| Orbiter Purge, Vent
and Drain System
The purge, vent and drain system on the orbiter is designed to perform the following functions: provide unpressurized compartments with gas purge for thermal conditioning and prevent accumulation of hazardous gases, vent the unpressurized compartments during ascent and entry, drain trapped fluids (water and hydraulic fluid) and condition window cavities to maintain visibility.
Three purge circuits are connected by the T-0 umbilical to ground equipment before launch during the preflight countdown and postlanding phases. Purge gas (cool, dry air and gaseous nitrogen) is provided to three sets of distribution plumbing: the forward fuselage, orbital maneuvering system/reaction control system pods, wings and vertical stabilizer; the midfuselage; and the aft fuselage. The purge gas makes all the unpressurized volumes inert, maintains constant humidity and temperature, forces out any hazardous gases and ensures that external contaminants cannot enter.
The active vent system provides the flow area to control pressure during purge, depressurization during ascent, molecular venting in orbit and repressurization during entry.
The vent and purge system is controlled exclusively through guidance, navigation and control software. The active ports are positioned by the software on the basis of mission time or mission events during ascent, entry and aborts and by crew inputs on the CRT and keyboard in the crew compartment flight deck.
There are 18 active vents in the orbiter fuselage, nine on each side. Each vent has a door that can be positioned for a specific purpose at various phases of flight. For identification, each door is numbered, starting at the nose of the orbiter. Each compartment has a dedicated vent on the left and right side of the orbiter for redundancy.
Internal vents are used to vent compartments that have no vent doors of their own, such as the nose wheel well, the two main wheel wells and the vertical tail section. Passive vents are used to back up vent 7 of the forward wing compartment, which responds to a delta pressure to open a check valve (passive vent) during ascent to vent the wing to the midbody if vent 7 fails; or, on descent, the midfuselage pressurizes the wing if vent 7 fails at a delta pressure of 0.72 to 1 psid. The aft bulkhead (X o 1307) has 14 one-way check valves that vent the payload bay into the aft fuselage at a delta pressure of 0.004 to 0.04 psid. Vent 8 vents the OMS/RCS pods, which are joined by a duct that enables the pod to vent through the opposite side of the vehicle if vent 8 fails to open.
All vent doors are driven by an electromechanical actuator. Vent doors located near each other share common actuators and controls. Vents 1 and 2, 4 and 7, and 8 and 9 share drive mechanisms on the left and right side. The 18 doors are divided into six groups of four ac motors each and are staggered so that all 24 motors do not run at the same time. All vent doors are driven inward, and each door has a pressure seal and thermal seal. The normal opening or closing time of a door with two motors operating is five seconds.
Vent doors 1, 2, 8 and 9 have purge positions that control flow from the forward and aft volumes, respectively. Vent 6 has two purge positions and a closed position that accommodates the different purge flow rates available to the payloads and payload bay. These doors are in the purge position before launch.
Two minutes 20 seconds before launch, the launch processing system reduces the purge flow in anticipation of closing vent 6. At T minus 35 seconds, vent 6 is closed. 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. Vents 3, 4, 5, 6 and 7 are closed to limit sound pressure levels in the payload bay. Vents 1, 2, 8 and 9 are opened. 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. At T plus 80 seconds, vent doors 8 and 9 are commanded closed to prevent hazardous gases from entering the aft fuselage; at T plus 122 seconds, vents 8 and 9 are commanded open. All vent doors remain open during the remainder of ascent and on-orbit operations.
In preparation for entry, the onboard operational sequence software (OPS 3) closes all vent doors. 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.
At the end of the mission, after the orbiter stops on the runway, vent doors 1, 2, 6, 8 and 9 are configured to their purge positions for ground cooling.
The purge and vent ducting is now made of Kevlar/epoxy (115 pieces up to 11 inches in diameter), which replaced the fiberglass or aluminum ducts and reduced the weight of the ducts 33 percent, or approximately 200 pounds.
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. The onboard orbiter sample lines duct the compartment gases to the ground support equipment at the T-0 right-hand umbilical panel and to the ground-based mass spectrometer for analysis at the launch pad.