Return to Human Space Flight home page

Engine Thrust Vector Control System

The engine TVC system consists of a gimbal ring assembly, two gimbal actuator assemblies and two gimbal actuator controllers. The engine gimbal ring assembly and gimbal actuator assemblies provide OMS TVC by gimbaling the engines in pitch and yaw. Each engine has a pitch actuator and a yaw actuator. Each actuator is extended or retracted by one of a pair of dual-redundant electric motors and is actuated by general-purpose computer control signals.

The gimbal ring assembly contains two mounting pads to attach the engine to the gimbal ring and two pads to attach the gimbal ring to the orbiter. The ring transmits engine thrust to the pod and orbiter.

The pitch and yaw gimbal actuator assembly for each OMS engine provides the force to gimbal the engines. Each actuator contains a primary and secondary motor and drive gears. The primary and secondary drive systems are isolated and are not operated concurrently. Each actuator consists of two redundant brushless dc motors and gear trains, a single jackscrew and nut-tube assembly and redundant linear position feedback transducers. A GPC position command signal from the primary electronic controller energizes the primary dc motor, which is coupled with a reduction gear and a no-back device. The output from the primary power train drives the jackscrew of the drive assembly, causing the nut-tube to translate (with the secondary power train at idle), which causes angular engine movement. If the primary power train is inoperative, a GPC position command from the secondary electronic controller energizes the secondary dc motor, providing linear travel by applying torque to the nut-tube through the spline that extends along the nut-tube for the stroke length of the unit. Rotation of the nut-tube about the stationary jackscrew causes the nut-tube to move along the screw. A no-back device in each drive system prevents backdriving of the standby system.

The electrical interface, power and electronic control elements for active and standby control channels are assembled in separate enclosures designated the active actuator controller and standby actuator controller. These are mounted on the OMS/RCS pod structure. The active and standby actuator controllers are electrically and mechanically interchangeable.

The gimbal assembly provides control angles of plus or minus 6 degrees in pitch and plus or minus 7 degrees in yaw with clearance provided for an additional 1 degree for snubbing and tolerances. The engine null position is with the engine nozzles up 15 degrees 49 seconds (as projected in the orbiter XZ plane) and outboard 6 degrees 30 seconds (measured in the 15-degree 49-second plane).

The thrust vector control command subsystem operating program processes and outputs pitch and yaw OMS engine actuator commands and the actuator power selection discretes. The OMS TVC command SOP is active during operational sequences, orbit insertion (OMS-1 and OMS-2), orbit coast, deorbit, deorbit coast and return-to-launch-site abort.

The flight crew can select either the primary or the secondary motors of the pitch and yaw actuators by item entry on the maneuver display or can select actuators off. The actuator command outputs are selected by the TVC command SOP depending on the flag that is present, i.e., major modes, deorbit maneuver, orbit coast, and RTLS abort, center-of-gravity trim and gimbal check. The deorbit maneuver coast flag causes the TVC command SOP to output I-loaded values to command the engines to the entry stowed position. The presence of the RTLS abort and center-of-gravity trim flags causes the engines to be commanded to a predefined position with the thrust vector through the center of gravity. The major mode RTLS flag by itself will cause the engines to be commanded to a stowed position for return-to-launch-site entry. The gimbal check flag causes the engines to be commanded to plus 7 degrees yaw and 6 degrees pitch, then to minus 7 degrees yaw and 6 degrees pitch, and back to zero degrees yaw and pitch. In the absence of these flags, the TVC command SOP will output the digital autopilot gimbal actuator commands to the engine actuators. The backup flight control system allows only manual TVC during a thrusting period, but it is otherwise similar.

The OMS TVC feedback SOP monitors the primary and secondary actuator selection discretes from the maneuver display and performs compensation on the selected pitch and yaw actuator feedback data. This data is output to the OMS actuator fault detection and identification and to the maneuver display. The OMS TVC feedback SOP is active during orbit insertion (OMS-1 and OMS-2), orbit coast, deorbit maneuver and deorbit maneuver coast. The present OMS gimbal positions can be monitored on the maneuver CRT display when this SOP is active and the primary or secondary actuator motors are selected.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
Web Accessibility and Policy Notices