I know that this is an oddly specific question, but it was something I was wondering

In my setting, lots of space navies field a hefty amount of kinetic weapons because energy weapons are highly power and heat inefficient, and missiles while being longer ranged and guided, don't have that initial electro-magnetic accelerated OOMPH

Now lets assume for the sake of the situation, two spacecraft for reasons unknown are forced to engage, and the first one opens up with a salvo with coilguns.

Barring the actual difficulties of firing the weapon, (Structure, Energy costs, Heat waste, etc..) the weapon has these stats:

  • 700mm Bore
  • Muzzle Velocity of 1,000km/s (1,000,000 m/s or 2,236,936.29 MPH)
  • Kinetic Slug Mass of 4535.92 kilograms

The estimated momentum from the fired projectile is roughly 564 kilotons if the newtons are converted.

I was wondering how a space faring civilization would attempt to deal with these weapons as they are commonplace within the universe. Would it be possible for navies to armor against them? Would it require good ol' evasive maneuvers? Handwaves like energy shields?

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    $\begingroup$ Can the navy include submarines? Seems an obvious solution. $\endgroup$ Commented Jan 20, 2021 at 22:12
  • $\begingroup$ Since the ships are engaging in combat and arming their weapons and firing thrusters, they would not be able to be stealthy like a submarine. Avoiding a fight is obviously a good way of not getting shot lol $\endgroup$ Commented Jan 20, 2021 at 22:25
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    $\begingroup$ @Tantalus'touch. : Space submarines! I imagine they would travel in subspace. They could stick a periscope back into real space to look around. $\endgroup$
    – Willk
    Commented Jan 20, 2021 at 22:25
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    $\begingroup$ 4535.92 kg is strangely specific and smacks of imperial conversion, which seems unlikely for a future spacecraft. Not an issue with the question as a whole, just something that leapt out at me. $\endgroup$
    – jdunlop
    Commented Jan 20, 2021 at 22:42
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    $\begingroup$ You might check out the "Trophy" active protection system used on military tanks. en.wikipedia.org/wiki/Trophy_(countermeasure) such a system could maybe be scaled up to handle a kinetic shell by disrupting the shell some distance from the protected target, spreading out the impact over a larger area. The same concept is used by the ISS space station to shield from micrometeorites, with a multi-layer shield system. $\endgroup$
    – Brianorca
    Commented Jan 20, 2021 at 23:08

3 Answers 3


You very much do not want to be hit by this projectile. I cannot stress this enough.

There are likely very few structures on Earth that could survive a direct hit from this Kinetic Impactor, and the ones that can are most certainly buried under many meters of solid ground.

Your best option is going to be to dodge. Electromagnetic acceleration via coil guns this powerful will give off a very distinct signature. Your ability to detect and attempt to dodge an incoming round will very much depend on your engagement distance and your ships evasive capabilities.

Given that your projectiles are moving a 1,000 km/s, any ships engaging at closer than 1,000 km had better hope they can move out of the way in less than one second.

If you cannot dodge, your second best bet is to attempt to deflect the round. Nudge its trajectory a few meters in the opposite direction from your evasive maneuvering and you might have just saved your whole ship.

Your two approaches to deflect the projectile are with your own kinetic impactor or with laser ablation. The key here is you aren't trying to stop the round, you want to apply a perpendicular acceleration to scoot it out of your way by the time it arrives.

That means pushing on one and only one side of the round. If you shoot at it from your ship, there's no way you can get enough of an angle on the incoming round to meaningfully alter its trajectory.

You also run the risk of fracturing the projectile into a cloud with mostly the same mass, moving with mostly the same velocity, but now much less predictably. Sustaining multiple hits from fragments might even be worse than a single direct hit.

If you focus a powerful enough laser on only one side of the round, you can begin pushing it in the opposite direction as bits of the projectile vaporize and push back against it. In order to meaningfully accelerate a 5000kg slug, its going to take a VERY powerful laser with VERY accurate targeting, especially if the round is rotating.

Even then I am not convinced you have enough time to meaningfully push the round out of the way.

