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> that we had landing orbital class boosters sorted out in 1991

SpaceX first stage boosters are not orbital class.

They reach around Mach 6 (Up to my knowledge) at top course and around Mach 3 on descend before the landing burn.

Valid criticism however would be that the DC-X was intended for a return head first with a belly flip. Not tail first like SpaceX boosters.

As such, it is much more similar to Starship spacecraft.

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An orbital class booster isn't a booster that goes to orbit, it's a booster that is part of an orbital rocket stack.

The important distinction is that the DC-X prototype was, at best, supersonic given the low altitude and flight time record, while any efficient booster powering an orbital spaceflight is easily going hypersonic by the time it's returning to Earth.

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