Wilson.Harvey@CMU-CS-IUS@sri-unix (11/22/82)
I remember a few years back, when the Apollo-Soyuz mission was about to take place, that one of the big problems with linking the two spacecraft was the Soviets used a different environment in their cabins. Namely, I thought that their cabin environment was close to 'normal' earth environment i .e. a mixture of Carbon Dioxide, Nitrogen, and Oxygen. I don't remember what the pressures were. I was just wondering why we use an environment of ~100% Oxygen and the Soviets used a different one. What are the benefits, if any, of the one over the other. (I do remember that the Soviets could perform welding experiments in space where we could not). Can anyone else confirm this, or am I totally out of the ballpark? Thanks, Wilson
DLENAHAN@USC-ISIE@sri-unix (11/23/82)
From: Den Lenahan <DLENAHAN at USC-ISIE> I recall one distinct disadvantage of 100% oxygen from about 1967 or so. Astronauts Grissom, Chaffe & White lost their lives in what should have been a small fire if not supported by the 100% O2 environment. I also recall that, after the accident, someone said that the same thing couldn't happen in zero-G. That it would have just (<just>?) been a flash, since no convection would occur in zero-G, and hence the fire would expend the local oxygen and quit burning. Also, had a similar accident occurred under a G'd environment, and had the explosive hatch bolts been in place, there was some speculation that the crew might have escaped. (Not during certain flight phases, obviously.) Dennis -------
henry (11/24/82)
American spacecraft have historically used pure oxygen as their internal atmosphere for three reasons: 1. You can get by with less pressure (since it's the partial pressure of oxygen that is the main consideration for breathing) and hence lighter cabin walls. 2. It is much easier to monitor the partial pressure of oxygen if you don't have large amounts of other gases. Monitoring is obviously necessary, and it is difficult to build a box that responds only to the oxygen pressure. Total pressure is easy to measure. I believe this problem has gotten easier in recent years with better sensor technology. 3. EVA suits must use the lowest pressure possible to keep the joints as flexible as possible -- the suits are pretty stiff even so -- and going from cabin atmosphere to suit atmosphere is simplest if they are as similar as possible. Even in the early American shots, the cabin pressure was somewhat higher than suit pressure, but pressure changes are much easier in a pure-oxygen atmosphere. No worries about the bends. My impression is that #1 was never a big thing and #2 was serious in the beginning but is not too much trouble now. #3 is still a nuisance: I believe that a shuttle EVA starts with an in-cabin preparation period during which the astronaut is breathing pure oxygen through a mask to get the damned nitrogen out of his body. Fire is not a serious risk to spacecraft in operation. Although I believe the nitrogen in normal air is considered to have some damping effect on fires, a similar partial pressure of oxygen means a similar degree of fire hazard. In addition, it is very difficult for a fire to keep going in free-fall, since there are no convection currents to keep a steady stream of air going into it. The Apollo fire would not have been serious in space. Problem is, on the ground the pressure inside a spacecraft must not be greatly lower than atmospheric, since the walls are not built to stand outside pressure. Before the Apollo fire, this meant that the Apollos carried 16 psi of pure oxygen when on the pad, and that *is* a major fire hazard. I believe this was changed to normal air, with a transition to low-pressure pure oxygen during climb.