The digital autopilot is the heart of flight control software. It is composed of several software modules that interpret maneuver requests; compare them to what the vehicle is doing; and generate commands for the appropriate effectors, as needed, to satisfy the requests. There are different DAPS for different flight phases and various modes and submodes within each.

At main engine cutoff, the transition digital autopilot becomes active, sending attitude hold commands to the reaction control system. External tank separation is automatically commanded 18 seconds after main engine cutoff, and the transition DAP immediately sets commands to fire the orbiter's minus Z RCS jets, causing the orbiter to translate in the minus Z direction. When a rate of negative 4 feet per second is reached, RCS fire commands are removed. The transition DAP is used from MECO until transition to OPS 2 (on orbit).

In the transition DAP mode, the external tank separation module initialized by the external tank separation sequencer compares Z-delta velocities from the DAP attitude processor with an initialized-loaded desired Z-delta velocity. Before this value is reached, the transition DAP and steering processor send commands to the RCS jet selection logic, which uses a table lookup technique for the primary RCS jets and commands 10 RCS jets to fire in the plus Z direction. When the desired Z-delta velocity is reached, the translation command is set to zero. Rotation commands are permitted during the external tank separation sequence.

The RCS jet selection module receives both RCS rotation and translation commands and outputs 75 discretes to the primary RCS jets to turn the 38 jets on or off by applying the table lookup technique.

The RCS reaction jet driver forward and aft assemblies provide the turn-on/turn-off jet selection logic signals to the RCS jets. There is also a driver redundancy management program that permits only ''good'' RCS jets to be turned on. The RCS jet yellow caution and warning light indicates a failed-on, failed-off or leaking RCS jet.

The 19 RCS jets thrusting in the plus or minus Z direction provide Z translation and roll or pitch rotation control and are considered independent of other axes. The 12 RCS jets thrusting in the plus or minus Y direction provide Y translation and yaw rotation only. The seven RCS jets thrusting in the plus or minus X direction provide X translation only.

Insertion flight control is accomplished using the transition DAP. The transition DAP uses commands from guidance for automatic maneuvers to orbital maneuvering system burn attitude using RCS jets. During OMS-1 and OMS-2 (or only OMS-1 in a direct insertion), the transition DAP uses the OMS engines and RCS jets, as required. The transition DAP also receives commands from the flight crew through the commander's THC and the commander's or pilot's RHC. The transition DAP then takes these commands and converts them into appropriate RCS commands. The transition DAP monitors the resultant attitude and attitude rates and sends the necessary commands to achieve the targeted attitude and attitude rate within premission-specified dead bands.

The transition DAP reconfiguration logic controls the moding, sequencing and initialization of the control law modules and sets gains, dead bands and rate limits. The steering processor is the interface between the guidance or manual steering commands and the transition DAP. The steering processor generates commands to the RCS processor, which generates the RCS jet command required to produce the commanded spacecraft translation and rotation using attitude and rotational rate signals or translation or rotation acceleration commands.

The flight crew interfaces with the transition DAP through the forward RHCs and THC and indirectly through entries to the OMS mnv exec CRT displays and the DAP panel push button light indicators on panel C3.

In the transition DAP, the commander's THC is active and totally independent of the DAP push button light indicators or RHC position or status. 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 jet firing. 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 receives priority.

For rotations, the flight crew can select either automatic or manual control through the use of the DAP panel push button light indicators or by moving the RHC. In manual, the capability exists to rotate in any axis in a pulse mode, in which each RHC deflection results in a single burst of jet fire, or in a discrete rate mode, in which RHC deflection results in a specified rate being commanded in that axis for the entire time the RHC is deflected. It is also possible to go to a free drift mode, in which no RCS jets are fired, or to an attitude hold mode, in which the DAP sends commands to maintain the current attitude with null rates within premission-specified dead bands. Also, if the RHC is deflected beyond a certain point, continuous RCS jet firings will result. In translation, movement of the THC results in continuous jet firings.

The orbital maneuvering system processor generates OMS engine gimbal actuator thrust vector control commands to produce the desired spacecraft/engine relationship for the commanded thrust direction.

