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

[The Romulan Republic]

For this scenario, it is the year 2120, and you are the newly appointed head of the Starship Design Committee for the Unified Sol Republic Space Force.

At present, nearly all of Earth (minus a few small, relatively impoverished hold-outs) is under the control of a single global government, run along broadly democratic socialist lines, with the capitol in Geneva. Their are also several space stations in orbit- they mostly are used for tourism, astronomical research, and maintenance of the orbital satellite network, as well as for customs inspection of any ships arriving at Earth from elsewhere in the Solar System.

In addition, their is a research base on the Moon (population about 200,000), another on Mars (about 100,000 residents), and several small, heavily automated asteroid mining facilities scattered around the inner Solar System and the asteroid belt. The first outpost was recently established in orbit of Jupiter, and the first fusion powered probe was launched toward Alpha Centauri ten years back (it should arrive in a few decades). Their is already serious talk (if the probe returns promising results) of launching a colony ship to the Centauri system.

Of course, the biggest event was the detection of a radio signal from another civilization, about twenty light years away.

Much of modern technology is automated, with AIs having far surpassed human capabilities (aside from a few anti-technology hold-outs, i.e. the future Amish, etc.). These machines are fortunately mostly benevolent in nature, although their have been tensions over the issue of AI independence vs. humans who resent having their jobs replaced and a society increasingly run by machines. However, the state bureaucracy employs a lot of people, mostly as a make-work program for people who's jobs have been automated away.

Not everything is positive, however. The Martian colony is largely self-sufficient in design, and many of its residents resent Earth rule. Reports have reached USR Intelligence that their is a growing Martian secession movement. More long-term, the discovery of sapient life elsewhere in the US has lead to concerns that a conflict might emerge with an extraterrestrial species in the future.

At present, the USR Space Force is a small fleet, more a Coast Guard than a navy. Given the near-unity of human space, their are few enemies left to fight- its work is mostly customs, anti-terrorism and anti-smuggling operations, and disaster response/search and rescue. It maintains space ports a training academy, and an HQ on Earth (in the American and Chinese provinces), as well as an orbital station and a pair of small observation posts/resupply bases in the asteroid belt, and a satellite network for surveilence and communications across the Inner Solar System (now expanding out to Jupiter). Its fleet consists mainly of a dozen transport shuttles (mostly based at Earth), four large, mostly automated transport/cargo vessels, and two squadrons of drone fighters based in Earth orbit for interception. The vessels are mostly armed with rail guns and point-defence lasers (point-defence lasers can also be found around all USR Space Force facilities), though the drone fighters can be equipped to carry tactical nuclear missiles.

However, the powers that be have dictated that greater offensive capability, particularly in deep space, is needed. Hence, you have been ordered to draw up a plan for a new Republic Cruiser. Its minimum specifications must include:

1. Ability to carry and fire nuclear missiles, as well as to shoot down inbound missiles.

2. Ability to intercept and destroy inbound vessels or missiles in the Outer Solar System.

3. Ability to conduct patrols out to Saturn, and to patrol for at least two years' duration without requiring resupply.

4. Ability to conduct boarding operations against a disabled space craft or space station, while limiting further collateral damage.

5. High automation is preferable, but so is the presence of at least minimal human crew.

6. While you have quite a bit of latitude regarding budget, the accountants would prefer if you try to keep the price tag down. The government wants to build and maintain at least four of these craft over the next decade, in addition to an initial test-bed/prototype, and they'd rather not have to cut back on social programs or existing defence programs. Money can be siphoned from the space exploration programs if necessary.

[Zeropoint]

with AIs having far surpassed human capabilities

I ask the AIs to design it for me, obviously. This is too important an effort to accept sub-standard work just to stroke the human ego.

[The Romulan Republic]

That's a fair point, and perhaps I should have hand-waved the AI such a society might have away, but I'm interested in hearing peoples' thoughts on what such a ship might be, regardless.

We'll say that the AIs will check over any design, but humans have to be included in the design process as a government make-work program.

[Starglider]

As the head of the starship design committee, I will not be designing anything. My job will be to mediate the office politics of the committee members who manage the department leads who set the direction for the actual senior designers. I will deliver a couple of inspiring kick-off speeches, choose which advanced research projects are ready to try and migrate into actual application in this design cycle, and spend most of the time arguing with political sponsors about why cutting the budget will make them less electable.

