Return
to Launch Site Abort Mode
The RTLS abort
mode is designed to allow the return of the orbiter, crew, and payload
to the launch site, Kennedy Space Center. approximately 25 minutes
after lift-off. The RTLS profile is designed to accommodate the
loss of thrust from one space shuttle main engine between liftoff
and approximately four minutes 20 seconds, at which time not enough
main propulsion system propellant remains to return to the launch
site.
An RTLS can
be considered to consist of three stages -- a powered stage, during
which the main engines are still thrusting; an ET separation phase;
and the glide phase, during which the orbiter glides to a landing
at the KSC. The powered RTLS phase begins with the crew selection
of the RTLS abort, which is done after SRB separation. The crew
selects the abort mode by positioning the abort rotary switch to
RTLS and depressing the abort push button. The time at which the
RTLS is selected depends on the reason for the abort. For example,
a three-engine RTLS is selected at the last moment, approximately
3 minutes, 34 seconds into the mission; whereas an RTLS chosen due
to an engine out at liftoff is selected at the earliest time, approximately
two minutes 20 seconds into the mission (after SOR separation).
After RTLS
is selected, the vehicle continues downrange to dissipate excess
main propulsion system propellant. The goal is to leave only enough
main propulsion system propellant to be able to turn the vehicle
around, fly back towards KSC and achieve the proper main engine
cutoff conditions so the vehicle can glide to the KSC after external
tank separation. During the downrange phase, a pitch-around maneuver
is initiated (the time depends in part on the time of a main engine
failure) to orient the orbiter/ external tank configuration to a
heads up attitude, pointing toward the launch site. At this time,
the vehicle is still moving away from the launch site, but the main
engines are now thrusting to null the downrange velocity. In addition,
excess orbital maneuvering system and reaction control system propellants
are dumped by continuous orbital maneuvering system and reaction
control system engine thrustings to improve the orbiter weight and
center of gravity for the glide phase and landing.
The vehicle
will reach the desired main engine cutoff point with less than 2
percent excess propellant remaining in the external tank. At main
engine cutoff minus 20 seconds, a pitch-down maneuver (called powered
pitch-down) takes the mated vehicle to the required external tank
separation attitude and pitch rate. After main engine cutoff has
been commanded, the external tank separation sequence begins, including
a reaction control system translation that ensures that the orbiter
does not recontact the external tank and that the orbiter has achieved
the necessary pitch attitude to begin the glide phase of the RTLS.
After the reaction
control system translation maneuver has been completed, the glide
phase of the RTLS begins. From then on, the RTLS is handled similarly
to a normal entry.
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