|After completing a space mission, the orbiter is returned to KSC to undergo preparations for its next flight in a sophisticated aircraft-like hangar called the Orbiter Processing Facility (OPF). Here, the vehicle is safed, residual propellants and other fluids are drained, and returning horizontal and middeck payloads are removed.
Any problems that may have occurred with orbiter systems and equipment on the previous mission are checked out and corrected. Equipment is repaired or replaced and extensively tested. Any modifications to the orbiter that are required for the next mission are also made in the OPF.
Orbiter refurbishment operations and processing for the next mission also begin in the OPF. Large horizontal payloads, such as Spacelab, are installed in the orbiter cargo bay. Vertical payloads are installed at the launch pad.
Following extensive testing and verification of all electrical and mechanical interfaces, the orbiter is transferred to the nearby Vehicle Assembly Building where it is mated to the external tank with attached solid rocket boosters. Then, the assembled Space Shuttle vehicle is carried to the launch pad by a large tracked vehicle called the crawler-transporter.
At the launch pad, final preflight and interface checks of the orbiter, its payloads and associated ground support equipment are conducted. After a positive Flight Readiness Review, the decision to launch is made and the final countdown begins.