Shuttle Reference Manual Space Shuttle Orbiter Systems Auxiliary Power Units

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The auxiliary power unit is a hydrazine-fueled, turbine-driven power unit that generates mechanical shaft power to drive a hydraulic pump that produces pressure for the orbiter's hydraulic system. There are three separate APUs, three hydraulic pumps and three hydraulic systems.

Each auxiliary power unit and its fuel system are located in the aft fuselage of the orbiter. They are identical but independent systems that are not interconnected. Each APU fuel system supplies storable liquid hydrazine fuel to its respective fuel pump, gas generator valve module and gas generator, which decomposes the fuel through catalytic action. The resultant hot gas drives a two-stage turbine. The turbine exhaust flow returns over the exterior of the gas generator, cooling it, and is then directed overboard through an exhaust duct at the upper portion of the aft fuselage near the vertical stabilizer. The turbine assembly provides mechanical power through a shaft to drive reduction gears in the gearbox. The gearbox drives a fuel pump, a hydraulic pump and a lube oil pump. The hydraulic pump supplies pressure to the hydraulic system. The fuel pump increases the fuel pressure at its outlet to sustain pressurized fuel to the gas generator valve module and gas generator. The lube oil system supplies lubricant to the gearbox reduction gears and uses the reduction gears as scavenge pumps to supply lube oil to the inlet of the lube oil pump to increase the pressure of the lube oil system.

The lube oil of each auxiliary power unit is circulated through a heat exchanger in a corresponding water spray boiler. Three water spray boilers, one for each APU, cool the lube oil systems. The hydraulic fluid of each hydraulic pump driven by an auxiliary power unit is also circulated through a hydraulic heat exchanger in a corresponding water spray boiler to cool hydraulic fluid during hydraulic system operation. The three water spray boilers are also located in the aft fuselage of the orbiter.

The three auxiliary power units, hydraulic pumps and water spray boilers are in operation five minutes before lift-off and throughout the launch phase. They are shut down after the first orbital maneuvering system thrusting period.

The hydraulic systems provide hydraulic pressure to position hydraulic actuators for thrust vector control by gimbaling the three main engines. The hydraulic system also operates the various propellant valves on the engines; controls the orbiter's aerosurfaces (elevons, body flap and rudder/speed brake); retracts the external tank/orbiter 17-inch liquid oxygen and liquid hydrogen disconnect umbilicals within the orbiter at external tank jettison; deploys the main and nose landing gear, main landing gear brakes and anti-skid devices; and enables nose wheel steering.

When the three auxiliary power units are started five minutes before lift-off, the hydraulic systems are used to position the three main engines for activation, control various propellant valves on the engines and position orbiter aerosurfaces. The hydraulic systems provide pressure for engine thrust vector control at launch through main engine cutoff, elevon load relief during ascent and retraction of the liquid oxygen and liquid hydrogen umbilicals at external tank jettison.

The auxiliary power units are not operated after the first OMS thrusting period because hydraulic power is no longer required. One power unit is operated briefly one day before deorbit to support checkout of the orbiter flight control system, which includes the orbiter aerosurfaces (elevons, rudder/speed brake and body flap).

One auxiliary power unit is restarted before the deorbit thrusting period. The two remaining units are started after the deorbit thrusting maneuver and operate continuously through entry, landing and landing rollout to provide hydraulic pressure for the following: 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; positioning of the three main engines after landing rollout; and maximum hydraulic pump operation reverification.

Each auxiliary power unit consists of a fuel tank, a fuel feed system, a system controller, an exhaust duct, lube oil cooling system, and fuel/lube oil vents and drains. Redundant electrical heater systems and insulation thermally control the system above 45 F to prevent fuel from freezing and to maintain required lube oil viscosity. Insulation is used on components containing hydrazine, lube oil or water to minimize electrical heater power requirements and to keep high surface temperatures within safe limits on the turbine and exhaust ducts.

