Some questions of 40K

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Re: Execution hour - tech discussion and analysis thread

Postby Emperor of Mankind » 2009-08-04 02:40pm

Greetings! Long time listener, first time caller. :D

- the palace shields evidently ARE quite similar to void shields, and torpedoes are "designed" to penetrate them "harmlessly" (implying little or no interaction, ie some sort of shield-penetrating countermeasure.) It is also implied that the torpedoes rely on the momentum of the high-density warhead to penetrate through hulls before detonating (Implying that torpedoes have a "kinetic impactor" element to the damage they inflict in addition to the warhead element.


So would you say, then, that this might imply that torpedoes have a capability to bypass shielding? As Grand Admiral Ancaris points out, slow moving attacks can bypass shields. But the text states that "the first missile hit the palace at tremendous speed", so it would seem that it didn't get past those shields due to moving slowly. It got past them some other way. So could we be looking at "shield breaker" torpedoes? Similar to the shield-breaker ammunition that Vindicare Assassins carry around for their Exitus Rifles?

And if so, how common would such torpedoes be? Do all torpedoes move at similar speeds, or are some much slower (and as such, capable of penetrating void shields without any "shield-breaking" effect)?

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Re: Execution hour - tech discussion and analysis thread

Postby Connor MacLeod » 2009-08-04 03:57pm

Hi. First off, please do not resurrect old threads to simply ask questions UNLESS I have contributed something new or updated them. This is technically necro-ing and could get my threads closed, which would annoy me to no end. If you have questions, use my private messaging function here to ask me questions. Or, start a new thread.

Secondly, I will answer your question. THIS TIME.

So would you say, then, that this might imply that torpedoes have a capability to bypass shielding?


Well, evidently they do. We just aren't sure what the mechanism for penetration is, and there's not any "single" consensus about how it does it.

As Grand Admiral Ancaris points out, slow moving attacks can bypass shields.


It could also get by the shields by being technobabble. Anything involving the tyranids is inherently technobabble. I dislike that statement because t have any real range (thousands/tens of thousands of km) it would have to be guided OR to travel at a high velocity that would strike in a few seconds at most, and the latter won't mesh with the "slow moving" bit.

Of course, alot of that is interpeting gameplay, so it may refer to some momentum-based "penetration" effect. Torpedoes are big, massive, and tough as nails. They're designed ot punch through hulls and they may be designed to brute force through shields as well (so much momentum they can knock out the generators or short them out.)

Or, it may be technobabble. I favor the latter, but as I said, my view is hardly the sole one.

But the text states that "the first missile hit the palace at tremendous speed", so it would seem that it didn't get past those shields due to moving slowly. It got past them some other way. So could we be looking at "shield breaker" torpedoes? Similar to the shield-breaker ammunition that Vindicare Assassins carry around for their Exitus Rifles?


Perhaps. I have noted that in Shadow Point fighteres were threatene dby destruction by the activation of void shields despite having little or no velocity, so there may be some shield penetration qality. Or there may not be, and its just a matter of momentum. Or, there may actually BE some speed based limitation on torpedoes. Or it may be a combination of effects.

And if so, how common would such torpedoes be? Do all torpedoes move at similar speeds, or are some much slower (and as such, capable of penetrating void shields without any "shield-breaking" effect)?


No way to tell. Given the way technolgoy in the Imperium works, it could even vary depending on the kind of shield or the kind of torpedo. Discussion of void shields occured in the 3rd/4th edition 40K analysis thread when I was covering the Orks, so you might want to check those out.

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Re: Execution hour - tech discussion and analysis thread

Postby Emperor of Mankind » 2009-08-04 04:57pm

Connor MacLeod wrote:Hi. First off, please do not resurrect old threads to simply ask questions UNLESS I have contributed something new or updated them. This is technically necro-ing and could get my threads closed, which would annoy me to no end. If you have questions, use my private messaging function here to ask me questions. Or, start a new thread.


Sorry. Didn't think it'd be a problem so long as the post was substantive (whereas most cases of necroing are people bumping old posts just to say "I agree" to something or something equally trivial), and I assumed it'd be less bothersome than starting a whole new thread just to ask. :(

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Re: Execution hour - tech discussion and analysis thread

Postby Ghost Rider » 2009-08-04 05:08pm

Dude...the thread is nearly three fucking years old.

Locking because really...if you've been lurking you'd at least see an occasional lock on threads that are a month old, unless they are threads of a very very specific nature.
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Re: Execution hour - tech discussion and analysis thread

Postby Feil » 2009-08-05 04:21pm

Connor MacLeod wrote:Of course, alot of that is interpeting gameplay, so it may refer to some momentum-based "penetration" effect. Torpedoes are big, massive, and tough as nails. They're designed ot punch through hulls and they may be designed to brute force through shields as well (so much momentum they can knock out the generators or short them out.)


