>>11548805Cont.
Things that would make reusable SSTO around Earth possible and practical;
Engines that could achieve 600 Isp or greater with a thrust to weight ratio comparable to chemical engines (at least 50:1).
Engines that could achieve TWR of 300 or greater, with efficiency in the same range as hydrolox engines (360 to 450, sea level to vacuum).
Structural materials that could partially or fully withstand reentry heating while having comparable specific strength to steel, AL-Li alloy, or carbon fiber composites.
Structural materials that still required TPS coatings but had specific strengths far in excess of steel, Al-Li alloy, or CFCs.
None of these technologies are very likely to be possible or feasible. Furthermore, literally every single one of these technologies would give you far greater payload mass fraction, robustness, and overall payload mass per launch, if used on a reusable TSTO instead of a reusable SSTO. Reusable TSTO is simply a better vehicle architecture given the high minimum delta V requirements.
With that being said, there are niches where SSTO makes sense. Unfortunately for advocates for SSTO, all of these niches are only found on other worlds, like the Moon and Mars. There's no need for a Moon Orbit shuttle vehicle to use multiple stages or boosters, because with a round trip delta V budget of only ~4 km/s, doing that single-stage is easy. Mars Orbit round-trip is actually slightly easier in terms of delta V, only requiring 3 km/s to reach low Mars orbit, and a tiny delta V in order to initiate aerobraking and reentry for propulsive landing (say another 500 m/s, for 3.5 km/s total). Forth these worlds, multi-stage vehicles would be used only for launching things away on interplanetary trajectories. In the case of Starship Super Heavy, the Super Heavy Booster would actually be able to drop Starship off fully loaded directly into low Mars orbit, AND return via aerodynamic reentry and propulsive landing.
