HSF - The Shuttle7.pdf

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Deorbit

Deorbit 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.

n returning home, the orbiter must be sufficiently decelerated by an OMS retrograde burn that whe

t 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 thevehicle's trajectory. For this maneuver, the orbiter is positioned in a tail-first thrusting attitude. Deor

hrusting 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 contro

or, in the event that both OMS engines malfunction, to plus X aft RCS jets.

Approximately 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 Freo

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 b

he 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 deorb

he 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

he 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

hrough the forward RCS engines, if required, and the two remaining APUs are started and remainoperating through entry and landing rollout. Thermal conditioning of the spacecraft's hydraulic fluid

system is also begun, if required.

The deorbit 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.

The deorbit and entry flight software is called OPS 3. Major mode 301 is a deorbit coast mode in

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which deorbit targets can be loaded, although the burn cannot be executed in this mode. This mod

s 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.

During the 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

hreshold. 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

MU 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

anding.

Another feature available during this phase is the software's computation of a statistical estimate o

he error in the state vector propagation, which is used later in flight when external sensor data areavailable. Also, in this phase, it is possible for the crew or the Mission Control Center to input a del

state vector to correct navigation.

Guidance during 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,

ncluding 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

anding 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 themusing 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

entry.

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.

Flight control during the deorbit phase is similar to that used during orbit insertion-i.e., the transition

DAP is once again in effect.

The flight crew interfaces with the guidance, navigation and flight control software during the deorb

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 maneuv

coast; whereas in major mode 302, it is deorbit maneuver execute. This display is identical in forma

o 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



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o 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.

Another CRT 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.

The flight crew's task during this phase includes entering the correct burn targets in the deorbitmaneuver execute display and maneuvering to burn attitude, either automatically or manually using

he 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.

n 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 RC

propellant remains for entry and that the orbiter center of gravity stays within limits.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002

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HSF - The Shuttle7.pdf