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
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.
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.
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.
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.
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
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
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.
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.
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
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.
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
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
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.
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.
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
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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.
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.