ANSWER from Jerry Jason on May 21, 1997:
This is a good question. The orbiter structure is divided into five major sections: the forward fuselage, which consists of upper and lower sections that fit clam-like around a pressurized crew compartment; wings; midfuselage; payloads bay doors; aft fuselage; and the vertical tail. The majority of the structures are of aluminum construction protected by reusable surface insulation.

The orbiter structure, which does not include the crew compartment, is designed to only to handle a .81 psia differential pressure. To prevent a structure failure due to a high differential pressure an elaborate vent system has been developed.

The active vent doors consists of 14 vent ports in the orbiter fuselage that allow the unpressurized compartments of the orbiter to vent to ambient as the vehicle travels and returns from sea-level pressure to orbital vacuum. The vent openings are sized according to the volume to be vented.

The active vent system is controlled by software in the GPC's (General Purpose Computers) on board the shuttle. Positioning of the vents can be performed by automatic software commands or keyboard item entries. Each orbiter compartment, with exception of the wings, has vents on both sides of the orbiter.

Vents doors 1 and 2 (left 1&2 and right 1&2) vent the forward RCS (Reaction Control System) and the forward fuselage compartment. Vent doors 3, 5 and 6 vent the midfuselage. Vent doors 8 and 9 vent the aft fuselage and OMS (orbital maneuver system)/RCS pods.

The vents are commanded open about 28 seconds before launch. The vents stay open the entire ascent and on orbit.

The vents are commanded closed approximately 25 minutes before the deorbit burn. The vents stay closed until about 7 minutes before landing to protect the structure form entry heating.

The crew compartment, which is supported within the froward fuselage at four attachment points, is welded to create a pressure-tight vessel. The compartment is pressurized to 14.7 +/- 0.2 psia and is maintained at an 80% nitrogen and 20% oxygen composition. The crew compartment is designed for 16 psia.