1. Auxiliary Power Unit

2. APU 7 APUs are onboard the STS –3 in Orbiter (triple redundant) –2 on each SRB (dual redundant) Used for hydraulic operations - high torque functions – that include: –Aero surfaces –Thrust vector control –Steering –Braking –SSME engine operations backup control

3. APU APU Major Components Turbine drive Speed reduction gearbox Hydraulic pump and controls Thermal conditioning –Water spray boilers –Heat exchanger –Electrical heaters

4. APU Systems

5. APU – Hydraulic System

6. APU Hydraulic system major components Main hydraulic pump Hydraulic fluid, fluid lines and fluid reservoir Accumulator Actuators Hydraulic fluid filters Control valves Heat exchanger (Freon loop) Electrical circulation pump Electrical heaters for low-temperature exposure in space

7. APU – Hydraulic System

8. STS Hydraulic System Actuators SSME thrust vector control (2 each engine) SSME propellant control valves Orbiter aerosurfaces –Elevons –Rudder –Speed brake –Body flap

9. APU – Hydraulic System STS Hydraulic System Actuators Umbilical door retraction (2) for LOX, LH2 feed lines after ET separation Main and nose wheel landing gear deployment Wheel brakes Nose wheel steering system SRBs (2 units on each) –Nozzle thrust vector control

10. APU Turbine The heart of the Auxiliary Power Unit is the single- stage (re-entry pass) turbine drive which consists of the gas generator, turbine, and exhaust subsystem The turbine drive generates the rotational torque to drive all of the Orbiter's hydraulic systems, as well as powering the internal fuel and lubrication pumps

11. APU Thermal Control Heat generation within the unit is considerable and requires an active cooling system Water Spray Boiler is used to remove heat from the APU lube oil and the hydraulic fluid –Circulated in separate lines in the WSB cooler Hydraulic lines also circulate through Freon external loop heat exchanger Because of the Orbiter's weight restrictions, the hydrazine fuel is limited to providing operation during launch, ascent, SSME purge, atmospheric flight, and landing, plus a safety reserve

12. APU Gas generator The APU gas generator converts the incoming liquid hydrazine fuel into a spray onto the catalyst bed which is a granular alumina coated with iridium, also known as Shell 405 The N 2 H 4 hydrazine molecules decompose into superheated nitrogen and ammonia gas –Ammonia partially decomposes into it constituent hydrogen and nitrogen in the second turbine stage

13. APU Gas generator Gas flow that drives the turbine and the resulting turbine speed is regulated by the APU controller Desired speed and torque is maintained by pulse- injection of the hydrazine into the injector

14. APU Gas generator Gas pressure in the combustion chamber is regulated by the fuel pulse frequency and the catalyst bed temperature For restarts from high temperatures, water must be injected into the gas generator Combustion chamber and exhaust shell is composed of titanium Design gas operating temperature is 1,300 o F in the first stage and 1,700 o F at the turbine

15. APU Gas generator Design gas pressure from combustion chamber to turbine is 1,300 psia Water cooling subsystem –Used for hot starts (less than 180 minute cooldown period) –Water injected into gas generator from a 415 in 3 storage tank

16. APU Turbine The APU drive turbine is a 14 cm (5.5 in) diameter axial, two-stage, impulse pressure-driven unit First-stage feed decomposes hydrazine into ammonia and nitrogen in an endothermic reaction Second pass converts ammonia into hydrogen and nitrogen which is an exothermic reaction

17. APU Turbine operations Operating speed - 74,160 rpm High speed operation - 81,360 rpm Secondary speed operation - 82,800 rpm Overspeed limit - 92,800 rpm Underspeed limit - 57,600 rpm

18. APU APU Turbine Drive stats Weight (without gearbox) - 88 lb Power output - 135 HP Fuel supply - 325 lb Maximum operation duration - approximately 120 min Fuel consumption - approximately 3 lb/min

