The first 42 seconds of the flight up to a height of about 3 kilometres was flown in abort mode one-alpha, meaning that the crew and equipment were poised to react in a certain way in the event of a launch vehicle failure. The escape tower above the command module would fire its large solid-fuelled motor, quickly pulling the CM up and away from the service module and the rest of the stack below. A smaller sideways-firing rocket motor at the top of the tower, the pitch control motor, would steer the CM eastward out over the ocean to ensure that it would not subsequently descend into the conflagration caused by the destruction of its launch vehicle. Safely clear, the CM would dump its manoeuvring fuel (nasty, toxic, highly corrosive stuff, which would otherwise present a danger to the recovery forces), jettison the escape system and the forward heatshield, then finally deploy its parachutes and make a normal landing in the ocean.
At the centre of the abort decision was the commander. From the launch pad to orbit, he closely monitored various lights and displays on the panel that supplied him with whatever information was relevant to making that decision. All the equipment that fed these displays, and sensed whether an emergency was imminent, was called the emergency detection system (EDS). It was decided that he would not react to a single cue, lest it be spurious, but if two cues from the EDS called for an abort, this was enough information for him to twist the T-handle in his left hand counterclockwise, activating the appropriate sequence to leave a malfunctioning launch vehicle behind and get to safety. There was also a set of abort conditions that could initiate an automatic abort. The idea of ending a half-billion-dollar mission on the behest of a few bits of hardware necessitated detection systems that were triple-redundant and were required to 'vote' electronically for an abort. The automatic portion of the EDS was switched off once the rocket was out of the thickest part of the atmosphere where situations could not develop so rapidly.
The EDS was responsible for lighting a cluster of indicators that showed whether each engine was running at full thrust, whether the rocket was veering too fast and whether the Saturn's guidance system still knew which way was up. In the latter case, from Apollo 11 onwards, if the commander saw that the Saturn was incapable of guiding itself, he had the option of twisting the T-handle clockwise to pass control of the entire rocket to the spacecraft, and if that was also failing, he could manually guide it to orbit. Another prominent light informed him when launch control in Florida, or a range safety officer, also in Florida, or mission control in Houston, believed an abort was advised. A gauge that normally showed the state of the spacecraft's main engine for the rest of the mission was pressed into service by the EDS to show the rocket's angle-of-attack; that is, it was moving cleanly through the air and not slipping sideways - a condition that could impose such aerodynamic stress as to cause the break-up of the vehicle's structure.
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