Compartment Cabin Pressurization
The cabin is pressurized to 14.7 psia, plus or minus 0.2 psia,
and maintained at an average 80-percent nitrogen and 20-percent
oxygen mixture by the air revitalization system. Oxygen partial
pressure is maintained between 2.95 and 3.45 psi, with sufficient
nitrogen pressure of 11.5 psia added to achieve the cabin total
pressure of 14.7 psia, plus or minus 0.2 psia.
The pressurization system
consists of two oxygen systems and two gaseous nitrogen systems.
The two oxygen systems are supplied by the PRSD oxygen system, which
is the same source that supplies oxygen to the orbiter fuel cell
power plants. The PRSD cryogenic supercritical oxygen storage system
is controlled by electrical heaters within the tanks and supplies
the oxygen to the ECLSS pressurization control system at a pressure
of 835 to 852 psia in a gaseous state. The gaseous nitrogen supply
system consists of two systems with two gaseous nitrogen tanks for
each system. The nitrogen storage tanks are serviced to a nominal
pressure of 2,964 psia at 80º F. If the auxiliary gaseous oxygen
supply tank is installed, it is serviced to 2,440 psia at 80º
F and stores 67.6 pounds of gaseous oxygen to provide high flow
along with gaseous nitrogen. It would maintain the crew cabin at
8 psi with oxygen partial pressure at 2 psia. For normal on-orbit
operations one oxygen and nitrogen supply system is used. For launch
and entry both oxygen and nitrogen supply systems are used in addition
to repressurization of the airlock.
The heart of the cabin pressurization is the nitrogen/oxygen
control and supply panels, the PPO 2 sensor, and crew cabin positive
and negative pressure relief valves. The nitrogen/oxygen control
panel selects and regulates primary (system 1) or secondary (system
2) oxygen and nitrogen. The primary and secondary nitrogen/oxygen
supply panels are located in the lower forward portion of the
midfuselage. The primary and secondary oxygen supply systems have
a crossover capability, as do the primary and secondary nitrogen
supply systems. If installed, the auxiliary oxygen supply system
is also controlled by the supply panel.
The oxygen and nitrogen supply systems provide the makeup cabin
oxygen gas consumed by the flight crew and nitrogen for pressurizing
the potable and waste water tanks and repressurizing the airlock.
An average of 1.76 pounds of oxygen is used per flight crew member
per day. Up to 7.7 pounds of nitrogen and 9 pounds of oxygen are
expected to be used per day for normal loss of crew cabin gas
to space and metabolic usage. The potable and waste water tanks
are pressurized to 17 psia.
Oxygen from the respective PRSD cryogenic oxygen supply system
is routed to the atmosphere pressure control oxygen sys tem 1
and system 2 supply valves. The atmosphere pressure control oxygen
system 1 and system 2 supply valves are controlled by a switch
on panel L2. When the switch is positioned to open, the corresponding
oxygen system valve opens to permit oxygen to flow through an
oxygen restrictor at a maximum flow of 20 pounds per hour and
to a heat exchanger in the Freon-21 coolant loop (oxygen system
1 through Freon coolant loop 1 and oxygen system 2 through Freon
coolant loop 2), which warms the oxygen supply to the oxygen regulator
of that system. A talkback indicator next to the switch indicates
op when the valve is open. When the atm press control O 2 sys
1 or sys 2 switch is positioned to close, the valve is closed,
isolating that oxygen supply system. The talkback indicator indicates
cl . A check valve downstream of the heat exchanger prevents oxygen
from flowing from one supply source to the other if the crossover
valves are open. Downstream of the oxygen check valve is a manifold
with an oxygen systems 1 and 2 crossover valve that would permit
system 1 to system 2 or vice versa. The crossover valves are controlled
by the atm press control O 2 xovr sys 1 and sys 2 switches on
panel L2. When the respective switch is positioned to open, that
oxygen supply system is directed to airlock supply oxygen 1 and
2 manual valves, airlock oxygen 1 and 2 extravehicular mobility
unit, and eight face mask outlets. If both atmosphere pressure
control oxygen pressure system 1 and system 2 crossover valves
are opened, oxygen supply systems 1 and 2 are interconnected.
When the respective atm press control O2 press sys 1 or sys 2
xover switch is positioned to close, that oxygen supply system
is isolated from the crossover feature.