Trying to launch an interceptor rocket for a close engagement (~1000km) is almost certainly unfeasible. Take all the problems with a kinetic interceptor and add more time required accelerate your missile.

Your rounds are slow enough, ONLY 0.3% the speed of light, that long range engagements will certainly revolve around detecting and dodging.

Even IF you had countermeasures in the form of super powerful lasers or hyper-fast missiles, those would be your primary close range armaments instead of your coil guns.

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    $\begingroup$ This answer is excellent. I'd like to add a source for some light reading on the subject if you're interested. $\endgroup$
    – SirTain
    Commented Jan 21, 2021 at 12:37

Attack the incoming slug with a missile or energy weapon.

/missiles while being longer ranged and guided/

That is what you want here. The kinetic projectile cannot change course. In space it will be stupidly easy to calculate that path. You will see the kinetic projectile coming thousands of kilometers away because even though it is fast it is not as fast as light. And there is no haze in space.,

This kinetic projectile is unarmored. It is a nice chunk but it is no battleship.

You could shoot it with an energy weapon. The energy weapon moves at light speed or close to it (particle beam) and will heat up the kinetic projectile. Keep it trained on the slug. The slug will keep coming, but as a molten blob or a cloud of gas.

You could take it out with one or more missiles which will be slower than a beam but which are synergistic with the beam. A radar guided missile will easily be able to hit it. The missile will disperse the mass which will then spread out as it travels. Fragments or molecules that still hit the target will hit with less force than the entire package would have imparted.

If you only have time for the beam that is ok. A kinetic impactor which has been liquified or gasified will splash against the target. Splash vigorously but still a splash.

Your battling spacefarers might prefer kinetic weaponry for offense but still have a complement of the other types you list for defense.

  • $\begingroup$ Thank you for the input, I find it very insightful $\endgroup$ Commented Jan 20, 2021 at 22:58
  • $\begingroup$ I am thinking of the defensive laser crew very pleased with how quick they heated the incoming projectile to glowing. When the missile splatters it they realize why. It was a metal shell full of water ice weighing only 500 kg. The 3500 kg projectile is 100 meters behind it, in its shadow. $\endgroup$
    – Willk
    Commented Jan 20, 2021 at 23:42
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    $\begingroup$ I can assure you that 4535 KG of molten metal at 1,000,000 m/s will do much more than splash against your spaceship's armor. You've effectively created a HEAT round. You would MUCH rather dodge or deflect the projectile than try to absorb ~600 kt TNT worth of kinetic energy. $\endgroup$
    – abestrange
    Commented Jan 21, 2021 at 1:10

This is a case of too much of a good thing. The kinetic energy of the projectile at 1,000,000 meters per second is absurdly high that that the physics of hypervelocity projectiles (https://en.wikipedia.org/wiki/Hypervelocity) comes into play and it will self-destruct on contact with anything physical because of the extreme energies involved. Therefore, it would be rather easy to defend against using the Whipple shield principle: https://en.wikipedia.org/wiki/Whipple_shield. Simply have a missile drop foil chaff in front of it, fire a blast of tiny pellets at it, probably even a cloud of sand would be sufficient to convert it to a ball of hot plasma well away from its intended target.

  • $\begingroup$ You really do not want to test your armor, whipple or otherwise, against that much energy. Your own links talk about dealing with velocity in the realm of ~10km/s. We're talking about 100x that fast. Debris between you and the projectile will certainly vaporize and apply a force against the round, but not nearly enough to halt or disperse the insane amount of momentum hurtling your way. $\endgroup$
    – abestrange
    Commented Jan 21, 2021 at 2:21
  • $\begingroup$ @abestrange You're not appreciating the absurd energy involved. The projectile possesses 564 kilotons of kinetic energy. 0.001 kiloton (1 ton of TNT) is easily enough to blow the projectile apart. The specific math depends on the shape of the projectile but, to approximate, if it were a perfect flat disk, impact with an object of 1/564000 = 0.0000017 of its area is enough to supply that energy. That's how tiny an object would need to be to shatter the thing and a 5 ton metal projectile isn't all that large. $\endgroup$ Commented Jan 21, 2021 at 2:39

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