AUXILIARY POWER UNITS

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.

The contractors involved with the auxiliary power unit system are Sundstrand Corp., Rockford, Ill. (APU and APU controller); TRW (was Pressure Systems Inc.), Los Angeles, Calif. (APU fuel and water tanks); SSP Products Inc., Burbank, Calif. (APU exhaust duct assembly); Sundstrand Data Control, Redmond, Wash. (APU heater thermostat); Cox and Co., New York, N.Y. (APU fuel tank, fuel and lube line heaters); Brunswick-Wintec, El Segundo, Calif. (APU fuel line filter); J.C. Carter Co., Costa Mesa, Calif. (APU servicing coupling); Wright Components Inc., Clifton Springs, N.J. (fuel pump seal cavity drain catch bottle, relief valve); Rocket Research Corp., Redmond, Wash. (APU gas generator); Eaton Controls Corp., Valencia, Calif. (fuel isolation valves); Lear Siegler, Elyria, Ohio (test-point couplings); Symetrics, Burbank, Calif. (oil couplings); Carleton Controls, East Aurora, N.Y. (water valves); Circle Seal Components, Buena Park, Calif. (manual drain valve); Aerodyne Controls Corp., Farmingdale, N.Y. (water relief valve).

WATER SPRAY BOILERS

The water spray boiler system consists of three identical independent water spray boilers, one for a corre sponding auxiliary power unit and hydraulic system. The boilers are located in the aft fuselage of the orbiter. Each water spray boiler cools the corre sponding power unit lube oil system and hydraulic system by spraying water onto their lines; the water then boils off, cooling the lube oil and hydraulic fluid. The water (steam) that boils off in each water spray boiler exits through its own steam duct, located to the right of the vertical stabilizer.

Each water spray boiler stores water in a bellows-type storage tank pressurized by gaseous nitrogen to provide positive water expulsion that feeds the boiler. The hydraulic fluid passes through the boiler three times, and the lube oil of the auxiliary power unit passes through the boiler twice in a set of tubes. The hydraulic fluid tubes are sprayed with water from three water spray bars, and two water spray bars spray the power unit lube oil. Separate water feed valves allow independent control of the hydraulic fluid spray bars and power unit lube oil spray bars. Redundant electrical controllers provide completely automatic operation.



The boiler system maintains auxiliary power unit lube oil temperature at approximately 250 F and the hydraulic fluid in the range of 210 to 220 F.

The three auxiliary power units and hydraulic pumps and water spray boilers are in operation five minutes before lift-off, operate throughout the launch phase and are shut down after the first orbital maneuvering system thrusting period. One power unit/hydraulic system and corresponding water spray boiler are operated briefly one day before deorbit during a checkout of the orbiter flight control system, which includes the orbiter aerosurfaces.

One auxiliary power unit is restarted before the deorbit thrusting maneuver. 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 positioning the orbiter aerosurfaces during the atmospheric flight portion of entry; deploying the nose and main landing gear, main landing gear braking and nose wheel steering; and positioning the three space shuttle main engines after landing rollout. The corresponding water spray boilers are also in operation during this period.

Each water spray boiler is 45 by 31 inches long by 19 inches wide. Each boiler, including controller and vent nozzle, weighs 181 pounds. They are mounted in the orbiter aft fuselage between X o 1340 and 1400, at Zo 488 minus 15, and at Y o plus 15. Insulation blankets cover each boiler. The boiler's water capacity is 142 pounds.

The gaseous nitrogen pressure for each water spray boiler is contained in a corresponding 6-inch spherical pressure vessel. The pressure vessel contains 0.77 pound of nitrogen at a nominal pressure of 2,400 psi at 70 F. The gaseous nitrogen storage system of each water spray boiler is directed to its corresponding water storage tank. Each storage vessel contains sufficient nitrogen gas to expel all the water from the tank and allow for relief valve venting during ascent.

The nitrogen shutoff valve between the pressure valve and water storage tank of each boiler permits the pressure to reach the nitrogen regulator and water tank or isolates the nitrogen supply from the water tank. Each nitrogen valve is controlled by its respective boiler N 2 supply 1, 2 or 3 switch on panel R2. The nitrogen shutoff valve, which is latched open or closed consists of two independent solenoid coils that permit valve control from either the primary or secondary controller.



