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Abort Once Around

The AOA abort mode would be used when vehicle performance has been lost to such an extent that either it is impossible to achieve a viable orbit or not enough OMS propellant is available to accomplish the OMS-1, OMS-2 and deorbit burns. AOA would also be used in cases in which a major system problem (cabin leak, loss of cooling) made it necessary to land quickly. In this abort, one OMS burn would be made to adjust the post-MECO orbit so that a second OMS burn would cause the vehicle to deorbit and land at the AOA landing site (Northrup, Edwards Air Force Base or Kennedy Space Center.. Thus, in an AOA, the orbiter would circle the Earth once and land approximately 90 minutes after lift-off.

Several options are available to perform an AOA. Selection of the OMS-1 targets would be based on whether the abort were caused by a MECO underspeed, a system problem or an OMS/RCS performance problem. Selection of the OMS-2 targets would depend on whether a MECO underspeed existed and its magnitude. (The AOA OMS-2 burn is really a deorbit burn.) One set of targets would result in a steeper trajectory than would the other as the vehicle enters the atmosphere (entry interface); thus, this trajectory is referred to as a steep AOA. This is a more normal trajectory and stays well within the vehicle's thermal limits after it penetrates the atmosphere. It would require more delta velocity and consequently more propellant for the deorbit burn. Thus, if the MECO underspeed were severe or if both OMS helium tanks had failed, the shallow AOA targets would be used, resulting in a more shallow trajectory at entry interface and placing the vehicle closer to the skip-out boundary and its thermal limits.

The flight crew would determine that an AOA is required by Mission Control Center call and by checking the OMS-1 target solution against the OMS targeting cue card. Depending on the kind of AOA required, the crew would load the required OMS-1 targets and execute the burn. They would then position the software mode to OPS 3 and load the appropriate OMS-2 (deorbit) targets. After the burn is executed, the flight crew would fly to a landing at the preplanned site, much as they would for a nominal entry.

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