Nuclear Thermal Rocket

Thermal Rockets work by heating up propellant in reactor core, vaporizing it and giving it both high temperature and velocity - and ejecting it behind the spaceship, propelling it forward. Nuclear Thermal Rockets use nuclear reactor to produce heat. While they consume fissile material when operating, typical nuclear reactor can burn for months to years before refuel becomes the issue.

Counter intuitively, heating up propellant will cool down the reactor. Most reactors are designed to compensate for that by speeding up the fission by speeding up the fission. Damage to reactor might lead to slower response and higher temperature fluctuations, resulting with both overcooling and overheating.

While powerful, easy to operate and maintain, Nuclear Thermal Rockets have poor propellant economy due to low exhaust velocity and specific impulses.