For the OMS thrusting period, the orbital state (position and vector) is produced by navigation incorporating inertial measurement unit delta velocities during powered and coasting flight. This state is sent to guidance, which uses target inputs through the CRT to compute thrust direction commands and commanded attitude for flight control and thrusting parameters for CRT display. Flight control converts the commands into OMS engine gimbal angles (thrust vector control) for an automatic thrusting period. OMS thrust vector control for normal two-engine thrusting is entered by depressing the orbital DAP auto push button light indicator with both RHCs within software detents. OMS manual thrust vector control for both OMS engines is entered by depressing the orbital man DAP push button light indicator or by moving the commander's or pilot's RHC out of detent; the flight crew supplies the rate commands to the TVC system instead of guidance. The manual RHC rotation requests are proportional to RHC deflections and are converted into gimbal angles. OMS thrust in either case is applied through the spacecraft's center of gravity.

The DAP controls the orbiter in response to automatic or manual commands during insertion and on orbit. The effectors used to produce control forces and moments on the orbiter are the two OMS engines and the 38 primary RCS engines. The forward and aft RCS engines also provide attitude control and three-axis translation during external tank separation, insertion and on-orbit maneuvers and roll control for a single-OMS-engine operation. The OMS provides propulsive and three-axis control for orbit insertion, orbit circularization, orbit transfer and rendezvous. Failure of a single OMS engine will not preclude a nominal orbit insertion.

Before OMS ignition, the spacecraft is maneuvered to the OMS ignition attitude by using the RHC and RCS jets, reducing transient fuel losses. Normally, the first OMS thrusting period raises the orbiter's low elliptical orbit after the external tank is jettisoned, and the second OMS thrusting places the spacecraft into the circular orbit designated for that mission. For orbital maneuvers that use the OMS, any delta velocities greater than 6 feet per second use the two OMS engines. Some missions use a direct insertion, requiring only one OMS thrusting period.

Automatic thrust vector control for one OMS engine is identical to that for two, except that the RCS processor is responsible for roll control. Single-OMS-engine thrust is also through the spacecraft's center of gravity, except when pitch or yaw rate commands are non-zero. If the left or right OMS engine fails, an OMS TVC red light on panel F7 will be illuminated.

Since an OMS cutoff is based on time rather than velocity, a velocity residual may exist following the cutoff. The residual is zeroed by the RCS through the THC.

The orbital flight control software includes an RCS DAP, an OMS TVC DAP and an attitude processor module to calculate vehicle attitude as well as logic to govern the selection of a DAP. The attitudes calculated by the attitude processor are displayed on the attitude display indicator along with another crew display, universal pointing, which is available in major mode 201 (orbit coast). The vehicle attitude is used by the DAP to determine attitude and rate errors.

The only time the RCS DAP is not used in OPS 2 is during an OMS burn. This DAP controls vehicle attitudes and rates through the use of RCS jet fire commands. Either the larger primary jets or the less powerful vernier jets are used for rotational maneuvers, depending on whether norm or vern is selected on the panel C3 orbital DAP panel. The choice of primary or vernier thrusters depends on fuel consumption considerations and how quickly the vehicle needs to be maneuvered to satisfy a mission objective.

The rotation rates and dead bands, translation rate and certain other DAP options can be changed by the flight crew during the orbit phase using the DAP CRT display. The flight crew can load the DAP with these options in two ways: one option set may be accessed by depressing the DAP A push button on the orbital DAP panel, the other by depressing the DAP B push button. For convenience, each planned DAP configuration is given a number and is referred to by that number and the DAP used to access it. Typically, the DAP A configurations will have larger dead bands and higher rates than the DAP B configurations. The wide dead bands are used to minimize fuel usage, while the tight dead bands allow greater precision in executing maneuvers or in holding attitude.

The RCS DAP can operate in both an automatic and a manual rotation mode, depending on whether the flight crew selects the auto or man push button light indicators on the orbital DAP panel. The manual mode is also accessed when the RHC is moved out of its detent (neutral) position. In both the automatic and manual modes, the rotation rate is controlled by the selection of DAP A or B and the information loaded in the DAP config display. In addition, in automatic, the DAP determines the required attitude to be achieved from universal pointing and then computes the RCS jet fire commands necessary to achieve these requirements within the current set of dead bands. In the manual rotation mode, the RCS DAP converts flight crew inputs with any of the three RHCs to RCS jet fire commands, depending on whether pulse, disc rate or accel is selected on the orbital DAP panel. Simply, when pulse is selected, a single burst of jet fire is produced with each RHC deflection. The resultant rotational rate is specified on the DAP config display. When disc rate is selected, jet firings continue to be made as long as the RHC is out of detent in order to maintain the rotational rate specified on the DAP config display. When accel is selected, continuous jet firings are made as long as the RHC is out of detent.

Another manual RCS DAP mode, local vertical/local horizontal, is used to maintain the current attitude with respect to the rotating LVLH reference frame. It is selected through the LVLH push button on the orbital DAP panel.

