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Watching Apollo 13 and rereading Lost Moon, Jim Lovell's exciting recollection of the events on the crippled spaceship, I began to wonder. Why does NASA maintain the gasses at the triple point? A pressure vessel designed to handle very high vacuum (which H2 and O2 triple points are near complete vacuum) and low temperatures is thicker and more dense then a pressure vessel designed to handle modest amounts of pressure. Why wouldn't they store these gasses at higher pressures, at or near atmospheric pressure?

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  • $\begingroup$ Which gasses are you referring to? Cabin refresh or thruster fuel? $\endgroup$ – Carl Witthoft Jul 7 '17 at 12:50
  • $\begingroup$ Both. In Apollo 13 it was used as fuel cell oxidizer as well as cabin refresh. But more research shows that H2 and O2 are stored at their triple point and used for main thruster fuel even on the space shuttle, so I figured I'd get whatever answers are available. $\endgroup$ – Mark Jul 7 '17 at 12:56
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If you want a few hundred pages of gory detail on gas management, I found an original research report here.

Looking at excerpts from a book on cryogenics, one thing that's pointed out is that the triple point provides a stable pressure inside the container even as energy is added or removed, because the energy just changes some of the material from one state to another. By comparison, a can of LOX needs pressure management because a little heat produces a gas head, and so on.

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  • $\begingroup$ That original research report just made my entire engineering schooling worth it. All over again. $\endgroup$ – Mark Jul 7 '17 at 15:50

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