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Apollo missions mostly launched very short after launch window opening.

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Table made using Apollo by the numbers.

Only Apollo 17 had a long launch delay of 2 hours 40 minutes, Apollo 14 of 40 minutes. Most delays were less than a second.

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According to the Apollo Program Summary Report, there was a failure in the countdown sequencer:

The launch countdown had proceeded smoothly until 30 seconds before the scheduled ignition when a failure in the automatic countdown sequencer occurred and delayed the launch 2 hours 40 minutes.

Section 14.2 of the mission report provides more detail:

The hold was caused when the terminal countdown sequencer failed to command pressurization of the S-IVB liquid oxygen tank. This command closes the liquid oxygen tank vent; opens the liquid oxygen tank pressurization valve; and arms the S-IVB liquid oxygen tank pressurized interlock. The tank was pressurized manually, thus satisfying the first two items, but the absence of the third item prevented actuation of the interlock in the S-IVB ready-to-launch logic train. The result was automatic cutoff at T-30 seconds. The launch was accomplished with the interlock bypassed by a jumper. Investigation indicates cause of failure to be a defective diode on a printed circuit card in the terminal countdown sequencer.

Another section claims that this was the only launch delay in the entire Apollo program caused by a hardware issue.

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  • $\begingroup$ @ Organic Marble This article says that the terminal countdown sequencer stopped because the S-IVB liquid oxygen tank failed to pressurize at the right time. If there's a malfunction, the countdown automatically stops. $\endgroup$ – Star Man Jul 20 '19 at 17:08
  • $\begingroup$ @StarMan I'm hinting at the fact that there's a much more interesting story here than "the launch was delayed because the clock stopped" $\endgroup$ – Organic Marble Jul 20 '19 at 17:09
  • $\begingroup$ See also Moonport pp. 525-526 ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790003956.pdf $\endgroup$ – Organic Marble Jul 20 '19 at 17:16
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    $\begingroup$ Answer has been updated. NTRS is acting flaky this morning, so I started the answer with the files I already had. The answer is now complete. $\endgroup$ – DrSheldon Jul 20 '19 at 17:17

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