19. APU APU Turbine Drive stats Weight (without gearbox) - 88 lb Power output - 135 HP Fuel supply - 325 lb Maximum operation duration - approximately 120 min Fuel consumption - approximately 3 lb/min

20. APU Exhaust subsystem Turbine exhaust from the three APUs is ducted overboard near the vertical tail root through three 6.4 cm (2.5") exhaust ducts The ducts are arranged with two on the left and one on the right so that the impulse gas flow has little, if any, net reaction force on the Orbiter Outlet gas composition: –H 2 - 8% –N 2 - 64% –NH 3 (ammonia) - 28%

21. APU Hydrazine fuel supply Hydrazine is supplied to the three Orbiter APUs with three 71.1 cm ( 28 in) diameter spherical tanks, one for each system Each tank contains a diaphragm which separates the hydrazine liquid from regulated, pressurized nitrogen

22. APU Hydrazine fuel supply The pressurized nitrogen provides a positive feed of the liquid in zero-gravity into the APU controller valves and the gas generator injector 325 lb fuel capacity each tank Provides 91 min operation each, plus contingency reserve

23. APU Hydrazine fuel supply The pressurized nitrogen provides a positive feed of the liquid in zero-gravity into the APU controller valves and the gas generator injector 325 lb fuel capacity each tank Provides 91 min operation each, plus contingency reserve

24. APU APU system controller Pulse signals for fuel flow into the APU gas generator are managed by the APU system controller Operation of the APU includes high and low speed regulation, as well as startup sequencing APU controller also regulates normal and hot starts, and manual operation –Also furnishes automatic and abnormal shutdown control

25. APU Gearbox The APU gearbox is used for reducing the high speed turbine rotation speeds to rates that are usable by the hydraulic pump, the lubrication pump and the fuel pump Turbine-to-hydraulic pump and turbine-to-fuel pump gear ratio is 19:1 Positive lube oil feed in zero-g is furnished by the gearbox design During APU operation, the gearbox is pressurized by nitrogen gas

26. APU Gearbox Lubrication pump operation - 11,860 rpm Lubricant - 6 lb Cooling is accomplished with lube oil circulation through Water Spray Boiler

27. APU Water Spray Boiler Three on each Orbiter (one each APU/hydraulic system) Located in the aft fuselage WSB includes: –Evaporator tank –WSB water tank 142 lb water capacity Pressurized nitrogen for positive feed

28. APU - WSB

30. APU APU Operations Total fuel capacity onboard the Orbiter for the APU operations is 325 lbs for each tank which is equivalent to 91 minutes of operation at a typical consumption rate of approximately 3 lb/min Since all three APUs run simultaneously, total APU operation time is also 91 minutes Prelaunch All three APUs are started 5 minutes before liftoff to power main engine thrust vectoring, SSME hydraulic backup control, and operate control surfaces

31. APU APU flight operations Ascent –Operations continue through main engine cutoff until first OMS burn –Umbilical doors closed after ET separation

32. APU APU flight operations SSME dump & purge –After MPS fuel dump, purge and stow operations completed, the APUs are shut down

33. APU APU flight operations On orbit - not used –Six hours after launch, the gas generator and fuel pump heaters are started and remain in operation for the remainder of the orbital mission –Fuel and water line heaters are also activated to prevent the lines from freezing as the auxiliary power units cool down –Hydraulic circulation pumps are activated to maintain the hydraulic system's set temperature range

34. APU APU flight operations Deorbit checkout –One APU started one day before deorbit to check Orbiter hydraulic flight control systems –Five minute checkout includes Water Spray Boiler (WSB) operation for cooling

35. APU APU flight operations Entry –Restarted before deorbit burn (1 APU only) –The other 2 are restarted after deorbit thrust at 13 minutes before entry interface and operate through the Orbiter's rollout and wheelstop

36. APU APU flight operations Landing & post-landing –After wheelstop, SSMEs are placed in stowed position and APUs are shut down

37. APU – Operational Controls

38. APU - Caution and Warning Panel