The APU fuel tanks are mounted on supports cantilevered from the sides of the internal portion of the aft fuselage. The fuel is hydrazine, a storable liquid fuel. The fuel tank, which incorporates a diaphragm at its center, is serviced with fuel on one side and the pressurant (gaseous nitrogen) on the other. The nitrogen is the force acting on the diaphragm (positive expulsion) to expel the fuel from the tank to the fuel distribution lines and maintain a positive fuel supply to the auxiliary power unit throughout its operation. Each typical fuel tank load is approximately 325 pounds. The fuel supply supports the nominal power unit operating time of 90 minutes in a mission or any defined abort mode, such as an abort once around, when the APUs run continuously for approximately 120 minutes. Under operating load conditions, an auxiliary power unit consumes approximately 3 pounds of fuel per hour.

Each of the three APU controllers is 6 inches wide, 7.5 inches high and 19 inches long. The rated horsepower of each unit is 135. Each unit weighs approximately 88 pounds; its controller weighs approximately 15 pounds.

The fuel tanks are 28-inch-diameter spheres. Fuel tanks 1 and 2 are located on the port (left), or minus Y, side of the orbiter's aft fuselage, and tank 3 is located on the starboard (right), or plus Y, side. Each fuel tank is serviced through its respective fill and drain service connections, located on the corresponding side of the aft fuselage. The gaseous nitrogen servicing connection for each fuel tank is located on the same panel as the fuel servicing connections on the corresponding side of the aft fuselage. The fuel tank's nitrogen gas pressure is determined by the propellant load.

Each fuel tank's temperature and gaseous nitrogen pressure is monitored and calculated through the onboard computer and transmitted to the APU fuel/H2O qty meters on panel F8. When the APU fuel/H2O switch on panel F8 is positioned to fuel , the quantity in APU fuel tanks 1, 2 and 3 is displayed simultaneously in percent. The fuel quantity of 100 percent on the meter is equivalent to 325 pounds.

The gaseous nitrogen pressure in each fuel tank exerts a force on the tank's diaphragm to expel the hydrazine fuel under pressure to the fuel distribution system. Filters are incorporated into each distribution line to remove any particles. The fuel distribution line branches into two parallel paths downstream of the filter. An isolation valve is installed in each parallel path, providing redundant paths to permit fuel flow to the auxiliary power unit or to isolate fuel from it.

Both valves in each APU fuel distribution system are controlled by the corresponding APU fuel tk vlv 1, 2, 3 switch on panel R2. They are energized open when the corresponding APU fuel tk vlv switch is positioned to open; both valves are closed when the switch is positioned to close.

Each valve has a reverse relief function to relieve pressure on fuel trapped in the fuel distribution line downstream of the fuel tank valves when both tanks' valves are closed. The valve relieves the downstream pressure when the pressure rate increases 40 psi to 200 psi above fuel tank pressure.

In the event that an APU fuel tk vlv switch is inadvertently left on after APU shutdown or an electrical short occurs within the valve's electrical coil, a system improvement provides additional protection to prevent overheating of the fuel isolation valves. Redundant temperature measurements (two per valve, four per auxiliary power unit) have been added to redundant electrical valve drivers and individual circuit breakers. The temperature measurements are displayed on the backup flight system's systems management display. If the temperature limits are exceeded, the flight crew responds by turning the applicable switch off or pulling the applicable circuit breaker.

Each auxiliary power unit has its own controller, which provides checkout logic before the APU is started. The controller detects malfunctions and controls the unit's turbine-speed gearbox pressurization and fuel pump/gas generator heaters when the auxiliary power unit is not in operation. Each controller is controlled by its corresponding APU cntlr pwr switch on panel R2. When the switch is positioned to on , 28-volt dc power is sent to that controller and auxiliary power unit. When the switch is positioned to off, electrical power is removed from that controller and APU.

