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 procédure avant EVA

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dominique M.




Nombre de messages : 1864
Localisation : val d'oise
Date d'inscription : 15/10/2005

MessageSujet: procédure avant EVA   Mar 22 Aoû 2006 - 17:16

dans son interview avant vol, Heidemarie Stefanyshyn-Piper explique la nouvelle procédure intitulée "Campout" qui permet de gagner du temps pour la dénitrogénation sans être obligé de baisser la pression dans toute la station:

Question:The preparation for the EVAs on this flight includes something new called the campout pre-breathe. What is that, how does it work and what’s the reason that we’re going to be doing this?

Well, the campout [is] interesting enough. It’s a new procedure -- we’ve never done it before. Actually it’s not a new procedure in the sense that someone just thought of it recently. It’s been on the books for a long time. Initially, that was going to be the way we were going to do EVAs on space station. One of the things about doing EVAs in space is you have the opposite effect of going diving. When you go diving you go from the Earth’s atmosphere [atmospheric pressure] to a higher atmosphere and so you’re, you’re, when you come back up you have to decompress. Well the same thing happens when you go out on a spacewalk because you’re inside the space station at 14.7 [pounds per square inch] nominally, and when you go out on your spacewalks, you, in the suit, you’re only down at 4.3 psi. So you’re, you know, almost down a third of your pressure. And so, you don’t want to have the risk of decompression sickness, which you would, you know, get from diving if you don’t decompress. So when we go out on our spacewalks we, in a sense, need to decompress. We’ve taken care of that on the space shuttle by bringing the pressure in the space shuttle down to 10.2 psi the day before the first EVA. At the lower pressure, you have the same amount of oxygen, so you have less nitrogen, so you’re able to off-gas that nitrogen. It’s not practical to bring the whole space station down to 10.2 psi because it’s just too big a volume. If you blow air overboard [to lower station pressure] well … that’s a consumable that you don’t have anymore. What we had been doing was known as the exercise protocol. The crew members going out for EVA would put on a mask and, the morning of EVA, they would get on a bicycle and while they were breathing 100 percent O2 and exercising. You would off-gas, because you’re breathing more during exercise and you’re breathing 100 percent O2. You flush out some of the nitrogen. The only problem with that is that it, the time that it took from the time that you woke up in the morning to the time you were able to go out the door was a significant amount of time. And so this campout protocol kind of mimics the 10.2 protocol that we use on shuttle. The EVA crewmembers get locked in the airlock the night before, they bring the pressure in the airlock down to 10.2 psi, and so instead of living in the shuttle at 10.2 you’re just living in the airlock at 10.2. Since you’re locked in there, it’s been given the name of campout, because you have to take your sleeping bags in there and you’re, in a sense, camping out. By doing that we’re able to start our EVAs one hour earlier, and so that means now we’re not crunched at the end of the day. When you come back in from an EVA you have to clean up your suits and put all your tools away, and all of that takes time. What happened in the past is that, that would run into the crew member’s sleep time. And we don’t want to compromise our crews for sleep because that’ll just lead to fatigue by the end of the mission. So in doing the campout, if we’re able to get out the door one hour earlier, then that gives us one more hour at the end of the day so we can finish all of our tasks and not have to run into our sleep period.
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