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From: Henry Spencer <>
Subject: Re: Apollo One Fire: Why pure oxygen?
Date: Tue, 19 May 1998 00:31:21 GMT

In article <6jq5gt$4emi$>,
Michael Gallagher <> wrote:
>For years, I'd heard that the reason pure oxygen was used for the Apollo
>One's atmosphere, as opposed to a mixture with inert gasses, was that it
>weighed less.  But I recently heard another explanation, that pure oxygen is
>"easier to establish and control" than a mixture of gasses.  Which is the
>correct answer? ...

Both.  If you keep CO2, water vapor, and miscellaneous minor contaminants
under control, it's very easy to sort out whether a pure-oxygen atmosphere
has sufficient oxygen content:  just measure the pressure.  With mixed-gas
atmospheres, you have to measure the oxygen percentage somehow, and doing
that continuously is a little tricky even today and was a whole lot tricky
in the early 1960s.  That, plus the need for separate supplies of nitrogen
and plumbing for them, made mixed-gas systems complicated and heavy by
comparison with pure-oxygen systems.  The higher pressure does also add
structural mass, but that was a lesser consideration (except perhaps in
the LM, where every gram of mass turned out to be precious).

Yet another contributing factor was that spacesuits -- especially 1960s
spacesuits -- necessarily run at the lowest possible pressure because
they're stiff enough even so.  That means pure oxygen in them, and that
means a difficult transition if the cabin atmosphere is mixed-gas.  Not
only is the change in atmosphere just plain awkward in a small capsule
with no separate airlock, but there is the threat of getting the bends
when the pressure is reduced.

One reason why Apollo used pure oxygen even on the pad was a fatal
accident early in the Mercury program.  Mercury used pure oxygen in space,
but at the time was trying to use normal air on the pad and in various
tests.  But normal air at in-space-pure-oxygen pressure isn't enough to
keep you alive, as a technician found out the hard way when somebody
screwed up the atmosphere transition in a test.  Using pure oxygen at all
times at least ensured that the cabin atmosphere was breathable at all
times; the increased fire hazard was overlooked, partly because official
fire-prevention policy was to ensure that no ignition sources were
present, which made flammability of materials a lesser issue.

Even after the fire, Apollo used pure oxygen in space.  It compromised
only in the cabin atmosphere used on the pad, which was about 40% nitrogen
(the smallest percentage that brought the fire hazard under control); the
astronauts were still breathing pure oxygen in their suits at all times.
The on-pad cabin atmosphere was still marginally breathable even at
in-space pressures, most of the plumbing and support hardware for it could
be left behind on the ground, and pure oxygen in the suits and in the
cabin after launch eliminated the problems with suit pressure.  Oh yes,
and the revised fire-prevention policy conceded the inevitability of
ignition sources and emphasized giving a fire no chance to spread.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

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