[FedRebel]

Presuming that the United States was a founding member of the USR, 'Project: Orion' is on the table

The presence of lunar infrastructure can make manufacturer easier, and could up the size.

The vanilla 1960's design would arguably be sufficient, 500 20MT nuclear warheads, 3 5inch guns, 6 Casaba-Howitzer launchers (more or less cheap disposable phasers), and 3 20mm CIWS turrets. (on paper it can stand up to a Galaxy Class in a fair fight.)

In 2120 no doubt practical pure fusion is viable, so update the 'Space Battleship' with a fusion propellant variant of Orion Drive, that will up performance over fission charges significantly.

The 5 inch guns would be replaced with Railguns (should be commonplace in 2120) of comparable if not better caliber. The CIWS, obviously traded for laser turrets.

No doubt tech would allow for Casaba-howitzer and missile yield to be upped drastically.

Again, in 2120, with Lunar infrastructure. We can scale things up significantly, add more CIWS/laser point defense emplacements, throw in more offensive weapons coverage, add "Rods from God" to the arsenal, etc.

[Knife]

I propose baby steps.

Step one the development of a small attack ship. Seeing how there isn't any real large ships to jump to a heavy dedicated warship is folly. I propose first to create a small-ish war ship as both test bed for needed systems and to train crew and develop tactics with current technology.

So the Fast Attack Ship (FAS) that is colloquially known as a Gunship will be developed. The gunship will bridge the gap of warship and police action ship that they have been doing, IE: anti piracy and SAR. Approximately 110 meters in length, the ship is driven by a magneto inertial fusion drive with a smaller version of the drive set up as a power plant and plasmoid generator. The power plant is located in a central engineering hull with the drive engine aft. A large ring dominates the aft of the craft and serves as both part of the fusion drive and the starting place for for the four pieces of Deuterium-Lithium mass that get injected into the drive by electromagnets that, along with the plasmoid that derives from the power plant, ignite the nuclear fusion that drives the ship. The large ring, usually referred to as the Drive Coil, also serves as an additional EM generator that forms a magnetic field around the ship that helps shield it from radiation and small particles during navigation.

As the nickname 'gunship' indicates, the main armament for the ship are 3 point defense turrets. Each turret has two sets of multi barreled 'gattling gun' style high rate of fire CIWS weapons firing a slug of caseless projectiles. One turret each to the port and starboard hull and one mounted to the nose of the vessel. The gunship also has 4 preloaded missile pods, 8 missile tubes per pod. Various load outs exist from all 32 slots being kilo to mega dial a yield fusion warheads, to various probes, relays, and buoys available.

Crew decks are heavily armored arms that go length wise down the dorsal and ventral hull with deck orientation built upon using a 1 G acceleration of the main drive as artificial gravity. In powered flight mode, 'up' is towards the bow and down is towards the engine nozzle in the stern. For non powered flight when the ship is either coasting, orbiting, or in dock, the two arms rotate out and spin for centripetal artificial gravity. In this mode only the 'bottom' decks receive around 1/3 G while the rest of the ship is in microgravity or zero G.

Gunships can operate either alone or in squadrons. Single ship operations include patrol, SAR, interdiction, customs, and reconnaissance. Squadron missions include escort, anti drone and anti missile defense, and area destruction missions via orbital bombardment.

NOTE: yes I am not a great modeler, images for you to get the idea.



Starboard side of gunship in powered flight mode.



Crew areas extended for rotational gravity.



Port side engineering hull with port weapons turret.

[Knife]

Forgot I could export to another program for a render. This should at least look better.

[The Romulan Republic]

Oh, nice gunship Knife.

And yes, its a good idea for an intermediary step.

[Knife]

https://www.youtube.com/watch?v=r-jlv1x3ov4

Stumbled across this the other day, well weeks ago, and thought it was need. Relevant part starts around 2:00. I envision a refined version of this for my gunship. Two actually, one small version in the engineering hull hooked up to basically a generator for power plus send the plasma it makes down range to the actual engine, and the second one for the actual propulsion. Seems from what I've read it's an open cycle too, mostly, so a significant amount of the heat is sent down range with the exhaust mass. So win win there. Fuel mass is low, thrust high. It's not like a torch ship or anything but still looks good on paper to my science layman eyes. Plus, it's fusion, so any damage to the system and it just stops. No huge kersplosion from volatile fuel.

