Post-Challenger Abort Enhancements
Before the Challenger disaster during STS-51-L, very limited ascent abort options existed. Failure of only a single SSME was survivable prior to about 350 seconds into the ascent. Two or three failed SSMEs prior to that point would mean loss of crew and vehicle (LOCV), since no bailout option existed. Two or three failed SSMEs while the SRBs were firing would probably have overstressed the struts attaching the orbiter to the external tank, causing vehicle breakup. For that reason, a Return To Launch Site (RTLS) abort was not possible in the event of two or three failed SSMEs. Studies showed an ocean ditching was not survivable. Furthermore, the loss of a second or third SSME at almost any time during an RTLS abort would have caused a LOCV.
After the loss of Challenger in STS-51-L, numerous abort enhancements were added. With those enhancements, the loss of two SSMEs was now survivable for the crew throughout the entire ascent, and the vehicle could survive and land for large portions of the ascent. Loss of three SSMEs was survivable for the crew for most of the ascent, although survival in the event of three failed SSMEs before T+90 seconds is questionable. However, it was conceivable that failure of three SSMEs just after liftoff might be survivable, since the SRBs provided enough thrust and steering authority to continue the ascent until a bailout or RTLS. The struts attaching the orbiter to the external tank were strengthened to better endure a multiple SSME failure.
A particular significant enhancement was bailout capability. This is not ejection as with a fighter plane, but an Inflight Crew Escape System (ICES). The vehicle was put in a stable glide on autopilot, the hatch was blown, and the crew slid out a pole to clear the orbiter's left wing. They would then parachute to earth or the sea. While this may at first appear only usable under rare conditions, there were many failure modes where reaching an emergency landing site was not possible yet the vehicle was still intact and under control. Before the Challenger disaster, this almost happened on STS-51-F, when a single SSME failed at about T+345 seconds. The orbiter in that case was also Challenger. A second SSME almost failed due to a spurious temperature reading; fortunately the engine shutdown was inhibited by a quick-thinking flight controller. If the second SSME failed within about 69 seconds of the first, there would have been insufficient energy to cross the Atlantic. Without bailout capability the entire crew would be lost. After the loss of Challenger, those types of failures were made survivable. To facilitate high altitude bailouts, the crew began wearing Advanced Crew Escape Suits during ascent and descent. Before the Challenger disaster, crews for operational missions wore only fabric flight suits.
Another post-Challenger enhancement was the addition of East Coast Abort Landings (ECAL). High-inclination launches (including all ISS missions) were now able to reach an emergency runway on the East Coast of the United States under certain conditions.
An ECAL abort was similar to RTLS, but instead of landing at the Kennedy Space Center, the orbiter would attempt to land at another site along the east coast of North America. Various emergency landing sites extended from South Carolina and Bermuda up into Newfoundland, Canada. ECAL was a contingency abort that was less desirable than an intact abort, primarily because there was so little time to choose the landing site and prepare for the orbiter's arrival. The ECAL emergency sites were not as well equipped to accommodate an orbiter landing as those prepared for an RTLS abort.
Numerous other abort refinements were added, mainly involving improved software for managing vehicle energy in various abort scenarios. These enabled a greater chance of reaching an emergency runway for various SSME failure scenarios.
Read more about this topic: Space Shuttle Abort Modes