Once developed under the guise of space exploration, he said, nuclear reactors could be used to drive dangerous space-based laser weapons. But Borowski and other nuclear and aerospace engineers call themselves pragmatists. The fact is that nuclear power can get human-crewed missions to the moon, Mars and elsewhere in the solar system faster, safer and cheaper than any other alternatives, they say. In a nuclear thermal rocket, this energy is used to heat hydrogen propellant, which is stored aboard the rocket as liquid in supercooled fuel tanks. The strength of nuclear propulsion is that it is more efficient than traditional chemically-propelled rockets.
All rockets require fuel.
Chemical rocket engines burn it, heating up the fuel and accelerating the combustion byproducts out a rocket nozzle. Nuclear thermal engines employ a very compact mass of nuclear fuel to release tremendous amounts of energy. That energy is used to heat lightweight hydrogen gas, and shoot it through a nozzle to get thrust. The nuclear reaction heats the hydrogen to much higher velocities than chemical combustion can. Or we can just decide to travel at the same speed as the chemical with the same payload and just require a lot less mass and maybe a smaller vehicle.