[Sky Captain]

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

[Shroom Man 777]

How about this design:

I would go crazy and make it some double-hulled thing where the external hull is mostly some huge ass aerogel armoring, a kinetic impact-absorbing AND direct energy weapon-dissimating marshmallow coating around the central manned or unmanned pod + propulsion system. The weapons would protrude through this soft puffy "armor"... so the central pod (be it spherical or cylindrical or rectangulocubical) would have "spokes" like a bicycle or something that reach THROUGH the marshmallow armoring. These spokes/turubles can be rigid but with joints, allowing for lines to provide power for the externally-mounted lasers, or for the PHOTONIC MASTS, sensors, comms or access ports for the external docking bays. And control of the engines. Missile systems can be pseudo-isolated externally, with the marshmallow enabling blowout, and the tubule-spoke can just be for command purposes.

It would be a space AMOEBA-ZEPPELIN.

Hmmm... more conservative pod-like vessels ala Knife's ship... the KNIFESHIP... could be the regular combat vehicles.

Mine... the reason for the large size and surface area of the puffy marshmallow armor, as opposed to miniaturization, would be... because it also carries valuable cargo? So it is either a HEIGHLINER carrying valuable spice or whatever fuels, or transporting troops or other assets, or it will poop out DRONES or can itself act as an area-control ship. Presumably for the KNIFESHIPS that use compact size rather than marshmallow armor.

SHROOMSHIPS.

The tubule-spokes could connect to really huge detachable external solar panels/radiators that can look like enormous sails or fins. So this puffy marshmallow ship can look like an enormous SUNFISH. A MOLA MOLA!

[FireNexus]

Do we have something like an Epstein Drive? Or even a less efficient and sustainable form of high-G fusion thrust? Because the design may change dramatically with the ability to sustain high G.

With an Epstein, I suspect your ultimate goal is to mass-produce the Donnager-class or something like it. Without, you probably want to get pretty close to it, but have a spin section so your marines can be in as much gravity as possible to keep them at peak physical condition. If you can't do well enough pharmacologically, that is.

[FireNexus]

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

Railguns are useful at close quarters where space-relevant explosives might nuke you. But if you can mass-produce ships automatically with minimal squishy crew, you might be best off to design them as expendable if they're unlucky enough to be fighting up close.

[Zixinus]

I would approach this more as a research project and pretty much just set out the task (read: the hard and important decisions) to people that not only are qualified to answer but would provide answers to questions I could barely begin to ask.

The thing is, I have no idea how space warfare really works. So the real question is finding out how it would work. Interview existing experts, create teams to work out ideal roles and such. Have the AIs run simulations.

Prioritize what role is most essential. Should the ships be enforcer units? Should they have room for further weapons and have existing capability in case aliens attack? Should we focus on providing powerful missiles or should we try some sort of long-range energy gun (x-ray laser? railguns?) ? Or both? Hell, can we even list the roles needed? Just how long range can we create for long-range weapons?

Because the more I think of it, that's kind of what is needed here. At most, current ships are equipped to have short-to-medium range warfare. You want to create something that trumps that, a ship that can dominate with just its very presence and attack other ships (or targets) before they are attacked. Plus be able to destroy any spaceship that decides to burn all of its fuel into a suicidal blind-attack.

A slap-on-my-knee priority listing:
1. Anti-relativistic weapons (alien attack, prevent interplenatary war from devestating Earth). Destruction of solid objects travelling at significant fraction of light-speed. Necessates powerful, long-range energy weapon as well as powerful detection capabilities, plus powerful engines.
2. Deep-space ship-on-ship warfare (alien visitors, rouge ships). With some focus on pin-point accuracy destruction for disabling. Good armor fit fit to stop human technology. If aliens have far more powerful weapons we are boned anyway.
3. Deep-space-to-planetary-orbit warfare (destroy orbital objects).
4. Away-team with lander (not just for disabled spaceships but in case of rescue missions).

I would start design around looking at what existing propulsion systems are available. After all, only that will give us good ideas what the spaceship will be capable of. Priorities:
1. Range (set out, needs to be independent).
2. Detection (a blind warship is useless, long-range awareness gives the most breathing room for whatever tactic is necessary).
3. Communication with other observer equipment with appropriate hardening for computer and signal warfare.
4. Anti-missile defenses (most likely and credible threat), long-range energy weapon paired with interceptor missiles
5. Protection from short-ranged human weapons. Either armor or other point-defences. Physical shield maybe?
6. Short-range warfare (low priority because other, smaller ships are better suited to it).
7. Away team.
8. Planetary bombardment.

