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From: tomm@asgb.UUCP (Tom Mackey)
Newsgroups: net.columbia
Subject: Re: Columbia's Tiled Damaged by Rain
Message-ID: <765@asgb.UUCP>
Date: Tue, 27-Aug-85 09:48:36 EDT
Article-I.D.: asgb.765
Posted: Tue Aug 27 09:48:36 1985
Date-Received: Thu, 29-Aug-85 23:25:31 EDT
References: <4010@alice.UUCP> <199@mot.UUCP> <521@calmasd.UUCP>
Reply-To: tomm@asgb.UUCP (Tom Mackey)
Organization: Burroughs Corp. ASG, Boulder Colo.
Lines: 59

I really did not intend that this discussion go on, but a followup to one of
my previous postings on the shuttle tiles might imply to the casual reader that
the tiles are made of polyurethane foam.  This IS NOT THE CASE.  Briefly, and
with more than a few omissions, the process of making a single tile goes
something like this (remember this was 7 or so years ago, so the process may
have changed somewhat since then):

A saturated (probably super-saturated) mixture containing silica crystals,
binder, and some other stuff is prepared and the silica crystals begin to
form long "strings".  The slurry goes through a sintering process which
compacts the material, removes the excess moisture, and leaves the tile
material with inter-woven strands of glass.

The block of material undergoes several cooking processes in micro-wave ovens
which removes all moisture (well, most of it, anyway).  The block is then ruff
sawed into tile-sized pieces.  I believe the blocks are X-Rayed somewhere in
here to weed out internally flawed tiles.

At this point, the material is snow white, and feels chalky to the touch.  It
is dense, with no visible air pockets.  If rubbed lightly with the finger, a
powdery residue (which is pure glass) is transfered to the finger.  The
N/C programmers used to use scrapped tiles as pencil holders, so you can imagine
how soft it is.  A pencil will push right in, as will a finger nail.  If
examined under high magnification, the material looks for all the world like
a gigantic snarl of lots and lots of pieces of fishing line.  The strands of
glass are completely interwoven, forming a dense, yet open and airy structure
that traps the air.  What you are rubbing off is the ends of millions of glass
strands.

Using various holding techniques, the tiles are machined.  Usually the IML
(Inner Mold Line - fits the outer aluminum skin of the ship) and sides are
machined first.  The tile is then placed with its neighbors in a AFA (Array
Frame Assembly), sucked down with a vacuum, and the OML (Outer Mold Line -
the aerodynamic surface) is machined.  This works for large numbers of
relatively flat tiles.  For the more complex tiles, as in the "candy cane"
tiles that cover the leading edge of the vertical stabilizer, individual
nesting fixtures are machined out of flawed tile material, the OML is machined
first, and sucked against the nesting fixture for IML and side machining.
Since all the data is available, the OML machining motions can be rotated
and translated, making the nesting fixture cheap and easy to build.  They
are usally thrown away after one use.

After inspection, the tiles are coated with the outer glaze and fired.  I
did not get to see this process, so I do not know what they looked like at
this point.  After firing, and there are several glaze/firing steps, the
IML and half of the sides are white, being coated with a clear glaze, and
the OML and the other half of the sides is either black or white, being
coated with either a black or an opaque white glaze.  A narrow strip is left
uncoated, so as to prevent the tiles from exploding in the vacuum of space.

I hope this clears up some misconceptions for some, provides some interesting
information, and didn't put anyone to sleep.  It sure is embarrasing for
your boss to walk by and find you with your nose jammed into the keyboard!


Tom Mackey   				   ihnp4!sabre!\
					hplabs!sdcrdcf!-bmcg!asgb!tomm
		    { ihnp4, ucbvax, allegra }!sdcsvax!/
Burroughs Distributed Systems Group 		     Boulder, Colorado