Bussard’s Fusion Ramjet: the Impossible Dream

Author: Geoffrey A. Landis

Description: “To dream the impossible dream… to reach the unreachable star…” — Joe Darion In 1960, physicist Robert Bussard proposed that the main problem of interstellar flight, the high mass ratio required for most propulsion systems, could be avoided if the fuel is not carried on the rocket, but instead fuel (and reaction mass) could be scooped up from the interstellar medium along the flight path of the vehicle, using a hypothetical electromagnetic scoop to gather hydrogen as fuel for a fusion reaction. Although Bussard’s original proposal was lacking in solutions to practical difficulties, the idea of an interstellar vehicle that did not carry fuel, and hence was capable of true relativistic velocities, caught the attention of the interstellar community, as well as science fiction writers. This system is not likely to be practical for a number of reasons, including the fact that the interstellar medium contains too low a density of material to be useful; the interstellar material is primarily hydrogen atoms, which cannot be used as the feedstock of any proposed fusion reactor or any hypothetical future fusion reactors based on known physics; and the fact that proposed scoop designs create more drag from the interstellar medium than the thrust produced by the interstellar hydrogen. Other investigators have proposed that that the collected material is not harvested as the energy source, but could be used as reaction mass; this concept is known as the “ram-augmented interstellar rocket.”  However, the difficulties of the low density of the interstellar remain. These problems can be alleviated if, instead of using the ambient interstellar medium, fuel is deliberately emplaced in the path of the spacecraft before the flight. Kare proposed that, at the velocities proposed for interstellar flight, nuclear fusion can be accomplished at the temperatures produced by impact, and proposed a propulsion system he tagged the “Bussard Buzz-bomb”. Analysis shows, however, that Kare’s initial concept is only plausible if the pellets can be pre-compressed before impact, a process which would have great difficulty to implement.  A more detailed variant of the impact-fusion runway will be presented, and the major difficulties to this approach discussed.