[Sky Captain]

Do we have something like an Epstein Drive? Or even a less efficient and sustainable form of high-G fusion thrust? Because the design may change dramatically with the ability to sustain high G.

With an Epstein, I suspect your ultimate goal is to mass-produce the Donnager-class or something like it. Without, you probably want to get pretty close to it, but have a spin section so your marines can be in as much gravity as possible to keep them at peak physical condition. If you can't do well enough pharmacologically, that is.

Yeah sustained high G propulsion would change things alot. If fusion drive don't scale down to something suitable for missiles it could make chemical powered missiles inefective at long range combat. Chemical fuel would allow around 10 km/s delta V in reasonably sized missile (something similar size to submarine launched ICBM). If warships have sustained acceleration of several G then few minute thrusting could make missiles miss entirely even with standoff warheads. Epstein drive could force combat down to shorter ranges where chemical missiles with high, but short duration acceleration can catch up and where lasers, particle beams and maybe railguns if you can get their muzzle velocity high enough also could be effective.

Railguns are useful at close quarters where space-relevant explosives might nuke you. But if you can mass-produce ships automatically with minimal squishy crew, you might be best off to design them as expendable if they're unlucky enough to be fighting up close.

If you are that close then particle beams and lasers also would be extremely effective. Particle beams have nice bonus that they can bypass armor and irradiate inside ships with lethal radiation doses.

[Beowulf]

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

Nothing says the railgun has to fire dumb slugs. They could be guided, in which case they have the advantage of leaving the ship at a relatively high velocity, even if you don't get as much spam as you could with missiles. You could even fit them with warheads, so they don't necessarily have to hit to cause damage, or can cause damage over a larger area.

---

I'll note that the specified tech that exist is around 95% of what you need to just build a carrier. Use the drones as the main striking power of the ship, and have the ship itself armed with self defense grade weaponry. The drones, being closer to the fight, will be able to conduct interceptions further out, making the ship more survivable. The drones can also carry and fire nuclear missiles. A sufficiently large carrier would make it possible to conduct patrols to Saturn. Onboard shuttles would allow boarding operations. The existing transports are already highly automated, allowing reduced crew. And because it's all based on existing technology, the price tag can be kept down, allowing more of them to be built.

Knife did have a good idea with the rotating crew decks. I would venture that they probably shouldn't be heavily armored, however, and instead be considered expendable in combat. Battlestations would be further inside the ship, where you don't have a weak link of a hinge holding the crew to ship.

[Knife]

Yeah, problem with long term space flight is going to be balancing the protection needed via radiation and enemy weapons and affects those have on mass for the engines. Plus, long patrols in zero gee are going to mess your personnel up. You're going to need a balance.

[B5B7]

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

Missiles in space have a problem called Delta V. Missiles in air and water can steer because they have a medium with which to easily interact to change direction. In space there is no such medium, and the only way a missile can change direction of motion is to change its heading and then do a long burn. Missiles have limited fuel and every course change it does will quickly reduce that fuel. Kinetic kill weapons have a good relative velocity versus target ship, and high probability hit calculations can be made, plus less detectable than missiles.
With real-world physics it is harder to evade kinetic projectile than a missile.

[Sky Captain]

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

Nothing says the railgun has to fire dumb slugs. They could be guided, in which case they have the advantage of leaving the ship at a relatively high velocity, even if you don't get as much spam as you could with missiles. You could even fit them with warheads, so they don't necessarily have to hit to cause damage, or can cause damage over a larger area.

It would need to be very large and heavy railgun system, you essentially are taking away the first stage of a missile and replacing it with railgun.

Missiles in space have a problem called Delta V. Missiles in air and water can steer because they have a medium with which to easily interact to change direction. In space there is no such medium, and the only way a missile can change direction of motion is to change its heading and then do a long burn. Missiles have limited fuel and every course change it does will quickly reduce that fuel. Kinetic kill weapons have a good relative velocity versus target ship, and high probability hit calculations can be made, plus less detectable than missiles.
With real-world physics it is harder to evade kinetic projectile than a missile.

