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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: Re: O2 pressure in space suits, etc
Message-ID: <4654@utzoo.UUCP>
Date: Sat, 17-Nov-84 20:04:31 EST
Article-I.D.: utzoo.4654
Posted: Sat Nov 17 20:04:31 1984
Date-Received: Sat, 17-Nov-84 20:04:31 EST
References: <38@decwrl.UUCP>
Organization: U of Toronto Zoology
Lines: 29

> As you may recall, the early capsules all ran on low pressure while in 
> space.  I presume that this was to save mass by allowing the walls to
> be thinner and keeping less mass of atmosphere within.  ...

A contributing reason was the difficulty of monitoring the partial
pressure of oxygen in a complex multi-gas mixture.  This has become much
easier with more modern sensors.

> For the shuttle, they had (I presume) enough mass to play around with that
> they decided to keep a 15 psi O2/N2 cabin.  Saves a lot of hassle, allows
> more shirtsleeves, less training, etc.

Actually, I believe the shuttle does not run a full 15 psi; they do run
at reduced pressure with a higher percentage of oxygen than normal air.
But it's not nearly as extreme.

> However, the EMUs (space suits) 
> were designed to run on 5 psi O2 (I presume for mass reasons).

The space suits run at something like 2 psi of pure oxygen, for a very
convincing reason:  the higher the internal pressure, the less flexible
the suits are.  Those suits are *very* stiff even at 2 psi; anything
worse is unacceptable.  So the transition from medium-pressure mixed
gases to low-pressure oxygen is unavoidable, and pre-breathing is needed
to get rid of the nitrogen.  NASA doesn't like the time wasted in
pre-breathing, but it's hard to avoid.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry