Return To Launch Site (RTLS) is one of four intact abort
modes that NASA
has planned for the Space Shuttle
in the event of a failure during launch. It is the least 'favored' in that it involves the most risk; however, until the Shuttle has reached an altitude and velocity where it would be able to make a Trans-Atlantic Abort Landing
(TAL), it may be the only option.
If a catastrophic failure occurs during the early First Stage Ascent, i.e. before SRB staging, the shuttle may not be able to continue. Examples might include catastrophic loss of orbiter cabin pressure, or failure of the orbiter cooling systems (which are critical for the survival of onboard systems and the crew outside the atmosphere, as well as during atmospheric heating induced by launch/re-entry). In this case, a Return To Launch Site is necessary. Abort modes are triggered by setting a rotary switch on the Orbiter main control panel, accessible to both Commander and Pilot positions. As planned, the abort would be initiated by the crew upon instructions from the Mission Control Center at Johnson Space Center.
In any case, if this procedure becomes necessary, it cannot be completed until the Solid Rocket Boosters have separated, since the SRBs cannot be shut down once ignited. The Shuttle would continue boosting with its main engines after separation until there was just enough fuel remaining in the external tank to return the Shuttle to its launch site (likely Launch Complex 39-A at Kennedy Space Center, where there is a runway the Shuttle typically uses for landings after missions).
Once this point is reached, the Shuttle begins a turn with a 5 degree per second slew rate until the orbiter is aligned to return to its launch site. It continues climbing until conditions are correct for external tank separation, at which point it drops the tank and heads for home. OMS fuel is dumped and/or moved around the orbiter to rebalance it for a landing. It then glides back to the KSC runway and lands there.
This abort mode involves the most significant deviation from normal procedures in order to safely land the orbiter; thus, it is the least 'preferred' mode. If the orbiter has reached a point in its ascent where possible, it will preferentially try a Trans-Atlantic Abort Landing.