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.
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