| Planning
for each space shuttle mission includes provisions for an unscheduled
landing at contingency landing sites in the U.S. and overseas.
Several unscheduled landing scenarios are possible, ranging from
adverse weather conditions at the primary and secondary landing
sites to mechanical problems during the ascent and mission phases
that would require emergency return of the orbiter and its crew.
The Transoceanic Abort Landing
(TAL) is one mode of unscheduled landing. The orbiter would
have to make an unscheduled landing if one or more of its three
main engines failed during ascent into orbit, or if a failure
of a major orbiter system, such as the cooling or cabin pressurization
systems, precluded satisfactory continuation of the mission.
Several unscheduled landing scenarios
are possible with abort modes available that include: Return
to Launch Site (RTLS); East Coast Abort Landing (ECAL) Site;
Transoceanic Abort Landing (TAL); Abort Once Around (AOA); and
Abort to Orbit (ATO). The abort mode would depend on when in
the ascent phase an abort became necessary.
The TAL abort mode was developed
to improve the options available if failure occurred after the
last opportunity for a safe Return To Launch Site (RTLS) or
East Coast Abort Landing (ECAL), but before the Abort Once Around
(AOA) option became available. A TAL would be declared between
roughly T+2:30 minutes (liftoff plus 2 minutes, 30 seconds)
and Main Engine Cutoff (MECO), about T+8:30 minutes, with the
exact time depending on the payload and mission profile.
 A
TAL would be made at one of three designated sites, one in France and two in Spain: Istres Air Base, France; Zaragoza Air Base, Spain; or Moron Air
Base, Spain.
Each TAL site is covered by a separate
international agreement. TAL sites are referred to as augmented
sites because they are equipped with shuttle-unique landing
aids and are staffed with NASA, contractor and Department of
Defense personnel during every launch.
Space shuttles are launched eastward
over the Atlantic Ocean from Kennedy Space Center in Florida for insertion into equatorial
orbits. Depending on mission requirements, an orbiter follows
an orbital insertion inclination between 28.5 degrees (low)
and 57.0 degrees (high) to the equator. All space shuttle launches to the International Space Station use an inclination of 51.6 degrees. The lower inclination
launches allowed for a higher maximum payload weight but are no longer used.
High or low inclination launches
require different contingency landing sites, with landing sites staffed to ensure there is acceptable weather
for a safe landing at a TAL site.
During a TAL abort, the orbiter
continues on a trajectory across the Atlantic to a predetermined
runway at one of the TAL sites. The sites NASA has designated
as TAL sites have been chosen in part because they are near
the nominal ascent ground track of the orbiter, which would
allow the most efficient use of main engine propellant and cross-range
steering capability.
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