Project Orion: Atom bombs as propellants. Those 50s guys had balls.

2007-05-04: Project Pluto

SLAM’s simple but revolutionary design called for the use of nuclear ramjet power, which would give the missile virtually unlimited range. Air forced into a duct as the missile flew would be heated by the reactor, causing it to expand, and exhaust out the back, providing thrust. Pluto’s namesake was Roman mythology’s ruler of the underworld — seemingly an apt inspiration for a locomotive-size missile that would travel at near-treetop level at 3x the speed of sound, tossing out hydrogen bombs as it roared overhead. Pluto’s designers calculated that its shock wave alone might kill people on the ground. Then there was the problem of fallout. In addition to gamma and neutron radiation from the unshielded reactor, Pluto’s nuclear ramjet would spew fission fragments out in its exhaust as it flew by

2014-11-23: The reason the Philae lander died after 60h is because the ESA couldn’t fit it with a nuclear battery, too much paranoia in Europe.
2017-12-04: A 10kw nuclear reactor for space exploration from nasa. bravo, especially considering the silliness of esa restrictions on nuclear propulsion in space.

2019-12-04: Pulsed Fission Fusion

Pulsed Fission-Fusion should be able to achieve 15 kW/kg and 30K seconds of ISP. This will be orders of magnitude improvement over competing systems such as nuclear electric, solar electric, and nuclear thermal propulsion that suffer from lower available power and inefficient thermodynamic cycles.

2022-01-30: How serious is NASA about nuclear?

Today’s push for nuclear power in space is a useful metric for measuring the seriousness of NASA’s—and the nation’s—lunar and Martian ambitions. In the context of human spaceflight, NASA has a well-known aversion to “new” (and thus presumably more risky) technology—but in this case, the “old” way makes an already perilous human endeavor needlessly difficult. For all the challenges of embracing nuclear power for pushing the horizon outward for humans in space, it is hard to make the case that tried-and-true chemical propulsion is easier or carries significantly less physical—and political—risk. Launching 10 International Space Stations’ worth of mass across 27 superheavy rocket launches for fuel alone for a single Mars mission would be a difficult pace for NASA to sustain. (That is more than 40 launches and at least $80b if the agency relies on the SLS.) And such a scenario assumes everything goes perfectly: sending help to a troubled crew on or around Mars would require 10s of additional fuel launches, and chemical propulsion allows very limited windows of opportunity for the liftoff of any rescue mission.

If, with a single technology, that alarmingly high number of ludicrously expensive launches could be cut down to 3—while also offering more chances to travel to Mars and back—how could a space agency that was earnest in its ambitions not pursue that approach? No miracles are necessary, and regulators and appropriators seem to agree that the time has come.

We can fly to Mars. Splitting atoms, it seems, is now the safest way to make that happen.