The oxygen supply systems are directed to their corresponding
oxygen regulator inlet manual valve. When the valve is manually
positioned to open on panel M010W, the oxygen supply system is
directed to its oxygen regulator, which reduces that oxygen supply
source pressure to 100 psi with a minimum flow rate capability
of 75 pounds per hour. Each regulator is a two-stage regulator
with the second stage functioning as a relief valve when the differential
pressure across the second stage is 215 psi. The relief pressure
is vented into the crew cabin. This regulated pressure passes
through another check valve and is directed to its 14.7-psi cabin
regulator inlet manual valve, 8-psi regulator and payload oxygen
manual valve for Spacelab (if the Spacelab pressurized module
is installed in the payload bay) on panel M010W. The check valve
on each oxygen supply system between the oxygen regulator and
14.7-psia cabin regulator prevents the reverse flow of oxygen
and nitrogen into the oxygen system since the 14.7-psi and 8-psi
regulators in each system control the oxygen and nitrogen flow
to the cabin as required to maintain the desired cabin pressure.
The two primary and two secondary
gaseous nitrogen supply tanks are constructed of filament-wound
Kevlar fiber with a titanium liner. Each nitrogen tank is serviced
to a nominal pressure of 3,300 psia at 80º F with a volume
of 8,181 cubic inches. The two nitrogen tanks in each system are
manifolded together. The primary and secondary nitrogen supply systems
are controlled by the atmosphere pressure control nitrogen supply
valves in each system. Each valve is controlled by its corresponding
atm press control N 2 sys 1 and 2 supply switch on panel L2. When
a supply switch is positioned to open, that nitrogen supply system
is directed to its corresponding atmosphere pressure control system
regulator inlet valve. An indicator adjacent to the switch indicates
barberpole when the motor-operated valve is in transit and op when
the supply valve is open. When the supply switch is positioned to
close, that nitrogen supply system is isolated from the nitrogen
system regulator inlet valve, and the talkback indicator indicates
The inlet valve in each nitrogen system is controlled by its
respective atm press control N 2 sys 1 and 2 reg inlet switch
on panel L2. When a reg inlet switch is positioned to open , that
system's nitrogen source pressure is directed to the system's
nitrogen regulator. A talkback indicator next to the reg inlet
switch indicates barberpole when the motor-operated valve is in
transit and op when the valve is open. When the reg inlet switch
is positioned to close, the nitrogen supply pressure is isolated
from the system's nitrogen regulator, and the talkback indicator
The nitrogen regulators in the primary and secondary supply system
reduce the pressure to 200 psi. Each nitrogen regulator is a two-stage
regulator with the second stage functioning as a relief valve.
The second stage relieves pressure overboard at 245 psi.
The regulated pressure of each nitrogen system is directed to
the nitrogen manual crossover valve, the water tank regulator
inlet valve and the oxygen and nitrogen controller valve in each
The nitrogen crossover manual valve connects both regulated nitrogen
systems when the valve is open and isolates the nitrogen supply
systems from each other when closed. A check valve between the
nitrogen regulator and nitrogen crossover valve in each nitrogen-regulated
supply line prevents flow from one nitrogen source supply pressure
to the other if the nitrogen crossover valve is open.
The partial pressure of oxygen in the flight crew cabin can be
controlled automatically by one of two oxygen and nitrogen controllers.
Two PPO2 sensors are located under the crew cabin flight deck
mission support console. The PPO 2 A and B sensors provide inputs
to the PPO2 control systems 1 and 2 controller and switches, respectively.
When a PPO2 contr switch is positioned to norm on panel M010W
and the atm press control PPO 2 snsr/vlv switch on panel L2 is
positioned to norm, electrical power is supplied to the corre
sponding atm press control O 2 /N 2 cntlr vlv switches on panel
L2 for system 1 or 2. When the atm press contlr vlv switch is
positioned to auto, electrical power automatically energizes or
de-energizes the corresponding nitrogen control valve and nitrogen-regulated
supply. When the corresponding PPO 2 sensor determines that oxygen
is required in the crew cabin to maintain the level at 3.5 psi,
the nitrogen supply valve is automatically closed. When the 200-psi
nitrogen supply in the manifold drops below 100 psi, the corresponding
oxygen supply system flows through its check valve and 14.7-psi
cabin regulator into the crew cabin. When the PPO2 sensor determines
that the oxygen in the crew cabin is at 3.2 psi, the corresponding
nitrogen supply system valve is automatically opened, the 200-psi
nitrogen enters the oxygen and nitrogen manifold and closes the
corresponding oxygen supply system check valve, and nitrogen flows
through the 14.7-psi regulator into the crew cabin. The open and
close positions of the O 2 /N 2 cntlr vlv sys 1 and 2 switch on
panel L2 permit the flight crew to control the nitrogen valve
in each system manually, and thus cabin pressure is controlled
manually. The reverse position of the PPO 2 snsr/vlv switch on
panel L2 allows controller B to system 1 and controller A to system
If the 14.7-psi cabin regulator inlet manual valves of systems
1 and 2 are closed on panel M010W, the crew module cabin pressure
will decrease to 8 psi. The PPO2 contr sys 1 and sys 2 switches
on panel M010W are positioned to emer for the corresponding nitrogen
system, which selects the 2.2-psi oxygen partial pressure. The
corresponding PPO 2 sensor and controller, through the corresponding
PPO 2 contr switch and the PPO2 snsr/vlv switch positioned to
norm , provide electrical inputs to the corresponding O 2 /N 2
cntrl vlv switch. The electrical output from the applicable O
2 /N 2 cntrl vlv switch controls the nitrogen valve in that supply
system in the same manner as in the 14.7-psi mode except that
the crew module cabin oxygen partial pressure is maintained at
The oxygen systems 1 and 2 and nitrogen systems 1 and 2 flows
are monitored and sent to the O 2 /N 2 flow rotary switch on panel
O1. The rotary switch permits system 1 oxygen or nitrogen or system
2 oxygen or nitrogen flow to be monitored on the flow meter on
panel O1 in pounds per hour.
