There are two self- contained,
independent HPUs on each SRB. Each HPU consists of an auxiliary
power unit, fuel supply module, hydraulic pump, hydraulic reservoir
and hydraulic fluid manifold assembly. The APUs are fueled by hydrazine
and generate mechanical shaft power to a hydraulic pump that produces
hydraulic pressure for the SRB hydraulic system. The two separate
HPUs and two hydraulic systems are located on the aft end of each
SRB between the SRB nozzle and aft skirt. The HPU components are
mounted on the aft skirt between the rock and tilt actuators. The
two systems operate from T minus 28 seconds until SRB separation
from the orbiter and external tank. The two independent hydraulic
systems are connected to the rock and tilt servoactuators.
The APU controller electronics
are located in the SRB aft integrated electronic assemblies on
the aft external tank attach rings.
The APUs and their fuel
systems are isolated from each other. Each fuel supply module
(tank) contains 22 pounds of hydrazine. The fuel tank is pressurized
with gaseous nitrogen at 400 psi, which provides the force to
expel (positive expulsion) the fuel from the tank to the fuel
distribution line, maintaining a positive fuel supply to the APU
throughout its operation.
The fuel isolation valve
is opened at APU startup to allow fuel to flow to the APU fuel
pump and control valves and then to the gas generator. The gas
generator's catalytic action decomposes the fuel and creates a
hot gas. It feeds the hot gas exhaust product to the APU two-
stage gas turbine. Fuel flows primarily through the startup bypass
line until the APU speed is such that the fuel pump outlet pressure
is greater than the bypass line's. Then all the fuel is supplied
to the fuel pump.
The APU turbine assembly
provides mechanical power to the APU gearbox. The gearbox drives
the APU fuel pump, hydraulic pump and lube oil pump. The APU lube
oil pump lubricates the gearbox. The turbine exhaust of each APU
flows over the exterior of the gas generator, cooling it, and
is then directed overboard through an exhaust duct.
When the APU speed reaches
100 percent, the APU primary control valve closes, and the APU
speed is controlled by the APU controller electronics. If the
primary control valve logic fails to the open state, the secondary
control valve assumes control of the APU at 112-percent speed.
Each HPU on an SRB is
connected to both servoactuators on that SRB. One HPU serves as
the primary hydraulic source for the servoactuator, and the other
HPU serves as the secondary hydraulics for the servoactuator.
Each sevoactuator has a switching valve that allows the secondary
hydraulics to power the actuator if the primary hydraulic pressure
drops below 2,050 psi. A switch contact on the switching valve
will close when the valve is in the secondary position. When the
valve is closed, a signal is sent to the APU controller that inhibits
the 100-percent APU speed control logic and enables the 112-percent
APU speed control logic. The 100-percent APU speed enables one
APU/HPU to supply sufficient operating hydraulic pressure to both
servoactuators of that SRB.
The APU 100-percent speed
corresponds to 72,000 rpm, 110-percent to 79,200 rpm, and 112-percent
to 80,640 rpm.
The hydraulic pump speed
is 3,600 rpm and supplies hydraulic pressure of 3,050, plus or
minus 50, psi. A high- pressure relief valve provides overpressure
protection to the hydraulic system and relieves at 3,750 psi.
The APUs/HPUs and hydraulic
systems are reusable for 20 missions.