It would require very high volume of fire. If a targeted ship can do several G burns potential space where it can be just after half a minute is fairly large and you would have to fill all that potential space with bullets to have a good probability of hit. At some point mass of all those bullets would equal many missiles. Another advantage of missiles is they don't create waste heat. Railguns with high rate of fire would generate a lot of waste heat from all kinds of inefficencies in nuclear reactor > electricity > pulsed power supply > railgun > fired bullet event chain. Heat radiators are big and fragile, very mass intensive to armor.

[Beowulf]

Nothing says the railgun has to fire dumb slugs. They could be guided, in which case they have the advantage of leaving the ship at a relatively high velocity, even if you don't get as much spam as you could with missiles. You could even fit them with warheads, so they don't necessarily have to hit to cause damage, or can cause damage over a larger area.

It would need to be very large and heavy railgun system, you essentially are taking away the first stage of a missile and replacing it with railgun.

Yes, you are. But since you'd be kicking it up to those velocities anyway, this isn't actually an all that great argument. It's already going fast, because dumb slugs are going to have to be fast to do damage. So adding a small mass penalty in order to improve the hit rate is probably a win.

Missiles in space have a problem called Delta V. Missiles in air and water can steer because they have a medium with which to easily interact to change direction. In space there is no such medium, and the only way a missile can change direction of motion is to change its heading and then do a long burn. Missiles have limited fuel and every course change it does will quickly reduce that fuel. Kinetic kill weapons have a good relative velocity versus target ship, and high probability hit calculations can be made, plus less detectable than missiles.
With real-world physics it is harder to evade kinetic projectile than a missile.

It would require very high volume of fire. If a targeted ship can do several G burns potential space where it can be just after half a minute is fairly large and you would have to fill all that potential space with bullets to have a good probability of hit. At some point mass of all those bullets would equal many missiles. Another advantage of missiles is they don't create waste heat. Railguns with high rate of fire would generate a lot of waste heat from all kinds of inefficencies in nuclear reactor > electricity > pulsed power supply > railgun > fired bullet event chain. Heat radiators are big and fragile, very mass intensive to armor.

It doesn't require as high a volume of fire if the KKW has some divert capability. Ships being able to do several G burns for significant periods of time implies fairly good propulsion, in which case you can equally well assume the guided rounds have the same propulsion (unless the propulsion has some economy of scale thing going on, like Orion drive). We don't actually know the drive tech, beyond something something fusion is possible. And heat radiators don't have to be fragile. Big, yes. But there's such things as droplet radiators. Or you could go open loop: just dump the heat into a working fluid you dump overboard. Open loop might have a lower mass cost than missiles.

[LaCroix]

Armament is an equation of mass&cost&heat vs damage&hit probablity per shot

Missiles: High/Very high mass and cost & Little to none heat per shot vs highest damage and lowest hit probablity (evasion & defensive fire) potential damage to yourself at close range
Ballistic guns (guided, maybe nuclear): Low/Medium mass and cost & Very Little heat buildup per shot vs moderate/high damage and low/moderate hit probablilty (evasion is a thing, but point defense less so), still potential damage to yourself at close range if nuclear
Railguns: Low mass (no propellant, just slug, guided), lower cost, but high heat buildup vs moderate damage and moderate hit propability (evasion risk, but little point defense risk), useful at short ranges, but with caveat on barrel heat issues/barrel life.
Lasers: Very low mass requirement (fuel burned for power), low cost (reacrot runs, anyway), heat buildup low to moderate vs low to moderate damage and very high hit probability (no air disturbing the beam, and even smoke dispensers are less useful in space where stuff dissipates quickly, and you only have a few meters of it, at most.) Only weapon with an actual range limit, depending on focusing ability.
Particle beams : low mass requirement (reactor fuel doubles as ammunition), no real cost added and heat buildup was there, anyway due to reactor running. Low to moderate damage with irradiation bonus, medium to high hit probability, depending of type.

If you look at it railguns for long distance and lasers for closer engagements/point defense are the best options for combat firepower. MW lasers are a thing, already, and the heat is not that bad to handle, you might be able to dump it into the fuel tank via cooling pumps as a buffer (since the fuel tank will usually less than full, it can handle a bit of thermal expansion of the fuel. might even be helpful fuels need to be above space temperature to be used in a reactor, anyway ), for later dissipation over the regular hull heat sinks. Same for the railgun barrels, capacitors. Energy is mostly a free byproduct on a ship with a fusion propulsion, anyway.