The RCS DAP has only a manual translation capability, which is executed through the forward or aft THC. Only the primary RCS jets are used. Deflections of the THC result in RCS jet firings based on the transition DAP mode push button light indicator selected on the orbital DAP panel. Pulse results in a single burst of jet fire. Norm results in continuous jet firings with a specified subset of the available jets. High results in all up-firing jets firing continuously in a Z translation. And low enables a special technique that accomplishes a Z translation using the forward- and aft-firing RCS jets in order to not fire directly toward a target (avoiding plume impingement and contamination of a target payload).

The OMS thrust vector control DAP is available when an OMS burn is executed in major mode 202 (maneuver execute) through the orbit mnvr exec display. The TVC DAP uses the guidance-generated velocity requirements and converts these into the appropriate OMS gimbal commands to achieve this target, assuming auto is selected on the orbital DAP panel. It generates the OMS fire commands; the OMS shutdown commands; and, if necessary due to OMS engine failure, required RCS commands to maintain attitude control. If manual is selected, the TVC DAP uses inputs from the RHC to control attitude during the burn.

As with the transition DAP, there are many subtleties in the operation of the orbital DAP.

There are 24 orbital DAP push button light indicators on panels C3 and A6. Assuming no electrical, computer bus (MDM) or hardware failures that could affect the operation of the push button light indicators, inputs made to one panel will be reflected in the configuration of the other panel. All of the push button light indicators are active in the orbital DAP, but only a subset of these are operational in the transition DAP or when the backup flight system is engaged. None of the push button light indicators are operational during ascent or entry. As with other aft flight deck controls, aft panel A6 push button light indicators are only operational while the vehicle is on orbit.

The orbital DAP select, control, RCS jets, and manual mode translation and rotation push button light indicators are illuminated by flight control when that mode is implemented in the flight control system in the transition or orbital DAP.

The orbital DAP select A or B push button light indicator selects the values the DAP will use from the DAP configuration parameter limits CRT display software loads. The values of attitude dead band, rate dead band and vehicle change in rotation rate are a function of the DAP selection (A or B) and RCS jet selection ( norm or vern ). The select A push button light indicator is illuminated when depressed and select B is extinguished. If select B is depressed, select B is illuminated and select A is extinguished.

When the automatic mode is selected by depressing the auto push button light indicator, the indicator is illuminated and the man push button light indicator is extinguished.

Automatic rotation commands are supplied by the universal pointing processor. The universal pointing processor, through the operational sequence display, provides three-axis automatic maneuver, tracking local vertical/local horizontal about any body vector, rotating about any body vector at the DAP discrete rate, and stopping any of these options and commanding attitude hold. The parameters of these maneuvers are displayed in current attitude, required attitude, attitude error and body rates. Either total or DAP attitude errors may be selected for display on the attitude display indicator error needles.

The automatic maneuver option is used to calculate a commanded vehicle attitude and angular rate or to hold a vehicle attitude. The desired inertial commanded and rotation attitude is input into the operational sequence display in pitch, yaw and roll. When the maneuver option is selected, universal pointing sends the required attitude increment and body rate to flight control, and flight control performs the maneuver when the DAP is in automatic.

The automatic rotation option calculates a rotation about a desired body axis. This option is used for passive thermal control, also known as barbecue. Pitch and yaw body components of the desired rotational axis are first input. The orbiter is maneuvered automatically or manually so that the rotational axis is oriented properly in inertial space. When the rotational option is selected, universal pointing will calculate the required body attitude and send it to flight control. Flight control performs the maneuver if the DAP is in automatic.

The LVLH automatic option calculates the attitude necessary to maintain LVLH with a desired body orientation. LVLH, which is available only on orbit in automatic or manual, calculates the attitude necessary to track the center of the Earth with a given body vector. First, pitch and yaw body components of the desired pointing vector are input. Then omicron roll angle about the body vector is input. When the LVLH option is selected, the required LVLH attitude and its associated maneuver are calculated and sent to flight control. When the LVLH attitude is reached, universal pointing will calculate an attitude and send it to flight control. Flight control performs the maneuver if the DAP is in automatic.

The automatic stop/attitude hold option cancels universal pointing processing of the automatic maneuver, rotation or LVLH options. When the stop/attitude hold option is selected, universal pointing will cancel the processing of the maneuver options and send the current attitude to flight control. When flight control is in automatic, attitude hold will be initiated about the current attitude.

