fisher@dvinci.DEC (Burns Fisher, MRO3-1/E13, DTN 231-4108) (11/14/84)
<Quoth the Raven, "Nevermore!"> I believe that the entire set of pressures/prebreath traces back to two design decisions: 1) Run the suits on low pressure 2) Put a nitrogen/oxygen atmosphere in the shuttle cabin 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. This required them to run on pure O2 in order to keep the partial pressure of oxygen high enough to support life. Originally, while the capsule was on the pad, they pressurized the thing to full atmospheric pressure with O2 so that it would not implode. Then came the Apollo I fire. In the rash of investigations that followed, they discovered that materials in the cabin had been qualified in pure O2 at 5 psi as it would be used in space, but noone had thought to test the stuff at 15 psi as it sat on the pad! My recollection is that after this they ran Apollos on O2/N2 while they were on the pad, with the astros breathing pure O2 through a facemask. As they rose through the atmosphere, the pressure dropped in the cabin, and the N2 was purged out, so that on orbit, they once again had 5 psi pure O2. 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. However, the EMUs (space suits) were designed to run on 5 psi O2 (I presume for mass reasons). This means that an astro who is going from one environment to the other must purge the N2 from his/her blood by pre-breathing pure O2. I don't believe the pre-breathing has any other purpose.
henry@utzoo.UUCP (Henry Spencer) (11/18/84)
> 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