If I remember correctly, SpaceX would be happy to experiment with nuclear-thermal propulsion but cited the lack of a engine test stand as reason why they aren't actively working on it. I'll see if I can find a quote for that. I am rather sure that it was by Gwynne Shotwell, COO of SpaceX. (Edit: progress! I think it's in a talk by her at MIT Road to Mars 2017. Too bad I cannot find a recording of that).
NERVA is another term to search for if you are interested in nuclear-thermal propulsion.
Given the regulatory delays and uncertainty of somewhat safer things like approval for orbital launches from Starbase, I imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade.
The regulatory environment is bad enough that I still expect this to eventually get cancelled again, only to be taken seriously when eventually another country is close to catching up technologically.
> imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade.
Or they might want to do it anyway knowing it would never be allowed to launch in order to drag the overton window in a more permissive direction.
It's tangentially related insofar as SpaceX says they're planning to go to Mars, and this NTR engine is also for going to Mars. But according to the DARPA announcement, DARPA determined that Falcon 9 doesn't presently support this sort of liquid hydrogen payload. They suggest Vulcan Centaur could do it with fairing modifications. (Vulcan Centaur hasn't flown yet. Where are the engines, Jeff??)
It's not really related as SpaceX has no plans to use them and isn't exactly interested in doing so as they don't see them as needed. Also NTR are kind of a tossup on efficiency as while you get somewhat better fuel efficiency, their mass is huge because you're lugging an entire nuclear reactor core along with with you. The thrust to weight ratio isn't great.
It's not just the shielding. An NTR engine needs a lot of the same plumbing that other engines need, including turbopumps, and they have additional cooling requirements because of the much hotter fuel. Add on to that the already very heavy Uranium and control rods.
Add on to that I'm not quite sure how you prevent the engine's nuclear reactor from going into meltdown once it shuts off. The residual heat from the decay products in the seconds to minutes after shutdown will be substantial and that heat needs to go somewhere or it'll cause a reactor meltdown the instant you shut off the engine. So you need all the hardware to dump heat somewhere (presumably radiators and a cooling system that pumps hydrogen through the reactor while it's shut off) so that's even more mass.
The only way NTR really makes sense to me is if your spacecraft is truly massive, but literally no one has anything like that even in planning stages.
The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures.
And the way they handle shut down is they continue a small flow of propellant through the engine until the core cools off and the hottest, shortest lived stuff decays away. NTRs usually run for a few hours at most, not years, so the decay heat a few minutes after shutdown isn’t that bad.
> The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures.
Pretty sure this can't be true. In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion.
I wasn't accounting for it, but I assumed it wouldn't be significant. The efficiencies claimed are over 2x better than chemical rockets. You don't get that much just from changing gasses.
Yes, you can. Hydrogen, for the same temperature, has a far higher speed of sound (which is close to the average speed of the gas molecules) than air or water vapor. This is why your voice is higher pitched when you breathe in helium (also a light gas like hydrogen).
It is true. Basic gas theory stuff, the average molecular speed (close to the speed of sound) at a given gas temperature is, to first order, higher for a lower molecular mass. Otherwise, why bother with such a difficult to store propellant which you’re not even extracting energy from (as the energy comes from the reactor, not the propellant as in chemical rockets)?
Chemical rockets reach over 3500 Kelvin, but Nerva only got to around 2300 Kelvin.
There isn't one really other than SpaceX COO (or was it Musk?) making a single passing reference to them in response to a question at a conference keynote a few years ago.
NERVA is another term to search for if you are interested in nuclear-thermal propulsion.
https://en.wikipedia.org/wiki/NERVA