank divisions and diesel, Fish fans and weed.

What it comes down to is that in space, speed is more valuable, and armor is of limited practicality. Tricking the enemy into missing with decoys and ECM is best, but being able to dodge is critical because your trajectory is totally predictable if you aren't expending delta-V to maneuver. But under no circumstances should you court being hit yourself if you don't have to, because being hit by an enemy weapon is far more likely to kill you than having 50 m/s less delta-V than you want. Worst case, that still leaves you with the option of decelerating enough that a rescue ship can catch you after the battle.


It depends on the defense/offensive balance. I know of no military platform today that operates on taking a round when they don’t have to. But I do know of many who expect to take lots of them due to the realities of their employment and prepare themselves thus.

The first German jet planes also took devastating losses because they kept getting shot up on takeoff and landing due to their short combat availability times. To the point where they were of doubtful utility against enemy piston-engine fighters with greater endurance.


There short combat availability times is not why they got shot down so much, it was because of the characteristics of their engines and avionics for tackoff and approach. Specifically, the Komet, landed dry via gliding. This is exactly why you might want to husband some propellant for later!

Running out of the ability to maneuver is death in space warfare, just as in air warfare. But for that exact reason, no weapon system will be very successful in the air OR in space if it lacks enough reserve maneuverability to make maneuvers while fighting. A combat aircraft whose options reduce entirely to "fly in straight line to target, drop bomb, return home" is going to get killed sooner or later. A combat spacecraft whose options reduce entirely to "accelerate to intercept enemy, coast ballistically, fire forward guns, pass the enemy, turn around, return home" is likewise going to get killed.


This is not a logical assumption if the enemy has to fight the same way. It depends on the circumstances of the universe. You continuously default to glass cannons, there is no need to do that every if it’s a probable state of affairs.

You don't have to pitch/yaw the ship quickly to get that new 15 m/s of velocity.


You do if you want it to be anything other than a degree or two off your nose.

Unlikely unless you're dealing with comically feeble weaponry. Again, trying to armor against serious weaponry in space, given the energies involved and the amount of mass it takes to armor a ship all over, is a losing game. You can mount stuff like Whipple shields and anti-radiation protection that minimizes the effect of lightweight stuff going for a 'cheap and easy' kill. But there are limits.


This is an unfounded statement. If we are talking kinetic slugs at 100km/s yes probably. But there are LOTS of armor options against slower kinetics, radiation, particle beams, and others.

Honestly, if you're worried about shots striking your engine bell, you should have started a turn to keep your engine bell away from the enemy in the first place. Before they got there, because they can't just magically teleport into a firing position on you. It's not World War Two and they can't swoop in from out of the sun and be hitting you from behind in seconds.


???

This is EXACTLY what I have been saying, but you have insisted over and over again that your main engine can be used to maneuver at will.

Stability is much more relevant to this discussion.

Remember your 6.2-meter radius thousand ton spherical ship? Try armoring it. You have a surface area of 482 square meters, which means that if your armor material masses one ton per square meter, you've increased the total mass of the ship by 50%, and reduced the mass fraction of the ship for purposes of delta-V calculations by a large factor.

Increased dry weight will not only eat into combat maneuverability (so that you get hit more often), but also your delta-V budget for deep space maneuvers. That's effectively reducing your cruising range and ability to get places, otherwise known as "so sorry, but we can't put this ship into Jupiter orbit in less than twelve months, so sorry about your pirate problem!"

To be fair, one ton of armor per square meter is pretty chunky- but it only translates as, say, four inches of steel plate. Yes, I know, you wouldn't actually use steel, the point is you have to use something, and since some of the threats (like radiation) are really only stopped by dense materials, you can't just use cotton candy or whatever. And while this level of armoring will stop some things (say, a one-gram fleck of shrapnel moving at 5 km/s), it won't stop just anything (say, a 100-gram fleck of shrapnel moving at 10 km/s). It's certainly not enough that you can be confident of laughing off a railgun to the face.


Whatever the armoring problem is with a sphere, is far worse for pretty much any other shape.

Of armor is a thing, spheres become a very viable hull shape.

So honestly I'd rather use those 482 tons of extra mass to add more fuel tanks (and thrusters) and further increase maneuverability, in order to avoid getting hit in the first place.

"Glass cannon" is totally a good description of space warfare fought with technologies we can realistically predict being invented in the foreseeable future. Including exotic stuff like fusion rockets and X-ray laser guns.


Again, if armor isn’t a thing then you are right. There are plenty of possible circumstances where it will be a thing.

That's because WWII battleships are less maneuverable than spacecraft- but still maneuverable enough to avoid the vast majority of enemy fire, despite being heavily armored!


Were they less maneuverable? Relative to the speeds and accuracies of what was being fired at them? I don’t think you should assume that.

Aircraft, by contrast, are almost totally unarmored, except for a handful of specific aircraft types designed to spend all their time loitering at or near ground level where any yutz with an AK can hit them. Because the mass penalty of armor plating against typical AA weapons (like a SAM the size of a telephone pole) is too great to offset the cost of extra fuel and munitions you could have carried.
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They aren’t using nuclear rockets for engines either…

Simon_Jester
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Re: Design a hard sci-fi-warship (RAR).

Postby Simon_Jester » 2017-02-20 03:33pm

I'm going to be honest, sheer frustration at the quote spaghetti is strangling my ability to even start writing a reply to this right now. The discussion has gotten so fragmented and the context so broken up that I know you've lost track of some elements of my point, Patroklos, and I wouldn't be the least bit surprised if I've lost track of some of yours.

Would you be open to rebooting our discussion a bit and having a moratorium on writing more than, say, five paragraphs at a time? It might hypothetically make the discussion take longer total because we'd have to go in series and not in parallel, but at least we wouldn't have to worry so much about stuff getting fragmented.

If you have a whole list of separate points to make, or if I do, we could maybe have actual numbered lists at the bottom of our posts.

How does that sound?
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