APU-Physics Project

[RobertVA 0]

The STS orbiter APUs are only operated during ascent, rentry and landing phases of the flight to operate flight controls, landing gear extension, and gimbaling of rocket engines. I'm not sure if a seperate system is used for the gimbals on the outlets at the base of the solid boosters. Since they are operated at such high altitudes, the orbiter must provide seperate fuel and oxidizer tanks to run the redundant APU turbines. The APUs and supply tanks are located in the space betwen the cargo bay and the main engines. Main engine propelents are pumped by seperate turbines that operate from materials diverted from the engine propelant supply, which is no longer available after the main engines are shut down.The APUs are shut down during orbital operations and restarted shortly before the deorbit "Orbiting Maneuvering System" burn. In orbit, electrical power is generated by a chemical process that produces water and electricity from hydrogen and oxygen in a unit refered to as a "Fuel Cell". The process is particularly convenient, since the water byproduct can be used for food preperation (rehyrating dried products), water for crew and eating utensil cleaning and the crew's drinking water supply. Excess heat is radiated to space by equipment located on the interior surface of the cargo bay doors, which must be opened soon after reaching orbit. If the cargo bay doors cannot be promptly opened, the flight has to be aborted and the orbiter has to promptly return to Earth. A malfunctioning cryogenic oxygen supply tank for a similar fuel cell system is what exploded in the Apollo 13 service module.

[Roger Mazengarb 0]

Tom, What you are describing is a ram air turbine ( RAT ) in modern parlance and no way does that constitute an APU. You must have airspeed for a rat to operate! A RAT is typically used as an emergency device. First generation jet aircraft ( and some later piston engined aircraft) required ground based generators driven by large GM diesels to supply 115/200 v ac power for maintenance/starting operations. Also a mobile gas turbine was required to supply air for the same reasons. This dependence on these units severly limited the operational use of the aircraft as it required airlines to have ground crews and equipment at all airports. A very expensive requirement. The APU (auxillary power unit) is used to overcome the previous limitations. Typically a modern APU consists of a small gas turbine engine ( electric starter) driving an accessory drive gearbox that has generators (AC) and a hydraulic pump, and also uses bleed air to supply electrical/hydraulic/pneumatic power to the aircraft systems for maintenance/starting.From memory the military were the originators of the APU so that they could scramble aircraft without need for ground support equipment. Of course civil aviation soon saw the dollars and cents advantage. Whilst an APU MAY (under some circumstances) be used as an emergency supply this is not it's main reason for being. Roger

[Guest glnflwrs]

Way to go Robert. We got an "A" huh?:>)Good work, glad I could help in some small way.Glenn