Realistically, what type of weapon is the best at delivering a specific radiation in space battle?

Question

It's 20 years into the future, and humanity is (trying to) reaching for the stars. And congratulation, we have met a lot of quirky alien civilization, as per SF tradition. Unfortunately, this isn't Star Trek, so conflicts do happen. Luckily, this isn't 40k either, so none of the star-faring nations are going for total war here. So long as human ships can prove themselves capable of fighting in a skirmish, the media (human and alien alike) is going to blow it up to the point everyone thinks we can handle a space war, and ironically, no full-scale war is going to happen.

The problem, of course, is that the majority of alien crafts are protected using energy shields, which render many types of weapon inefficient, however. As far as that future scientific understanding goes, the best way to beat them is with a large dose of neutron radiation to penetrate and destroy the shield mechanism underneath, which weapons would be the best to do so?

We are assuming that with futuristic technologies, anything modern piece of technology on Earth can be mass-produced reliably, at record-level performance, in space. The neutron source is standardized, the same amount of reaction material create the same impulse of radiation, regardless of weapon. Costs, weight, and other factors will be considered, but only as secondary, after 2 main criteria:

1. Damage: The amount of radiation delivered to the target
2. Combat usability: How good that bundle of radiation is at getting to a moving (though still limited to dogfight combat speed, since the enemy also want to shoot, too, and their targeting system is not noticeably better than ours) target in space

The 2 ideas I have in mind are missiles and beam weapons. Of the 2, missiles with neutron bomb warheads seem like it should do more damage (it is a modern weapon, after all), but also seem a little bit too easy to counter (speed, point-defense weapon, etc.). Whereas it is impossible to dodge a neutron beam, but people keep them in labs, and not battlefields for a reason, right? Is there any other way to deliver radiation accurately to a distant spot in a vacuum?

Answer 1

I'd discard the idea of neutron beams, unfortunately. Whilst a working neutron beam would be an extremely effective weapon regardless of whether the target is shielded or not, making such a thing seems somewhat implausible outwidth a magical way to accelerate neutrons.

Firstly, generating such a beam is problematic. Once you've generated it, focussing it is problematic... modern spallation sources, for example, aren't particularly collimated, eg. they fire the damn things out in all sorts of directions, and fancy grazing-incidence mirrors can only really work on neutrons that are already heading in roughly the right direction so you'll lose a lot of neutrons and generate a lot of waste heat. Even once you've focussed it, you'll still have limited range due to thermal blooming (basically, neutrons in the beam bumping into each other causing the beam to spread out). No handwavium neutron accelerator, no neutron beam.

Now, lets talk about neutron bombs. An excellent way to generate an awful lot of neutrons, in a military context, is to use a fusion explosion. D-T fusion, one of the easiest reactions to spark off, generates over 15 times more neutrons for a given mass of fuel than U-235, and the neutrons it generates are much higher energy (about 15 times more again).

Problem: fusion reactions are difficult to kick off. Modern nuclear weapons do this with the aid of a fission primary, but the size of the fission primary is limited by the critical mass of fissile material which can often be many kilos. You also need a very carefully controlled detonation of an implosion-type fission device, which just increases the hassle.

How do you make a fission explosion with a tiny subcritical mass? Well, one way to do this is to use a very small amount of antimatter. Have a look at the ICAN-II antimatter-catalysed microfission rocket design. A tiny pulse of antiprotons (less than a femtogram) is fired into a 3 gram uranium pellet, causing it to fission and release a healthy number of gigajoules of energy. This can be used as the primary in a more conventional (but miniaturised) 2-stage Teller-Ulam thermonuclear weapon, with a radiation case chosen to be as neutron-transparent as possible. Now you have the basis for a tiny neutron source, maybe only a kilo. You don't need access to massive amounts of exotic materials, like plutonium or antimatter... a few kilos of uranium and a nanogram of antimatter will supply thousands of micronukes. You just need to supply the tritium to get the party going.

For your next trick, you will need a high-velocity projectile weapon, such as a railgun, coilgun or powerful rocket engine (though the previous two are likely to be more economical here). The projectiles won't travel at a significant speed of light, unlike a laser or neutron beam. Make up for this by firing Quite A Lot of them. Most of these projectiles will be dumb metal slugs, but some, say, one in every hundred, are your micronukes. Throw a lot of rounds down range. Ships have a limited ability to dodge, especially when the incoming fire is a big cloud of hypervelocity crud. They have a limited ability to intercept these weapons with point-defense fire... they're small, hard to see via any spectrum and coming in fast. And they have to shoot all of them, because the one they let though might be the micronuke.

After a few skirmishes with your new toys, maybe you'll be able to collect the weapons that the aliens use to oppose shields, and use something sensible instead...

Answer 2

A fission or fusion nuclear reactor or some form of high powered cyclotron or synchrotron aimed at a solid (spallation) target would be capable of producing a large neutron flux of the order of 10^15 neutrons / square cm for the reactors and in the case of the spallation target method a neutron flux in excess of 10^17 n/square cm is possible.

It is also possible although difficult to focus a neutron beam. So a large synchrotron projecting into a spallation target with a specialist Wolter optics for focusing neutron should do the trick. However is would be very heavy, very large, very energy intensive to run and very expensive as well.

Answer 3

It's time for my favourite space weapon.

Sandcasters!

A sandcaster missile is about as simple as it gets. Fill a regular missile with sand, put a conventional explosive in the middle so the sand disperses nicely, shoot it towards them at relativistic speeds, and blow it up outside of their point-defense range.

It doesn't even have to be sand. It can be lead ball bearings, for extra kinetic energy, or even...

Tiny neutron bombs...

Design a small neutron bomb that explodes on impact. Pack them into your sandcasters. follow above procedure.

You can't dodge them all. Or see it coming, since the light informing you of the missile's presence doesn't appear long before the payload. Send regular sandcasters after your neutron ones if you want to actually blow anything up.

In regards to "dogfight speeds", shoot the missiles from very far away.