We're building thermonuclear spaceships again-this time for real
Briefly

Phoebus 2A, the most powerful space nuclear reactor ever made, confirmed capabilities for Mars mission but didn't proceed due to size, cost, and political factors like Nixon's view on space exploration limitations.
Nuclear thermal rockets (NTRs) utilized liquid hydrogen passing through a nuclear reactor core for efficient thrust without combustion, significantly enhancing fuel efficiency over chemical rockets.
Read at Ars Technica
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