|Thrust Vector Control
Each SRB has two hydraulic
gimbal servoactuators: one for rock and one for tilt. The servoactuators
provide the force and control to gimbal the nozzle for thrust
The space shuttle ascent
thrust vector control portion of the flight control system directs
the thrust of the three shuttle main engines and the two SRB nozzles
to control shuttle attitude and trajectory during lift- off and
ascent. Commands from the guidance system are transmitted to the
ATVC drivers, which transmit signals proportional to the commands
to each servoactuator of the main engines and SRBs. Four independent
flight control system channels and four ATVC channels control
six main engine and four SRB ATVC drivers, with each driver controlling
one hydraulic port on each main and SRB servoactuator.
Each SRB servoactuator
consists of four independent, two- stage servovalves that receive
signals from the drivers. Each servovalve controls one power spool
in each actuator, which positions an actuator ram and the nozzle
to control the direction of thrust.
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 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 a selector valve to isolate and remove
the defective servovalve hydraulic pressure, permitting the remaining
channels and servovalves to control the actuator ram spool.
Failure monitors are
provided for each channel to indicate which channel has been bypassed.
An isolation valve on each channel provides the capability of
resetting a failed or bypassed channel.
Each actuator ram is
equipped with transducers for position feedback to the thrust
vector control system. Within each servoactuator ram is a splashdown
load relief assembly to cushion the nozzle at water splashdown
and prevent damage to the nozzle flexible bearing.