There are two
speed brake/thrust controllers: one on the left side of the flight
deck forward on panel L2 for the commander and one on the right
side of the center console on panel C3 for the pilot. The SBTCs
serve two distinct functions: during ascent, the pilot's speed brake/thrust
controller may be used to vary the thrust level of the three SSMEs.
During entry, the commander's or pilot's speed brake/thrust controller
may be used to control aerodynamic drag (hence airspeed) by opening
or closing the speed brake.
a takeover switch (with three contacts) on each SBTC switches to
manual control of the SSME thrust level setting (pilot's only) or
speed brake position. Each SBTC contains three transducers-channels
1, 2 and 3, which produce a voltage proportional to the deflection.
Redundancy management selects the output.
In the case
of the SSME thrust-level setting, the top half of both spd bk/throt
push button light indicators on panels F2 and F4 will be illuminated,
indicating auto . Only the pilot's SBTC can be enabled for manual
throttle control. The pilot depresses the takeover push button on
the SBTC, causing the general-purpose computer throttle command
to be frozen at its current value. While depressing the takeover
button, the pilot moves the SBTC to match the frozen GPC command.
Manual control is established when the match is within 4 percent.
When the match is achieved, the spd bk/throt man push button light
indicator on panel F4 will be illuminated and the auto light extinguished.
The takeover push button is then released. If the takeover push
button is released before a match is achieved, the system reverts
to GPC auto commands. Under manual throttle command, depressing
either or both push button light indicators on panel F2 or F4 will
cause the system to revert to the GPC auto commands, extinguishing
the pilot's man light and illuminating the auto lights on panels
F2 and F4. Transferring back to auto during a return-to-launch-site
abort leaves the throttle at the last-commanded manual setting until
3 g's, and the vehicle is held at 3 g's.
If the speed
brake mode is in automatic and the commander or pilot wishes to
control the speed brake manually, momentarily depressing the takeover
push button grants control of the SBTC to the crew member who depressed
the switch. The speed brake is driven to the position currently
commanded by the SBTC. The spd bk/throt man push button light indicator
on panel F2 will be illuminated if the commander takes control,
extinguishing the auto light. If the pilot takes control, the spd
bk/throt man push button light indicator on panel F4 will be illuminated,
and the commander's light will be extinguished. To place the speed
brake under software control, either or both spd bk/throt push button
indicators on panel F2 or F4 can be depressed, and the auto lights
on panels F2 and F4 will be illuminated.
At the forward
setting, the SSME thrust level is the greatest, and the speed brake
is closed. Rotating the SBTC back decreases the SSME thrust level
or opens the speed brake.
The SBTC SOP
converts the selected SSME throttle command to a setting in percent
and the selected speed brake command from volts to degrees. In addition,
the SBTC SOP selects the speed brake command from the SBTC whose
takeover button was depressed last. If both takeover buttons are
depressed simultaneously, the commander's SBTC is given control.
If redundancy management declares an SBTC bad, the command is frozen.
SBTC is powered by the flt cntlr on/off switch on panel F7 when
positioned to on . The pilot's SBTC is powered when the flt cntlr
on/off switch on panel F8 is positioned to on .
The SBTC contractor
is Honeywell Inc., Clearwater, Fla.