system consists of three independent systems. Each of the three
auxiliary power units provides mechanical shaft power to drive a
hydraulic pump, and each of the three hydraulic pumps provides the
hydraulic pressure for the respective hydraulic system.
systems are designated 1, 2 and 3. Each of the three independent
hydraulic systems consists of a main hydraulic pump, hydraulic reservoir,
hydraulic bootstrap accumulator, hydraulic filters, control valves,
hydraulic/ Freon-21 heat exchanger, electrical circulation pump
and electrical heaters.
system provides hydraulic pressure for positioning of hydraulic
actuators for (1) thrust vector control of the three space shuttle
main engines by gimbaling the three SSMEs, (2) propel lant control
of various valves on the SSMEs, (3) con trol of the orbiter aerosurfaces
(elevons, body flap, rudder/speed brake), (4) retraction of the
external tank/orbiter 17-inch liquid oxygen and liquid hydrogen
disconnect umbilicals within the orbiter at external tank jettison,
(5) main and nose landing gear deployment, (6) main landing gear
brakes and anti-skid and (7) nose wheel steering.
When the three
APUs are started five minutes before lift-off, the hydraulic systems
position the three SSMEs for start, control various propellant valves
on the SSMEs and position orbiter aerosurfaces. The hydraulic systems
provide pressure for SSME thrust vector control at launch through
SSME propellant venting, elevon load relief during ascent and retraction
of the liquid oxygen and liquid hydrogen umbilicals at external
systems are not operated after the first orbital maneuvering system
thrusting period because hydraulic functions are no longer required.
One hydraulic/APU system is operated briefly one day before deorbit
to support a checkout of the orbiter flight control system, which
includes the orbiter aerosurfaces (elevons, rudder/speed brake and
system is activated before the deorbit thrusting period; and the
two remaining systems are activated after the deorbit thrusting
maneuver and operate continuously through entry, landing and landing
rollout to provide hydraulic power for positioning of the orbiter
aerosurfaces during the atmospheric flight portion of entry, deployment
of the nose and main landing gear, main landing gear brakes and
anti-skid, nose wheel steering and positioning of the three SSMEs
after landing rollout.
When the hydraulic/APU
systems are in operation, the corresponding water spray boilers
cool the APU lube oil system and hydraulic systems. On orbit, because
hydraulic power is no longer required, each hydraulic system's fluid
is circulated periodically by an electric-motor-driven circulation
pump to absorb heat from the Freon-21 hydraulic system heat exchanger
and distribute it to active areas of that hydraulic system. Electrical
heaters are provided in areas of the hydraulic systems that cannot
be warmed by fluid circulation on orbit.
system is capable of operation when exposed to forces or conditions
caused by acceleration, deceleration, normal gravity, zero gravity,
hard vacuum and temperatures encountered during on-orbit dormant
The main hydraulic
pump for each hydraulic system is a variable displacement type.
Each operates at approximately 3,900 rpm when driven by the corresponding
Each main hydraulic
pump has an electrically operated depressurization valve. The depressurization
valve for each pump is controlled by its corresponding hyd main
press 1, 2 or 3 switch on panel R2. When the switch is positioned
to low , the depressurization valve is energized to reduce the main
hydraulic pump discharge pressure from its nominal range of 2,900
to 3,100 psi output to a nominal range of 500 to 1,000 psi to reduce
the APU torque requirements during the start of the APU.
start of each APU, the corresponding APU/hyd ready to start 1, 2,
3 talkback indicator on panel R2 should indicate gray. For the talkback
indicator to indicate gray, the corre sponding hydraulic system
hyd main pump press switch on panel R2 must be in low , the corresponding
boiler cntlr/pwr/htr switch on panel R2 must be in the A or B position,
the corresponding boiler cntlr switch on panel R2 must be on, the
corresponding boiler N 2 supply switch on panel R2 must be on, and
the boiler-ready signal, which consists of four parameters-boiler
steam vent nozzle above 130 F, nitrogen valve open, bypass valve
powered and boiler enabled-must be present.
