Auxiliary Power Units

There hasn't been a whole lot of exciting stuff going on. Mostly just writing closures and putting run books together for the launch next week. So here we are, 6 posts deep and it's evident what department I work in but it was never really explained what the system does. So here you go...

The auxiliary power unit (APU) is a hydrazine fueled, turbine driven power unit which produces mechanical shaft power to a hydraulic pump which produces pressure for the orbiter's hydraulic system. There are three APUs on the orbiter. Located in the aft compartment of the orbiter, APUs 1 and 2 are located on the left side and APU 3 is on the right. The three systems are identical but independent.

Each fuel system supplies stored liquid hydrazine to the fuel pump, gas generator valve module, and gas generator. The fuel decomposes extremely rapidly in a catalytic action. The resulting gas is used to drive a two-stage turbine. The gas returns over the the gas chamber to cool it down and then exits through exhaust ports on the top of that aft of the orbiter.

The lube oil of each APU is circulated through a heat exchanger in a respective water spray boiler which cool the lube oil systems. The hydraulic fluid sees the same process to cool it as well.

The APU system is started 5 minutes before launch and remains online through the launch and until the first orbital maneuvering system (OMS) thrusting period where they are shut down until the orbiter is ready to make a return to Earth. During launch the hydraulic fluid powered by the APU applies pressure to actuators for thrust vectoring by gimbaling of the three main engines, supplies load relief to the elevons during the ascent phase, and retracts the fuel and oxidizer umbilicals from the external tank when it is ready to be jettisoned.

About a day before deorbit, one APU is restarted to support checkout of the flight controls which include the elevons, rudder, speed brake, and body flap. The other two APUs are restarted after the deorbit and function through entry, landing, and landing rollout for hydraulic pressure for positioning of the aerosurfaces during atmospheric flight, deployment of the landing gear, brakes, anti-skid, nose wheel steering, and positioning of the three main engines.

The fuel tanks hold about 325 pounds of hydrazine which is enough for the 90 minutes per mission the APUs would normally operate with reserve in the event of an abort once around when the APUs would be active for 120 minutes. They consume about 3 pounds of fuel per hour.





Technical data collected from: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-apu.html

Pictures from: http://www.columbiassacrifice.com/&0_shttlovrvw.htm