MSC's Spacecraft Integration Branch proposed an Apollo 'X' spacecraft to be used in Earth orbit for biomedical and scientific missions of extended duration. The spacecraft would consist of the lunar Apollo spacecraft and its systems, with minimum modifications consisting- of redundancies and spares. The concept provided for a first-phase mission which would consider the Apollo 'X' a two-man Earth-orbiting laboratory for a period of 14 to 45 days. The spacecraft would be boosted into a 370-km orbit by a Saturn IB launch vehicle.

Variations of configurations under consideration provided for Configuration A, a two-man crew, 14- to 45- day mission, no lab module; Configuration B, a three-man crew, 45-day mission, single lab module; Configuration C, a three-man crew, 45-day mission, dependent systems double lab module; and Configuration D, a three-man crew, 120-day mission, independent systems lab module.