When an APU
has been started, the corresponding hyd main pump press switch is
positioned from low to norm . This de-energizes the respective depressurization
valve, allowing that hydraulic pump to increase its outlet pressure
from 500 to 1,000 psi to 2,900 to 3,100 psi. Each hydraulic pump
is a variable displacement type that provides zero to 63 gallons
per minute at 3,000 psi nominal with the APU at normal speed and
69.6 gallons per minute at 3,000 psi nominal with the APU at high
fluid going out to the system passes through a 5-micron filter before
entering the hydraulic system, and all fluid passes through a 15-micron
filter before entering the reservoir.
relief valve in the filter module for each hydraulic system relieves
the hydraulic pump supply line pressure into the return line in
the event the supply line pressure exceeds 3,850 psid.
sensor in the filter module for each hydraulic system monitors the
hydraulic system source pressure and displays the pressure on the
hydraulic pressure 1, 2 and 3 meters on panel F8. The same hydraulic
pressure sensor for each system also provides an input to the yellow
hyd press caution and warning light on panel F7 if the hydraulic
pressure of system 1, 2 or 3 is below 2,400 psi. The red backup
caution and warning alarm light on panel F7 will also be illuminated
if the hydraulic pressure of system 1, 2 or 3 is at 2,400 psi.
reservoir bootstrap accumulator in each hydraulic system bootstrap
circuit assures adequate pressure at the inlet of the main hydraulic
pump and circulation pump in that system through the use of a differential
area piston (41-1 area ratio between the reservoir side and accumulator
side). When the main hydraulic pump is in operation, the high-pressure
side of the piston and the bootstrap accumulator are pressurized
to 3,000 psig from the main pump discharge line. When the main hydraulic
pump is shut down, the priority valve closes and the bootstrap accumulator
maintains a pressure of approximately 2,500 psi. The 2,500 psi on
the high side results in a main pump inlet (low side) pressure of
40 to 60 psia. The minimum inlet pressure to assure a reliable main
pump start is 20 psia (which corresponds to a high-pressure side
of 800 psi). This prevents the main pump from cavitating (not drawing
hydraulic fluid), which could damage the pump.
in each reservoir is 8 gallons. The hydraulic fluid specification
is MIL-H-83282, which is a synthetic hydrocarbon (to reduce fire
hazards). The reservoir provides for volumetric expansion and contraction.
The quantity of each reservoir is monitored in percent on the hydraulic
quantity 1, 2, 3 meters on panel F8. A pressure relief valve in
each reservoir protects the reservoir from overpressurization and
relieves at 120 psid.
is a piston type precharged with gaseous nitrogen at 1,650 to 1,750
psi. The gaseous nitrogen capacity of each accumulator is 96 cubic
inches, and the hydraulic volume is 51 cubic inches.
When each APU/main
hydraulic pump and water spray boiler is in operation, each hydraulic
fluid system directs through its corresponding water spray boiler.
The hydraulic fluid is directed through the water spray boiler for
cooling when the hydraulic fluid temperature of that system reaches
210 F. When the hydraulic fluid temperature of that system decreases
to 190 F, the hydraulic fluid bypasses the boiler. This is automatically
accomplished by the hydraulic bypass valve in the water spray boiler.
bypass valve in each hydraulic fluid system directs the hydraulic
fluid through the hydraulic/ Freon-21 coolant heat exchanger, if
the fluid's temperature is less than 105 F, and bypasses the fluid
around the hydraulic/ Freon-21 coolant heat exchanger if the temperature
is greater than 115 F.
