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Hydraulic System

The hydraulic 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.

The hydraulic 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.

Each hydraulic 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 tank jettison.

The hydraulic/APU 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 body flap).

One hydraulic/APU 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.

Each hydraulic 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 conditions.

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 APU.

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.

Before the 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 speed.

All hydraulic 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.

A high-pressure 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.

A pressure 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.

A hydraulic 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.

The quantity 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.

The accumulator 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.

Another temperature-controlled 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.

The aerosurfaces (elevons, rudder/speed brake and body flap) are powered by the hydraulic system, and the movement of the applicable aerosurface is accomplished mechanically.

Each elevon 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.

The rudder/speed 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.

The priority 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 SOP.

If two pressure transducer readings are lost, redundancy management will pass the remaining value to the hydraulic SOP unaltered.

Redundancy 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 bad.

Redundancy 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.

Manual crew 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.

Each hydraulic 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, if required.

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.

After external 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 op.

Only hydraulic 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 steering.

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 braking distance.

A circulation 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.

The electrical 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.

The circulation 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 circulation pump.

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.

The hydraulic 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.

The normally 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.

The normally 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 flight crew.

The activation/deactivation 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.

During entry, 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) as possible.

Insulation 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 fluid areas.

Redundant electrical 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.

Redundant electrical 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.

The hydraulics 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.

The hydraulics 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.

The return 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.

Because of 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.

Unless all three hydraulic main pump pressures are greater than 2,800 psi by T minus four minutes, the automatic launch sequencer will abort the launch.

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 up sufficiently.

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 off.

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.

After touchdown, 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.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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