Nuclear thermal rocket
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.