(elevons, rudder/speed brake and body flap) are powered by the hydraulic
system, and the movement of the applicable aerosurface is accomplished
can be positioned by any of the three hydraulic systems. For each
elevon, one hydraulic system is designated as the primary system,
and the other two systems are standby 1 and standby 2. Switching
valves are located in each elevon actuator. If the primary system
pressure drops to around 1,200 to 1,500 psig, the switching valve
will switch that elevon actuator to standby 1; and if that system
pressure drops to around 1,200 to 1,500 psig, the switching valve
will switch that elevon actuator to standby 2.
brake is driven by six hydraulic motors, contained in a power drive
unit. Three motors power the rudder, and three power the speed brake
function. Each motor in its group is supplied by a different hydraulic
system. The outputs of the three motors are combined in a planetary
gear train, and the rudder and speed brake functions are summed
in a mixing gear train. Loss of one hydraulic system results in
loss of one motor. Because of the velocity summary nature of the
gearbox, loss of two hydraulic systems results in about half the
design speed output from the gearbox.
The body flap
operation is similar to that of the rudder/speed brake.
rate-limiting system provides an automatic management of the loads
on the remaining hydraulic system or systems if one or two hydraulic
systems are lost for ascent or entry. The PRL system assigns relative
priorities to the various flight controls of the orbiter and limits
the demand on the hydraulic system by reducing the rate of movement
of the control effectors. The PRL software is part of the guidance,
navigation and control computer's digital autopilot software. The
PRL system is automatically informed of the loss of a hydraulic
system by a hydraulic-pressure-based redundancy management scheme
in the GN&C; computer software.
For each hydraulic
system, the RM selection filter software receives three hydraulic
main pump outlet pressure readings from three separate pressure
transducers in each system by way of three different flight-critical
aft multiplexers/demultiplexers. The selection filter selects the
middle value, which it passes on to the hydraulic subsystem operating
program. The SOP declares the hydraulic system failed if the pressure
reading it gets from redundancy management is less than 1,706 psia.
The hydraulic SOP then reports to the DAP PRL program how many good
hydraulic systems are left and which systems are bad.
The PRL software
establishes the elevons and rudder at a higher priority than the
speed brake when the flow demand for all three systems cannot be
met. In addition, for loss of one or two hydraulic systems, PRL
will reduce the maximum rate of movement of the elevons to reduce
the hydraulic flow demand. For loss of one hydraulic system, the
reduction in elevon rates is approximately 4 percent (the body flap
rate is not limited). For loss of two hydraulic systems, the reduction
in elevon rates from normal rates is approximately 46 percent.
If one pressure
transducer reading is lost because of an aft MDM failure, the redundancy
management selection filter will take the remaining two readings,
calculate an average and pass this average value on to the hydraulic
If two pressure
transducer readings are lost, redundancy management will pass the
remaining value to the hydraulic SOP unaltered.
management also looks at the difference between the two readings
when only two readings are involved. If the difference between the
two pressures is greater than 250 psi, redundancy management will
declare a miscompare, set a flag in the software declaring the data
to be bad and pass this flag to the hydraulic SOP. When the hydraulic
SOP sees the bad-data flag, it will ignore the current pressure
value that redundancy management is sending it and use the last
pressure value redundancy management sent before the data were declared
management also looks at the differences among the three readings.
If one reading differs from the other two readings by greater than
250 psi, a miscompare is declared and that reading is no longer
used. The remaining two readings are averaged.
inputs to PRL can become necessary if an unlikely series of MDM
failures, pressure transducer failures and hydraulic system failures
on a given hydraulic system leads the hydraulic SOP to an incorrect
conclusion regarding the status of that system.
system is supplied or isolated to the space shuttle main engines'
engine valve hydraulic actuators, SSME thrust vector control pitch
and yaw actuators and umbilical retract actuators by the main propulsion
system/thrust vector control isol (isolation) vlv 1, 2 and 3 switches
on panel R4. When the corresponding MPS/TVC isol vlv switch is positioned
to open, the corre sponding hydraulic source pressure is supplied
to the SSME thrust vector control and umbilical actuators; when
the switch is positioned to close, the hydraulic system is isolated
from those functions. A talkback indicator located above the respective
switch indicates op when that valve is open and cl when it is closed.
