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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 Gs 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.
There are 38 primary Reaction Control System (RCS) engines and six vernier RCS engines located on the orbiter. The first use of selected primary reaction control system engines occurs at orbiter/external tank separation. The selected primary reaction control system engines are used in the separation sequence to provide an attitude hold for separation. Then they move the orbiter away from the external tank to ensure orbiter clearance from the arc of the rotating external tank. Finally, they return to an attitude hold prior to the initiation of the firing of the Orbital Maneuvering System (OMS) engines to place the orbiter into orbit.
The primary and/or vernier RCS engines are used normally on orbit to provide attitude pitch, roll and yaw (rotational) maneuvers as well as translation maneuvers.
During the initial entry sequence, selected primary RCS engines are used to control the orbiter's attitude (pitch, roll and yaw). As aerodynamic pressure builds up, the orbiter flight control surfaces become active and the primary reaction control system engines are inhibited.
The two OMS engines are used to place the orbiter on orbit, for major velocity maneuvers on orbit and to slow the orbiter for reentry, called the deorbit maneuver. Normally, two OMS engine thrusting sequences are used to place the orbiter on orbit, and only one thrusting sequence is used for deorbit. The deorbit maneuver decreases the orbital velocity by approximately 300 fps (205 mph).
In some missions, only one OMS thrusting sequence is used to place the orbiter on orbit. This is referred to as direct insertion. Direct insertion is a technique used in some missions where there are high-performance requirements, such as a heavy payload or a high orbital altitude. This technique uses the Space Shuttle main engines to achieve the desired apogee (high point in an orbit) altitude, thus conserving orbital maneuvering system propellants. Following jettison of the external tank, only one OMS thrusting sequence is required to establish the desired orbit altitude.
For deorbit, the orbiter is rotated tail first in the direction of the velocity by the primary reaction control system engines. Then the OMS engines are used to decrease the orbiter's velocity.
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