The TAL abort
mode was developed to improve the options available when a main
engine fails after the last RTLS opportunity but before the first
time that an AOA can be accomplished with only two main engines
or when a major orbiter system failure, for example, a large cabin
pressure leak or cooling system failure, occurs after the last
RTLS opportunity, making it imperative to land as quickly as possible.
In a TAL
abort, the vehicle continues on a ballistic trajectory across
the Atlantic Ocean to land at a predetermined runway. Landing
occurs approximately 45 minutes after launch. The landing site
is selected near the nominal ascent ground track of the orbiter
in order to make the most efficient use of space shuttle main
engine propellant. The landing site also must have the necessary
runway length, weather conditions and U.S. State Department approval.
Currently, the three landing sites that have been identified for
a due east launch are Moron, Spain; Banjul, The Gambia; and Ben
the TAL abort mode, the crew must place the abort rotary switch
in the TAL/AOA position and depress the abort push button before
main engine cutoff. (Depressing it after main engine cutoff selects
the AOA abort mode.) The TAL abort mode begins sending commands
to steer the vehicle toward the plane of the landing site. It
also rolls the vehicle heads up before main engine cutoff and
sends commands to begin an orbital maneuvering system propellant
dump (by burning the propellants through the orbital maneuvering
system engines and the reaction control system engines). This
dump is necessary to increase vehicle performance (by decreasing
weight), to place the center of gravity in the proper place for
vehicle control, and to decrease the vehicle's landing weight.
TAL is handled
like a nominal entry.