[cwojno]

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.

[mrfusion]

What happens when the hydrogen runs out? Control rods to slow the reactor? Or eject it?

[giantrobot]

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.

[m4rtink]

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.

[giantrobot]

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.

[ianai]

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.

[kadoban]

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.

[valuearb]

Cooling is the Achilles heel of NTR, requiring massive radiators to remain functional over a long burn.