An APU/hyd ready to start talkback indicator for each auxiliary power unit is located on panel R2. The talkback indicator signals gray when that auxiliary power unit hydraulic system is ready to start, that is, when the APU gas generator temperature is above 190 F, APU turbine speed is less than 80 percent, APU gearbox pressure is above 5.5 psi, water spray boiler controller is ready, corresponding APU fuel tank isolation valves are open and corresponding hydraulic main pump is depressurized. When the auxiliary power unit is started and its turbine speed is greater than 80 percent of normal speed, the corresponding indicator shows a barberpole image.

An APU control 1, 2, 3 switch is located on panel R2 for each auxiliary power unit. When the switch is positioned to start/run , the corresponding APU controller activates the start of that unit and removes electrical power automatically from the unit's gas generator and fuel pump heaters. The off position of each switch removes the start signal from the corresponding APU controller.

To start the auxiliary power unit, fuel expelled from the hydrazine tank flows through the open tank valves and filter to the gas generator valve module, which contains a primary and secondary fuel control valve in series. The primary pulse control valve is normally open and the secondary pulse control valve is energized open. Fuel flowing through the pump bypass valve is directed to the gas generator, for the fuel pump is not being driven at that moment by the APU turbine. The fuel in the gas generator decomposes through catalytic reaction, creates hot gas and directs the hot gas to the two-stage turbine, which begins to rotate. The turbine's mechanical shaft drives the reduction gears, rotating the fuel pump, lube oil pump and hydraulic pump. The fuel pump increases the fuel pressure at its outlet and sustains pressurized fuel to the gas generator valve module and gas generator. The turbine must come up to speed in 9.5 seconds or the APU controller automatically shuts the auxiliary power unit down.

The startup logic delays the APU underspeed logic check for 9.5 seconds after the start command is issued, allowing the APU to reach normal speed before the shutdown logic begins checking for a speed lower than 80 percent.

When the upper APU turbine speed is reached, the primary fuel control valve closes the fuel supply off to the gas generator and routes the fuel through the bypass line back to the fuel pump inlet. When the lower turbine speed is reached, the primary fuel control valve opens, permitting fuel to the gas generator, and closes the fuel off to the bypass line. Thus, the primary fuel valve pulses to maintain auxiliary power unit speed. The frequency and duration of the primary fuel control valve pulses are a function of the hydraulic load on the unit. The secondary fuel control valve normally stays fully open during the operation of the primary. If the primary valve loses power, it goes to the fully open position and the secondary valve begins pulsing and controlling APU speed. If the secondary valve loses power at any time, the APU is shut down. If the auxiliary power unit is taken to a high speed (by the APU select switch on panel R2), the primary valve is unpow ered and goes to the fully open position while the secondary valve controls the unit's speed.

Each APU fuel pump is a fixed-displacement, gear-type pump that discharges fuel at approximately 1,400 psi to 1,500 psi and operates at approximately 3,918 rpm. A fuel filter is located at the fuel pump outlet, and a relief valve relieves at approximately 1,725 psi back to the pump inlet if the filter becomes clogged.

As stated previously, each fuel pump is driven by the turbine through the reduction gearbox. The fuel pump reduction gear is located in the lube oil system gearbox, and a shaft from the reduction gear drives the fuel pump. Seals are installed on the shaft to contain any leakage of fuel or lube oil. If leakage occurs through the seals, it is directed to a drain line that runs to a 500-cubic-centimeter catch bottle for each auxiliary power unit. If the catch bottle were overfilled, it would relieve overboard at approximately 28 psia through a drain port. The flight crew can monitor the catch bottle's line pressure on the CRT.

Each gas generator consists of a bed of Shell 405 catalyst in a pressure chamber, mounted inside the APU exhaust chamber. When the hydrazine fuel comes into contact with the catalyst, it undergoes an exothermic reaction, decomposing into a hot gas at approximately 1,700 F. The gas expands rapidly and makes two passes through the two-stage turbine wheel, passes over the outside gas generator chamber and exits overboard through its own independent exhaust duct, located near the base of the vertical stabilizer. The temperature of the hot gas at the exhaust duct is approximately 1,000 F.

