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From: clj@ksr.com (Chris Jones)
Newsgroups: sci.space.shuttle
Subject: Re: Shuttle scrubbed again
Message-ID: <830@ksr.com>
Date: 28 Sep 90 17:37:15 GMT
References: <1990Sep17.224055.1343@murdoch.acc.Virginia.EDU> <993@dg.dg.com> <1990Sep25.152824.15327@murdoch.acc.Virginia.EDU> <1001@dg.dg.com>
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Reply-To: clj@ksr.com (Chris Jones)
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In-reply-to: ahughes@dg-rtp.dg.com (Arch Hughes)

In article <1001@dg.dg.com>, ahughes@dg-rtp (Arch Hughes) writes:
>In article <1990Sep25.152824.15327@murdoch.acc.Virginia.EDU>,
>gsh7w@astsun.astro.Virginia.EDU (Greg Hennessy) writes:
>|> In article <993@dg.dg.com> ahughes@dg-rtp.dg.com (Arch Hughes)
>writes:
>|> #Anyone know what concentrations of hydrogen are capable of causing a
>|> #real explosion?          
>|> 
>|> About 4 percent, or 40,000 ppm. The limit for a shuttle launch is
>|> 1,000 ppm.
>
>Why is there such a great gap between the launch limit and the 4% 
>concentration?  If the limit was 2% (20K ppm) wouldn't there be ample
>time to scrub as the limit was exceeded, and vent the gas?

According to the 24 September 1990 AW&ST, Crippen raised the limit from 600 ppm
to 1000 ppm (when the hydrogran was in its "fast flow" mode; the limit is lower
when the flow is slower), and some advisors were holding out for 1400 ppm.  In
the event, the hydrogen concentration peaked at 4000 ppm.  "There is some
belief among NASA's shuttle contractors that the agency is being overly
conservative in its assessment of the hydrogen danger."  (When do you want me
to launch, April?)  "One said, ''NASA is looking for a pure machine.  They'll
never find it.''"

Also, NASA is willing to go to 10000 ppm, and would have no qualms about doing
troubleshooting at that level.  This is in keeping with the safety factor of four
which they apparently use in many shuttle systems.  They just don't want to
launch in that condition.
--
Chris Jones    clj@ksr.com    {world,uunet,harvard}!ksr!clj