Presumably you already know this, but if you or others don't...: if a weapon has enough momentum, it will defeat shielding, even if the shields could absorb and dissipate all the kinetic energy and mass-energy of the projectile safely. Momentum is conserved. In order to stop the projectile, the shield generator has to transfer the projectile's momentum to the ship. That means that any defender has a choice:

1: take the impact on the armored hull
2: take the impact on the weakest link in the chain of momentum transfers between the shield generator and the ship; then, if that fails to stop the projectile, take the impact on the hull anyway.

Unless the shield generator has many dispersed points which soak up momentum and very strong physical structure to withstand the impact, it makes perfect sense for a shield to have a safety shutoff sensor that flickers the shield off and on for a second when it starts getting pushed back into itself by the momentum of an impacting object, so that the projectile strikes the armored hull (which is designed to withstand such impacts) in stead of crushing the comparatively fragile shield generators as it passes through, then striking the hull anyway.

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Re: Execution hour - tech discussion and analysis thread

Postby Steel » 2009-08-05 09:38pm

Feil wrote:
Connor MacLeod wrote:Of course, alot of that is interpeting gameplay, so it may refer to some momentum-based "penetration" effect. Torpedoes are big, massive, and tough as nails. They're designed ot punch through hulls and they may be designed to brute force through shields as well (so much momentum they can knock out the generators or short them out.)


Presumably you already know this, but if you or others don't...: if a weapon has enough momentum, it will defeat shielding, even if the shields could absorb and dissipate all the kinetic energy and mass-energy of the projectile safely. Momentum is conserved. In order to stop the projectile, the shield generator has to transfer the projectile's momentum to the ship. That means that any defender has a choice:

1: take the impact on the armored hull
2: take the impact on the weakest link in the chain of momentum transfers between the shield generator and the ship; then, if that fails to stop the projectile, take the impact on the hull anyway.

Unless the shield generator has many dispersed points which soak up momentum and very strong physical structure to withstand the impact, it makes perfect sense for a shield to have a safety shutoff sensor that flickers the shield off and on for a second when it starts getting pushed back into itself by the momentum of an impacting object, so that the projectile strikes the armored hull (which is designed to withstand such impacts) in stead of crushing the comparatively fragile shield generators as it passes through, then striking the hull anyway.


But thats not a good plan at all...

Option 1: Die
Option 2: 90% chance of generators failing and then you die, 10% chance shield generators hold (and perhaps suffer damage) and you dont all die.

Pretty sure option 2 dominates option 1 there.

It isnt sensible to try and save your defensive systems at the cost of the thing they are there to defend.

Within the whole theory of shield generator momentum handling to consider, rather than just force imparted on generator by high momentum impactor, another thing to consider is impacts not aligned straight towards the generator which willl impart torque on generator.

To take a rough analogy, consider a bike wheel on a large fixed axle, where the wheel cannot rotate relative to the axle and the axle is fixed in place. In you were to push on the edge of the wheel towards the centre of the wheel then is is possible with a certain amount of force to crush the spokes. However if you instead try and spin the wheel it would be much easier to snap the spokes with the turning force.

If we have the situation of a fixed generator of a certain size w projecting its shield out to a certain range r, then oblique impactors will have a leverage advantage scaling as r/w. It is way to late to bother with a proper analysis tonight, but this could be a reason to make your shields as close to the hull as possible and have many shield generators spread over the ship. If you were to have a single generator in the middle of your ship and the shields right on the hull then someone aiming to graze the edge of your ship would actually be able to put much more stress on the single larger generator than if you had multiple smaller ones closer to the edges. The smaller ones would be structurally weaker than the single larger one and thus more vulnerable to a single high momentum projectile, but would not have the weakenss of giving such a big leaverage advantage to the projectile.

Large bubble shields would be horrifically vulnerable to this.
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Re: Execution hour - tech discussion and analysis thread

Postby lPeregrine » 2009-08-06 03:15am

Steel wrote:That means that any defender has a choice:

1: take the impact on the armored hull
2: take the impact on the weakest link in the chain of momentum transfers between the shield generator and the ship; then, if that fails to stop the projectile, take the impact on the hull anyway.




The assumption here is that allowing the shot to hit the hull is instant death. If we assume that it isn't, taking it on the shields has two significant advantages:


1) Your shields are still intact (once you raise them again). If you let the shot pass through, your shields are still there to defend against the next shot, but if you trash your shield generators failing to stop it, the rest of the shots are going to hit your armor. This is a big problem if only a small percentage of shots have enough momentum to punch right through the shields, as you just left yourself open to a lot of fire you could have stopped.


