Space Shuttle program March 25, 1979, and was first launched on April 12, 1981—the 20th anniversary of Yuri Gagarin's space flight—with a crew of two. "Challenger" (OV-099) was delivered to KSC in July 1982, "Discovery" (OV-103) in November 1983, "Atlantis" (OV-104) in April 1985 and "Endeavour" in May 1991. "Challenger" was originally built and used as a Structural Test Article (STA-099), but was converted to a complete orbiter when this was found to be less expensive than converting "Enterprise" from its Approach and Landing Test configuration into a spaceworthy vehicle. On April 24, 1990, "Discovery" carried the Hubble Space Telescope into space during
Space Shuttle two different upper stages developed for the STS (single-stage Payload Assist Module or two-stage Inertial Upper Stage). The Space Shuttle was stacked in the Vehicle Assembly Building, and the stack mounted on a mobile launch platform held down by four frangible nuts on each SRB, which were detonated at launch. The Shuttle stack launched vertically like a conventional rocket. It lifted off under the power of its two SRBs and three main engines, which were fueled by liquid hydrogen and liquid oxygen from the ET. The Space Shuttle had a two-stage ascent. The SRBs provided additional thrust during liftoff and