Flight Controls
Aerosurface Servoamplifiers - Receive commands during atmospheric flight, causing aerosurface deflections.
Aerosurface servoamplifiers are electronic devices that receive aerosurface commands during atmospheric flight from the flight control system software and electrically position hydraulic valves in aerosurface actuators, causing aerosurface deflections.
Each aerosurface is driven by a hydraulic actuator controlled by a redundant set of electrically driven valves (ports). There are four of these valves for each aerosurface actuator, except the body flap, which has only three. These valves are controlled by the selected ASAs.
There are four ASAs located in aft avionics bays 4, 5 and 6. Each ASA commands one valve for each aerosurface, except the body flap. ASA 4 does not command the body flap.
In addition to the command channels from the ASAs to the control valves, there are data feedback channels to the ASAs from the aerosurface actuators. Each aerosurface has four associated position feedback transducers that are summed with the position command to provide a servoloop closure for one of the four independent servoloops associated with the elevons, rudder and speed brake. The body flap utilizes only three servoloops. The path from an ASA to its associated servovalve in the actuators and from the aerosurface feedback transducers to an ASA is called a flight control channel; there are, thus, four flight control channels, except for the body flap.
Each of the four elevons located on the trailing edges has an associated servoactuator that positions it. Each servoactuator is supplied with hydraulic pressure from the three orbiter hydraulic systems. A switching valve is used to control the hydraulic system that becomes the source of hydraulic pressure for that servoactuator. The valve allows a primary source of pressure (P1) to be supplied to that servoactuator. If the primary hydraulic pressure drops to around 1,200 to 1,500 psig, the switching valve allows the first standby hydraulic pressure (P2) to supply that servoactuator. If the first standby hydraulic pressure drops to around 1,200 to 1,500 psig, the secondary standby hydraulic source pressure (P3) is then supplied to that servoactuator. The yellow hyd press caution and warning light will be illuminated on panel F7 if the hydraulic pressure of system 1, 2 or 3 is below 2,400 psi and will also illuminate the red backup caution and warning alarm light on panel F7.
Each elevon servoactuator receives command signals from each of the four ASAs. Each actuator is composed of four two-stage servovalves that drive a modulating piston. Each piston is summed on a common mechanical shaft, creating a force to position a power spool that controls the flow of hydraulic fluid to the actuator power ram, controlling the direction of ram movement, thus driving the elevon to the desired position. When the desired position is reached, the power spool positions the mechanical shaft to block the hydraulic pressure to the hydraulically operated ram, locking the ram at that position. If a problem develops within a servovalve or it is commanded to a position different than the positions of the other three within an actuator, secondary delta pressure should begin to rise to 2,200 psi. Once the secondary delta pressure is at or above 2,200 psia for more than 120 seconds, the corresponding ASA sends an isolation command to the servovalve, opening the isolation valve, bypassing the hydraulic pressure to the servovalve, and causing its commanded pressure to the power spool to drop to zero, effectively removing it from operation. The pressure differential is sensed by a primary linear differential pressure transducer across the modulating piston when the respective FCS channel switch on panel C3 is in auto . This automatic function prevents excessive transient motion to that aerosurface, which could result in loss of the orbiter due to slow manual redundancy.
The FCS channel yellow caution and warning light on panel F7 will be illuminated to inform the flight crew of a failed channel. A red FCS saturation caution and warning light on panel F7 will be illuminated if one of the four elevons is set at more than plus 15 degrees or less than minus 20 degrees.
There are four FCS channel switches on panel C3- FCS channels 1, 2, 3 and 4; each has an override, auto and off position. The switch for a channel controls the channel for the elevons, rudder/speed brake and body flap, except channel 4, which has no body flap commands. When an FCS channel switch is in auto and that channel was bypassed, it can be reset by positioning the applicable switch to override . When an FCS channel switch is positioned to off , that channel is bypassed.
In each elevon servoactuator ram, there are four linear ram position transducers and four linear ram secondary differential pressure transducers. The ram linear transducers provide position feedback to the corresponding servoloop in the ASA, which is then summed with the position command to close the servoloop. This feedback is then summed with the elevon ram linear secondary differential pressures to develop an electrohydraulic valve drive current that is proportional to the error signal in order to position the ram. The maximum elevon deflection rate is 20 degrees per second.
During ascent, the elevons are deflected to reduce wing loads caused by rapid acceleration through the lower atmosphere. In this scheme, the inboard and outboard elevons are deflected together. By the time the vehicle reaches approximately Mach 2.5, the elevons have reached a null position, where they remain. This is accomplished by the initialized-loaded program.
