![]() The abort modes covered a wide range of potential problems, but the most commonly expected problem was a main engine failure, causing the vehicle to have insufficient thrust to achieve its planned orbit. The choice of abort mode depended on how urgent the situation was and what emergency landing site could be reached. There were five abort modes available during ascent, divided into the categories of intact aborts and contingency aborts. If an event requiring an abort happened after SRB ignition, it was not possible to begin the abort until after SRB burnout and separation, about two minutes after launch. Once the shuttle's SRBs were ignited, the vehicle was committed to liftoff. This was called a "redundant set launch sequencer (RSLS) abort", and occurred five times: STS-41-D, STS-51-F, STS-55, STS-51, and STS-68. If an anomaly was detected, the engines would be shut down automatically and the countdown terminated before ignition of the solid rocket boosters (SRBs) at T = 0 seconds. The three Space Shuttle main engines (SSMEs) were ignited roughly 6.6 seconds before liftoff, and computers monitored their performance as they increased thrust. Crew bailout was still possible in some situations in which the orbiter could not land on a runway. An abort during ascent that would result in the orbiter returning to a runway or to an orbit lower than planned was called an "intact abort", while an abort in which the orbiter would be unable to reach a runway, or any abort involving the failure of more than one main engine, was called a "contingency abort". A pad abort occurred after ignition of the shuttle's main engines but prior to liftoff. Space Shuttle abort modes were procedures by which the nominal launch of the NASA Space Shuttle could be terminated. NASA Space Shuttle safety procedures Abort modes available depending on engine failure time.
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