The MPS/TVC isol vlv 1, 2 and 3 switches are open during prelaunch
and ascent and are closed after SSME propellant dump and stow. They
remain closed except to reposition the SSMEs after deorbit thrusting,
The three SSMEs
and their associated controllers provide the positioning of the
individual hydraulic actuators, which control each SSME oxidizer
preburner oxidizer valve, main oxidizer valve, chamber coolant valve,
fuel preburner oxidizer valve and the main fuel valve. These valves
are commanded open for SSME ignition and are sustained in the open
position through ascent. These valves are commanded closed hydraulically
at main engine cutoff. After SSME shutdown and external tank separation,
these valves are sequenced open for SSME propellant dump and purge
and then sequenced closed for the remainder of the mission. Hydraulic
system 1 supplies SSME 1, hydraulic system 2 supplies SSME 2 and
hydraulic system 3 supplies SSME 3. If the corresponding hydraulic
pressure drops below approximately 1,700 psig, a shuttle valve will
shut off the hydraulic inlet and outlet to all five control valves.
This is called a ''soft lockup'' and freezes that SSME at its current
throttle setting. The soft lockup is reversible if that hydraulic
system recovers pressure. However, if that SSME receives a command
from its electronic controller to change throttle settings while
in soft lockup, it enters an irreversible ''hard lockup'' condition
and is held at that throttle setting for the rest of that SSME thrusting
period. With the hydraulic system failed, if that SSME is required
to shut down before or at MECO, shutdown is accomplished by a backup
pneumatic (helium) system.
Each SSME is
provided with thrust vector control by a pitch and yaw actuator,
which is controlled by the ascent thrust vector control system.
Each actuator is powered hydraulically for mechanically gimbaling
the SSME for start and launch position and for thrust vector control
during ascent. A switching valve is located at each actuator. A
primary and secondary hydraulic system is supplied to each switching
valve. If the primary hydraulic system at that switching valve drops
below approximately 1,500 psig, the switching valve automatically
switches that actuator to its secondary hydraulic system. After
MECO, the actuators will position the SSMEs to the dump position
for SSME propellant dump in order to minimize attitude disturbance.
After propellant dump, the actuators will position the SSMEs to
the stowed position for minimum aerodynamic interference for entry.
tank separation and SSME propellant dump and purge, the orbiter
liquid oxygen and liquid hydrogen umbilicals at the external tank/orbiter
interface are retracted and locked by three hydraulic actuators
at each umbilical. The two umbilicals are retracted to permit the
closure of the two external tank/orbiter umbilical doors in the
bottom aft fuselage in preparation for entry. Hydraulic system 1
source pressure is supplied to one actuator at each umbilical, hydraulic
system 2 source pressure is supplied to a second actuator at each
umbilical and hydraulic system 3 source pressure is supplied to
a third actuator at each umbilical.
There are three
landing gear hydraulic isolation valve ( LG hyd isol vlv) switches
on panel R4 for hydraulic systems 1, 2 and 3. The LG hyd isol vlv
1 switch positioned to close isolates hydraulic system 1 source
pressure from the nose and main landing gear deployment uplock hook
actuators and strut actuators, nose wheel steering actuator and
main landing gear brake control valves. A talkback indicator next
to the switch indicates cl when the valve is closed. The landing
gear isolation valves will not close or open unless the pressure
in that system is at least 100 psi. When the LG hyd isol vlv 1 switch
is positioned to open, hydraulic system 1 source pressure is supplied
to the main landing gear brake control valves and to the normally
closed extend valve. The normally closed extend valve is not energized
until a gear down command is initiated by the commander or pilot
on panel F6 or panel F8. The talkback indicator would indicate op
. In order to prevent inadvertent nose and main landing gear deployment,
the LG hyd isol vlv 1 switch is left in the cl position.