Turbine exhaust gas temperature, lube oil temperature and fuel pressure for each auxiliary power unit are transmitted to panel F8. The three-position APU switch permits the exhaust gas temperature, fuel pressure, and lube oil temperature of the respective units to be displayed on the APU EGT (exhaust gas temperature), fuel press and oil temp meters on panel F8. The APU temp yellow caution and warning light on panel F7 is illuminated if the APU 1, 2 or 3 lube oil temperature is above 290 F.

Each auxiliary power unit controller controls the speed of each unit upon the activation of the APU select switch for each APU on panel R2. The norm position controls the speed at 74,160 rpm, 103 percent, plus or minus 8 percent. The high position controls the speed at 81,360 rpm, 113 percent, plus or minus 8 percent, with a second backup of 82,800 rpm, 115 percent, plus or minus 8 percent.

The APU auto shutdown switch on panel R2 enables the automatic shutdown feature in all three APU controllers. When the switch is positioned to enable, each controller monitors its corresponding APU speed. If that APU speed falls below 57,600 rpm (80 percent) or rises above 92,880 rpm (129 percent), the controller automatically shuts down that unit. Each shutdown command closes that unit's secondary fuel valve and the tank isolation valves. The APU overspeed yellow caution and warning light on panel F7 is illuminated if APU 1, 2 or 3's turbine speed is above 92,880 rpm (129 percent). The APU underspeed yellow caution and warning light on panel F7 is illuminated if the APU 1, 2 or 3 turbine speed is less than 57,600 rpm (80 percent).

In the event of an overspeed or underspeed shutdown, the hyd press yellow caution and warning light on panel F7 also is illuminated for the corresponding hydraulic system. Because of APU shutdown, the corresponding hydraulic pump is inoperative. The yellow hyd press caution and warning light is illuminated when hydraulic system 1, 2 or 3 drops below 2,800 psi.

When the APU auto shutdown switch on panel R2 is positioned to inhibit, the automatic shutdown sequence for all three auxiliary power unit controllers is inhibited, and the 9.5-second speed time delay for all three units is inhibited when the APU control 1, 2 or 3 switch is set to the start/run position. The caution and warning alert light is illuminated and a tone is generated, even though the APU auto shutdown switch is in inhibit.

The start oride/run position of each APU control switch on panel R2 overrides the APU prestart conditions (gas generator temperature above 190 F, turbine speed less than 80 percent and gearbox pressure above 5.5 psi) to permit a start of the respective unit if one or more of the prestart conditions are not met. This switch also is activates the APU gas generator active cooling system, which provides the capability to restart a hot auxiliary power unit. The restart is inhibited for 209 seconds after the switch is positioned, during which time the gas generator is cooled by water flowing through its cooling passages, when the normal cool-down time of approximately three hours for the gas generator is not available.

As stated previously, each APU turbine imparts the mechanical drive to the gearbox to drive the lube oil pump at 12,215 rpm. The lube oil system of each unit is a scavenger-type system with a fixed-displacement pump. The system is pressurized with gaseous nitrogen to provide adequate suction pressure to start the lube oil pump under zero-gravity conditions. Each lube oil system has its own nitrogen gas storage vessel, which is pressurized to approximately 140 psia. The pressurization system for each lube oil system has a valve controlled by its corresponding APU controller. The gaseous nitrogen pressurization valve for each power unit is energized open by its corresponding controller when the gearbox pressure is below 4.5 psi, plus or minus 1.5 psi, to ensure that gearbox pressure is sufficiently above the requirements for proper scavenging and lube pump operation.

