The space shuttle ascent thrust
vector control portion of the flight control system directs the
thrust of the three main engines and two solid rocket boosters to
control attitude and trajectory during lift-off and first-stage
ascent and the main engines alone during second-stage ascent.
Ascent thrust vector control is provided by avionics hardware
packages that supply gimbal commands and fault detection for each
hydraulic gimbal actuator. The MPS ATVC packages are located in
the three aft avionics bays in the orbiter aft fuselage and are
cooled by cold plates and the Freon-21 system. The associated
flight aft multiplexers/demultiplexers are also located in the
aft avionics bays.
The MPS TVC command flow starts in the general-purpose computers,
in which the flight control system generates the TVC position
commands, and terminates at the SSME servoactuators, where the
actuators gimbal the SSMEs in response to the commands. All the
MPS TVC position commands generated by the flight control system
are issued to the MPS TVC command subsystem operating program,
which processes and disburses them to their corresponding flight
aft MDMs. The flight aft MDMs separate these linear discrete commands
and disburse them to ATVC channels, which generate equivalent
command analog voltages for each command issued. These voltages
are, in turn, sent to the servoactuators, commanding the SSME
hydraulic actuators to extend or retract, thus gimbaling the main
engines to which they are fastened.
Six MPS TVC actuators respond to the command voltages issued
by four ATVC channels. Each ATVC channel has six MPS drivers and
four SRB drivers. Each actuator receives four identical command
voltages from four different MPS drivers, each located in different
Each main engine servoactuator consists of four independent,
two-stage servovalves, which receive signals from the drivers.
Each servovalve controls one power spool in each actuator, which
positions an actuator ram and the engine to control thrust direction.
The four servovalves in each actuator provide a force-summed
majority voting arrangement to position the power spool. With
four identical commands to the four servovalves, the actuator's
force-sum action prevents a single erroneous command from affecting
power ram motion. If the erroneous command persists for more than
a predetermined time, differential pressure sensing activates
an isolation driver, which energizes an isolation valve that isolates
the defective servovalve and removes hydraulic pressure, permitting
the remaining channels and servovalves to control the actuator
ram spool provided the FCS channel 1, 2, 3, 4 switch on panel
C3 is in the auto position. A second failure would isolate the
defective servovalve and remove hydraulic pressure in the same
manner as the first failure, leaving only two channels remaining.
Failure monitors are provided for each channel on the CRT and
backup caution and warning light to indicate which channel has
been bypassed for the MPS and/or SRB. If the FCS channel 1, 2,
3, or 4 switch on panel C3 is positioned to off, that ATVC channel
is isolated from its servovalve on all MPS and SRB actuators.
The override position of the FCS channel 1, 2, 3, 4 switch inhibits
the isolation valve driver from energizing the isolation valve
for its respective channel and provides the capability of resetting
a failed or bypassed channel.
The ATVC 1, 2, 3, 4 power switch is located on panel O17. The
on position enables the ATVC channel selected; off disables the
Each actuator ram is equipped with transducers for position feedback
to the TVC system.
The SSME servoactuators change each main engine's thrust vector
direction as needed during the flight sequence. The three pitch
actuators gimbal the engine up or down a maximum of 10 degrees
30 minutes from the installed null position. The three yaw actuators
gimbal the engine left or right a maximum of 8 degrees 30 minutes
from the installed position. The installed null position for the
left and right main engines is 10 degrees up from the X axis in
a negative Z direction and 3 degrees 30 minutes outboard from
an engine centerline parallel to the X axis. The center engine's
installed null position is 16 degrees above the X axis for pitch
and on the X axis for yaw. When any engine is installed in the
null position, the other engines cannot collide with it.
The minimum gimbal rate is 10 degrees per second; the maximum
rate is 20 degrees per second.
There are three actuator sizes for the main engines. The piston
area of the one upper pitch actuator is 24.8 square inches, its
stroke is 10.8 inches, it has a peak flow of 50 gallons per minute,
and it weighs 265 pounds. The piston area of the two lower pitch
actuators is 20 square inches, their stroke is 10.8 inches, their
peak flow is 45 gallons per minute, and they weigh 245 pounds.
All three yaw actuators have a piston area of 20 square inches,
a stroke of 8.8 inches and a peak flow of 45 gallons per minute
and weigh 240 pounds.