A new NASA NIAC (NASA Innovative Advanced Concepts) project is creating a buildable near term design for a nuclear fission fragment rocket. It would enable manned mission to Mars with 90 day travel times.
The fission fragment system would give experience in a technology which could eventually enable interstellar rockets with speeds of 10% of the speed of light.
A mature and advanced fission fragment rocket using hundreds of tons of fissile fuel and a 200 gigawatt reactor could have a design that could reach about 10% of light speed.
The proposed designs for Fission Fragment Rocket Engines are prohibitively massive, have significant thermal constraints, or require implementing complex designs, such as dusty plasma levitation, which limits the near-term viability. NASA NIAC researchers propose to develop a small prototype low-density nuclear reactor core and convert the nuclear energy stored in a fissile material into a high velocity rocket exhaust and electrical power for spacecraft payloads.