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Saturn INT-21

The Saturn INT-21 was a study for an American orbital launch vehicle of the 1970s. It was derived from the Saturn V rocket used for the Apollo program. It was never launched.

A related variant was launched once, from the Kennedy Space Center, Florida carrying the Skylab space station into orbit, at 17:30 GMT, on May 14, 1973. This version was intended to be used for other flights in the Apollo Applications Program, and would have also been used to launch other American space stations, including Skylab B. This version kept the IU at the top of the OWS while INT-21 would have relocated the IU to the top of the S-II to allow INT-21 to carry payloads not based on the S-IVB.

The INT-21 and Saturn V had common first and second stages, but the INT-21 lacked a third stage. The Instrument unit was to be relocated to the second stage.