guidance, navigation and flight control software operates through
the transition DAP to provide maneuvering of the spacecraft to the
OMS deorbit ignition attitude, OMS thrusting commands, OMS engine
gimbaling for thrust vector control and RCS thrusting commands,
in conjunction with use of the DAP similar to that for orbit insertion.
home, the orbiter must be sufficiently decelerated by an OMS retrograde
burn that when it enters the atmosphere, it maintains control and
glides to the landing site. For the nominal end of mission, a retrofiring
of approximately 2.5 minutes is performed at the appropriate point
in the vehicle's trajectory. For this maneuver, the orbiter is positioned
in a tail-first thrusting attitude. Deorbit thrusting is nominally
accomplished with the two OMS engines and must establish the proper
entry velocity and range conditions. It is possible to downmode
to one OMS engine (with RCS roll control) or, in the event that
both OMS engines malfunction, to plus X aft RCS jets.
four hours before deorbit, the environmental control and life support
system radiator bypass/flash evaporator system is checked out, since
the flash evaporator is used to cool the Freon-21 coolant loops
when the ECLSS radiators are deactivated and the payload bay doors
are closed. The high-load evaporator cools the coolant loops until
the ECLSS ammonia boilers are activated by the GPCs at an altitude
of some 140,000 feet. The orbiter IMUs are aligned, the star trackers
are deactivated and the star tracker doors are closed.
About one hour
before deorbit, the crew members take their seats. The spacecraft
is then manually maneuvered using the RCS jets to the deorbit attitude
(retrograde). About 30 minutes before deorbit, the OMS is prepared
for deorbit thrusting. This consists of OMS thrust vector control
gimbal checks, OMS data checks, orbiter vent door closure and single
auxiliary power unit start. At the completion of the single OMS
deorbit burn, the crew manually maneuvers the spacecraft to the
required entry attitude (nose first) using the RCS jets. The propellants
remaining in the forward RCS are dumped through the forward RCS
engines, if required, and the two remaining APUs are started and
remain operating through entry and landing rollout. Thermal conditioning
of the spacecraft's hydraulic fluid system is also begun, if required.
phase of the mission includes the deorbit burn preparations, including
the loading of burn targets and maneuvering to burn attitude; the
execution and monitoring of the burn; reconfiguration after the
burn; and a coast mode until the atmosphere (and dynamic pressure
buildup) is reached at approximately 400,000 feet. This is called
the entry interface.
and entry flight software is called OPS 3. Major mode 301 is a deorbit
coast mode in which deorbit targets can be loaded, although the
burn cannot be executed in this mode. This mode is necessary to
execution of the burn. After the burn, a software transition is
made to another coast mode, major mode 303, which is used to prepare
for penetration into the atmosphere.
deorbit phase, navigation again propagates the orbiter state vector
based upon a drag model or upon inertial measurement unit data if
sensed vehicle accelerations are above a specified threshold. During
OPS 3, navigation maintains and propagates three orbiter state vectors,
each based on a different IMU. From these three state vectors, a
single orbiter state vector is calculated using a mid value selection
process and is passed on for use by guidance, flight control, dedicated
display and CRT display software. Three separate state vectors are
propagated to protect the onboard software from problems resulting
from two IMU data failures. In such a case, once the bad IMU is
detected and deselected, the state vector associated with the remaining
good IMU will not have been polluted. This three-state vector system
is used only during OPS 3 since this phase is most critical with
respect to navigation errors and their effects on vehicle control
and an accurate landing.
available during this phase is the software's computation of a statistical
estimate of the error in the state vector propagation, which is
used later in flight when external sensor data are available. Also,
in this phase, it is possible for the crew or the Mission Control
Center to input a delta state vector to correct navigation.
deorbit is similar to that used in the orbit insertion phase. The
PEG 4 scheme is used to target the deorbit burn and guide the vehicle
during the burn, although the required conditions are different.
The deorbit burn targets are for the proper conditions for entry
interface, including altitude, position with respect to the Earth
and thus the landing site, and satisfaction of certain velocity/flight
path angle constraints. Together these ensure that the vehicle can
glide to the landing site within thermal limits. Deorbit burn targets
are specified before flight for a nominal mission, but it is possible
for the ground to uplink changes or for the flight crew to recompute
them using an onboard hand-held calculator program. It is also possible
to specify that OMS fuel be wasted during the burn (burned out of
plane) to establish an acceptable orbiter center of gravity for
The crew is
responsible for loading these targets on the deorbit maneuver execute
display. Guidance then computes the necessary vehicle attitude to
be established before the burn and displays it to the crew. As in
OPS 1, it is possible to load an external delta-velocity (PEG 7)
target, but this option is not normally used.
during the deorbit phase is similar to that used during orbit insertion-i.e.,
the transition DAP is once again in effect.
crew interfaces with the guidance, navigation and flight control
software during the deorbit phase via CRT display inputs, RHC/THC
maneuvers and ADI monitoring. The major CRT display used is the
deorbit maneuver execute. In major modes 301 and 303, the display
is deorbit maneuver coast; whereas in major mode 302, it is deorbit
maneuver execute. This display is identical in format to the OMS-1
maneuver execute and orbit maneuver execute displays, although there
are some differences in its capabilities between OPS 1, 2 and 3.
It is used to set up and target the OMS burn, to specify fuel to
be wasted during the burn, to display the required burn attitude,
to initiate an automatic maneuver to that attitude and to monitor
the progress of the burn.
display available during the deorbit phase is the horizontal situation.
During deorbit preparation, the crew may verify that the display
is ready for use during entry (correct runway selection, altimeter
setting, etc.), but its other capabilities are not utilized until
after entry interface.
crew's task during this phase includes entering the correct burn
targets in the deorbit maneuver execute display and maneuvering
to burn attitude, either automatically or manually using the RHC.
The burn itself is typically executed in auto, and the flight crew's
task is to monitor the burn's progress in terms of velocities gained
and OMS performance.
In cases of
OMS failures (engine, propellant tank, data path), the flight crew
must be prepared to reconfigure the system to ensure that the burn
can safely continue to completion, that sufficient RCS propellant
remains for entry and that the orbiter center of gravity stays within