From IEEE Spectrum, August 7:
The engine would drastically shorten trips to the moon and Mars
If you want to fly to Mars, you have to pick your departure date carefully. The ideal launch windows only come around every 26 months, and those launch windows are narrow because the planets need to be in alignment. Literally.
A fast rocket could widen those windows, shorten the trip’s duration, and spare time-sensitive cargo as well as passengers. The trouble is that the speed of today’s chemical rockets is limited by the fuel and oxygen they can carry.
Instead, you could use nuclear power—not a mere radioactive heat source, the kind that might power the weak ion propulsion of a long-term space probe, but an actual fission reactor. Such a furnace could expand a trickle of 20-kelvin liquid hydrogen into a tornado of 2,700-kelvin gas, enabling a manageable amount of propellent to provide powerful thrust halfway to Mars, then to reverse the thrust to decelerate.
That is precisely what NASA and DARPA want to build, first as a prototype, then as a moon rocket, and finally as an interplanetary vehicle. On 26 July, the agencies disclosed details of the project, a partnership with Lockheed Martin and BWX Technologies, a reactor company based in Lynchburg, Va. They give the project the Harry Potterish name of DRACO, for Demonstration Rocket for Agile Cislunar Operations.
The plan is to test the prototype in space beginning in late 2026. That’s a very short order, eased in part by combining what would normally be the second and third phases of development. The speedup is possible because the prototype “incorporates a lot of heritage hardware from past deep-space missions,” says Tabitha Dodson, the DRACO program manager at DARPA. “We wanted to have a highly reliable space platform, with everything that’s not the engine low risk.”....
....MUCH MORE
What about translunar? Oops. Never mind. Here's the U.S. Air Force Research Laboratory with:
A Primer on Cislunar Space