Space Shuttle abort modes were procedures by which the nominal launch of the NASA Space Shuttle could be terminated. A pad abort occurred after ignition of the shuttle's main engines but prior to liftoff. 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". Crew bailout was still possible in some situations in which the orbiter could not land on a runway. Redundant set launch sequencer abort The three Space Shuttle main engines (SSMEs) were ignited roughly 6.6 seconds before liftoff, and computers monitored their performance as they increased thrust. 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. 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. Ascent abort modes Once the shuttle's SRBs were ignited, the vehicle was committed to liftoff. 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. There were five abort modes available during ascent, divided into the categories of intact aborts and contingency aborts. The choice of abort mode depended on how urgent the situation was and what emergency landing site could be reached. 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. Other possible failures not involving the engines but necessitating an abort included a multiple auxiliary power unit (APU) failure, a progressive hydraulic failure, a cabin leak, and an external tank leak. Intact abort modes There were four intact abort modes for the Space Shuttle. Intact aborts were designed to provide a safe return of the orbiter to a planned landing site or to a lower orbit than that which had been planned for the mission. Return to launch site Return to launch site (RTLS) was the first abort mode available and could be selected just after SRB jettison. The