The LG hyd
isol vlv 2 and 3 switches on panel R4 positioned to close isolate
the corresponding hydraulic system from only the main landing gear
brakes. The adjacent talkback indicator would indicate cl. When
the switches are positioned to open , the corresponding hydraulic
system source pressure is available to the main landing gear brake
control valves. The corresponding talkback indicator would indicate
system 1 is used for the deployment of the nose and main landing
gear and nose wheel steering. When the nose and main landing gear
down command is initiated by the commander or pilot on panel F6
or F8, hydraulic system 1 pressure is directed to the nose and main
landing gear uplock hook actuators and strut actuators (provided
the LG hyd isol vlv 1 switch is in the open position) to actuate
the mechanical uplock hook for each landing gear and allow the gear
to be deployed and also provide hydraulic system 1 source pressure
to the nose wheel steering actuator. The main landing gear brake
control valves receive hydraulic system 1 source pressure when the
LG hyd isol vlv 1 is positioned to open . If hydraulic system 1
source pressure is unavailable, a pyrotechnic initiator attached
to the nose and main landing gear uplock actuator automatically,
one second after the gear down command, deploys the landing gear,
actuates the mechanical uplock hook for each landing gear and allows
the gear to be deployed. Because of the unavailability of hydraulic
system 1 source pressure, powered nose wheel steering would not
be functional; however, directional control of the orbiter can be
maintained by differential braking to caster the nose wheel for
The main landing
gear brakes utilize hydraulic systems 1 and 2 as the primary source
of hydraulic power and system 3 as a standby source of hydraulic
power. Each of the four main landing gear wheel brake assemblies
receives pressure from two different hydraulic systems in two separate
brake chambers. One chamber receives hydraulic source pressure from
hydraulic system 1 and the other chamber from hydraulic system 2.
In the event of the loss of system 1 or 2 source pressure, switching
valves provide automatic switching to the standby hydraulic system
3 when the active hydraulic system source pressure drops below approximately
1,000 psi. If hydraulic system 1 is unavailable, there is no effect
to the braking system because standby system 3 would be automatically
switched to replace system 1. Loss of hydraulic system 1 or 2 or
both would also have no effect on the braking system because standby
system 3 would automatically be switched to replace system 1 or
2 or both. Loss of hydraulic system 1 and 3 would cause the loss
of half the braking power on each wheel and would require additional
braking distance. Loss of hydraulic systems 2 and 3 would also cause
the loss of half the braking power on each wheel, requiring additional
pump in each hydraulic system consists of a high-pressure and low-pressure,
two-stage gear pump driven by a 28-volt dc induction electric motor
with a self-contained inverter. Protection against excessive electronic
component temperature is provided by directing the inlet fluid flow
around these components and through the electric motor before it
enters the pumps. The low-pressure stage is rated at 2.9 gallons
per minute at 350 psi. The circulation pumps in each hydraulic system
maintain the desired hydraulic fluid temperatures during prelaunch
activities before auxiliary power unit start and provide orbital
thermal control of the hydraulic fluid by transferring heat from
the active thermal control system Freon-21 coolant loop/hydraulic
heat exchanger to that hydraulic system. After landing and rollout,
the circulation pump in each hydraulic system provides thermal conditioning
of the hydraulic fluid after APU shutdown through the water spray
boiler to limit hydraulic fluid temperature rise due to heat soakback.
