The Bussard ramjet method of spacecraft propulsion was proposed in 1960 by the physicist Robert W. Bussard and popularized by Carl Sagan in the television series and subsequent book Cosmos as a variant of a fusion rocket capable of fast interstellar spaceflight. It would use a large scoop (on the order of miles) to compress hydrogen from the interstellar medium and fuse it. To save mass, some people have suggested using a magnetic field for a scoop. More recent calculations have shown that the drag of a scoop would be more than the thrust generated by the fusion reaction. However, the calculations (by Robert Zubrin and an associate) inspired the idea of a magnetic parachute or sail. Sagan called the construction of such a ship "engineering on the scale of small worlds".

There may be practical modifications of this concept. For example, perhaps one could shoot nuggets of fuel in front of a spacecraft from a fixed base, and then the spacecraft would not have to accelerate its own fuel. More speculatively, if the hydrogen was somehow fed into the engine and fused without being accelerated to the spacecraft's current velocity first, there would be no drag. A problem that must be overcome is that most interstellar hydrogen is ordinary protium, instead of the easier-to-fuse deuterium and tritium isotopes, and so makes a poor fusion fuel; it is possible that this could be overcome by using a carbon–nitrogen–oxygen catalysed nuclear cycle. Potential relative velocities of such a ship are theorized to exceed 16 per cent (0.16) of the speed of light.

The ramjet has been used occasionally in science fiction: Poul Anderson's 1970 novel Tau Zero is about a journey through the universe in a Bussard ramjet, and they play a key role in several stories set in Larry Niven's Known Space fictional universe. In the Star Trek fictional universe vessels commonly have magnetic hydrogen collectors. In the computer game series Marathon, Mars's moon Deimos was outfitted with a ramjet and converted into a generation ship.

The massive Seedships Calypso and Tantalus, from the PC game Alien Legacy, are equipped with Bussard ramjets.