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.
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