In the event of pressure loss in the bootstrap accumulator due to
leakage on orbit, an unloader valve at the circulation pump directs
the high-pressure stage pump to deliver 0.1 gallon per minute at
a discharge pressure of up to 2,500 psi to repressurize the accumulator
to greater than 2,563 psi and then redirects the high-pressure output
to combine with the low-pressure output.
power for each circulation pump is supplied by the hyd circ pump
power 1, 2 and 3 switches on panel A12. Circulation pump 1 can receive
power from main bus A or B, circulation pump 2 can receive power
from main bus B or C, and circulation pump 3 can receive power from
main bus C or A.
pump for each hydraulic system is controlled by the hyd circ pump
1, 2 and 3 switches on panel R2. The on position provides the electrical
power to its corresponding circulation pump, provided that the corresponding
APU start/run switch on panel R2 is not in the start/run or start
oride/run position. The off position removes electrical power from
the corresponding circulation pump. The GPC position allows the
general-purpose computer to automatically control the corresponding
The GPC position
of the hyd circ pump 1, 2 and 3 switches on panel R2 permits the
activation or deactivation of the corresponding circulation pump
according to the control program in the onboard computer based on
certain hydraulic system line temperatures. The program activates
the appropriate circulation pump when any of a hydraulic system's
control temperatures drop below zero degree F and deactivates the
circulation pump when all of the control temperatures for that hydraulic
system are greater than 20 F.
circulation pump for a hydraulic system circulates the corresponding
fluid system to the flight control system aerosurfaces. In order
to circulate the fluid for the landing gear system, the LG hyd isol
vlv switches on panel R4 must be positioned to GPC or open. The
GPC position allows automatic computer control of the valves, whereas
the open position enables manual control of the valves in conjunction
with GPC control of the circulation pump. Note that hydraulic systems
2 and 3 provide fluid circulation to only the main landing gear
brakes and that circulation dead-ends at the brake control valves,
but system 1 is for gear deployment and main landing gear brakes.
As stated previously, the LG hyd isol vlv 1 switch is left closed
to prevent inadvertent gear deployment.
open hydraulic system 1 redundant shutoff valve is a backup to the
retract/circulation valve to prevent hydraulic pressure from being
directed to the retract side of the nose and main landing gear uplock
hook actuators and strut actuators if the retract/circulation valve
fails open during nose and main landing gear deployment.
closed hydraulic system 1 dump valve is energized open to allow
hydraulic system 1 fluid to return from the nose and main landing
gear areas when deployment of the landing gear is commanded by the
limits of the hydraulic fluid circulation systems can be changed
during the mission by the flight crew or the Mission Control Center
in Houston. The program also includes a timer to limit the maximum
time a circulation pump will run and a priority system that automatically
monitors hydraulic bootstrap pressure, which would allow all three
circulation pumps to be on at the same time. The software timers
allow this software to be used in contingency situations for ''time-controlled''
circulation pump operations in order to periodically boost an accumulator
that is losing hydraulic fluid through a leaking priority valve
or unloader valve.
if required, the LG hyd isol vlv 1, 2 and 3 switches are positioned
to GPC . At 19,000 feet per second, the landing gear isolation valve
automatic opening sequence begins under guidance, navigation and
control software control. If the landing gear isolation valve is
not opened automatically, the flight crew will be requested by the
Mission Control Center to open the valve by positioning the applicable
LG hyd isol vlv switch to open . Landing gear isolation valve 2
is automatically opened six minutes and 37 seconds later, followed
by the automatic opening of landing gear isolation valve 1 when
the orbiter's velocity is 800 feet per second or less. Landing gear
isolation valve 3 is automatically opened at ground speed enable.
Landing gear isolation valve 1 opens next to last to ensure that
an inadvertent gear deployment would occur as late (low airspeed)
and electrical heaters are installed on the portions of the hydraulic
systems that are not adequately thermally conditioned by the individual
hydraulic circulation pump system because of stagnant hydraulic
heaters are installed on the body flap differential gearbox, rudder/speed
brake mixer gearbox, the four elevon actuators, the aft fuselage
body flap A and B seal cavity drain line and rudder/speed brake
cavity drain line. The hydraulic heater switches are located on
panel A12. There are hydraulic heater switches for the rudder/speed
brake, body flap, elevon and aft fuselage. The auto A or B switch
for the rudder/speed brake, body flap, elevon and aft fuselage permits
the corresponding main bus A or B to power redundant heaters at
each location. Thermostats in each electrical A or B system cycle
the heaters automatically off or on. The off position of the applicable
switch removes electrical power from that heater system.
