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Abort to Orbit

The ATO would be used to boost the orbiter to a safe orbital altitude if performance has been lost and it is impossible to reach the planned altitude. If a main engine fails in a region that results in a MECO underspeed, the Mission Control Center would determine that an abort mode is necessary and would inform the crew. In addition, the crew can verify the nominal OMS burn solutions on the OMS-1 maneuver execute display and burn them instead of the nominal targets. Similarly, they would load the ATO OMS-2 burn targets and use them for OMS-2. This results in the orbiter being placed in a circular orbit.

Another reason for an ATO is loss of OMS performance due to various failures, such as loss of two OMS engines, loss of one OMS propellant tank or loss of main bus A and B electrical power. In these cases, the OMS-1 burn would be delayed about 10 minutes, and the ATO OMS-1 targets would be used. This would result in an approximately 105- by 80-nautical-mile (120- by 92-statute-mile) orbit, which is considered safe for 24 hours. Thus, an OMS-2 burn would not be necessary immediately. The delayed ATO would be accomplished by loading the delayed ATO targets in the OMS-1 maneuver execute display and performing only the OMS-1 burn. If an underspeed existed at MECO so that an ATO or worse abort would be required, the OMS-1 burn must not be delayed. An underspeed would result in the apogee moving close to the post-MECO orbiter position. Delaying OMS-1 would cause the burn to be performed after apogee, which requires increased delta velocity and, thus, more propellant.

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