The rudder/speed brake, which consists of upper and lower panels, is located on the trailing edge of the orbiter's vertical stabilizer. One servoactuator positions the panels together to act as a rudder; another opens the panels at the rudder's flared end so it functions as a speed brake.
The rudder and speed brake servoactuator receives four command signals from the four ASAs. Each servoactuator is composed of four two-stage servovalves that function like those of the elevons. The exception is that the rudder's power spool controls the flow of hydraulic fluid to the rudder's three reversible hydraulic motors and the power spool for the speed brake controls the flow of hydraulic fluid in the speed brake's three hydraulic reversible motors. Each rudder and speed brake hydraulic motor receives hydraulic pressure from only one of the orbiter's hydraulic systems. Each hydraulic motor has a hydraulic brake. When the motor is supplied with hydraulic pressure, the motor's brake is released. When the hydraulic pressure is blocked to that hydraulic motor, the hydraulic brake is applied, holding that motor and the corresponding aerosurface at that position.
The three hydraulic motors provide output to the rudder differential gearbox, which is connected to a mixer gearbox that drives rotary shafts. These rotary shafts drive four rotary actuators, which position the rudder panels.
The three speed brake hydraulic motors provide power output to the speed brake differential gearbox, which is connected to the same mixer gearbox as that of the rudder. This gearbox drives rotary shafts, which drive the same four rotary actuators involved with the rudder. Within each of the four rotary actuators, planetary gears blend the rudder positioning with the opening of the rudder flared ends.
There are four rotary position transducers on the rudder differential gearbox output and one differential linear position transducer in each rudder servoactuator. The rotary position transducers provide position feedback to the corresponding servoloop in the ASA. The feedback is summed with the linear differential pressures that develop the electrohydraulic valve drive current in proportion to the error signal in order to position the rudder.
There are also four rotary position transducers on the speed brake differential gearbox output and one differential linear pressure transducer in each speed brake servoactuator.
The rotary position transducers provide position feedback to the corresponding servoloop in the ASA, which is summed with the position command to close the servoloop. These are then summed with the linear differential pressures that develop the electrohydraulic valve drive current in proportion to the error signal to position the speed brake.
If a problem occurs in one of the four rudder or speed brake servoactuator channels, the corresponding linear differential pressure transducer will cause the corresponding ASA to signal a solenoid isolation valve to remove the pressure from the failed channel and bypass it if that FCS channel switch is in auto . The FCS channel switches' override and off positions and the FCS channel caution and warning light function the same as for the elevons. The hyd press light indicates a hydraulic failure. The rudder deflection rate is a maximum of 14 degrees per second. The speed brake deflection rate is approximately 10 degrees per second. If two of the three hydraulic motors fail in the rudder or speed brake, about half the design speed output will result from the corresponding gearbox due to its velocity summary nature.
Three servoactuators at the lower aft end of the fuselage are used to position the body flap; each is supplied with hydraulic pressure from an orbiter hydraulic system and has a solenoid-operated enable valve controlled by one of the three ASAs (the fourth ASA is not used for the body flap commands). Each solenoid-operated enable valve supplies hydraulic pressure from one orbiter hydraulic system to a corresponding solenoid-operated pilot valve, which is, in turn, controlled by one of the three ASAs. When the individual pilot valve receives a command signal from its corresponding ASA, it positions a common mechanical shaft in the control valve, allowing hydraulic pressure to be supplied to the hydraulic motors (normally one pilot valve is enabled and moves the other two). The hydraulic motors are reversible, allowing the body flap to be positioned up or down. The hydraulic brake associated with each hydraulic motor releases the hydraulic motor for rotation. When the desired body flap position is reached, the control valves block the hydraulic pressure to the hydraulic motor and apply the hydraulic brake, holding that hydraulic motor at that position. Each hydraulic motor provides the power output to a differential gearbox, which drives a rotary shaft, and four rotary actuators, which position the body flap. The rotary position transducer associated with each rotary actuator provides position feedback to the ASAs; the fourth ASA is used to provide position feedback to the flight control system software.
If the FCS channel switches are in auto , the ASAs will isolate a body flap channel through the solenoid-operated enable valve if the corresponding solenoid-operated pilot valve malfunctions or the control valve associated with the pilot valve does not provide the proper response and allows the hydraulic pressure fluid to recirculate. The FCS channel switches and FCS channel caution and warning light function the same as for the elevons. If the hydraulic system associated with the hydraulic motor fails, the remaining two hydraulic motors will position the body flap, and the hyd press caution and warning light will be illuminated. The body flap deflection rate is approximately 4.5 degrees per second.
