Robert Bussard first proposed the Nuclear-Fusion Interstellar Ramjet in 1960 and it caught the imagination of researchers (like Carl Sagan) and fiction writers (like Larry Niven & Poul Anderson) alike. Basically Bussard proposed to scoop up the interstellar medium and fuse it for propulsion, thus allowing a rocket to refuel for its entire journey. A 1,000 ton rocket could theoretically scoop propellant and fly at 1 g ‘forever’ – at least until drag became equal to its thrust.
A problem arose – hydrogen is very hard to fuse all by itself. The reaction rate of proton-proton fusion at “low” (i.e. an achieveable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.
How do we make the reaction go faster? Physicist Daniel Whitmire proposed we burn the hydrogen via the well-known CNO Bi-Cycle. Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry”.
The CNO-cycle was first proposed by Hans Bethe as the means by which the Sun makes heat & light. It’s one means by which the Sun does so, but proton-proton fusion is dominant at the lower temperatures in the Sun and lower mass stars. Slightly bigger stars are predominantly powered by the CNO cycle and as the Sun evolves and its core contracts as helium builds up, it too will burn mostly via the CNO cycle.
Whitmire’s paper gives a rough guide to how well the CNO cycle improves a ramjet’s power-levels and shows, reasonably, that a Catalytic Nuclear Ramjet could propel a ship to the stars…
…the rest is an exercise for the student.