During Space Shuttle buildup operations inside the Vehicle Assembly Building, solid rocket booster segments are transferred from nearby assembly and checkout facilities, hoisted onto a Mobile Launcher Platform in High Bays 1 or 3, and mated to form two complete boosters. After checkout and inspection in High Bays 2 or 4, the external tank transfers to High Bays 1 or 3 to be attached to the solid rocket boosters. The orbiter, the final element, gets towed from the Orbiter Processing Facility to the Vehicle Assembly Building transfer aisle, raised to a vertical position by overhead cranes, lowered onto the Mobile Launcher Platform, and mated to the other components.

With the assembly and checkout operations complete, the huge outer doors of the high bay open to permit the Crawler-Transporter to enter and move under the Mobile Launcher Platform holding the assembled Shuttle. The high bay doors are 456 feet (139 meters) high. The lower section is 152 feet (46 meters) wide and 114 feet (35 meters) high, with four door panels that move horizontally. The upper door is 342 feet (104 meters) high and 76 feet (23 meters) wide, with seven door panels that move vertically.