I wish I could find the book 📕 I read around 1974 where someone proposed a “doughnut” spacecraft in which a high powered magnetic field would compress hydrogen gas enough to cause fusion reactions and thus propel the ship at some fraction of the speed of light. It relied on interstellar hydrogen gas in order to work.

I think their drawings depicted something like a gigantic ramjet engine, with crew quarters and ship control areas embedded inside the walls of the engine.

Almost fifty years later, I would like to see whether the idea could still be viable.