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As portrayed in the movie Apollo 13, Fred Haise tried to spook his crewmates by trigerring the cabin repressurization valve, which creates a banging sound. Indeed, every Apollo crew reported hearing this sound from that valve:

All crews reported that reaction control firings were much more audible in the lunar module than in the command module. Crews also reported hearing the sharp shotgunlike report made by the closure of the cabin repressurization valve, the glycol pump whine, the grinding of the S-band antenna, and several pyrotechnic firings. Although sometimes annoying, these noise cues were often helpful as indications of proper system functioning.

Apollo Program Summary Report, April 1975, p. 6-6

Why would this valve create such a sound? Is it an example of a water hammer effect?

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  • $\begingroup$ Argh, I feel like I just ran across some information on this in the last week or so while looking up something else. $\endgroup$ – Russell Borogove Nov 20 '18 at 2:19
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Odds are it was the spring slamming the valve closed.

Green arrows are the flow path. Red arrow shows the spring.

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Diagram from Apollo Spacecraft News Reference page 122

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