4 ms·
There is still a propellant problem. The other possibility for nuclear radiation drives is reflection. It’s much less efficient per watt because the momentum to
by jl2718 6y ago
There is still a propellant problem. The other possibility for nuclear radiation drives is reflection. It’s much less efficient per watt because the momentum to energy ratio decreases as energy per mass increases, but the critical metric for long-term (nonrelativistic) space travel is momentum transfer per mass ejected, which is just the speed of the propellant medium, maximum c for photons, so reflection is preferred over expelled gas.
A few careers ago (NASA), I designed (not built) something called a duoplasmatron breeder reactor, which is a reactor driven remotely by a tritium ion beam. The nuclear radiation reflector would look something like a parabolic layered medium (gold, CVD diamond, Si3N4, carbon composite) to achieve a wide range of photon and neutron reflectivity with high heat tolerance. A second parabolic thermal reflector baffle may provide additional boost from anything that gets absorbed as heat.
So... if anybody is doing this sort of thing now, I’d be very interested to hear about it.
- lliamander 6y ago> duoplasmatron breeder reactor, which is a reactor driven remotely by a tritium ion beam. This reads like it's straight out of Star Trek ;). Seriously though, one of the things I like about HN is the chance to hear from people with relevant backgrounds who don't otherwise have a major platform to share their perspective.
- skykooler 6y agoHow do you reflect neutrons? I thought they could only be absorbed?
- xxpor 6y agoLots of ways :) A common material is Tungsten Carbide, which was what caused the accidents with https://en.wikipedia.org/wiki/Demon_core https://en.wikipedia.org/wiki/Demon_core But here's an overview: https://en.wikipedia.org/wiki/Neutron_reflector https://en.wikipedia.org/wiki/Neutron_reflector Reflectors are important because they allow you to have a quantity of radioactive material that's not supercritical on its own, but becomes supercritical when surrounded with the reflector. This allows controlling when things happen (unless you screw up like the accidents above)
- jl2718 6y agoThe original intent of the duoplasmatron breeder was to eliminate the dangers of going supercritical, actually to eliminate HE fissile material entirely from the bulk, as the plutonium would have to be neutron-bred by the fusion reactions triggered by the tritium beam. Also short pulse control. This was mainly to make it safe and cheap. You could also just use a subcritical mass of HE-fissile material surrounded by a neutron reflector with aperture in the direction of thrust. Recommended only if you are certain to be far enough away from humans to avoid recovery and conversion to a weapon. Aliens too; they might send one back. There is a design for a safer and simpler cylindrical slow-burn wave reactor that may also work for low levels of neutron propulsion.
- keyle 6y agoAside, I was terribly disappointed to not read "rocket scientist" in your profile :-)
- seph-reed 6y agoMy physics isn't great, but I have questions based off what I have (mis) learned: 1. If you jettison a 1kg rock at 99.999% the speed of light, it'll not really be 1kg anymore because of something with energy and mass being the same near the speed of light? 2. At that point you'd be jettisoning raw energy? 3. Could you make a single molecule act like a very heavy object this way? 4. What would happen if that single molecule hit a planet? 5. Would fusion (or fission) be capable of creating more mass through velocity than the weight of the reactor? As in, using a couple molecules to create enough energy to speed up a molecule so much that it acts massive.
- ianai 6y agoc is the speed energy travels at and less than c is the speed matter travels at in the universe. You’re right that ultimately matter is energy, but you typically have to introduce it to it’s antiparticle to turn it into energy. (I’m less clear on how you go from energy to mass. I think you can knock a photon into an electron with enough force to escape its nucleus but you’re not going to get new matter out of it. I wonder whether further knowledge of the Higgs Boson would give us some ideas in this area.) So photons can’t transmit forces dependent upon mass - but they can transmit momentum. I remember being told this is what keeps a stars gravity from collapsing the matter into a singularity or otherwise blowing apart. I think you’re probably right that speeding matter up to significant fractions of c would increase its effect on your opposite velocity, but it’s probably hard to benefit from as an engineered system. Like having a particle accelerator the size of our orbit around the sun to make meaningful thrust-hard.
- btilly 6y agoAnswers. 1. The relativistic mass is the rest mass / sqrt(1 - (v/c)^2) which in this case is 223.607356769625 kg. The kinetic energy is the change in mass, so 222.607356769625 kg which through the conversion E = mc^2 is about 2 * 10^19 Joules. The momentum is mass times velocity which is about 6.7 * 10^10. 2. No. You'd be jettisoning a very nasty rock. That will immediately explode on impact. 3. There is no limit to the mass it could have. 4. The first thing it hit, would convert into highly energized particles still moving in the same direction, which would hit new particles and so on. The result of the cascade will be to generate a shower of high energy particles with the same energy and momentum as the original. Or in short, BOOM! 5. Because of conservation of energy/mass, no. Even with 100% efficiency, the reactor would have to lose the same amount of mass that shows up as kinetic energy in the ejected particle.