10 ms·
But concretely, what is the reaction mass made of and what is the most reasonable mechanism for propelling that mass with the energy of the nuclear reaction?
by mac01021 6y ago
But concretely, what is the reaction mass made of and what is the most reasonable mechanism for propelling that mass with the energy of the nuclear reaction?
- xupybd 6y agoNormally liquid hydrogen. Just pass the hydrogen past the reactor. Cooling the reactor and heating the hydrogen. The expansion due to heat send the hydrogen out of the nozzle at high speed, creating thrust. More efficient than a chemical hydrogen and oxygen rocket due to the high energy to weight ratio you get from nuclear fuel.
- digdigdag 6y agoWhat happens when the rector runs out of fuel? Control rods permanently insert or it just, you know, proceeds to melt down once arrived at Mars?
- lumost 6y agoGood design would either reuse the reactor for other purposes, or ensure it runs out of fuel at roughly the same time as the rocket runs out of reaction mass.
- db48x 6y agoIf it's out of fuel then it can no longer generate any heat. The position of the control rods wouldn't matter, and it couldn't melt down. If you're referring to reaction mass (the hydrogen gas), then yes, you would need to insert the control rods to stop the reaction. Or land the reactor and use it to power your Mars colony. If you do the latter, then you will have had to build it with another cooling mechanism in mind. That could increase the mass of the reactor and reduce your payload capacity, so you might not do it. Instead, you might include just enough uranium to make the trip, and no more.
- Dylan16807 6y agoNobody's going to make a reactor that can't turn off.
- dajomu 6y agoWhy would it melt down if the fuel has run out?
- dmurray 6y agoHow much more efficient is it? The fact that you have to carry propellant with you anyway seems like a big factor in favour of traditional rockets, where the fuel becomes the propellant. And burning the hydrogen way hotter seems like a big engineering problem - rocket motors already run at temperatures which pose materials science problems, increase that a few thousand degrees and you might as well say we should run the spaceship on fusion power. I don't doubt that the physics works, and it's important to note that say a 2x gain in thrust-to-mass ratio would lead to enormous advances in trips out of Earth's gravitational well (10x? 100x?) due to the tyranny of the rocket equation. But I'm curious whether this is the 2x gain of replacing oxygen+hydrogen with hydrogen, plus a big complex reactor, or the millionfold gain of replacing hydrogen fuels with uranium. Edit: TFA says twice the specific impulse, and presumably that's the optimistic estimate. But still very good!
- giantrobot 6y agoThe gains from an NTR over chemical rocket has to do with the temperature, thus velocity, of the exhaust.
- valuearb 6y agoNTR is not anywhere near 2x chemical because of parasitic losses, in fact it may only barely offer higher net DeltaV. NTR requires heavy engines, heavy shielding and heavy radiators to keep cool. The final NERVA prototype was as close to a functional NTR as ever built, and it massed 40,000 lbs while only generated 55,000 lbs of thrust, with a maximum ISP of 710 seconds. A SpaceX Raptor only has a Mac ISP of 380 seconds, but masses only 3,000 lbs, and produces 500,000 lbs of thrust. Add another 50,000 lbs dead mass to the NTR for shielding and cooling, and you see why Raptor will get humans to Mars well before any NTR and just as quickly.
- dr_zoidberg 6y agoTWR is only important when you have to climb up the gravity well. Once you're up in orbit, high ISP is king to get you anywhere. Other comments already highlighted that NTRs are a middle ground between superefficient, ultra-high ISP electric drives, and very high TWR chemical rockets. So that's where this sits: high enough TWR to be practical, but way better ISP than chemical (though still far from electrical).
- deleted 6y ago[deleted]
- credit_guy 6y agoMost likely the reaction mass is hydrogen. The NERVA project [1] used it, and the mechanism was quite straightforward: heat the hydrogen to 2250 deg Celsius and let it go. The vacuum specific impulse obtained by NERVA was 841 s (the Space Shuttle main engine had a specific impulse of 453 s) [1] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
- valuearb 6y agoThis is another shortcoming of NTRs, as Hydrogen is tiny and slippery, leading to significant amounts of leakage over long trips.