Running a direct output fusion rocket in the atmosphere would be tough-- all the reactors we know how to build, or can vaguely guess how to build, run at small

Running a direct output fusion rocket in the atmosphere would be tough-- all the reactors we know how to build, or can vaguely guess how to build, run at small fractions of one atmospheric pressure. If you have fusion engines, you would just be doing all the construction in orbit anyway, rather than boosting stuff from the surface of the orbit.