Whether SSTO or air-breathing a/c : We don’t want big zoom-zooms leading to boom-booms.
(I’ve just reminded myself of an early Boeing SST conceptual diagram marked up with personal observations by one of the principal engineers. He drew an arrow pointing at the aircraft’s nose and with this written in: “Nice and pointy.”)
To put a finer point on it – even setting aside the other practical logistical hurdles like cryogenic storage – if you’re flying a presumptively reusable hypersonic vehicle out into the field, you’ve only got two options:
Carry your return fuel with you, which, well, there goes your payload and your fuel efficiency on the trip out, or
Have fuel available in the field, which, if the presumption is that you’re landing in the middle of nowhere, how did the return fuel come to be stored there in the first place?
IANARS, but this circle sure seems impossible to square.