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Overview
Selection of an ascent abort mode may become necessary if there
is a failure that affects vehicle
performance, such as the failure of a main engine or an orbital
maneuvering system failure. Other
failures dictating early termination of a flight, such as a
cabin leak, might require the selection of an
abort mode.
There are two basic types of ascent abort modes for space
shuttle missions: intact aborts and
contingency aborts. An intact abort would provide a safe return
of the orbiter to a planned landing
site, while a contingency abort is designed to permit the crew
to survive following more severefailures when an intact abort is
not possible. A contingency abort would generally result in a
ditch
operation.
There are four types of intact abort modes: return to launch
site, transatlantic landing, abort to orbit
and abort once around.
The ATO mode is designed to allow the vehicle to achieve a
temporary orbit that is lower than the
nominal orbit. This mode requires less performance and permits
time to evaluate problems and to
choose either an early deorbit burn or an OMS maneuver to raise
the orbit and continue the missio
The AOA would permit the vehicle to fly once around the Earth
and make a normal entry and
anding. This mode generally involves two OMS burns, with the
second burn being a deorbit
maneuver. There are two types of AOA trajectories: a normal AOA
and a shallow AOA (which is
considered only for contingency aborts). The entry trajectory
for the normal AOA is similar to the
nominal entry trajectory. The shallow AOA results in a flatter
entry trajectory, which is less desirabl
han that of the normal AOA but uses less propellant in the OMS
maneuvers. The shallow entry
rajectory is less desirable because it exposes the vehicle to a
longer period of atmospheric heatin
and to less predictable aerodynamic drag forces.
The TLA abort is designed to permit an intact landing on the
other side of the Atlantic Ocean. This
mode would result in a ballistic trajectory, which does not
require an OMS maneuver.
The RTLS would require flying downrange to dissipate propellant
and turning around under power
return directly to a landing at or near the launch site.
There is a definite order of preference for the various abort
modes. The type of failure and the time
of the failure would determine which type of abort is selected.
In cases where performance loss is
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he only factor, the preferred modes, in order, would be ATO,
AOA, TLA and RTLS. The mode
chosen would be the highest one that could be completed with the
remaining vehicle performance.
For certain support system failures, such as cabin leaks or
vehicle cooling problems, the preferred
mode might be the one that would end the mission most quickly.
In these cases, TLA or RTLS mig
be preferable to AOA or ATO. A contingency abort would never be
chosen if another abort option
existed.
The Mission Control Center is primarily responsible for calling
aborts, since the controllers have
more precise knowledge of the orbiter's state vector (through
the use of sophisticated tracking
equipment and ground computer resources) than the crew can
obtain from the onboard navigation
system. Before MECO, Mission Control periodically calls the crew
to tell them which abort mode is
(or is not) available. If ground communications are lost, the
flight crew has onboard methods, such
as cue cards, dedicated displays and GN&C; CRT display
information, from which to determine the
current abort region.
The abort mode selected would depend on the cause and timing of
the failure causing the abort an
on which mode is safest or improves the chances for mission
success. If the emergency is a main
engine failure, the flight crew and MCC would select the best
option available at the time of thefailure. If the problem is a
system failure that jeopardizes the vehicle, the abort mode that
would
result in the earliest vehicle landing would be chosen. RTLS and
TLA would be the quickest option
(35 minutes), whereas an AOA would require approximately 90
minutes. Which of these is selected
would depend on the time of the failure with three good main
engines.
The flight crew would select the abort mode by positioning the
abort mode switch on panel F6 and
depressing the abort push button on the panel. This switch is a
rotary model with off , RTLS , TAL-
AOA-S (shallow) and ATO positions.
Curator: Kim Dismukes | Responsible NASA Official: John Ira
Petty | Updated: 04/07/2002
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