Cold gas thrusters (DIY by inflating a balloon and then letting go) operate on the same concept: the energy stored in the compressed gas is released through the

Cold gas thrusters (DIY by inflating a balloon and then letting go) operate on the same concept: the energy stored in the compressed gas is released through the nozzle, providing thrust. All other types of thermal rockets simply provide a way for this energy to be replenished, either by external (laser, solar, microwave) or internal (nuclear) sources. The US built nuclear thermal rockets from 1955 through the 1970s. The Soviets experimented with a few for their manned moon missions. Nuclear thermal rockets heat their reaction mass with a nuclear reactor. No nuclear thermal rocket has flown, but NASA keeps tinkering with derivatives of the Rover/NERVA programs.