Nuclear thermal rocket

A nuclear thermal rocket (abbreviated to NTR) is a type of rocket engine which heats a propellant (usually liquid hydrogen) using a nuclear reactor. This is different from chemical rockets, which burn their propellant to heat it. NTRs have higher exhaust velocities than chemical rockets, high enough to triple a spacecraft's payload capacity (the amount of cargo or astronauts they can carry to their destination).

Simple diagram of a nuclear thermal rocket. This variant uses a turbopump to move fuel from the fuel tank to the reactor.

NTRs have the potential to be very fuel efficient while still providing good thrust, but their thrust-to-weight ratio is very small. A rocket's thrust-to-weight ratio (TWR) is the proportion between the amount of thrust the spacecraft can make and how much the spacecraft weighs. For example, a spacecraft that weighs 100 pounds and can produce 10 pounds of thrust would have a TWR of 1:10. An average chemical rocket has a TWR of about 70:1, but an average NTR has a TWR of 7:1. This means that NTRs have to burn for a long time to get the same acceleration as a chemical rocket. This is balanced by their high fuel efficiency.

NTRs have not been tested in space yet, but a special program named DRACO is going to attempt to do so.