Nuclear Propulsion in Space

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 [link] . 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. [link] Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen [link] . Old timey video explaining it [this video] . 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 ( [link] ) 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 s