Author Topic: Pressure garments: internal / external pressures  (Read 19553 times)

Offline Obviousman

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Pressure garments: internal / external pressures
« on: August 29, 2009, 03:38:50 AM »
I saw a question about the Apollo 1 suits that got me wondering: the David Clark A1C suits worn by the Apollo 1 crew were designed to maintain an internal atmosphere of roughly 100% O2 at about 3.5 PSIA.

In the case of the AS204 fire, when they had faceplates down / suit sealed and the spacecraft cabin pressure was at about 16.7 PSIA of about 100% O2, what was the pressure inside the suit?

I've looked at the Apollo Operations Handbook section of the report (Appendix C-1, AOH section 2.7) but can't figure it out from the diagramme.
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Offline ijuin

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Re: Pressure garments: internal / external pressures
« Reply #1 on: August 30, 2009, 01:50:50 AM »
I am not certain, but I think that when the cabin is pressurized, suits are kept at just barely above cabin pressure in order to make mobility easier than when the suits are at their full relative pressure of 3.5 or so PSI above the surroundings.

Offline evancise

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Re: Pressure garments: internal / external pressures
« Reply #2 on: August 30, 2009, 08:22:01 AM »
I don't know specifically about the Apollo suits but when they do checkouts of the EMUs (spacesuits) on Shuttle and Station, they pressurized them to only about 1 psid (1 psi above cabin pressure).  (Sorry I don't know the mmHg or kPa conversion off hand.)

Offline Obviousman

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Re: Pressure garments: internal / external pressures
« Reply #3 on: August 31, 2009, 03:08:08 AM »
In the case of Apollo 1, when they had the capsule pressurised to 16.7 PSI at 100% O2, how did they avoid having the crew exposed to high levels of O2 for too long? 14.7 PSI = 1 Atmosphere = 760 mmHg. oxygen toxicity starts at about 425 mmHg, so the crew should only be exposed to O2 levels above that for short periods.

So in the Apollo 1 case, they should have been exposed at a reduced pressure or at least at a reduced partial pressure of O2. I can't see how the regulator / ECS did it.

The way to do it would be to seal the suit, and run at a lower internal (suit) pressure and thus a lower O2pp... but would the higher external (cabin) pressure cause the suit to "cling"? Strapped in for a CDDT, etc, I wouldn't see a big problem... but I'd like to deal in facts, not guesses, and I don't understand.

Help!
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Offline ijuin

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Re: Pressure garments: internal / external pressures
« Reply #4 on: September 01, 2009, 03:22:45 AM »
Having less pressure in the suit than in the cabin would have the same effect as breathing air at too low a pressure when diving under water--your lungs would collapse as your rib cage gets compressed by hundreds of newtons of external pressure.

Given the problems with breathing high pressure oxygen (nevermind the fire hazard), I really don't get why they didn't just use compressed air for the test.