Each ASA is hard-wired to a flight MDM. Flight control commands originate from guidance software or from controllers. These inputs go to the flight control software, where they are augmented and then routed to the ASAs.
There are several subsystem operating programs associated with the ASA commands and data. The SOPs convert elevon, rudder and speed brake commands from flight control software from degrees to millivolts; set commands to body flap valves based on an enable command from body flap redundancy management and up/down commands from flight control; convert position feedback to degrees for the elevons, rudder, speed brake and body flap; compute elevator position from elevon position feedbacks; calculate body flap and speed brake deflections as percentages; calculate elevon and rudder positions for display on the surface position indicator; monitor the FCS channel switches and if any are positioned to override, set the override command for that ASA; monitor hydraulic system pressures for failures; and rate-limit aileron and elevator commands according to the number of failures.
Each ASA is mounted on a cold plate and cooled by the Freon-21 coolant loops. Each is 20 inches long, 6.4 inches high and 9.12 inches wide and weighs 30.2 pounds.
The ASA contractor is Honeywell Inc., Clearwater, Fla.
Body Flap Switches - Provide manual control for positioning body flap during entry.
There are two body flap switches: one for the commander on panel L2 and one for the pilot on panel C3. Each switch is a lever-locked switch spring loaded to the center position. The body flap switches provide manual control for positioning the body flap for SSME thermal protection and for reducing elevon deflections during the entry phase.
The body flap can be switched from its automatic mode to its manual mode by moving either switch from the auto/off position to the up or down position. These are momentary switch positions; when released, the switch returns to auto/off . The white body flap man (lower half) of the push button light indicator on panel F2 or F4 is illuminated, indicating manual control of the body flap. To regain automatic control, the body flap push button light indicator on panel F2 or F4 is depressed, extinguishing the man white light and illuminating the auto white light. The push button indicator can also be depressed to man for manual body flap control. The push button light indicator is triply redundant.
The up and down positions of each switch have two power supplies from a control bus.
If the commander and pilot generate conflicting commands, a body-flap-up command will be output to flight control because up has priority.
Control Stick Steering Push Button Light Indicators - Indicate control stick mode.
The pitch auto, CSS and roll/yaw auto , CSS push button light indicators are located on panel F2 for the commander and on panel F4 for the pilot. Each push button light indicator is triply redundant. During entry, depressing a CSS push button light indicator will mode flight control to augmented manual in the corresponding axis, illuminate both CSS lights and extinguish both auto lights for that axis. Depressing a CSS push button light indicator to auto will return flight control in that axis to auto, extinguish both CSS lights and illuminate both auto lights. During ascent, depressing any of the four CSS push button light indicators will mode flight control to augmented manual in all axes, illuminate all four CSS push button light indicators and extinguish all four auto lights. Depressing any of the four push button light indicators to auto will mode flight control to automatic, illuminate all four auto push button light indicators and extinguish the CSS push button light indicators.
Flight Control System Hardware - Hard-wired to one of eight flight-critical MDMs.
Each piece of GN&C; hardware is hard-wired to one of eight flight-critical multiplexers/demultiplexers, which are connected to each of the five GPCs by data buses. Each GPC is assigned to command on one or more data buses; assignments can be changed.
A sensor, controller or flight control effector cannot be assigned or rerouted to another MDM during flight, but the MDM it is wired to can be assigned to a different GPC. Each multiple unit of each type of GN&C; hardware is hard-wired to a different MDM. For example, there are four accelerometer assemblies on board the orbiter. AA 1 is wired to forward flight MDM 1 and is part of string 1, AA 2 is wired to FF MDM 2 on string 2, AA 3 is wired to FF MDM 3 on string 3, and AA 4 is wired to FF MDM 4 on string 4.
Rotational Hand Controller - Used by flight crew to gimbal engines and OMS/RCS systems.
There are three rotational hand controllers on the orbiter crew compartment flight deck: one at the commander's station, one at the pilot's station and one at the aft flight deck station. Each RHC controls vehicle rotation about three axes: roll, pitch and yaw. During ascent, the commander's and pilot's RHCs may be used to gimbal the SSMEs and SRBs. For insertion and deorbit, the commander's and pilot's RHCs may be used to gimbal the orbital maneuvering system engines and to command thrusting of the reaction control system engines. On orbit, the commander's, pilot's and aft flight station RHCs may be used to command RCS engine thrusting. During entry, the commander's and pilot's RHCs may be used to command RCS engine thrusting during the early portion of entry and may be used to position the orbiter elevons in roll and pitch axes in the latter portion of entry.