My armament proposal:
Build big main weapon(s) along ship axis, and a couple of lower powered turrets (YAL-1 on steroids) as secondaries for closer range and multiple opponent engagement.
An additional array of high kw/low Mw lasers (THAL sized or up) for point defense is a very effective shield against impactors in space. They are small and could be employed in turrets, and without air dissipation could engage incoming fire very far out.
A higher weapon count (especiall with lightspeed weapons as lasers) will allow for a quick disabling of enemy weapon emplacements once they are identified, and reduces the risk of losing all of your own in one salvo.

The secondaries and point defense weaponry will also work fine (less overkill) for small opponents and space trash / small asteroid removal, if need be.

Add some nuclear tipped shells for the railgun and maybe a dozen big nuclear missiles as a firepower buff against big and less agile targets (like an incoming asteroid or ships too big to maneuver a lot even with a torch drive), and you get a relatively small ship with a pretty wide arc of engagement scenarios.

[Khaat]

Kind of a fan of an engine-boosted "comms laser" (The Warriors, Niven), plus point defense lasers or missiles, "hot junk droppers" (flares/chaff). Decoy balutes [inflatable decoy ships] (Aliens: Colonial Marines Technical Manual).

Possibly more along the lines of autonomous/semi-autonomous satellites than ships. If the AIs are any good, that's what they would recommend: humans are only good as corpsicles in space combat.

That would mean nuke-boosted lasers (not drive-system boosted) one-shot killer satellites, probably operating in an AI-designed multi-layered defense network. Maybe with a back-up swarm/impactor satellite network element.

And the window-dressing nuke-missile boat/human-showpiece (with boarding killer spiderbots.) The Captain will be pretty. The 1st Officer will be pretty. Hell, the entire crew of 7-10 will be "pretty 20-30 somethings" (or contemporary equivalent), so in the season finale, when the [[Spoiler

]], we definitely get renewed for another three seasons.

[Sky Captain]

Nothing says the railgun has to fire dumb slugs. They could be guided, in which case they have the advantage of leaving the ship at a relatively high velocity, even if you don't get as much spam as you could with missiles. You could even fit them with warheads, so they don't necessarily have to hit to cause damage, or can cause damage over a larger area.

It would need to be very large and heavy railgun system, you essentially are taking away the first stage of a missile and replacing it with railgun.

Yes, you are. But since you'd be kicking it up to those velocities anyway, this isn't actually an all that great argument. It's already going fast, because dumb slugs are going to have to be fast to do damage. So adding a small mass penalty in order to improve the hit rate is probably a win.

Missiles in space have a problem called Delta V. Missiles in air and water can steer because they have a medium with which to easily interact to change direction. In space there is no such medium, and the only way a missile can change direction of motion is to change its heading and then do a long burn. Missiles have limited fuel and every course change it does will quickly reduce that fuel. Kinetic kill weapons have a good relative velocity versus target ship, and high probability hit calculations can be made, plus less detectable than missiles.
With real-world physics it is harder to evade kinetic projectile than a missile.

It would require very high volume of fire. If a targeted ship can do several G burns potential space where it can be just after half a minute is fairly large and you would have to fill all that potential space with bullets to have a good probability of hit. At some point mass of all those bullets would equal many missiles. Another advantage of missiles is they don't create waste heat. Railguns with high rate of fire would generate a lot of waste heat from all kinds of inefficencies in nuclear reactor > electricity > pulsed power supply > railgun > fired bullet event chain. Heat radiators are big and fragile, very mass intensive to armor.

It doesn't require as high a volume of fire if the KKW has some divert capability. Ships being able to do several G burns for significant periods of time implies fairly good propulsion, in which case you can equally well assume the guided rounds have the same propulsion (unless the propulsion has some economy of scale thing going on, like Orion drive). We don't actually know the drive tech, beyond something something fusion is possible. And heat radiators don't have to be fragile. Big, yes. But there's such things as droplet radiators. Or you could go open loop: just dump the heat into a working fluid you dump overboard. Open loop might have a lower mass cost than missiles.

