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

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

APU Systems

APU – Hydraulic System

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

APU – Hydraulic System

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

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

APU Turbine
The heart of the
Auxiliary Power
Unit is the singlestage (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

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

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

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 pulseinjection of the hydrazine into the injector

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

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

APU
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

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

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

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

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
– N
2
- 8%
- 64%
– NH
3
(ammonia) - 28%

APU
• 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

APU
• 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

APU
• 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

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

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

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

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

APU - WSB

APU - WSB

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

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

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

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

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

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

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

APU – Operational Controls

APU - Caution and Warning Panel