Selection of an ascent abort mode may become necessary if there
is a failure that affects vehicle performance, such as the failure
of a Space Shuttle main engine or an orbital maneuvering system.
Other failures requiring 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. Intact aborts are designed to provide a safe return
of the orbiter to a planned landing site. Contingency aborts are
designed to permit flight crew survival following more severe failures
when an intact abort is not possible. A contingency abort would
generally result in a ditch operation.
There are four types of intact aborts:
- Abort To Orbit
- 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 allows time to evaluate problems and then
choose either an early deorbit maneuver or an orbital maneuvering
system thrusting maneuver to raise the orbit and continue the
- Abort Once Around
- The AOA is designed to allow the vehicle to fly once around
the Earth and make a normal entry and landing. This mode generally
involves two orbital maneuvering system thrusting sequences, with
the second sequence being a deorbit maneuver. The entry sequence
would be similar to a normal entry.
- Transatlantic Landing
- The TAL mode is designed to permit an intact landing on the
other side of the Atlantic Ocean. This mode results in a ballistic
trajectory, which does not require an orbital maneuvering system
- Return to Launch Site
- The RTLS mode involves flying downrange to dissipate propellant
and then turning around under power to 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 determine which
type of abort is selected. In cases where performance loss is the
only factor, the preferred modes would be ATO, AOA, TAL and RTLS,
in that order. The mode chosen is the highest one that can be completed
with the remaining vehicle performance. In the case of some support
system failures, such as cabin leaks or vehicle cooling problems,
the preferred mode might be the one that will end the mission most
quickly. In these cases, TAL or RTLS might be preferable to AOA
or ATO. A contingency abort is never chosen if another abort option
The Mission Control Center-Houston is prime for calling these aborts
because it has a more precise knowledge of the orbiter's position
than the crew can obtain from onboard systems. Before main engine
cutoff, Mission Control makes periodic calls to 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 display information, to determine the current
Which abort mode is selected depends on the cause and timing of
the failure causing the abort and which mode is safest or improves
mission success. If the problem is a Space Shuttle main engine failure,
the flight crew and Mission Control Center select the best option
available at the time a space shuttle main engine fails.
If the problem is a system failure that jeopardizes the vehicle,
the fastest abort mode that results in the earliest vehicle landing
is chosen. RTLS and TAL are the quickest options (35 minutes), whereas
an AOA requires approximately 90 minutes. Which of these is elected
depends on the time of the failure with three good Space Shuttle
The flight crew selects the abort mode by positioning an abort
mode switch and depressing an abort push button.
Contingency aborts are caused by loss of more than one main engine
or failures in other systems. Loss of one main engine while another
is stuck at a low thrust setting may also necessitate a contingency
abort. Such an abort would maintain orbiter integrity for in-flight
crew escape if a landing cannot be achieved at a suitable landing
Contingency aborts due to system failures other than those involving
the main engines would normally result in an intact recovery of
vehicle and crew. Loss of more than one main engine may, depending
on engine failure times, result in a safe runway landing. However,
in most three-engine-out cases during ascent, the orbiter would
have to be ditched. The in-flight crew escape system would be used
before ditching the orbiter.