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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 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 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 mission.

The AOA would permit the vehicle to fly once around the Earth and make a normal entry and landing. 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 desirable than that of the normal AOA but uses less propellant in the OMS maneuvers. The shallow entry trajectory is less desirable because it exposes the vehicle to a longer period of atmospheric heating 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 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 would determine which type of abort is selected. In cases where performance loss is the 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 might 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 and 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 the failure. 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 options (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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