Single stage to orbit (from Earth) is not as efficient as two stages to orbit, and that's the reason it's not done. If SpaceX manages to deliver Starship and r

Single stage to orbit (from Earth) is not as efficient as two stages to orbit, and that's the reason it's not done. If SpaceX manages to deliver Starship and reuse it and its booster, that will be a huge improvement over any possible reusable SSTO. SSTO or otherwise, reentry and landing are difficult enough, but SSTO adds other difficult problems.