The APUs that have been
in use to date have a limited life. Each unit was refurbished after
25 hours of operation because of cracks in the turbine housing,
degradation of the gas generator catalyst (which varied up to approximately
30 hours of operation) and operation of the gas generator valve
module (which also varied up to approximately 30 hours of operation).
The remaining parts of the APU were qualified for 40 hours of operation.
Improved auxiliary power units are scheduled for delivery in late
1988. A new turbine housing increases the life of the housing to
75 hours of operation (50 missions); 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 that was required previously for each APU upon shutdown after
the first OMS thrusting period 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
leaking into the lube oil system.
The deletion of the water spray system for the gas generator valve
module and fuel pump for each APU results in a weight reduction
of approximately 150 pounds for each orbiter. Upon the delivery
of the improved units, the life-limited APUs will be refurbished
to the upgraded design.
In the event that a fuel tank valve switch in an auxiliary power
unit is inadvertently left on or an electrical short occurs within
the valve electrical coil, additional protection is provided to
prevent overheating of the fuel isolation valves.