Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!texbell!nuchat!steve
From: steve@nuchat.UUCP (Steve Nuchia)
Newsgroups: sci.space.shuttle
Subject: Re: Shuttle Status for 09/25/89 (Forwarded)
Keywords: firex
Message-ID: <15054@nuchat.UUCP>
Date: 28 Sep 89 07:07:48 GMT
References: <32507@ames.arc.nasa.gov> <238@tness7.UUCP> <15035@nuchat.UUCP> <5442@hplabsb.HP.COM>
Reply-To: steve@nuchat.UUCP (Steve Nuchia)
Organization: Houston Public Access
Lines: 34

In article <5442@hplabsb.HP.COM> dsmith@hplabsb.UUCP (David Smith) writes:
>I had presumed that there would be concern that if water soaked into
>the tiles, and didn't fully evaporate before re-entry, that flash
>evaporation could break tiles.  I had read of such concerns before
>with respect to the shuttle sitting out in the rain.  Is this truly
>a concern, or am I all wet?  :-)

My understanding of it is that the tiles, officially called the
Thermal Protection System (in NASAese, if it doesn't have three
words in its name you have to add "system") are designed to deal
with rain as long as they are intact.  There is concern about raindrops
encountered aloft, either during atmospheric ascent or descent or
ferry operations, causing pitting of the tile surface.  The tiles
are not homogenous by the way -- they consist of a non-porous ceramic
"glaze" layer encapsulating a very porous ceramic inside.

The tiles are laid over a backing that is basically a felt of
some kind.  Excess water in the felt could lead to steam explosions
damaging tiles, as could steam explosions in individual tiles.
Fortunately the whole system gets a good soaking in hard vacuum
on a normal mission, which should make any water in it sublimate
harmlessly.  I don't know the numbers but without running the
calculations carefully I'd be concerned that an abort during
the first few orbits (or partial orbit) might carry a steam
explosion risk.  Such an abort could occur if the orbiter fails
to achieve a safe orbit or for a medical emergency, for instance.

I'd guess that the forces involved in flash evaporation of water
at ordinary temperatures in vacuum would not be great enough to
damage an otherwise nominal tile.
-- 
Steve Nuchia	      South Coast Computing Services
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