The pump increases the lube oil pressure to approximately 77 psi and directs the lube oil system through the corresponding APU/hydraulic water spray boiler for cooling and returns the lube oil to the accumulators and gearbox. The two accumulators in each lube oil system allow thermal expansion of the lube oil, accommodate gas initially trapped in the external lube circuit, maintain lube oil pressure at a minimum of approximately 15 psia and act as a zero-gravity, all-altitude lube reservoir.

The lube oil pump outlet pressure at approximately 45 psia, outlet temperature at approximately 270 F and return temperature from the water spray boiler at approximately 250 F for each auxiliary power unit are transmitted to the CRT. The lube oil temperature of each APU is also monitored on panel F8 through the select 1, 2, 3 switch on panel F8.

One gas generator valve module injector water cooling system serves all three auxiliary power units. It is used only when the normal cool-down period of 180 minutes is not available. The water cooling system sprays water to reduce the temperature of the gas generator bed to less than 450 F in the event that a hot auxiliary power unit must be restarted after it has been recently shut down. The water cooling ensures that no hydrazine will detonate at APU startup because of heat soakback in the gas generator. The injector is cooled by circulating water through it. The water from the gas generator injector is exhausted into the aft fuselage.

A single water tank located in the aft fuselage of the orbiter serves all three APUs. The water tank is 9.4 inches in diameter and loaded with 6 pounds (plus or minus 0.5 pound) of water. The water tank is pressurized with gaseous nitrogen at a nominal pressure of 85 psi. The pressure acts on a diaphragm to expel the water through three 0.25-inch-diameter lines to three control valves. When the APU control switch on panel R2 for APU 1, 2 or 3 is positioned to start oride/run , that APU controller opens the water valve of that unit for 209 seconds (plus or minus five seconds) and directs the water into the gas generator to cool it. Regardless of the reason that start oride/run is selected for an APU, the water for that unit operates for 209 seconds (plus or minus five seconds). If the catalytic bed temperature of an APU is above 400 F from heat soakback, if the catalytic bed heater temperature is above 430 F or if the gearbox pressure is low, the flight crew starts that unit in the start oride/run position and the water valve for that unit is opened for 209 seconds (plus or minus five seconds). When the timer in that unit controller times out, its control valve is closed and the power unit starts.

The water tank supply is sufficient for about four hot starts, one hot start per APU, plus one extra. The unit's injector temperature can be monitored on the CRT. The APU gas generator water cooling system will not be activated when the APU control 1, 2, 3 switch on panel R2 is positioned to start/run.

The APU heater tank/fuel line/H 2 O sys 1A, lB, 2A, 2B, 3A, 3B switches on panel A12 operate the thermostatically controlled heaters located on the corresponding APU fuel system and water system. The fuel tank, fuel line and water line heaters for each auxiliary power unit are divided into redundant A and B systems for each unit. For example, for APU 1, 1A and 1B, the 1A switch controls the A heaters and the thermostats provide automatic control. Only one set of heaters is used at a time. The 1B switch controls the 1B heaters and the thermostats provide automatic control. The APU fuel tank and line heater thermostats maintain the temperatures between a nominal 55 F and 65 F. The water system heater thermostats maintain the temperatures between 80 F and 90 F. The off position of each switch removes power from the respective heater circuits.

The APU heater gas gen/fuel pump 1, 2, 3 switches on panel A12 operate thermostatically controlled heaters located on the corresponding auxiliary power unit. The thermostats control a series of heaters on the gas generator valve module, fuel pump, and all the fuel lines and the water lines from the fuel pump spray manifold to the gas generator valve module. The heaters are divided into redundant A and B systems for each APU. The auto A switch controls the A heater, and the A thermostat automatically controls the corresponding APU gas generator heater, keeping the gas generator in the temperature range of 360 F to 425 F while the auxiliary power unit is not operating. The gas generator temperature range ensures efficient APU startup through efficient catalytic reaction. The auto A switch also controls the A heater, and the thermostat automatically controls the corresponding APU fuel pump heater to keep the fuel pump temperature in the range of 80 F to 100 F while the auxiliary power unit is not operating. The auto B switch position provides the same capability for the B heater system. The gas generator and fuel pump heaters are automatically deactivated by the corresponding controller at APU start. The off position of each switch removes power from the respective heater circuits.

