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The main propulsion system, assisted by the two solid rocket boosters during the initial phases of the ascent trajectory, provides the velocity increment from lift-off to a predetermined velocity increment before orbit insertion. The two SRBs are jettisoned after their fuel has been expended, but the MPS continues to thrust until the predetermined velocity is achieved. At that time, main engine cutoff is initiated. The external tank is jettisoned, and the orbital maneuvering system is ignited to provide the final velocity increment for orbital insertion. The magnitude of the velocity increment supplied by the OMS depends on payload weight, mission trajectory and system limitations.

Coincident with the start of the OMS thrusting maneuver (which settles the MPS propellants), the remaining liquid oxygen propellant in the orbiter feed system and space shuttle main engines is dumped through the nozzles of the three SSMEs. At the same time, the remaining liquid hydrogen propellant in the orbiter feed system and SSMEs is dumped overboard through the hydrogen fill and drain valves for six seconds. Then the hydrogen inboard fill and drain valve is closed, and the hydrogen recirculation valve is opened, continuing the dump. The hydrogen flows through the engine hydrogen bleed valves to the orbiter hydrogen MPS line between the inboard and outboard hydrogen fill and drain valves, and the remaining hydrogen is dumped through the outboard fill and drain valve for approximately 120 seconds.

During on-orbit operations, the flight crew vacuum inerts the MPS by opening the liquid oxygen and liquid hydrogen fill and drain valves, which allows the remaining propellants to be vented to space.

Before entry, the flight crew repressurizes the MPS propellant lines with helium to prevent contaminants from being drawn into the lines during entry and to maintain internal positive pressure. MPS helium is also used to purge the spacecraft's aft fuselage. The last activity involving the MPS occurs at the end of the landing rollout. At that time, the helium remaining in onboard helium storage tanks is released into the MPS to provide an inert atmosphere for safety.

The MPS consists of the following major subsystems: three SSMEs, three SSME controllers, the external tank, the orbiter MPS propellant management subsystem and helium subsystem, four ascent thrust vector control units, and six SSME hydraulic servoactuators.

The main engines are reusable, high-performance, liquid-propellant rocket engines with variable thrust. The propellant fuel is liquid hydrogen and the oxidizer is liquid oxygen. The propellant is carried in separate tanks in the external tank and supplied to the main engines under pressure. Each engine can be gimbaled plus or minus 10.5 degrees in the yaw axis and plus or minus 10.5 degrees in the pitch axis for thrust vector control by hydraulically powered gimbal actuators.

The main engines can be throttled over a range of 65 to 109 percent of their rated power level in 1-percent increments. A value of 100 percent corresponds to a thrust level of 375,000 pounds at sea level and 470,000 pounds in a vacuum. A value of 104 percent corresponds to 393,800 pounds at sea level and 488,800 pounds in a vacuum; 109 percent corresponds to 417,300 pounds at sea level and 513,250 pounds in a vacuum.

At sea level, the engine throttling range is reduced due to flow separation in the nozzle, prohibiting operation of the engine at its 65-percent throttle setting, referred to as minimum power level. All three main engines receive the same throttle command at the same time. Normally, these come automatically from the orbiter general-purpose computers through the engine controllers. During certain contingency situations, manual control of engine throttling is possible through the speed brake/thrust controller handle. The throttling ability reduces vehicle loads during maximum aerodynamic pressure and limits vehicle acceleration to 3 g's maximum during boost.

Each engine is designed for 7.5 hours of operation over a life span of 55 starts. Throughout the throttling range, the ratio of the liquid oxygen-liquid hydrogen mixture is 6-to-1. Each nozzle area ratio is 77.5-to-1. The engines are 14 feet long and 7.5 feet in diameter at the nozzle exit.

The SSME controllers are digital, computer system, electronic packages mounted on the SSMEs. They operate in conjunction with engine sensors, valve actuators and spark igniters to provide a self-contained system for monitoring engine control, checkout and status. Each controller is attached to the forward end of the SSME.

Engine data and status collected by each controller are transmitted to the engine interface unit, which is mounted in the orbiter. There is one EIU for each main engine. The EIU transmits commands from the orbiter GPCs to the main engine controller. When engine data and status are received by the EIU, the data are held in a buffer until the EIU receives a request for data from the computers.

Three orbiter hydraulic systems provide hydraulic pressure to position the SSME servoactuators for thrust vector control during the ascent phase of the mission in addition to performing other functions in the main propulsion system. The three orbiter auxiliary power units provide mechanical shaft power through a gear train to drive the hydraulic pumps that provide hydraulic pressure to their respective hydraulic systems.

The ascent thrust vector control units receive commands from the orbiter GPCs and send commands to the engine gimbal actuators. The units are electronics packages (four in all) mounted in the orbiter's aft fuselage avionics bays. Hydraulic isolation commands are directed to engine gimbal actuators that indicate faulty servovalve position. In conjunction with this, a servovalve isolation signal is transmitted to the computers.

The SSME hydraulic servoactuators are used to gimbal the main engine. There are two actuators per engine, one for pitch motion and one for yaw motion. They convert electrical commands received from the orbiter GPCs and position servovalves, which direct hydraulic pressure to a piston that converts the pressure into a mechanical force that is used to gimbal the SSMEs. The hydraulic pressure status of each servovalve is transmitted to the ATVC units.

The orbiter MPS propellant management subsystem consists of the manifolds, distribution lines and valves by which the liquid propellants pass from the external tank to the main engines and the gaseous propellants pass from the main engines to the external tank. The SSMEs' gaseous propellants are used to pressurize the external tank. All the valves in the propellant management subsystem are under direct control of the orbiter GPCs and are either electrically or pneumatically actuated.

The orbiter MPS helium subsystem consists of a series of helium supply tanks and regulators, check valves, distribution lines and control valves. The subsystem supplies the helium used within the engine to purge the high-pressure oxidizer turbopump intermediate seal and preburner oxidizer domes and to actuate valves during emergency pneumatic shutdown. The balance of the helium is used to actuate all the pneumatically operated valves within the propellant management subsystem and to pressurize the propellant lines before re-entry.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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