Unlike most things at Beyond NERVA, the nuclear thermal rocket (NTR) has been built and tested by both the US and the USSR. Both programs looked at a solid core NTR, the simplest type of nuclear rocket. In a nuclear thermal rocket, the heat from a reactor is used to directly heat a propellant, which is pushed out a nozzle at the back of the spacecraft. This propellant isn’t burned, just heated, so there’s no need for the fuel to even be able to burn. However, the lighter the propellant, the more efficient the rocket, since the same amount of energy can make a smaller molecule travel faster than a larger one, and this is proportional to the atomic mass of the molecules that make up the propellant. Because of this, hydrogen is usually chosen since it’s very lightweight, but ammonia, CO2, and others have been proposed over the years as well.