In the 1960s, Lockheed and the U.S. Air Force’s Flight Dynamics Laboratory (FDL) explored several design concepts for hypersonic flight. Three principal vehicles, the FDL-5, FDL-6 and FDL-7, were based on 70-degree triangles and had different designs.
The FDL-5 had variable-geometry wings for controlled landings, and one proposal called for it to be carried aloft by a C-5 Galaxy transport and then released at high altitude.
The Air Force Rocket Propulsion Laboratory also outlined an “Air Force Sortie Space System” that had three major parts: a launch platform, drop tanks, and a space vehicle.
Which was a 747 with liquid oxygen and liquid hydrogen storage tanks inside its fuselage and hydrogen pumped into afterburners for thrust augmentation. Another option involved a vehicle sized to fit inside the shuttle bay.