2) You're taking the hit on your armor, which is designed to absorb damage, not on the shield generators inside your ship. Which would you rather have, your armor blown off, or the shield generators trashed and who knows what other delicate stuff destroyed by the flying debris coming from a direction that isn't armored?

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Re: Some questions of 40K

Postby Coalition » 2009-08-06 07:59am

I'd try to take as much of it as I could on the shields, for the following reasons:

1) Each Joule my shield generators handle is one less Joule hitting the ship directly (and hopefully the generators are braced to transfer momentum to the structure properly instead of breaking loose)
2) Each Joule transferred to momentum in step #1 helps with my velocity away from the incoming, reducing relative velocity between my ship and the object for when the object hits the hull (amount reduced will depend on impact energy vs hull mass)

#1 will handle an amount of impact energy depending on my tech base and design. High tech and low energy means the shields handle it nicely, while low tech and high energy means that #2 comes into play, and my ship gets a ding, dent, a hole, or does a bug on windshield impression.

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Re: Execution hour - tech discussion and analysis thread

Postby Fingolfin_Noldor » 2009-08-06 08:20am

lPeregrine wrote:The assumption here is that allowing the shot to hit the hull is instant death. If we assume that it isn't, taking it on the shields has two significant advantages:


1) Your shields are still intact (once you raise them again). If you let the shot pass through, your shields are still there to defend against the next shot, but if you trash your shield generators failing to stop it, the rest of the shots are going to hit your armor. This is a big problem if only a small percentage of shots have enough momentum to punch right through the shields, as you just left yourself open to a lot of fire you could have stopped.


2) You're taking the hit on your armor, which is designed to absorb damage, not on the shield generators inside your ship. Which would you rather have, your armor blown off, or the shield generators trashed and who knows what other delicate stuff destroyed by the flying debris coming from a direction that isn't armored?

Some torpedoes are specifically armor penetrating, and if there were ever repeat instances of the torpedoes seen in Battle for the Abyss where the torpedoes penetrated armor without trouble and set a fire to the inner decks, I'd rather take my chances.
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Re: Some questions of 40K

Postby Enosh » 2009-08-06 12:16pm

that are specific melta torpedoes iirc, who melt the hull beacose of the extreme heat and then the whole molten metal fills the ship burning everything, quite fun
don't know how common they are

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Re: Some questions of 40K

Postby Feil » 2009-08-06 01:34pm

Coalition wrote:I'd try to take as much of it as I could on the shields, for the following reasons:

1) Each Joule my shield generators handle is one less Joule hitting the ship directly (and hopefully the generators are braced to transfer momentum to the structure properly instead of breaking loose)
2) Each Joule transferred to momentum in step #1 helps with my velocity away from the incoming, reducing relative velocity between my ship and the object for when the object hits the hull (amount reduced will depend on impact energy vs hull mass)

#1 will handle an amount of impact energy depending on my tech base and design. High tech and low energy means the shields handle it nicely, while low tech and high energy means that #2 comes into play, and my ship gets a ding, dent, a hole, or does a bug on windshield impression.


This plan is mechanically identical to the shields having a safety shutoff threshold after they are pushed towards their limit. We already know that shields don't shut off for any impactor. If nothing else, light at the levels of firepower thrown around by 40k ships has significant momentum, and shields stop that just fine.

Incidentally, momentum doesn't come in joules, and kinetic energy can't be 'transferred' to momentum. An object has a certain amount of kinetic energy and a certain amount of momentum; they are related by Ek = Pv and vary accordingly in any collision. In an inelastic collision (like a torpedo ripping a hole in the side of a spaceship), the vector sum of the objects' momentum is conserved but their kinetic energy is not (the Ek is transferred to other forms), which might be what you're talking about..?

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Re: Some questions of 40K

Postby Coalition » 2009-08-07 07:27am

Feil wrote:This plan is mechanically identical to the shields having a safety shutoff threshold after they are pushed towards their limit. We already know that shields don't shut off for any impactor. If nothing else, light at the levels of firepower thrown around by 40k ships has significant momentum, and shields stop that just fine.

Incidentally, momentum doesn't come in joules, and kinetic energy can't be 'transferred' to momentum. An object has a certain amount of kinetic energy and a certain amount of momentum; they are related by Ek = Pv and vary accordingly in any collision. In an inelastic collision (like a torpedo ripping a hole in the side of a spaceship), the vector sum of the objects' momentum is conserved but their kinetic energy is not (the Ek is transferred to other forms), which might be what you're talking about..?


