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
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
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
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
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.