A single-stage regulator is installed between the nitrogen pressure shutoff valve and the water storage tank. The gaseous nitrogen regulator for each water spray boiler regulates the high-pressure nitrogen between 24.5 and 26 psig as it flows to the water storage tank.

A relief valve is incorporated inside each nitrogen regulator to prevent the water storage tank pressure from exceeding 33.5 psig from heat soakback during operations or in the event of a failed-open nitrogen regulator. The gaseous nitrogen relief valve opens between 30 to 33.5 psig.

The water supply for each boiler is stored in a positive-displacement aluminum tank containing a welded metal bellows separating the stored water inside the bellows from the nitrogen expulsion gas.

Non-redundant pressure and temperature sensors located downstream from the gaseous nitrogen pressure vessel and on the water tank for each boiler transmit the pressures and temperatures through the A controller to the systems management general-purpose computer. The computer computes the pressure, volume and temperature and transmits the water tank quantity to panel F8 for each boiler. The APU fuel/H 2 O qty switch on panel F8 is positioned to allow the water quantity of each boiler to be displayed on the APU fuel/H2O qty 1, 2 or 3 meter. Thus, water quantity is available only when the A controller is powered.

Downstream of the water storage tank, the feedwater lines to each water boiler split into two parallel lines: one line goes to the hydraulic fluid flow section and one to the lube oil section of the auxiliary power unit. A hydraulic-fluid water feed valve is installed in the water line to the hydraulic fluid section, and a power unit lube oil water feed valve is installed in the water line to the lube oil section of the power unit. Each valve is controlled independently by the boiler controller.

The two boiler controllers are operated by the respective boiler cntlr pwr/htr 1, 2 and 3 switches on panel R2. When the applicable switch is positioned to A, the A controller for that boiler is powered; if it is positioned to B, the B controller is powered. The off position of the applicable switch removes electrical power from both controllers.

The boiler cntlr 1, 2 and 3 switches on panel R2 enable (provide the automatic control functions) the specific controller A or B selected for that boiler by the boiler cntlr pwr/htr 1, 2 and 3 switches on panel R2. When the applicable controller A or B is enabled for that boiler, a ready signal is transmitted to the corre sponding APU/hyd ready to start talkback indicator (along with other prerequisites from the auxiliary power unit and hydraulic system) on panel R2 if the following additional conditions are met: gaseous nitrogen shutoff valve is open, steam vent nozzle temperature is greater than 130 F, and hydraulic fluid bypass valve is in the correct position with regard to the hydraulic fluid temperature.

The core of each water spray boiler is a stainless steel crimped-tube bundle. The hydraulic fluid section is divided into three 17-inch-long passes of smooth tubes (first pass-234 tubes, second pass-224 tubes and third pass-214 tubes). The lube oil section of the auxiliary power unit comprises two passes with 103 crimped tubes in its first pass and 81 smooth tubes in the second pass. The tubes are 0.0125 of an inch in diameter with a wall thickness of 0.010 of an inch. Crimps located every 0.24 of an inch break up the internal boundary layer and promote enhanced turbulent heat transfer. Although the second pass is primarily a low-pressure drop return section, approximately 15 percent of the unit's lube oil heat transfer occurs there.

Three connected spray bars feed the hydraulic fluid section, while two spray bars feed the power unit's lube oil section in each boiler.

When the orbiter is in the vertical position on the launch pad, each boiler is loaded with up to 3.5 pounds of water, which is referred to as pool mode operation. When each auxiliary power unit/hydraulic system and water spray boiler is in operation five minutes before lift-off, the power unit tube bundle and hydraulic tube bundle are immersed in the boiler water precharge pool mode operation. Liquid level sensors in each water boiler prevent the water feed valves from pulsing to avoid water spillage or loss. As the vehicle ascends during launch, the lube oil system of the auxiliary power unit heats up, eventually the boiler water precharge boils off, and the boiler goes into the spray mode. The hydraulic fluid usually does not heat up enough during ascent to require spray cooling.