Scaling would be an issue, all plausible fusion drive concepts are big and expensive systems, if we try to stick to plausible technology it is unlikely that fusion drive would scale down to small missiles and gun shells. A large fusion powered missile bus may be possible with big fusion first stage that contains several high acceleration chemical missiles with standoff warheads. If this is somewhat correct then missile with Casaba howitzer warhead would be devastating weapon capable of striking from several thousand km distance, or even more for more advanced models. Lasers would have hard time intercepting missiles that have to come only within 5000 to 10 000 km distance to deliver killing shot. Only good defence may be defensive missiles with similar warheads to intercept incoming shipkillers outside standoff range.

In this context railguns may be useful as launchers for casaba howitzer warheads since you don't need much terminal propulsion if 1000 km from target is good enough if math works out that railgun is better from cost and mass perspective than extra boost stage for each warhead.

[Simon_Jester]

Not everything is positive, however. The Martian colony is largely self-sufficient in design, and many of its residents resent Earth rule. Reports have reached USR Intelligence that their is a growing Martian secession movement. More long-term, the discovery of sapient life elsewhere in the US has lead to concerns that a conflict might emerge with an extraterrestrial species in the future.

Does the united Earth government actually intend to fight if the Martian colony secedes? This is an important consideration. In a society where the world is fairly wealthy, and there aren't megacorporations dominating events on Mars or whatever... There's no obvious reason NOT to negotiate a degree of independence, even a very large degree.

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ON THE ORION DRIVE BATTLESHIP PROPOSAL

Presuming that the United States was a founding member of the USR, 'Project: Orion' is on the table

The presence of lunar infrastructure can make manufacturer easier, and could up the size.

The vanilla 1960's design would arguably be sufficient, 500 20MT nuclear warheads, 3 5inch guns, 6 Casaba-Howitzer launchers (more or less cheap disposable phasers), and 3 20mm CIWS turrets.

This fails design requirements (1b) and (4).

The proposed CIWS and gun mounts are inadequate for the stated need for missile defense. If we can design nuclear shaped charges so can a hypothetical enemy, which means they will have standoff range against this ship of, oh... I don't know, 10-100 kilometers. Purely ballistic antimissile defenses start looking inadequate towards the outer limit of that range.

The twenty-megaton bombs are too heavy unless you're using them as propulsion bombs; the extra mass will impair their ability to be placed on a viable delivery platform.

Also, are we going to have to launch these things from the ground? Launching Orion drive ships from the ground is a very bad idea. On the Moon it's less stupid, granted.

(on paper it can stand up to a Galaxy Class in a fair fight.)

...Seriously? Anything from Star Trek, or for that matter most other fictional soft-SF settings, could outmaneuver such a ship, and its projectile weapons, like its feet were nailed to the floor. Casaba Howitzer's range against soft-SF targets isn't amazing either. The jet angle for a shaped nuclear charge is narrow- but it's a cone, not a straight line, and the energy you actually deliver to the target drops off with the square of the distance.

In 2120 no doubt practical pure fusion is viable, so update the 'Space Battleship' with a fusion propellant variant of Orion Drive, that will up performance over fission charges significantly.

The 5 inch guns would be replaced with Railguns (should be commonplace in 2120) of comparable if not better caliber. The CIWS, obviously traded for laser turrets.

Caliber of the railgun is largely irrelevant; the rounds don't have to be very big to hit hard enough that they overcome any foreseeable protection scheme. Plus, muzzle velocity is really important. The probability of you actually hitting a maneuvering target with a bullet in space, at long enough ranges that missing is even possible, drops off very fast. In fact, it's inversely proportionate to the fourth power of the time it takes your bullet to get there. Double the range, and you will hit 1/16 as often. Yes, I can prove that with math.

[Incidentally this is also the main reason to use guided missiles; they don't have that problem]

No doubt tech would allow for Casaba-howitzer and missile yield to be upped drastically.

You'd do better to focus on narrowing the jet angle. Shaped nuclear charges are about the only counter you have to improved antimissile defense, but they're desirable for their standoff range, not their yield. This is a hard-SF setting; if a fifty-kiloton nuclear shaped charge doesn't have enough energy to destroy whatever you were shooting at, you probably shouldn't have pissed it off in the first place.

Again, in 2120, with Lunar infrastructure. We can scale things up significantly, add more CIWS/laser point defense emplacements,

Okay, now this is a good idea. Mainly because by 2120, point defense lasers are likely to be strictly superior to ballistic systems for antimissile defense.

throw in more offensive weapons coverage,

More weapons is almost certainly not a good idea compared to better weapons, or spending the extra tonnage on better propulsion, better crew endurance, and so on.

add "Rods from God" to the arsenal, etc.