PPO2 sensors A and B monitor the oxygen partial pressure and
transmit the signal to the PPO 2 sensor select switch on panel
O1. When the switch is positioned to sensor A, oxygen partial
pressure from sensor A is monitored on the PPO 2 meter on panel
O1 in psia. If the switch is set on sensor B, oxygen partial pressure
from sensor B is monitored. The cabin pressure sensor transmits
directly to the cabin press meter on panel O1 and is monitored
The red cabin atm caution and warning light on panel F7 is illuminated
for any of the following monitored parameters:
Cabin pressure below 14.0 psia or above 15.4 psia.
PPO2 below 2.8 psia or above 3.6 psia.
Oxygen flow rate above 5 pounds per hour.
Nitrogen flow rate above 5 pounds per hour.
A klaxon will sound in the crew cabin and the master alarm push
button light indicators will be illuminated if the change in pressure
versus change in time decreases at a rate of 0.05 psi per minute
or greater. The normal cabin dP/dT is zero psi per minute, plus
or minus 0.01 psi, for all normal operations.
The temperature and pressure of the primary and secondary nitrogen
and emergency oxygen tanks are monitored and transmitted to the
systems management computer. This information is used to compute
oxygen and nitrogen quantities.
The two cabin relief valves are in parallel to provide overpres
surization protection of the crew module cabin above 16 psid.
Each cabin relief valve is controlled by its corresponding switch
on panel L2. The cabin relief A switch controls cabin relief A,
and the cabin relief B switch controls cabin relief B. When the
switch is positioned to enable, the corresponding motor-operated
valve allows the cabin pressure to a corresponding positive pressure
relief valve that relieves at 16 psid and reseats at 15.5 psid.
The relief valve maximum flow capability is 150 pounds per hour.
A talkback indicator above the respective switch indicates barberpole
when the motor-operated valve is in transit and op when the motor-operated
valve is open. When the switch is positioned to close , the corresponding
motor-operated valve isolates cabin pressure from the relief valve,
and the talkback indicator indicates cl.
The crew module cabin vent isolation valve and cabin vent valve
are in series to vent the crew cabin to ambient pressure. Approximately
one hour and 30 minutes before lift-off, the crew module cabin
is pressurized to approximately 16.7 psi for leak checks of the
crew cabin. The cabin vent isolation valve is controlled by the
cabin vent , vent isol switch on panel L2, and the cabin vent
valve is controlled by the cabin vent, vent switch on panel L2.
Each switch is positioned to open to control its respective motor-operated
valve. When both valves are open, the cabin pressure is vented
into the midfuselage. The maximum flow capability through the
valves at 0.2 psid is 900 pounds per hour. A talkback indicator
above each switch indicates the position of the respective valve-barberpole
when the valve is in transit and op when it is open.
If the crew cabin pressure is lower than the pressure outside
the crew cabin, two negative pressure relief valves in parallel
will open at 0.2 to 0.7 psid, permitting flow of ambient pressure
into the crew cabin. The maximum flow rate at 0.5 psid is zero
to 654 pounds per hour.
The manual water tank nitrogen regulator inlet valve in each
nitrogen-regulated supply system permits nitrogen to flow to its
corresponding regulator and water tank nitrogen regulator isolation
manual valve. The inlet and isolation manual valves are on panel
M010W. The water regulator in each nitrogen system reduces the
200-psi supply pressure to 16 psi. Each regulator is a two-stage
regulator with the second stage relieving pressure into the crew
cabin at a differential pressure of 20 psi. The nitrogen pressurization
system for the potable and waste water tanks is discussed later
in this section.