Saturn INT-21

The Saturn INT-21 was a study for an American orbital launch vehicle of the 1970s, a proposed shortened member of the Saturn V family. It would have used the first and second stages of the Saturn V rocket from the Apollo program but left out the third stage, and the guidance unit would have moved from the top of the third stage to the top of the second stage. In that form it was capable of placing about 115,900 kilograms (255,500 pounds) into low Earth orbit. The INT-21 itself was never flown. A related variant was launched once, from the Kennedy Space Center in Florida, carrying the Skylab space station into orbit at 17:30 UTC on May 14, 1973. Because Skylab was built from an S-IVB stage, the guidance unit did not need to move in that version. The configuration was meant to serve other flights in the Apollo Applications Program and to launch further American space stations, including Skylab B.