In case you didn't know, SSTO capable flying boat space shuttles are actually a thing in Jerry Pournelle's CoDominium universe: the reasoning being that when CoDominium Marines need to make planetfall on a habitable world, they can do it even if there is no runway available (whether it's in enemy hands, bombed to craters, or doesn't even exist), even if this rules out inland landings and requires mild sea state. Also, being flying boats means that the shuttles can be HUEG (so that the Marines can bring all their heavy equipment along) without caring for things like ground pressure (well, you then need deeper water to land them instead of shallows, and getting things to beach from the shuttle might become an issue, but there you go).