This is essentially the what you are doing. Like if you sat on a chair and aimed a fire extinguisher you'd go flying (cold gas thruster, typically used in RCS). You'd go faster if you super heated the gas before it exited the nozzle.
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?
Normally 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.
How 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!
NTR 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.
TWR 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).
NTR is not competitive with chemical rockets in any actual application. Dry mass has to triple at a minimum, your propellant evaporates over long trips, and NTR engines don’t have enough thrust to land on Mars or even the moon, requiring the additional dry mass and complications of specialized landers.
We need a huge step forward in NTR before it’s going to be useful at all.
Those already take the slow boat Hohmann transfer orbit to maximize payload capacity. Starship might be able to send 150 tons that way to the surface itself. And building bigger starships is easier than building NTRs.
This sounds like a matter of scale. A nuclear spaceship (a theoretical one, like all of them) with twice the thrust wouldn't be carrying twice the dead weight.
So at some level of scale this outperforms a traditional rocket, and your oddly impassioned argument about why SpaceX is so much better is just relevant to particular use cases rather than spaceship design in general.
Scaling up low thrust to weight engines with large cooling and shielding requirements doesn’t create many mass efficiencies. Maybe in the shielding, but that’s the least of your concerns.
The other problem is that low thrust to weight means an NTR spade ship can’t land on Mars, or on any body with a significant gravity well. So you need to bring chemical rocket landers, increasing your mass duplication and tech complications.
A multipurpose chemical rocket powered space ship Luke Star Ship is far more practical and nearly as fast.
If 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.
Good 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.
Most 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)
This article didn't explain it at all, so you're right to ask.
tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel.
The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse
Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design.
Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase.
Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft.
Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.
The primary failsafe mode for an NTR would be to insert control rods to stop fission. Without the hydrogen the core wouldn't be able to cool itself and would melt down. There are however NTR core designs with closed circuit cooling. The core would be kept at a low critical state (hot but not melting) and circulate a coolant through the core and into a generator and from there to radiator panels. When the NTR wasn't providing thrust it would provide electrical power. When thrust is needed the coolant loop would cut off and hydrogen would be pumped through the core. Provided no mechanical breakdown in the coolant/generator loop an NTR could provide power for years.
I guess one could still build single burn/single use reactors. The thing would be a bit lighter than one that can survive multiple burns & it should not pose a Hazzard as long as you plan the resulting orbit of the discarded reactor accordingly.
For a Hohmann transfer orbit you need at least two burns, the perigee burn to put you into the elliptical transfer orbit and the apogee burn to circularize that orbit at your destination. Even free return trajectories can require a secondary burn. So in many situations throwing your engines away is not a great idea.
An NTR can be designed such that the engine and spacecraft "chassis" are reusable over multiple missions. NASA has/has an NTR concept with such a reusable vehicle. The fuel tanks are disposable and slot into the central frame like AA batteries. The crew portion would be a TransHab-like habitation module with a docked crew capsule and Mars lander. Propellant tanks would be disposed of during the mission and the vehicle parked in Earth orbit between missions. For a new mission propellant tanks would be fitted along with a new crew and off it goes. It's an interesting design but a little passed the current bleeding edge of in-orbit construction.
Uhh space itself is about 2.7K. Seems like cooling shouldn’t be a problem. There’s probably all sorts of ways to avoid a problem. Even just using a different element altogether.
You'd think cooling would be no problem, but it turns out that when your only option is to radiate away heat, that's _very_ slow. We get spoiled here on Earth by conduction and convection, both much easier and faster.
