The three Space Shuttle main engines were ignited roughly 6.6 seconds before liftoff, and computers monitored their performance as they built up 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 happened five times: STS-41-D, STS-51-F, STS-51, STS-55, and STS-68.