11
$\begingroup$

I'm trying to work through the mechanics of hard sci-fi space combat, and I came across the Casaba Howitzer.

The basic idea is a nuclear shaped charge. The blast from a nuclear bomb is partially contained into a cone / beam of plasma which can do some serious damage, according to the numbers in the link above. For reference, a one-megaton Casaba could theoretically apply an energy of 6.7MJ/m^2 to a target 1000 km away, or vaporize half a meter of aluminum at a range of 50 km.

The first two obvious ways of applying this concept are A: Put them in a missile which gets close to the target, and detonates outside the target's point defense grid, or B: Use very large versions for long-range combat. Even if you're too far away for the blast to destroy the enemy's armor, it can still wreck havoc on exposed sensors and other delicate components, as well as contribute to overheating the target.

While the idea is cool, I don't really like its implications for my 'realistic' space combat system. However, it occurred to me that the key word in the description of the Casaba's energy delivery method is 'plasma'. The Casaba's casing is vaporized into plasma, which is hurled at the target at near-relativistic speeds; but plasma, by definition, is charged, which means you should be able to repel it with a magnetic field. I know that we're working on magnetic fields for modern spaceships, which can theoretically protect from solar radiation for a fairly minimal energy cost. How much can such a system be scaled up? Could you build a version that could effectively protect against a Casaba? How much energy would it require?

Also, shields in soft sci-fi lose integrity / require more power the more they're hit. Would a magnetic shield hit by a Casaba somehow 'lose integrity', or would it continue to function just fine as long as it's fed enough power to repel damage to the ship?

$\endgroup$
  • 1
    $\begingroup$ If your defender is going to generate an enormous magnetic field, seems you don't really need the Casaba after all - just an otherwise inert magnet as the warhead. Let the defender's own shield draw it in, and the high-speed collision will splash the target. Or a pair of missiles - one Casaba, one magnetic, splash the target either way. $\endgroup$ – user535733 Aug 15 '19 at 2:47
8
$\begingroup$

A charged particle passing through the magnetic field has no work done to it. It’s trajectory changes but not its energy.

The magnetic field won’t alter the path of neutral atoms, neutrons, or light produced by the Casaba Howitzer. And, plasma’s are a state of matter dominated by their internal electric and magnetic fields. The individual atoms of plasma are constantly transitioning between neutral and ionized states — the emission of photons by plasma is dominated by recombination processes.

As a consequence, the very long mean free path of plasma’s in outer space and recombination, high-velocity neutral particles will traverse the magnetic shield. I suspect the net effect will be that the force of the detonation will be blunted and impact a larger surface area of the ship, weakening the attack.

I think one odd consequence of the Casaba detonation would be that the EMP generated by the explosion would couple into the system generating the magnetic shield. This might cause the magnetic shield to become momentarily stronger, or it might become drastically weaker. This effect might provide you with your need for the magnetic shield to need more power when screening an attack since either more energy needs to be sourced to strength the field or power needs to be drained from the system to prevent overloads from damaging the integrity of the shield

$\endgroup$
  • $\begingroup$ So, you're saying that plasma particles are constantly shifting between neutral and ionized states, and that they stay in one state for long enough that neutral particles will pass through the magnetic field and hit the hull before they're likely to shift back to charged and be deflected? $\endgroup$ – FlyingLemmingSoup Aug 15 '19 at 4:48
  • 1
    $\begingroup$ "Magnetic fields are conservative. This means a charged particle passing through the magnetic field has no work done to it."-- The second sentence is correct, but that's not what it means for a vector field to be conservative. A conservative vector field means that it can be written as the gradient of a scalar function which the magnetic field cannot because it has a non-zero curl. In fact, the magnetic field is solenoidal, which is basically the opposite of being conservative. $\endgroup$ – el duderino Aug 15 '19 at 14:12
  • $\begingroup$ @FlyingLemmingSoup plasma particles are charged ions (+) and electrons(-), it's not technically feasible to completely separate them, they are only kept apart by the plasma's high temperature. But eventually some ions and electrons recombine to neutral atoms and stop being affected by magnetic field. $\endgroup$ – Juraj Aug 15 '19 at 15:24
2
$\begingroup$

You need to narrow the question down some. You need to decide how strong the casaba howitzers are, and at what range. And you also need to decide how strong the ship hulls are. To be effective, the shields need to be strong enough to disperse the plasma enough to reduce the penetration of the beam to less than the thickness of the hull.

