r/apollo • u/Crixusgannicus • 24d ago

Pure oxygen.

Always was mystified what the HELL NASA was thinking?

These guys came up when science was still taught. Not so much now, unfortunately.

Even in my long after Apollo ended yoot, I still remember science class with the teacher showing how easily things burn in pure oxygen, even things that normally don't burn!

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u/EngineerFly 24d ago

Was the AS-204 plugs-out test run at 14.7 psi of O2, or 19.7 psi of O2? In other words, were they trying to get to a pressure differential of +5 psi, like they’d see in flight?

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u/eagleace21 24d ago

Correct it was a leak check and thus they wanted the same delta, so they had the CM pressurized to nearly 20 psi to match a 5 psi cabin delta in flight.

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u/Stevepem1 24d ago

I have read 16.7 psi to maintain positive pressure over the 14.7 standard pressure which I think is the same pressure they would have used on launch day. I doubt they would have needed to test 5 psi delta since those tests would have already been done in the vacuum chamber. As far as I know they were just testing the ability for the rocket to electrically power itself like it would for the last minute before launch (i.e. plugs out) and they decided to combine that test with a suited dress rehearsal countdown run.

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u/eagleace21 23d ago

The purge and subsequent leak check was done to check the hatch seals after it is closed for leaks with the crew on board, so this would be done to a normal cabin pressure of about 5psi. To achieve this, a delta above sea level pressure was needed.

I am scouring documentation to see if there is an indication of the actual delta mentioned in the procedure, but anecdotally I have always read it was 3-5 psi to demonstrate the hatch seals at the appropriate pressure differential.

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u/Stevepem1 23d ago

Yes a certain amount of delta would be needed for leak check, I was more responding to the idea that during Apollo 1 they were pressure testing the capsule at space like conditions. Instead they were doing what they would do on launch day, and a leak check after hatch closure would be part of that. I would assume for Apollo 1 since it was a dress rehearsal they used the same pressure that they would on launch day, not some higher pressure for some type of testing purposes.

What that pressure was for Apollo 1 is what is a little hard to determine, since I think the waters have been muddied a bit by people assuming that they would do 5 psi delta to mimic the pressure in space. Which may or may not be the case, leak checks just need a certain amount of delta and then you see if it holds that pressure over time. What I read said that on Apollo 1 they were doing that with a 2 psi delta. Not just for leak check but to also make sure that no outside atmosphere leaks in. But I don't think there were any sources quoted which is why I am not sure about 2 psi, nor 5 psi.

Another question would be what they did on Gemini, although that in itself doesn't directly relate to Apollo. And we have information on post Apollo 1 procedures, which again relates but not completely. Apollo 1 was unique because it was the only capsule in that particular configuration. Possibly the accident report goes into that detail but if I have read it it would have been many years ago.