At T minus 25 seconds, the onboard general-purpose computers are enabled and take over the sequences. At T minus 10 seconds, the vents are configured for launch. If a launch abort occurs from T minus 10 seconds to T minus zero, the vent doors reposition to the prelaunch configuration. At T minus four seconds, any vent door out of configuration and not overridden causes the onboard GPCs to call a hold. At T plus 10 seconds, all vent doors are commanded open.
All vent doors remain open during the remainder of ascent and on-orbit operations.
The doors remain closed until the velocity of the orbiter reaches 2,400 feet per second, when all vents are opened by the onboard GPCs. The window cavity conditioning system prevents moisture from entering into the windshields and the cavities of the overhead and payload-viewing windows. It also depressurizes and repressurizes these cavities during flight and supplies the purge conditioning to dry them during ground operations. The side hatch window is self-contained. A hazardous gas detection system detects hazardous levels of explosive or toxic gases.