The enabled controller of the operating water spray boiler monitors the hydraulic fluid and lube oil outlet temperature from the auxiliary power unit. The hydraulic fluid outlet temperature controls the hydraulic-fluid water feed valve, and the power unit's lube oil outlet temperature controls the lube oil water feed valve. Signals are based on a comparison of the hydraulic system fluid temperature to its 208 F set point and of the lube oil of the power unit to its 250 F set point. When the respective water feed valve opens, instantaneous flows of 10 pounds per minute maximum through the hydraulic section and 5 pounds per minute maximum through the lube oil section of the power unit enter the water boiler through the corresponding spray bars to begin evaporative cooling of the hydraulic fluid and auxiliary power unit lube oil. The steam is vented out through the overboard steam vent.

The separate water feed valves modulate the water flow to each section of the tube bundle core in each water spray boiler independently in 200-millisecond pulses that vary from one pulse every 10 seconds to one pulse every 0.25 of a second.

Because of the unique hydraulic system fluid flows, control valves are located in the hydraulic system fluid line section of each water spray boiler. Normally, hydraulic system fluid flows at up to 21 gallons per minute; however, the hydraulic system experiences one- to two-second flow spikes at up to 63 gallons per minute. If these spikes were to pass through the boiler, pressure drop would increase ninefold and the boiler would limit the flow of the hydraulic system. To prevent this, a relief function is provided by a spring-loaded poppet valve that opens when the hydraulic fluid pressure drop exceeds 48 psi and is capable of flowing 43 gallons per minute at a differential pressure of 50 psi across the boiler. A temperature controller diverter valve allows the hydraulic fluid to bypass the boiler when the fluid temperature decreases to 190 F. At 210 F, the controller commands the diverter valve to direct the fluid through the boiler. When the hydraulic fluid cools to 190 F, the controller again commands the diverter valve to route the fluid around the boiler.

Each water boiler, water tank and steam vent is equipped with electrical heaters to prevent freeze-up in orbit. The water tank and boiler electrical heaters are activated by the corresponding boiler cntlr pwr/htr 1, 2 and 3 switches on panel R2. The A or B position of each switch selects the A or B heater system and is automatically controlled by the corresponding A or B controller. The steam vent heaters are also activated by the boiler cntlr pwr/htr 1, 2 and 3 switches but only if the boiler cntlr 1, 2 or 3 switch on panel R2 is on. The water tank and boiler heaters are cycled on at 50 F and off at 55 F. The steam vent heaters are not operated continuously in orbit; they are activated approximately two hours before auxiliary power unit startup. The steam vent heaters are cycled on at 150 F and cycled off at 175 F.

When the auxiliary power unit/hydraulic combination is started for atmospheric entry and the hydraulic fluid and power unit lube oil flow commences and fluid temperatures rise, spraying is initiated as required. During the lower part of entry, when the boiler temperature reaches 188 F, the water spray boiler returns to the pool mode. The spray bars begin discharging excess water to fill the boiler. When the water reaches the liquid level sensors, the spray is turned off so that the boiler is not overfilled. During entry, because the orbiter's orientation is different from that of launch, the boiler can hold up to 14 pounds of water.

Each water spray boiler transmits data to the systems management summary CRT for display. Data displayed for each boiler consist of water quantity, gaseous nitrogen pressure, gaseous nitrogen regulated pressure, bypass valve status, gaseous nitrogen and water tank temperature, and boiler temperature.

The boiler system 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 auxiliary power unit activation. The boiler system controllers activate heaters on the water tank, boiler and steam vent to ensure 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 power unit prestart sequence-confirming that the water spray boiler is activated-activates the auxiliary power unit 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 auxiliary power unit fuel tank valves and looks for three APU ready to start gray talkbacks. At T minus five minutes, the pilot starts the three power units by setting the APU cntl switches to start/run and checks the hydraulic pressure gauges for an indication of approximately 600 to 1,000 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 through the first orbital maneuvering system 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 auxiliary power units are left running but the hydraulic pumps are depressurized to reduce power unit fuel consumption. The auxiliary power units are left running to avoid restarting hot power units for deorbit and re-entry.

