NASA NIAC phase 1 studies development of a nuclear fission fragment rocket engine (FFRE) that is exponentially more propellent efficient than rocket engines currently used to power today's space vehicles and could achieve very high specific impulse (>100,000 sec) at high power density (>kW/kg).
Fission Fragment rockets are several proposed nuclear rocket designs made since the 1980s with the highest potential theoretical top speed. The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The designs can, in theory, produce very high specific impulse while still being well within the abilities of current technologies. The new aerogel design proposal seems to be the most buildable version yet. It should be smaller, cheaper and simpler to make.
Here is a video from 5 years ago on fission fragment rockets.