Single-stage-to-orbit
SSTOs have been supposed to hold many advantages over rockets. The main one is their reusability. Most launch systems are expendable (meaning that they are used once and not reused or recovered), with some exception of partially reusable launch systems (such as NASA's Space Shuttle or SpaceX's Falcon Heavy). Such partially reusable launch systems have significantly reduced many peoples' interest in SSTOs, because they dramatically lower launch costs using already proven technology that doesn't need expensive development.