The nine rotation push button light indicators on panel C3 or A6 are meaningless when auto is selected. The transition or orbital DAP can be switched to manual by depressing the man push button light indicator or by positioning the RHC out of detent while operating in the automatic mode. In the manual mode, the man light is illuminated and the auto light is extinguished. When the manual mode is selected, the nine rotation roll, pitch and yaw push button light indicators determine the kind of control the RHC will provide.

There are three RCS rotation submodes available in the orbital DAP: automatic, manual or LVLH. LVLH is not available in the transition DAP; thus the choice in transition DAP is automatic or manual. Within each of these submodes are submodes that depend on conditions, such as the DAP push button light indicator configuration and the RHC state. Manual RCS translation modes are independent.

Depressing the RCS jets norm push button enables the primary RCS jets for rotational and translational firings and disables the vernier RCS jets. The RCS jets norm light is illuminated and the RCS jets vern light is extinguished.

Depressing the RCS jets vern push button enables the vernier RCS jets for rotational (not translational) RCS firings and disables the primary RCS jets. The RCS jets vern light is illuminated and the RCS jets norm light is extinguished.

The manual mode rotation, roll, pitch and yaw push button light indicators are used on orbit and during transition DAP operations. The RCS rotation is selected on an axis-by-axis basis. For example, pitch could be in discrete rate, yaw in acceleration and roll in pulse.

Depressing the rotation disc rate push button for an axis causes the appropriate primary or vernier RCS jets to fire to attain a predetermined rotational rate in that axis while the RHC is out of detent. When the RHC is returned to detent, the rate is nulled and attitude hold is re-established. When depressed, the disc rate push button light indicator is illuminated and the accel or pulse indicators are extinguished for that axis. Rotation units are in degrees per second.

Depressing the rotation accel push button for an axis causes the primary or vernier jets to fire when the RHC is out of detent, producing a moment with the same sense as the RHC deflection in that axis. The jets remain on as long as the RHC is out of detent and shut off when the RHC is returned to detent, allowing attitude to drift freely. When depressed, the accel push button light indicator is illuminated and the disc rate or pulse push button light indicators are extinguished for that axis. The accel push button light indicator is not functional in the transition DAP, but the same effect can be achieved by taking the RHC beyond the softstop in any mode.

Depressing the rotation pulse push button for an axis causes the primary or vernier jets to fire for preset increments in response to each deflection of the RHC in that axis. No further firing occurs until the RHC is returned to detent and is again deflected, allowing attitude to drift. When depressed, the pulse push button light indicator is illuminated and the disc rate or accel push button light indicators are extinguished for that axis. The units for rotation pulse are in degrees per second, and the resulting vehicle rate will be a product of the pulse size and number of pulses.

Depressing the LVLH push button light indicator places the DAP in a manual mode. With rotation in discrete rate for all three axes and the RHC in detent, LVLH hold will be maintained and the LVLH push button light indicator will remain on.

The manual mode translation push button light indicators are used on orbit, and mixed modes are permitted on an axis-by-axis basis. Except for the case of the automatic minus Z external tank separation firing, all RCS translations must be performed manually. In transition DAP, none of the translation push button light indicators will be illuminated. Depressing the translation high push button light indicator causes all nine up-firing primary plus Z jets to be fired as long as the THC is held out of detent in that axis. For a minus Z translation, the high Z mode functions identically to the normal Z and low Z. The Z high push button light indicators are illuminated and the low Z , Z norm and Z pulse push button light indicators are extinguished.

Depressing the low Z push button inhibits all up-firing jets in order to prevent plume damage to payloads or injury to extra-vehicular activity crew members. If a plus Z translation is requested, plus X and minus X jets are fired simultaneously, producing a downward translation because the X jets are oriented in such a way that they have small plus Z thrust components. Minus Z uses the six down-firing jets with the selected high or norm Z. The jets continue to fire as long as the THC is out of detent. The low Z push button light indicators are illuminated and the Z high push button light indicator is extinguished.

Depressing the norm push button light indicator for an axis causes the appropriate primary jets in that axis to be fired for as long as the THC is out of detent. This mode is used for most RCS translations and propellant dumps. The norm push button light indicators are illuminated and the pulse and Z high indicators are extinguished.

Depressing the pulse push button light indicator for an axis causes the appropriate primary jets to fire for a preset increment in response to each deflection of the THC. The firing duration is a function of pulse size. No further firing occurs until the THC is returned to detent and is again deflected. The pulse push button light indicators are illuminated and the norm or Z high indicators are extinguished.