Six hours after lift-off, the heater gas generator/fuel pump heaters of the auxiliary power units are activated to operate for the remainder of the orbital mission. The fuel and water line heaters of the power units are also activated to prevent the lines from freezing as the 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 the lines. Because these valves will not open or close unless the pressure in the line is at least 100 psi, the main hydraulic pump or hydraulic circulation pump must 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 boiler steam vents.

While the vehicle is in orbit, the hydraulic circulation pumps are in the GPC mode (controlled by the general-purpose computers): 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 during checkout.) The associated water spray boiler controller is activated, and landing gear isolation valves 2 and 3 are closed. Then one auxiliary power unit (selected by the Mission Control Center) is started. The hydraulic main pump is set to normal pressure (approximately 3,000 psi), and aerosurface drive is checked. After about five minutes, the checks are complete and the power unit is shut down. Normally, the auxiliary power 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 auxiliary power unit is shut down.

At 2.5 hours before the deorbit thrusting period, the steam vent heaters of the water spray boiler are activated to prepare the boiler systems for operation during 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 water tanks of the boiler systems are pressurized, the APU controllers are activated, and the main hydraulic pumps are set at low pressure. The pilot opens the fuel tank valves of the auxiliary power units and looks for three gray APU/hyd rdy talkbacks. The pilot then closes the fuel tank valves. This procedure takes place while the pilot is in contact with the ground so that flight controllers can observe the status of the auxiliary power units. Five minutes before the deorbit thrusting period, one 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. This auxiliary power unit 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 auxiliary power units are started and all three hydraulic pumps are pressurized (norm). Two space shuttle main engine hydraulic isolation valves are cycled open, 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 conducted 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 power units 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 moved to the transport position. At this point, the hydraulic systems are no longer needed, and the auxiliary power units and water spray boilers are shut down.

The contractor for the water spray boilers is Hamilton Standard Division, United Technologies Corp., of Windsor Locks, Conn.

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.

The contractors involved with the hydraulic systems are Arkwin Industries, Westbury, N.Y. (hydraulic reservoir, filter and control valves); Purolator Inc., Newbury Park, Calif. (hydraulic filter module); Parker-Hannifin Corp., Irvine, Calif. (hydraulic accumulator); Abex Corp., Aerospace Division, Oxnard, Calif. (hydraulic pump); Crissair Inc., El Segundo, Calif. (hydraulic check valve and flow restrictor); Hi-Temp Insulation Inc., Camarillo, Calif. (hydraulic blanket and line insulation); Bertea Corp., Irvine, Calif. (external tank umbilical retractor actuator, main and nose landing gear strut uplock actuator); Lear Siegler, Elyria, Ohio (hydraulic water spray boiler disconnect); Moog Inc., East Aurora, N.Y. (main engine gimbal servoactuators and elevon servoactuators); Pneu Devices, Goleta, Calif. (electric-motor-driven circulation pump); Pneu Draulics, Montclair, Calif. (priority valve and hydraulic thermal control shutoff valve); Resistoflex, Roseland, N.J. (hydraulic system line connectors); Sterer Engineering and Manufacturing, Los Angeles, Calif. (hydraulic landing gear solenoid shutoff valve, nose landing gear steering and damping system, three-way solenoid operating valve landing gear uplock and control valves); Sundstrand, Rockford, Ill. (rudder/speed brake actuation unit and body flap actuation unit); Symet rics, Canoga Park, Calif. (hydraulic quick disconnects); Titeflex Division, Springfield, Mass. (hydraulic system hose); Whittaker Corp., North Hollywood, Calif. (hydraulic accumulator dump valve); Wright Components Inc., Clifton Springs, N.J. (hydraulic latching solenoid valve); Hamilton Standard Division of United Technologies Corp., Windsor Locks, Conn. (water boiler hydraulic thermal unit and ground support equipment hydraulic cart); Cox and Co., New York, N.Y. (electrical heaters for components and lines).

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