The lube oil system lines on each auxiliary power unit also have a heater system. These heaters are controlled by the APU heater lube oil line 1, 2, 3 switches on panel A12. The lube oil line heaters for each auxiliary power unit are also divided into an A and B system: e.g., for APU 1, auto A and auto B. The auto A switch controls the A heater, and the thermostat automatically controls the corresponding lube oil system heater, maintaining the lube oil line in the temperature range of 55 F to 65 F. The auto B switch position provides the same capability to the B heater system. The off position of each switch removes power from the respective heater circuits.

The life of the auxiliary power units used to date is limited. Refurbishment of each was required after 20 hours of operation, degradation of the gas generator catalyst varied up to approximately 40 hours of operation, and operation of the gas generator valve module also varied up to approximately 30 hours of operation. The remaining parts were qualified for 40 hours of operation.

Improved APUs are scheduled for delivery in late 1988. A new turbine housing has an increased life of 75 hours of operation (50 missions), and a new gas generator increases its life to 75 hours. A new standoff design of the gas generator valve module and fuel pump deletes the requirement for a water spray system previously required for each APU upon shutdown after the ascent or orbital checkout; and the addition of a third seal in the middle of the two existing seals for the shaft of the fuel pump/lube oil system (previously only two seals were located on the shaft, one on the fuel pump side and one on the gearbox lube oil side) reduces the probability of hydrazine leakage into the lube oil system.

With the improved auxiliary power units, the deletion of the water spray system for the gas generator valve module and fuel pump on each unit results in a weight reduction of approximately 150 pounds for each orbiter.

Upon delivery of the improved APUs, the limited-life APUs will be refurbished to the upgraded design.

The fuel pump and gas generator valve module on the limited-life APUs are cooled by a separate water spray system after APU shutdown following the first OMS thrusting period and orbital checkout. The water spray system cooling prevents hydrazine decomposition in the fuel pump and gas generator valve module caused by heat soakback. The water spray cooling system consists of primary and secondary independent water supply systems for each APU. Each water system consists of a 16.5-inch-diameter tank, a 0.25-inch-diameter line to each APU, control valves and electrical heaters. Each water tank is loaded with 21 pounds (plus or minus 1 pound) of water. Each tank is pressurized with gaseous nitrogen between 50 psi and 59 psi. The gaseous nitrogen pressure acts on a diaphragm in each tank to expel the water into the lines to the control valves. When the limited-life units are shut down, the APU fuel pump/vlv cool A or B switch on panel R2 is positioned to auto . With the A switch on auto, the 150 F to 160 F thermostats on each APU, through the timer in the water controller, open control valve A on each unit to permit water to spray onto the valve module and fuel pump for 1.25 seconds, then close valve A for four seconds, etc. The cooling system is activated for two hours and 45 minutes after APU shutdown. The B switch controls valve B in the same manner. Nitrogen pressurization in each water tank is referred to as a blowdown system (pressure decay continues until the water is expelled from each tank). The water is exhausted into the aft fuselage compartment.

The water spray boiler controllers are powered up at launch minus four hours. The boiler water tanks are pressured at T minus one hour and 10 minutes in preparation for APU activation. The controllers activate heaters on the water tank, boiler and steam vent to assure that the water spray boiler is ready to operate for launch.

