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A large railgun mounted on the spine of a mega-scale starship (20-32km) or an antimatter-powered missile launched outside the ship? For clarification's sake, when I say "safer" I'm referring to "safer for the ship firing the weapon" rather than anything else.

I figure the railgun has more combat advantages, as the projectile spends less time in open space accelerating compared to a missile, which makes it less vulnerable to premature detection and destruction by laser-intercept weapons. Additionally, its heat signature is masked by the larger heat signature of the ship, giving the enemy less forewarning that the weapon is being fired as opposed to a missile, whose engines would appear as their own distinct heat signatures apart from the ship. On the flip side however, a missile would not impart any of its destructive energy on the ship, which is a problem a railgun would certainly have. That being said, storing missiles that are fueled by antimatter is also pretty far from safe (although a ship this size would certainly need it as fuel anyway).

So which is safer for the ship firing them while still being effective in combat, missiles or railguns?

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  • $\begingroup$ This question is about starship-launched missiles & not relativistic kill vehicles (RKVs). Railguns firing projectiles or antimatter-powered missiles capable of accelerating to relativistic velocities would be dangerous for starships launching them. I guess editing the title of your question. Delete the RKV reference and substitute "combat weapons" or whatever you think best suits your question. $\endgroup$ – a4android Oct 20 '16 at 8:41
  • $\begingroup$ RKVs are things that have to accelerate for decades or centuries to get up to speed and mass megatons. they are not things you can fire from ships, but rather are starships themselves that don't bother to slow down. $\endgroup$ – John Meacham Oct 20 '16 at 9:49
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If you have antimatter technology, use it as a warhead. The damage done by exploding matter-antimatter would be enormously large compared to any high speed projectiles.

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    $\begingroup$ Actually, nothing beats speed, at only a bit over 3/4 the speed of light, your kinetic energy becomes greater than the energy released by total conversion of matter to energy, making your warhead out of antimatter only marginally increases yield. Might as well just make it a tad faster. $\endgroup$ – John Meacham Oct 20 '16 at 9:42
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    $\begingroup$ Its not the total energy, its the damage done. At relativistic speed the projectile will just go right through the ship, leaving a small hole and wasting most of the energy, maybe never hitting a vital component. A stationary bomb up aghinst the hull would cause a much bigger hole and damage $\endgroup$ – Innovine Oct 20 '16 at 10:52
  • $\begingroup$ Also, plenty of things can beat speed. The mass being annihilated, for one. Compare several kilos of matter-antimatter being reacted together, compared to a dust particle at .999c. Maybe with equal mass the kinetic energy wins out, but "nothing beats speed" is incorrect. $\endgroup$ – Innovine Oct 20 '16 at 10:56
  • $\begingroup$ @Innovine That's why the military uses shotgun-style railguns, with a sabot and multiple projectiles. $\endgroup$ – TheZouave Oct 20 '16 at 12:01
  • $\begingroup$ Yes, exclusively. $\endgroup$ – Innovine Oct 20 '16 at 18:55
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You can actually get the best of both worlds by combining the two weapons - Railmissiles.

But what is a "railmissile"? A Railmissile is a railgun in missile form - by attaching enough high tech capacitors and some short range maneuvering thrusters to a small railgun, you can fire the "missile" away from the ship and have it maneuver into a position in which the railgun portion can fire its one shot without affecting the ship.

Assuming the railmissile was designed properly, the body of it can be retrieved and refueled + recharged in order for reuse.

In fact, if you design the projectiles properly you may even be able to launch Antimatter warheads at railgun level speeds.

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  • $\begingroup$ Your definition of 'railmissle' sounds like 'gun buddy' to me, in which case, why would it be limited to 1 shot/target? I'd imagine a railmissle being a missile that is launched via railgun, and arms soon after it leaves the launcher. $\endgroup$ – Tezra Oct 20 '16 at 15:30
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    $\begingroup$ How could you recover the missile body if it is zipping away from the battle? If it can slow down and return then you have thruster tech so advanced the rail launching part is irrelevant. If you can catch it with another ship then you STILL have thruster tech that makes the missile moot. It's like the difference in trying to catch bullets versus baseballs while on motorcycles. $\endgroup$ – Jason K Oct 20 '16 at 16:10
  • $\begingroup$ @JasonK are you familiar with how early naval cannons were designed to be able to roll backwards when fired? By designing the railmissile with the same concept in mind, we can stop the missile from zipping away from the battle after shooting the rail. Instead of having it attached to the body of the missile, you release the part that needs to move backwards due to recoil - That way, the missile body remains intact, and stays near where it maneuvered to. $\endgroup$ – Aify Oct 20 '16 at 17:42
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    $\begingroup$ @Aify so the rail launched missile stops all of its forward velocity (relative to the launching ship) when it fires it's payload? That would be some interesting calculations between payload mass, missile mass, and fuel distribution. The missile shell is probably so light and flimsy (all the expensive stuff went off in the payload) that it would be considered disposable. $\endgroup$ – Jason K Oct 21 '16 at 13:18
  • $\begingroup$ @JasonK Like I said, the re usability aspect depends on how the railmissile was designed. The fact remains that the concept would allow a ship to fire railgun shots without suffering any of the recoil effects typically attached to a railgun $\endgroup$ – Aify Oct 22 '16 at 6:33
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As noted, an RKKV is a starship in its own right, and accelerating to relativistic velocity could take decades or centuries. RKKV's are generally considered to be "planet busters", and once up to speed, won't be able to manoeuvre to hit a small target capable of independent acceleration (you can calculate the orbit of a planet decades to centuries in advance).