Human factors dictate that an RHC deflection produce a rotation in the same direction as the flight crew member's line of sight. The aft flight station RHC is used only on orbit. An aft sense -Z switch on panel A6 selects the line-of-sight reference about the minus Z axis (overhead windows), and the -X position selects the line-of-sight reference about the minus X axis (aft windows) in order for aft RHC commands to be correctly transformed to give the desired orbiter movement.
Several switches are located on the RHC. A backup flight system (BFS) mode button on the commander's and pilot's RHCs engage the BFS when depressed. The commander's, pilot's, and aft flight station RHCs have a two-contact trim switch that can be pushed forward or aft to add a trim rate to the RHC pitch command; pushing it left or right adds a roll trim rate. The aft RHC's trim switch is inactive. The communications switch on each RHC is a push-to-talk switch that enables voice transmission when the switch is depressed.
Each RHC contains nine transducers: three redundant transducers sense pitch deflection, three sense roll deflection and three sense yaw deflection. The transducers produce an electrical signal proportional to the deflection of the RHC. The three transducers are called channels 1, 2 and 3; the channel selected by redundancy management provides the command. Each channel is powered by a separate power supply in its associated display driver unit. Each controller is triply redundant; thus, it takes only one good signal from a controller for the controller to operate.
Each RHC has an initial dead band of 0.25 of a degree in all three axes. To move the RHC beyond the dead band, an additional force is required. When the amount of deflection reaches a certain level, called the softstop, a step increase in the force required for further deflection occurs. When a software detent position is exceeded, that RHC assumes control.
The softstop occurs at 19.5 degrees in the roll and pitch axes and at 9.5 to 10.5 degrees in the yaw axis. To reach the softstop in the roll axes, 40.95 inch-pounds of static torque deflection are required; 38.2 inch-pounds are needed in pitch and 7 inch-pounds in yaw.
The mechanical hardstop that can be obtained in an axis is 24.3 degrees in the roll and pitch axis and 14.3 degrees in the yaw axis.
Software normally flows from the RHCs to the flight control system through redundancy management and a SOP before it is passed to the aerojet digital autopilot.
In a nominal mission, the flight crew has manual control of the RHC during every major mode except terminal countdown. When an RHC deflection exceeds the detent in an axis, the RHC SOP generates a discrete signal that converts the RHC from the automatic mode to control stick steering, or hot stick. However, during ascent when the ascent digital autopilot is active, a CSS pitch and/or roll/yaw mode push button light indicator on panel F2 or F4 must be depressed in order for manual inputs to be implemented into the flight control system from the commander's or pilot's RHC. When a CSS pitch or roll/yaw push button light indicator is depressed on panels F2 or F4, the white light for the push button indicator will be illuminated and that axis will be downmoded from automatic to CSS.
When the flight crew commands three-axis motion using the RHC, the GPCs process the RHC and motion sensor commands; and the flight control system interprets the RHC motions (fore and aft, right and left, clockwise and counterclockwise) as rate commands in pitch, roll and yaw and then processes the flight control law (equations) to enhance control response and stability. If conflicting commands are given, no commands result.
During orbital flight, any one of the three stations can input three-axis control commands to the flight control system. During entry and landing, the commander and pilot have two-axis (roll and pitch only) capability. Roll, pitch and yaw aerosurface deflection trim is controlled by the panel trim switches, while roll and pitch vehicle rate trim is controlled with the trim switches on the RHC. For a return-to-launch-site abort, both the commander's and pilot's RHC have three-axis capability during major mode 601 and roll and pitch during major modes 602 and 603.
The commander's RHC is powered when the flt cntlr (controller) on/off switch on panel F7 is positioned to on . The pilot's RHC is powered when the flt cntlr on/off switch on panel F8 is positioned to on. The aft RHC is powered when the flt cntlr on/off switch on panel A6 is positioned to on .
If a malfunction occurs in the commander's or pilot's RHC, the red RHC caution and warning light on panel F7 is illuminated.
The RHC contractor is Honeywell Inc., Clearwater, Fla.
Rudder Pedals - Command orbiter rotation about the yaw axis by positioning the rudder during atmospheric flight.
