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 Guerir, Morocco.
To select 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.