Auxiliary power unit start is delayed as long as possible to save fuel. At T minus six minutes, the pilot begins the prestart sequence. The pilot confirms that the water spray boiler is activated, then activates the APU controllers and depressurizes the main hydraulic pump. Depressurizing the main pump reduces the starting torque on the auxiliary power unit. The pilot then opens the fuel tank valves and looks for three APU ready-to-start indications (gray talkbacks). At T minus five minutes, the pilot starts the three units by setting 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 looks for approximately 3,000 psi on the gauges. All three hydraulic main pump pressures must be greater than 2,800 psi by T minus four minutes, or the automatic launch sequencer will abort the launch.

The auxiliary power units operate during the ascent phase and continue to operate through the first OMS thrusting period. At the conclusion of the main engine purge, dump and stow sequence, the auxiliary power units and water spray boilers are shut down. The same sequence applies for a delayed OMS-1 thrusting period. If an abort once around is declared, the APUs are left running, but the hydraulic pumps are depressurized to reduce fuel consumption. The units are left running to avoid having to restart hot APUs for deorbit and re-entry.

Six hours after lift-off or as soon as they are required, depending on the environment, the gas generator/fuel pump heaters are activated and are in operation for the remainder of the orbital mission. The fuel and water line heaters are also activated to prevent the lines from freezing as the auxiliary power units cool down.

A few hours after lift-off, the landing gear isolation valves on hydraulic systems 2 and 3 are opened so that the pumps can circulate hydraulic fluid through these lines. The valves will not open or close unless the pressure in the line is at least 100 psi, which requires the main hydraulic pump or hydraulic circulation pump to be active. The hydraulic system 1 landing gear isolation valve is left closed because of the danger of inadvertently lowering the landing gear while the vehicle is in orbit.

Two hours after lift-off, the steam vent heaters of the water spray boilers are turned on and left on for about 1.5 hours to eliminate all ice from the steam vents.

While the vehicle is in orbit, the hydraulic circulation pumps are in the GPC mode-automatically activated when hydraulic line temperatures become too low and automatically deactivated when the lines warm up sufficiently.

On the day before deorbit, one auxiliary power unit is started to supply hydraulic pressure for checkout of the flight control system. (Hydraulic pressure is needed to move the orbiter aerosurfaces as part of this checkout.) The associated water spray boiler controller is activated, landing gear isolation valves 2 and 3 are closed, and one APU (selected by the Mission Control Center) is started. The hydraulic main pump is set to normal pressure (approximately 3,000 psi), and aerosurface drive checks are made. After about five minutes, the checks are complete and the APU is shut down. Normally, the unit does not run long enough to require water spray boiler operation. The landing gear isolation valves on hydraulic systems 2 and 3 are reopened after the APU is shut down.

At 2.5 hours before the deorbit thrusting period, the boilers' steam vent heaters are activated to prepare the system for operation during atmospheric 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 set to low pressure. The pilot opens the fuel tank valves and looks for three gray APU/hyd rdy talkbacks. The pilot then closes the fuel tank valves. This procedure takes place while the crew is in contact with the ground so that flight controllers can observe APU status. Five minutes before the deorbit thrusting period, one auxiliary power unit (selected by Mission Control) is started to ensure that at least one unit will be operating for entry. The hydraulic pump is left in low-pressure operation. The APU operates through the deorbit burn. At 13 minutes before entry interface (400,000-foot altitude), while the orbiter is still in free fall, the other two APUs are started and all three hydraulic pumps are pressurized to normal. Two main engine hydraulic isolation valves are cycled open and then closed to ensure that the engines 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 done to test the response of the auxiliary power units and hydraulic pumps under high load (i.e., high flow demand) conditions. This test cycles 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 main engine hydraulic isolation valves are opened again and the engines are set to the transport position. At this point, the hydraulic systems are no longer needed; thus, the auxiliary power units and water spray boilers are shut down.

Each auxiliary power unit transmits data to the systems management summary CRT for display. Data displayed for each APU consist of exhaust gas temperature, lube oil inlet/outlet, gas generator bed/injector temperatures, speed, fuel quantity, pump leak, oil outlet pressure and fuel tank valves' status.