heaters are installed on the main landing gear hydraulic flexible
lines located on the back side of each main landing gear strut between
the brake module and brakes. These heaters are required because
the hydraulic fluid systems are dead-ended and cannot be circulated
with the circulation pumps. In addition, on OV-103 and OV-104, the
hydraulic system 1 lines to the nose landing gear are located in
a tunnel between the crew compartment and forward fuselage. The
passive thermal control system on OV-103 and OV-104 is attached
to the crew compartment, and this leaves the hydraulic system 1
lines to the nose landing gear exposed to environmental temperatures,
thus requiring electrical heaters on the lines in the tunnel. The
passive thermal control system on OV-102 is attached to the inner
portion of the forward fuselage rather than the crew compartment;
thus, no heaters are required on the hydraulic system 1 lines to
the nose landing gear on OV-102.
brake heater A, B and C switches on panel R4 enable the heater circuits.
On OV-103 and OV-104, the hydraulics brake heater A, B and C switches
provide electrical power from the corresponding main bus A, B and
C to the redundant heaters on the main landing gear flexible lines
and the hydraulic system 1 lines in the tunnel between the crew
compartment and forward fuselage leading to the nose landing gear.
Thermostats on each electrical A, B and C system cycle the heaters
automatically off or on for the brake systems.
brake heater A, B and C switches on panel R4 enable the heater circuits
on only the main landing gear hydraulic flexible lines on OV-102.
line of each hydraulic system is directed to its respective water
spray boiler. One WSB for each hydraulic system provides the expendable
heat sink for each orbiter hydraulic system and each of the APU
lube oil systems during prelaunch, the boost phase, on-orbit checkout,
deorbit and entry through rollout and landing.
the unique hydraulic system fluid flows, hydraulic fluid control
valves are located in the return line of the hydraulic system to
the WSB. Normally, the hydraulic system fluid flows at up to 21
gallons per minute; however, the hydraulic system experiences one-
to two-second flow spikes of up to 63 gallons per minute. If these
spikes were to pass through the WSB, pressure drop would increase
ninefold and the WSB would flow-limit the hydraulic system. To prevent
this, a relief function is provided by a spring-loaded poppet valve
that opens when the hydraulic fluid's pressure exceeds 48 psi and
is capable of producing a flow of 43 gallons per minute at 50 psid
across the WSB. A hydraulic bypass valve allows the hydraulic fluid
to bypass the boiler when the hydraulic fluid has increased to 210
F. At 210 F, the controller commands the bypass valve to direct
the hydraulic fluid through the WSB. When the hydraulic fluid cools
to 190 F, the controller commands the bypass valve to direct the
fluid around the WSB.
The WSB controllers
are powered up at launch minus four hours. The WSB water tanks are
pressurized at T minus one hour and 10 minutes in preparation for
activating the auxiliary power units. The WSB controllers activate
heaters on the water tank, boiler and steam vent to assure that
the WSB is ready to operate for launch.
APU start is
delayed as long as possible to save fuel. At T minus six minutes,
the pilot begins the APU prestart sequence. The pilot confirms that
the WSB is activated, then activates the APU controllers and depressurizes
the main hydraulic pump. Depressurizing the main pump will reduce
the starting torque on the APU. The pilot then opens the APU fuel
tank valves and looks for three ready-to-start indications (gray
talkbacks). At T minus five minutes, the pilot starts the three
APUs by taking the APU cntl switches to start/run and checks the
hydraulic pressure gauges for an indication of approximately 900
psi. Then the pilot pressurizes the main pump and verifies approximately
3,000 psi on the gauges.
three hydraulic main pump pressures are greater than 2,800 psi by
T minus four minutes, the automatic launch sequencer will abort
The APUs operate
during the ascent phase and continue to operate through the first
orbital maneuvering system thrusting period. At the conclusion of
the space shuttle main engine purge, dump and stow sequence, the
APUs and WSBs are shut down. The same sequence applies for a delayed
OMS-1 thrusting period. If an abort once around has been declared,
the APUs are left running, but the hydraulic pumps are depressurized
to reduce APU fuel consumption. Leaving the APUs running avoids
having to restart hot APUs for deorbit and re-entry.
