Orbiter Hydraulic Systems

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 systems.

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.