The only time that Thor-type systems make sense as a weapon is if you're putting them in orbit in order to bombard targets on Earth with it. I figure your government isn't planning to shoot at itself. Aside from that, direct fire from railguns or laser mounts would be quite adequate.

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On Shroom Space-Amoeba-Ship Thing

How about this design:

I would go crazy and make it some double-hulled thing where the external hull is mostly some huge ass aerogel armoring, a kinetic impact-absorbing AND direct energy weapon-dissimating marshmallow coating around the central manned or unmanned pod + propulsion system. The weapons would protrude through this soft puffy "armor"... so the central pod (be it spherical or cylindrical or rectangulocubical) would have "spokes" like a bicycle or something that reach THROUGH the marshmallow armoring. These spokes/turubles can be rigid but with joints, allowing for lines to provide power for the externally-mounted lasers, or for the PHOTONIC MASTS, sensors, comms or access ports for the external docking bays. And control of the engines. Missile systems can be pseudo-isolated externally, with the marshmallow enabling blowout, and the tubule-spoke can just be for command purposes.

It would be a space AMOEBA-ZEPPELIN...

The tubule-spokes could connect to really huge detachable external solar panels/radiators that can look like enormous sails or fins. So this puffy marshmallow ship can look like an enormous SUNFISH. A MOLA MOLA!

This is actually probably a better idea than it sounds. You might want an outer layer of plating and a rigid framework just so there's something to bolt stuff like radar dishes to, so that your amoeba is more like a vertebrate with an internal skellington, and less like a beached jellyfish. But wrapping the core of the ship in layers of low-density ablative materials may well be a good way to provide protection. And while mass is precious in space, volume really doesn't matter very much, except if you're trying to dodge ballistic weapons like bullets in which case you want a small hitbox.

Which might be a good time to use drone ships anyway. This goes well with the idea of designing a space carrier, actually. You're a big target, but a reasonably well protected one, and you try to stay out of range while drones destroy whatever dangerous targets you can't afford to get too close to.

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OTHER STUFF

Would railguns actually be good idea? They are large, heavy systems requiring a lot of electrical energy. For the mass taken by railgun and hefty power system for it you could carry a lot of missiles. Missiles are guided so at least there is good possibility of actually scoring a hit on evading target. With railguns it would be down to luck to hit evading target at more than few hundred km distance. Missile with nuke pumped laser or Cassaba howitzer warhead to avoid worst of enemy point defences would be ultimate hard sci fi anti ship weapon.

Lasers or particle beams may be good for point defence and soft kill like when suspicious freighter refuse to dock with space patrol for inspection and similar circumstances when pinpoint accuraccy is required and total destruction of the target should be avoided.

Missiles in space have a problem called Delta V. Missiles in air and water can steer because they have a medium with which to easily interact to change direction. In space there is no such medium, and the only way a missile can change direction of motion is to change its heading and then do a long burn.

The catch is that enemies also have the ability to do engine burns to evade a target. Beyond point blank range (where the enemy lacks the ability to take effective evasive action), an unguided projectile becomes very unlikely to actually hit the target.

Don't think of kinetic projectiles in terms of bullets fired from a gun at your opponent. Think of them in terms of rocks thrown at an opponent. It usually isn't that hard to dodge a thrown rock at long range, if you're alert. And this doesn't even rely on being able to see it coming, because an unguided bullet can be thrown off by purely random course changes. All the enemy has to do is make sure their hundred meter ship is, say, 500 meters from where it was "supposed" to be when the bullet arrives.

This was actually a serious problem with real life naval warfare during the World Wars. Gun ranges increased to where it took a minute or more for shots fired by one ship to reach the location of another ship, and ships could easily zig-zag back and forth enough to throw off each other's aim. As a result, hit rates declined to the single digit percent level, and sometimes even worse- and that was with ships that could only evade in two dimensions!

[Solauren]

Well, you don't need areodynamics in space, so really, all you need is a big flying box with thick outer walls, and covered in weapons "just in case".

In truth, if it never needs to land, a sphere is the perfect shape. You can put thrusters all around it for maximum maneuverability.
Same with guns.