At six hours
after lift-off, the APU heater gas generator/fuel pump heaters are
activated and operate for the remainder of the on-orbit mission.
The APU fuel and water line heaters are also activated to prevent
freezing of these lines as the APUs cool down.
A few hours
after lift-off, the landing gear isolation valves on hydraulic systems
2 and 3 are opened so that the circulation pumps can circulate hydraulic
fluid through these systems. These valves will not open or close
unless the pressure in the line is at least 100 psi, requiring the
main hydraulic pump or hydraulic circulation pump to be active.
The hydraulic system 1 landing gear isolation valve is left closed.
Two hours after
lift-off, the WSB steam vent heaters are turned on and left on for
about 1.5 hours to eliminate all ice from the WSB steam vents.
While the vehicle
is in orbit, the hydraulic circulation pumps are in the GPC mode
and are automatically activated when hydraulic line temperatures
become too low and automatically deactivated when the lines warm
On the day
before deorbit, one hydraulic/APU system is started in order to
have hydraulic power to check out the flight control system. Hydraulic
power is needed to move the orbiter aerosurfaces as part of this
checkout. The associated WSB controller is activated, landing gear
isolation valves 2 and 3 are closed, and one APU (selected by the
Mission Control Center) is started up. The hydraulic main pump is
taken to normal pressure, and aerosurface drive checks are done.
After about five minutes, the checks are complete and the APU is
shut down. Normally, the APU does not run long enough to require
WSB operation. The landing gear isolation valves on hydraulic systems
2 and 3 are re opened after the APU is shut down.
At 2.5 hours
before the deorbit thrusting period, the WSB steam vent heaters
are activated to prepare the WSB for operation during the entry.
At about the same time, the landing gear isolation valves on hydraulic
systems 2 and 3 are closed, and the circulation pumps are turned
At 45 minutes
before deorbit, the WSB water tanks are pressurized, the APU controllers
are activated, and the main hydraulic pumps are commanded to low
pressure. The pilot opens the APU fuel tank valves and verifies
three gray APU/hyd rdy talkbacks. The pilot then recloses the fuel
tank valves. This procedure is run while in contact with the ground
so that flight controllers can observe APU status. Five minutes
before the deorbit thrusting period, one APU (selected by Mission
Control) is started in order to assure that at least one APU will
be operating for entry. The hydraulic pump is left in low. This
APU operates through the deorbit burn. At 13 minutes before entry
interface (entry interface is, by definition, 400,000 feet altitude),
while the orbiter is still in free fall, the other two APUs are
started, and all three hydraulic pumps are pressurized (norm). Any
two SSME hydraulic isolation valves are cycled opened for 10 seconds
and then closed in order to ensure that the SSMEs are stowed for
entry. Two minutes later, if required, the aerosurfaces are put
through an automatic cycle sequence to make sure warm hydraulic
fluid is available in the aerosurface drive units.
a hydraulic load test may be performed to test the response of the
APUs and hydraulic pumps under high load (i.e., high flow demand)
conditions. This test consists of cycling the orbiter aerosurfaces
with one hydraulic system at a time in depressed mode (the remaining
two APUs and hydraulic pumps have to drive all the aerosurfaces).
This is typically done on the first flight of a new vehicle. Then
the SSME hydraulic isolation valves are opened again and the SSMEs
are positioned to the transport position. At this point, the hydraulic
systems are no longer needed, and the APUs and WSBs are shut down.