Bussard ramjet

The Bussard ramjet design was proposed in 1960 by the physicist Robert W. Bussard as a variant of a fusion rocket. It would use a large scoop 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 energy generated by the fusion reaction. However, the calculations (by Robert Zubrin and an associate) inspired the idea of a magnetic parachute or sail -- when life gives you lemons, make lemonade.

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 which will have to be overcome is that most interstellar hydrogen is ordinary hydrogen-1, 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.