Why Pure O2 at 5 PSI?

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Proponent!!!N0_SP@M!!!@gmx.net - 29 Nov 2005 10:45 GMT

Why were early US manned spacecraft pressurized with pure oxygen at 5
PSI when the partial pressure of oxygen at sea level is only about 3
PSI?

I'd have thought that a lower pressure would 1) allow a lighter
structure, and 2) make space suits significantly more flexible when
pressurized, and 3) present a slightly lesser fire hazard.

The only benefit I can think of for the higher pressure is that in the
event of a leak in the cabin, one would have a bit more time before the
crew lost consciousness. But given the fact that oxygen for several
cabin changes was probably on-board anyway, this seems a very small
advantage. And I suppose it might be slightly easier to keep
electronics cool with the higher pressure.

Just curious.

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Jeff Findley - 30 Nov 2005 16:10 GMT

> Why were early US manned spacecraft pressurized with pure oxygen at 5
> PSI when the partial pressure of oxygen at sea level is only about 3
[quoted text clipped - 12 lines]
>
> Just curious.

I'm sure this doesn't completely answer your question, but you may want to
read this:

http://www.hq.nasa.gov/office/pao/History/SP-4003/ch6-4.htm

Jeff

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Proponent!!!N0_SP@M!!!@gmx.net - 05 Dec 2005 10:45 GMT

> I'm sure this doesn't completely answer your question, but you may want to
> read this:
>
> http://www.hq.nasa.gov/office/pao/History/SP-4003/ch6-4.htm

That's interesting. It contains the following summary of reasons for
choosing 5 PSI:

"This pressure level was chosen as the best compromise to provide (1)
necessary oxygen partial pressure, (2) efficient use of supply for
emergency modes of operation, (3) a pressure offering small
differential change during cabin decompression emergencies, and (4) a
level for which decompression sickness would be minimal."

Item (1) requires a pressure of at least 3 PSI, but does not favor
higher pressures.

I presume the decompression referred to in items (3) and (4) relates to
the change in pressure experienced by the astronaut when cabin pressure
is lost. Pressurization would then be provided by the suit. Hence,
the suit's operating pressure must not be too much lower than the
cabin's. But this still doesn't explain the choice of 5 PSI for the
cabin.

The meaning of item (2) is not clear to me.

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Heinrich Zinndorf-Linker (zili@home) - 07 Dec 2005 20:57 GMT

Am 5 Dec 2005 02:45:23 -0800 schrieb "Proponent!!!N0_SP@M!!!@gmx.net":

>That's interesting. It contains the following summary of reasons for
>choosing 5 PSI:

[...]

>"This pressure level was chosen as the best compromise to provide (1)
>necessary oxygen partial pressure, (2) efficient use of supply for
>emergency modes of operation, (3) a pressure offering small
>differential change during cabin decompression emergencies, and (4) a
>level for which decompression sickness would be minimal."
>The meaning of item (2) is not clear to me.

item 2 provides some reserve in case of a slow decompression, where
you are NOT instantly at the least survivable o2 partial pressure. You
just need some time to do emergency actions (plug a hole, close an
accidetial open valve or whatever else). And a bit higher pressure
might give you that time...

cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)

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John Schilling - 07 Dec 2005 22:22 GMT

>> I'm sure this doesn't completely answer your question, but you may want to
>> read this:
[quoted text clipped - 12 lines]
>Item (1) requires a pressure of at least 3 PSI, but does not favor
>higher pressures.

Actually, it does. The ideal partial pressure of oxygen in a breathing
mix may be 3 psi, but that's partial pressure *in the lungs*. The air
in your lungs consists of ~1 psi carbon dioxide and water vapor being
exhaled, and the balance whatever you were inhaling.

So, at sea level, 1 psi CO2/H2O and 13.7 psi air, giving 2.87 psi oxygen.

Breathing 3 psi O2, your lungs see 1 psi CO2/H2O and 2 psi oxygen. That's
about what you get on a 10,000 foot mountain, and where people start running
into problems.

4 psi oxygen roughly duplicates sea-level air in that respect, and 5 psi
gives you a safety margin.

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Space Forum / Space Flight / December 2005

Why Pure O2 at 5 PSI?