When We Left The Moon

posted in: Project Apollo | 3

44 years ago, Apollo 17 left the Moon, and so far no human has been back. We watched the lunar liftoff live. This is how…

3 Responses

  1. Roger A. Krupski
    | Reply

    In almost every liftoff from the moon, there is a peculiar sound that’s occurs precisely at T-0. The sound is sort of like the crack of a baseball bat heard from far away… or maybe a champagne bottle cork popping. I’ve asked and searched everywhere and cannot seem to find out what the sound is.

    For example, the Apollo 17 liftoff goes like so: “engine arm is ascent – ok I’m gonna get the Pro – 99 proceeded 3..2..1..[the crack sound]…ignition! We’re on our way Houston!”

    The Apollo 14 launch: “ok the abort stage is set – ascent engine is armed – 6..5..4..Pro..3..2..1..[the crack sound]..0..we have ignition – whew what a liftoff!”

    Apollo 15: “abort stage – engine armed ascent – 99 – Pro – [the crack sound] – good liftoff – automatic”

    Apollo 11: “9…8…7…6…5… abort stage – engine arm ascent – proceed – [the crack sound] – (radio static) – …beautiful! ..26..36 feet per second up… standby for the pitchover…”

    I’ve guessed that maybe it’s an electrical transient from the pyro firings to separate the stages or maybe an actual mechanical bang from the explosive devices, but I really don’t know and have not been able to learn what the sound is. And since (as I understand it) the explosive devices had their own batteries and power buses, that makes the idea of an electrical transient going from one system to another seem unlikely.

    If anyone knows and can tell me what makes that sound, I would be grateful.

    • markwgray
      | Reply

      I’m virtually certain it is the pyros firing the guillotine devices that cut all the connections between the stages. They are located just under the LM cabin, and with the astronaut’s mics on vox they would create a loud enough mechanical sound to be picked up. I’ll see if I can get confirmation of this.

  2. Roger A. Krupski
    | Reply

    Mark, thanks for the reply. What you said makes sense in that the astronaut(s) are talking before and after “the sound” so their vox would have been triggered and able to pick up the sound.

    Along this same line of thought, I wonder if the bangs of RCS firings could be heard during some periods when the mike was open on vox? I would think the fuel and ox solenoid valves would make quite a racket, especially being mechanically mounted on a large “speaker diaphragm” (the LM frame and skin itself), not to mention possibly a bang caused by RCS firing itself (not any sort of “exhaust” sound which would need atmosphere to transmit, but a sound caused by the sudden application of stress to the LM body by the thrust of the engine).


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