Atomic Rockets has a conventional weapons page with the equations and calculations necessary to work out the sorts of effects you want. Once you get into this, you will discover that extreme energies can only be reached using improbably large devices (I may have mad a math error, but when I tried the math for a relativistic launcher to propel a spacecraft at a high fraction of c, the launcher ran from Mercury to the outer gas giants...)

For more reasonable combat weapons, you still run into the issue of efficiency and heat rejection. Carrying a super sized laser, coilgun, particle beam accelerator etc. not only requires a super sized power plant to energize the weapon, but a means of rejecting the waste heat from both the reactor and the weapon itself. This means the spaceship must carry large radiators (and sites like Atomic Rockets and Tough SF have worked examples to help you figure out how much radiating surface you will need).

enter image description here

Realistic military spacecraft from "Children of a Dead Earth"

The only way to get around the heat rejection issue is to carry and fire missiles. While missiles take up space inside your spacecraft and there is the inherent danger of all the propellant and warheads being aboard, you could theoretically "ripple fire" all your missiles at once, ensuring you overwhelm your opponent or potentially devastating a planet. High rates of fire with energy weapons either need you to extend a large radiator, or stop firing to allow the systems to cool down.

enter image description here

VLS in action

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Definitely the missile.

This is what happens if you use the railgun

  1. Antimatter generator annihilates matter and antimatter
  2. Energy from annihilation is converted into electricity
  3. The energy is used to accelerate projectile using railgun

So you not only need same amount of antimatter as for the missile. You also need machinery to convert that antimatter into energy of motion for the projectile. So why go through that kind of problems and not just throw that antimatter at enemy and let it annihilate itself.

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  • $\begingroup$ You need electricity anyway on the ship so the machinery are already aboard. Maybe they just need to be a little bigger, but I don't see the problem to have them on the ship $\endgroup$ – Gianluca Oct 20 '16 at 10:15
  • $\begingroup$ Energy needed to fire the railgun at same destructive strength is few orders of magnitude bigger than what spaceship would need to keep itself running. IMO. $\endgroup$ – Euphoric Oct 20 '16 at 11:12
  • $\begingroup$ Well, to be entirely fair, this ship would already have to have produced way more power in the form of kinetic energy to get up to speed, so the railgun wouldn't actually use more than what the ship's already using/used. That being said, the ship's engine is a psuedo-reactionless drive similar to a Q-thruster specifically BECAUSE it's so massive and requires so much energy to get up to speed, so maybe the two aren't so comparable. $\endgroup$ – Z.Schroeder Oct 20 '16 at 12:59
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There are a host of particular features that I envision for a RKKV which are not otherwise well-written about, so I want to quality my answer by saying that I have not seen these details described in other places.

  • Nano-particles make the most sense as the projectile. This comes from a balance between minimizing penetration depth, avoiding unwanted disruptive interactions along the way, manageability, and charge-to-mass ratio
  • The particle would be moved along in a particle accelerator
  • The energy source for the acceleration would be a capacitor bank onboard the ship, heat energy is deposited directly into them and the magnetic coils (not much because of superconductors, most energy goes into the projectile)
  • To the question, considering the safety aspects, engineers would either strive very hard to get it up-to-speed in a linear accelerator, or they would not put critical equipment in the free-path trajectory in a circular accelerator

The best analog to the safety situation on Earth would be the LHC. There are genuine safety concerns for the case where the confining coils lose power, and then the beam of particles plows into the wall and penetrating into the surrounding soil.

To be perfectly blunt about a battle situation, the biggest problem might be the need for such a large power plant and sufficient capacitive storage to generate and hold enough energy to power this weapon. Whatever ship contained the weapon would not be able to move very fast due to this extremely heavy loading. Yield will scale up in direct proportion to its mass. So in a "battle" (if such a thing can ever happen in space in the traditional form), the ship would be a sitting duck for enemy fire because it would have very poor ability to maneuver itself.

Capacitors are very good at fast energy release, so I would favor the case of the linear accelerator. You impart and electrical charge to the particle, and then dump all the juice you have into it within a lightning-fast process. If someone went wrong technically, it would still exit along its intended trajectory, but not at full energy.

Since the particle is small, you would be looking at a highly relativistic speed like 0.999c. As such, it is almost perfectly invulnerable to premature detection. Those killed in the initial blast it causes will die with literally no warning.

This would be safer than a missile, but anytime you cary an enormous amount of energy onboard, you risk premature release of that energy, and having those capacitors accidentally discharge could effectively blow its own crew up. Since no energy storage is perfect, I don't think you can ever fully eliminate such possibilities, even if it is well-engineered and the chances of it happening are remote.

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