Wasn't the "super cheap fusion launch" predicated on discovering a non-Newtonian rocket engine? As long as you're suffering under the third law the idea of a super cheap launch is a pipe dream. This is also why concepts of reaching distant solar systems by maintaining 1G of thrust for the entire trip don't work. Plug the numbers for that into the rocket equation and even with absurd ISP values it requires more mass than is available in our solar system for a trip to even just Proxima Centauri.

The only maybe possible exception would be to pump so much energy into your reaction mass that it gains a significant amount of mass from relativity, but even that is engineering in the "it's probably easier to build a space elevator" realm.