I have right here the book "Spaceflight in the era of aero-space planes".

Plase B of the shuttle study (1970-71) wanted to demonstrate the feasability of 2 manned and winged rockets launched vertically. The first stage would go to an altitude of 80 km before separating from the orbiter and fly back to the launch site.

It revealed something drastic to the engineers: Could such a vehicle be built? The problem was integrating LH2 and LO2 tanks inside the orbiter. If the orbiter mass was heavier than expected, stretching it would require a total redesign or reoptimization of both the orbiter AND the first stage, such are the problems of fully reusable launch systems.

Besides, even much later in the Shuttle launch preparations (in fact, too late to return back to the drawing boards), the tiles became the shuttle's weakest link...and incidentally, the Shuttle's center of gravity at landing forced a redesign even later of the Spacelab pressurized modules (putting them as far away as possible from the flight deck).