Alternate-history space shuttle. The 1968 Starclipper proposal used a lifting-body shuttle with a drop tank (no boosters). Smaller and more nimble than the shuttle we eventually built.
Usonian's Skylab 1973 is not absolutely required, but highly recommended.Velcro Rockets is optional (used by one scenario).
Kev33's 747 SCA is optional (used by one scenario).