The three orbiter hydraulic
systems supply hydraulic pressure to the main propulsion system
for providing thrust vector control and actuating engine valves
on each SSME.
The three hydraulic supply systems are distributed to the MPS
TVC valves. These valves are controlled by hydraulics MPS/TVC
1, 2, 3 switches on panel R4. A valve is opened by positioning
its respective switch to open. The talkback indicator above each
switch indicates op or cl for open and close.
When the three MPS TVC hydraulic isolation valves are opened,
hydraulic pressure actuates the engine main fuel valve, the main
oxidizer valve, the fuel preburner oxidizer valve, the oxidizer
preburner oxidizer valve and the chamber coolant valve. All hydraulically
actuated engine valves on an engine receive hydraulic pressure
from the same hydraulic system. The left engine valves are actuated
by hydraulic system 2, the center engine valves are actuated by
hydraulic system 1, and the right engine valves are actuated by
hydraulic system 3. Each engine valve actuator is controlled by
dual-redundant signals: channel A/engine servovalve 1 and channel
B/engine servovalve 2 from that engine controller electronics.
As a backup, all of the hydraulically actuated engine valves on
an engine are supplied with helium pressure from the helium subsystem
left, center and right engine helium tank supply system. In the
event of a hydraulic lockup in an engine, helium pressure is used
to actuate the engine's propellant valves to their fully closed
position when the engine is shut down.
Hydraulic lockup is a condition in which all of the propellant
valves on an engine are hydraulically locked in a fixed position.
This is a built-in protective response of the MPS propellant valve
actuator/control circuit. It takes effect any time low hydraulic
pressure or loss of control of one or more propellant valve actuators
renders closed-loop control of engine thrust or propellant mixture
ratio impossible. Hydraulic lockup allows an engine to continue
to thrust in a safe manner under conditions that normally would
require that the engine be shut down; however, the affected engine
will continue to operate at approximately the throttle level in
effect at the time hydraulic lockup occurred. Once an engine is
in a hydraulic lockup, any subsequent shutoff commands, whether
nominal or premature, will cause a pneumatic helium shutdown.
Hydraulic lockup does not affect the capability of the engine
controller to monitor critical operating parameters or issue an
automatic shutdown if an operating limit is out of tolerance;
however, the engine shutdown would be accomplished pneumatically.
The three MPS thrust vector control valves must also be opened
to supply hydraulic pressure to the six main engine TVC actuators.
There are two servoactuators per SSME: one for yaw and one for
pitch. Each actuator is fastened to the orbiter thrust structure
and to the powerhead of one of the three SSMEs. The two actuators
per engine provide attitude control and trajectory shaping by
gimbaling the SSMEs in conjunction with the solid rocket boosters
during first-stage ascent and without the SRBs during second-stage
ascent. Each SSME servoactuator receives hydraulic pressure from
two of the three orbiter hydraulic systems; one system is the
primary system and the other is a standby system. Each servoactuator
has its own hydraulic switching valve. The switching valve receives
hydraulic pressure from two of the three orbiter hydraulic systems
and provides a single source to the actuator. Normally, the primary
hydraulic supply is directed to the actuator; however, if the
primary system were to fail and lose hydraulic pressure, the switching
valve would automatically switch over to the standby system, and
the actuator would continue to function on the standby system.
The left engine's pitch actuator utilizes hydraulic system 2 as
the primary and hydraulic system 1 as the standby. The engine's
yaw actuator utilizes hydraulic system 1 as the primary and hydraulic
system 2 as the standby. The center engine's pitch actuator utilizes
hydraulic system 1 as the primary and hydraulic system 3 as the
standby, and the yaw actuator utilizes hydraulic system 3 as the
primary and hydraulic system 1 as the standby. The right engine's
pitch actuator utilizes hydraulic system 3 as the primary and
hydraulic system 2 as the standby. Its yaw actuator utilizes hydraulic
system 2 as the primary and hydraulic system 3 as the standby.
The hydraulic systems are distributed among the actuators and
engine valves to equalize the hydraulic work load among the three
The hydraulic MPS/TVC isol vlv sys1, sys2, sys3 switches on panel
R4 are positioned to close during on-orbit operations to protect
against hydraulic leaks downstream of the isolation valves. In
addition, there is no requirement to gimbal the main engines from
the stow position. During on-orbit operations when the MPS TVC
valves are closed, the hydraulic pressure supply and return lines
within each MPS TVC component are interconnected to enable hydraulic
fluid to circulate for thermal conditioning.