There are two pairs of rudder pedals: one each for the commander and pilot. The commander's and pilot's rudder pedals are mechanically linked so that movement on one side moves the other side. When a pedal is depressed, it moves a mechanical input arm in a rudder pedal transducer assembly. Each RPTA contains three transducers-channels 1, 2 and 3-and generates an electrical signal proportional to the rudder pedal deflection. An artificial feel is provided in the rudder pedal assemblies.
The rudder pedals command orbiter rotation about the yaw axis by positioning the rudder during atmospheric flight. In atmospheric flight, flight control software performs automatic turn coordination; thus the rudder pedals are not used until the wings are level before touchdown.
The RPTA SOP converts the selected left and right commands from volts to degrees; selects the largest of the left and right commands for output to flight control software after applying a dead band; and if redundancy management declares an RPTA bad, sets that RPTA to zero.
The rudder pedals can be adjusted 3.25 inches forward or aft from the neutral position in 0.81-inch increments (nine positions). The breakout force is 10 pounds. A pedal force of 70 pounds is required to depress a pedal to its maximum forward or aft position.
The rudder pedals provide two additional functions unrelated to software after touchdown. Rudder pedal deflections provide nose wheel steering, and depressing the upper portion of the pedals by applying toe pressure provides braking. Differential braking may be used for nose wheel steering.
The commander's RPTA is powered when the flt cntlr on/off switch on panel F7 is positioned to on . The pilot's RPTA is powered when the flt cntlr on/off switch on panel F8 is positioned to on .
The RPTA contractor is Honeywell Inc., Clearwater, Fla.
Translational Hand Controller - Used for manual control of translation along the longitudinal, lateral, and vertical axes to control RCS.
There are two translational hand controllers: one at the commander's station and one at the aft flight deck station. The commander's THC is active during orbit insertion, on orbit and during deorbit. The aft flight deck station THC is active only on orbit. The THCs are used for manual control of translation along the longitudinal (X), lateral (Y) and vertical (Z) vehicle axes using the RCS.
Each THC contains six three-contact switches, one in the plus and minus directions for each axis. Moving the THC to the right commands translation along the plus Y axis and closes three switch contacts (referred to as channels 1, 2 and 3). Redundancy management then selects the channel and provides the command.
An aft sense switch on panel A6 selects the line-of-sight reference along the minus X or minus Z axis of the orbiter for the aft THC. The aft sense switch must be in the -X position for aft windows and -Z for the overhead windows in order for the aft THC commands to be correctly transformed to give the desired orbiter movement.
The normal displacement of a THC is 0.5 of an inch from the center null position in both directions along each of the three THC axes. A force of 2 pounds is required to deflect either THC 0.5 of an inch in all axes.
The redundant signals from the forward and aft THC pass through a redundant management process and a SOP before being passed to the flight control system. If both the forward and aft THCs generate conflicting translation commands, the output translation command is given.
In what is referred to as the transition digital autopilot mode, the commander's THC is active and totally independent of the flight control orbital digital autopilot ( DAP ) push buttons on panel C3 or A6 or the position or status of the RHC. Whenever the commander's THC is out of detent plus or minus X, Y or Z, translation acceleration commands are sent directly to the RCS jet selection logic for continuous RCS thrusting periods. Rotational commands may be sent simultaneously with translation commands within the limits of the RCS jet selection logic; if both plus X and minus Z translations are commanded simultaneously, plus X translation is given priority.
The commander's THC is powered when the flt cntlr on/off switch on panel F7 is positioned to on . The aft THC is powered when the flt cntlr on/off switch on panel A6 is positioned to on .
The THC contractor is Honeywell Inc., Clearwater, Fla.
Trim Switches - Used to move the aerosurfaces in roll, pitch and yaw.
The commander's trim roll l (left), r (right); the yaw trim l, r; and trim pitch up, down switches are located on panel L2. The pilot's switches are located on panel C3. The commander's trim switches on panel L2 are enabled when the trim panel on/off switch on the left side of panel F3 is positioned to on . The pilot's trim switches on panel C3 are enabled when the trim panel on/off switch on the right side of panel F3 is positioned to on. The corresponding trim RHC/panel enable/inhibit switch must be in enable in order for trimming to take place.
Each of the three trim switches on panels L2 and C3 are spring loaded to the center off position.
Redundancy management processes the two sets of switches. If two switches generate opposing commands, the resultant trim command in that axis is zero.