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.
See also: spacecraft propulsion