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With the Saturn V, there were two critical moments during launch that I am curious about:

  1. The vehicle COMMITs to liftoff
  2. The vehicle LIFTS OFF

LIFTOFF is fairly obvious: the booster has risen some small distance above the launch pad, and is basically on its way (but hasn't yet cleared the launch tower).

COMMIT was the point, a moment before LIFTOFF, at which the vehicle was committed to lift-off. In other words, no matter what else happened, it either rose from the launch pad, or things crashed and burned (and the LES likely saw use). Turning off the engines no longer caused it to settle back nicely on the pad with just a few tons less fuel than it had a few seconds earlier.

So much for pretext for my actual questions, which I believe to be highly related:

  • What specific event or condition defined the COMMIT state of the vehicle? In other words, what caused the vehicle to become committed to lift-off, as opposed to not being committed to lift-off a moment earlier?
  • What specific events, possibly limited to those visible to an ideal external observer (in the physics sense), occured in between COMMIT and LIFTOFF?
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This article might offer some clarity. The Saturn V was held down to the pad until T=0. The engines started up several seconds before. The engines took a few seconds to build up to and stabilize at lift-off thrust. During that time, if an anomaly is detected, the engines could be shut down, leaving the vehicle still safely anchored to the pad. Once the hold-downs are released, the option to shut down on the pad is gone because the vehicle will lift off, hence "launch commit". The lift-off call is a confirmation that lift-off has occurred.

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    $\begingroup$ @MichaelKjörling Here is a wikipedia article which also describes the launch sequence. The "commit" call is apparently how it was announced that hold downs were released - because the vehicle was committed to launch. $\endgroup$
    – Anthony X
    Aug 14, 2016 at 15:38
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    $\begingroup$ So, COMMIT means "lifting off" and LIFTOFF means "yep, can confirm, that worked, we've got some separation here now boys". $\endgroup$
    – Lightness Races in Orbit
    Aug 14, 2016 at 19:55
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    $\begingroup$ The practical result being before COMMIT an abort of an anomaly not resulting in RUD would involve shutting the engines and the astronauts climbing down the ladders to the command center. After COMMIT, the abort would inevitably involve activation of LES and landing a couple kilometers from the launch site. $\endgroup$
    – SF.
    Feb 21, 2019 at 10:42
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All five F-1 engines' chamber pressures must be at 100% (985 PSIA) before commit to launch. Four hold down arms have a broaching system incorporated in them to gradually transfer thrust loads to vehicle initial acceleration at liftoff to prevent POGO. Once hold down arms begin travel launch is committed, not stoppable. Launch escape tower is now the only astronaut means of safe escape in the event of vehicle failure until just after second stage ignition.

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    $\begingroup$ Welcome to Space Exploration! Nice answer, to improve it further, please add some source link :) $\endgroup$
    – BlueCoder
    Feb 21, 2019 at 7:46

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