Shuttle
Reference Manual
Orbiter
Purge, Vent and Drain System
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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.
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