Sixty Seconds

Charles Duke was born on 3 October 1935. On 20 July 1969 he was the voice in Houston calling out the fuel during the last two minutes of Apollo 11's landing: "60 seconds", then "30 seconds". Here are those two and a half minutes in real time, with the clock Houston was counting and the one slosh in the tanks had shifted.

"Bingo" meant land within 20 seconds or abort. Houston started its 94-second count when the low-fuel light latched. Slosh set that light off early, so the real margin was longer: by Wikipedia's account about 50 seconds at touchdown instead of 25. The second clock adds the 25-second difference. The altitude tape follows Buzz Aldrin's calls and is drawn straight between them.

The arithmetic checks out

Paul Fjeld's note in the Apollo Lunar Surface Journal says the quantity light latched at 102:44:31 and "started a 94-second countdown to a 'Bingo' fuel call". That puts Bingo at 102:46:05. Duke's "60 seconds" at 102:45:02 and "30 seconds" at 102:45:31 point to 102:46:02 and 102:46:01, within a few seconds of it. Aldrin called "Contact light" at 102:45:40, 25 seconds before Bingo. That is Wikipedia's figure for the margin the crew and controllers saw: "enough fuel for another 25 seconds of powered flight before an abort without touchdown would have become unsafe".

The Journal commentary slips in one place. A note earlier in the descent says slosh "at 102:44:45, will cause the propellant low-level sensor to latch nearly 30 seconds early". 102:44:45 is when Aldrin called "Quantity light"; Fjeld's own note puts the latch fourteen seconds earlier, at 102:44:31. The page uses Fjeld's time.

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