From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Apollo fire
Date: Thu, 12 Nov 1992 17:52:59 GMT
Organization: U of Toronto Zoology

In article <BxLwzr.3L3.1@cs.cmu.edu> GE777010@brownvm.brown.edu (Bill Collins) writes:
>One of the main reasons that the fire was so devastating was the fact
>that at the time Apollo capsules had 100% oxygen atmospheres at normal
>atmospheric pressure (14.7 lbs./sq.inch).  That's a lot of oxidizing
>agent to help with combustion!  After Apollo was allowed to fly subsequent
>to the Apollo1 fire the pressure was dropped to 5.3 lbs/sq. inch (am I
>right, Henry?).

Not really.  Having checked some references...

The Apollo spacecraft, like Mercury and Gemini spacecraft, ran on about
5psi of pure oxygen in space; it was designed that way from the start and
this was not changed after the fire.  The fire hazard in that atmosphere
is not significantly worse than in 14.7psi air, and in any case, the lack
of convection in free fall greatly reduces fire risk (which is nice of it,
since it also reduces the effectiveness of fire extinguishers).  A one-gas
life-support system is a good deal simpler and lighter than a two-gas
system, the lower pressure reduces structural loads, and prebreathing for
spacewalks would have been very awkward for Apollo.

The problem was that the capsule had to be pressurized to 14.7psi, or
preferably slightly more, while on the ground; it was not built to stand
external pressure exceeding internal pressure.

Now, that pressurization didn't have to use pure oxygen.  In fact, Mercury
used normal air for ground pressurization, although I believe the astronaut
breathed pure oxygen at all times.  However, it turned out that there were
risks in this approach:  a technician was killed in an accident which would
not have happened with pure oxygen.  (I don't have details, but I would
guess a pressure-chamber test went awry and he ended up breathing air at
low pressure.)  It was decided to use pure oxygen for ground pressurization
as a result.  Nobody noticed that this greatly increased the fire hazard,
partly because the fire-prevention philosophy at the time was to ensure
that there were no ignition sources to start a fire.

In the wake of the Apollo fire, this was all re-assessed.  It became clear
that there had been many potential ignition sources in the capsule, to the
point where there was no possibility of determining which caused the fire.
While a lot of this was due to sloppy manufacturing, it was decided that
the basic philosophy would have to change:  presence of ignition sources
would henceforth be taken for granted, so fires would have to be contained
rather than prevented.  A much more determined effort was made to exclude
flammable materials from the cabin, but it proved impossible to make it
fireproof for 14.7psi+ of pure oxygen.  Max Faget suggested using a mix
containing some nitrogen, as a compromise between avoiding the bends and
reducing fire hazard, and the final ground atmosphere was 60% oxygen and
40% nitrogen.
-- 
MS-DOS is the OS/360 of the 1980s.      | Henry Spencer @ U of Toronto Zoology
              -Hal W. Hardenbergh (1985)|  henry@zoo.toronto.edu  utzoo!henry
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