You are right. I was trying to spell out the steps, and used the wrong terms. Conceded.

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Re: Some questions of 40K

Postby Fingolfin_Noldor » 2009-08-07 01:31pm

Photon beams with lots of photons themselves have quite a large collective momentum, so how does that get dealt with? Especially if we are talking about terrawatt to pentawatt beams, and no one is even talking about wavelength.
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Re: Some questions of 40K

Postby Feil » 2009-08-07 02:28pm

Fingolfin_Noldor wrote:Photon beams with lots of photons themselves have quite a large collective momentum, so how does that get dealt with? Especially if we are talking about terrawatt to pentawatt beams, and no one is even talking about wavelength.


First, wavelength and quantity of photons is irrelevant if we know the power of the beam. Quantum theory of light gives Momentum = E/c for a light pulse (being the sum of all the individual photons), with E the energy delivered over a given time, which implies F = Power/c. Converting from the power of a light pulse to the force it imparts on an object is as simple as dividing by 3E8.

A 1 petawatt beam will impart about 1 million newtons if all the energy is absorbed (more if it is reflected: force, like momentum is a vector). If the beam duration is 1 second, the momentum is 1 million kgm/s. Not trivial, but not a lot, either: this is the momentum of a 60-tonne loaded semi truck traveling at 40 miles per hour.

Comparatively, torpedoes in 40k are fucking huge. They have a volume somewhere on the order of 6E3m^3 (being 60 meters long and fairly thick). Assuming the torpedo has the density of aluminum it will weight 1.62E7 kg. It is hard, approaching impossible, to approximate the impact force for an inelastic collision without knowing the exact specifics of deformation, penetration distance, etc. However, momentum is easy to calculate: to have more momentum than the 1-second petawatt beam, our torpedo needs to be moving at... 0.6 m/s.

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Re: Some questions of 40K

Postby Feil » 2009-08-07 02:41pm

Note that, generally speaking, a moving physical body will impart considerably more impulse than a light beam per unit momentum, because very few high-speed impacts of (comparatively) small bodies into (comparatively) large bodies are stretched out over a full second. A 1-second pulse from a petawatt laser might be safely reflected by an extremely reflective mirror surface built over a concrete wall, (especially if the beam diameter was the size of a semi truck), while the semi truck smashing into the wall at 80 miles per hour (reflection doubles momentum so I doubled the truck's velocity to compensate) would smash it to bits.

EDIT: "dividing by 3E8" in the above post should be "dividing by 3E8m/s"

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Re: Some questions of 40K

Postby Fingolfin_Noldor » 2009-08-07 03:26pm

Feil wrote:Note that, generally speaking, a moving physical body will impart considerably more impulse than a light beam per unit momentum, because very few high-speed impacts of (comparatively) small bodies into (comparatively) large bodies are stretched out over a full second. A 1-second pulse from a petawatt laser might be safely reflected by an extremely reflective mirror surface built over a concrete wall, (especially if the beam diameter was the size of a semi truck), while the semi truck smashing into the wall at 80 miles per hour (reflection doubles momentum so I doubled the truck's velocity to compensate) would smash it to bits.

EDIT: "dividing by 3E8" in the above post should be "dividing by 3E8m/s"

A few nitpicks. 1 Pentawatt = 10^15 W. E = pc => p = 10^15/(3 * 10^8) = 3.33*10^6 kg m s^(-1). Moreover, a light pulse does not deliver a continues stream of energy, but the peak power is often many times that of the average power depending on pulse width and total energy of the pulse. Peak power is what does the most damage, and the high peak power is more desirable compared against average power. I have yet to hear of void shields acting as reflectors, so therefore the shield has to absorb all that momentum. If I wanted to inflict maximum damage, I'd use a laser that delivers as narrow a pulse (attoseconds and below), and as much energy per pulse, and deliver multiple pulses. I could get as much as a peak power of 1000 times or more times going by here: http://www.rp-photonics.com/peak_power.html

And why do I mention wavelength? Because not everything works independent of wavelength and most things in nature rarely does.
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Re: Some questions of 40K

Postby Feil » 2009-08-07 04:55pm

The word you're looking for is petawatt, not pentawatt. Penta means five; peta means quadrillion.

You are correct, I was off by a factor of 3.

Note that reflection doubles momentum transfer as compared to a , because momentum is a vector quantity.

You are correct about peak power being higher than average power. I overlooked that fact and stand corrected.

That said, P=E/c => F=Power/c are easy formulas to work with and easily show that in order for a light pulse to carry an impulse similar to a high-velocity impact of a large, heavy object, they would have to be far more powerful than the kinds of beams 40k ships are capable of producing.


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