In my opinion though, I don't think magnetic shields will be effective, because magnetic fields can't really be directed. By this I mean they're not a beam, they're a field that will surround whatever generated them. This means you can't focus your efforts on creating the field in front of the missile. You'll end up generating a field around your whole ship, wasting most of the energy. ~50% of your energy will be spent keeping the field up on the starboard side, even though the enemy is only on the port side. Think about how much tonnage must be spent on shield generators, most of which will be dead weight most of the time.

I think you're better off putting your tonnage into bigger and more accurate point defense weapons so you can shoot down the casaba howitzers before they get in range. Plus PD weapons work on targets that aren't casaba howitzers as well; traditional missiles, boarding craft, crippled enemies you don't want to waste ordnance on, etc. Basically, I think a ship that relied on magnetic shields could be built, but it would be a bad ship overall.

$\endgroup$
  • $\begingroup$ You make a good point about the inflexibility of magnetic shields, but I think that's a problem that could be solved with clever engineering; mounting the generators on articulated armatures that can shift to cover a particular side, for instance. Or simply mounting the shields and the lion's share of the armor on one side and gambling on being able to always make that side face the enemy. $\endgroup$ – FlyingLemmingSoup Aug 15 '19 at 4:51
  • $\begingroup$ @FlyingLemmingSoup if you use orion drives, your thruster and your shielding are all part of the same mechanism. You're well placed to a) protect yourself against incoming nuclear blasts and b) run away, all at the same time. $\endgroup$ – Starfish Prime Aug 15 '19 at 20:58
  • $\begingroup$ @StarfishPrime if you have orion drives, you don't need casaba howitzers, you just need bomb-pumped lasers. $\endgroup$ – Monty Wild Aug 15 '19 at 22:10
  • $\begingroup$ @MontyWild if you have orion drives, you have the technology to make casaba howitzers already. Making bomb pumped lasers on the other hand, especially ones that you can aim, is a rather more difficult proposal. $\endgroup$ – Starfish Prime Aug 16 '19 at 10:17
1
$\begingroup$

(OP here, I just got around to actually making an account, so I'm responding to my own question.)

Allright, follow on question: The general consensus seems to be that the magnetic shield MIGHT work, IF the plasma beam generated by the Casaba is still, well, plasma; meaning that it's positively charged from the heat of the blast stripping electrons away from their nuclei.

The article linked in my original post gave some sample numbers for particle speed and effective range, and it looks as though there's pretty much no situation where a Casaba's effective range will be far enough for the plasma beam to take more than about 1/10th of a second to reach the target, probably less than that.

So, how long does it take for the beam of positively charged particle to cool down into a beam of neutrally charged particles? If it's longer than about 1/10th of a second, then a magnetic shield is theoretically effective.

EDIT: This also provides a solution to another issue of mine: How to protect against engine wash. If you want a starship with engines powerful enough to provide 1-5 G's of acceleration, while also providing several hundred km/s of delta-v at around 20% of the ship's total mass being propellant, then you pretty much need fusion rockets that output enough energy to be comparable to the solar power Earth receives from the sun. This would make engine wash a fairly viable weapon, which I kinda don't want it to be. Having magnetic shields to protect against the plasma from such engines would be logical and convenient.

$\endgroup$
  • 1
    $\begingroup$ You can contact the mods and ask them to merge your accounts, so all your posts are in one place. It would also allow you to accept an answer, which I don't think you can do from your new account. $\endgroup$ – F1Krazy Aug 16 '19 at 5:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.