For anyone who wants a refresher on the UNS nuclear thermal rocket propulsion program from the '60s, here's the final report. It's a very good read: https://nt

For anyone who wants a refresher on the UNS nuclear thermal rocket propulsion program from the '60s, here's the final report. It's a very good read: https://ntrs.nasa.gov/api/citations/19920005899/downloads/19920005899.pdf https://ntrs.nasa.gov/api/citations/19920005899/downloads/19... For this new initiative, I can see 2 big differences. On one hand they will use medium enriched uranium (up to 20%), while the original program used highly enriched one (85%), so it's going to be more challenging to extract as much power from a compact reactor. On the other hand, in 2023 we have computers and nuclear reactor simulation software that was not available in 1960, so that could provide a huge advantage. The biggest challenge I see is how to power down the engine. You can't simply flip a switch a turn off a nuclear reactor. You can stick in all the control rods, and the fission stops in less than a second, but the fission products will continue to generate heat for a few hours, initially at about 10% of the full reactor power. During this cool-down period, you will use the same coolant (hydrogen) as during normal operation, but you won't get the same exhaust velocity. With a traditional chemical rocket you don't have this problem. If you get a specific impulse of 450 s, you always get that. If a nuclear thermal rocket can get 900 s running at full throttle, but it goes down to 100 seconds in cool down mode, the main advantage might not be so great after all.