Sequential Events Control Subsystem
The Sequential Events Control Subsystem (SECS) is responsible for controlling events which must happen in a pre-programmed sequence, for example parachute deployment during landing, or jettisoning the Service Module.
Launch Escape System
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The Launch Escape System (LES) is responsible for returning the crew safely to Earth in the event of an abort during the launch. If the Launch Escape Tower is still attached to the Command Module then the escape motor in the tower will be fired to pull the CM away from the Saturn stack, and then after the CM is in a stable situation the Earth Landing System will take over for an automated landing.
Earth Landing System
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The Earth Landing System (ELS) is responsible for the sequential events required to land the crew safely on Earth, for example jettisoning the apex cover and deploying the parachutes.
Since these events are irreversible and could be fatal if they occurred at the wrong time or didn't occur at the correct time, NASA went to a significant amount of trouble to ensure that the ELS couldn't be activated accidentally and would operate reliably when it was activated.
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