From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Re: Shuttle thermal tiles
X-Added: Forwarded by Space Digest
Organization: National Institute of Standards and Technology formerly National Bureau of Standards
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Fri, 18 Dec 1992 05:38:17 GMT


-From: shafer@rigel.dfrf.nasa.gov (Mary Shafer)
-Subject: Re: Shuttle thermal tiles
-Date: 17 Dec 92 01:45:28 GMT
-Organization: NASA Dryden, Edwards, Cal.

-On 12 Dec 92 01:24:21 GMT, roberts@cmr.ncsl.nist.gov (John Roberts) said:

-J> The Shuttle launch towers have movable covers which protect the orbiter
-J> from damage by hail and severe storms.

-Dryden flew a number of shuttle tiles (current and proposed) on the
-aerodynamic test fixture on the F-104G.  The tiles are undamaged by
-snow and devastated by rain, with hail being not quite as bad as
-rain.  

That's a strange result - this summer I drove through a hail storm
(small hailstones, but a tremendous number of them), and was very impressed
with the impact properties of the hail. It was also raining heavily, but
I believe sometimes there's hail without rain in localized areas.

I suppose in principle you *could* fly through a rainstorm and still land
safely, but it would be a very expensive landing. :-)

-Of course you can cover the vehicle on the pad, but you can't
-fly through rain or hail without etching the tiles down to the felt
-pads they're glued to.

Apparently gentle rain doesn't hurt the Shuttle on the pad. I don't know
to what extent increased weight might be a concern. I believe they spray
water repellant on the tiles prior to launch to minimize water absorption.
The mechanism of damage in flight must be primarily one of abrasion.

Do you happen to recall what kind of glue is used? I got the impression
that it's some variation of RTV (silicone).

-J> It's interesting to recall the extreme concern over the thermal tiles
-J> before STS-1. Many people were convinced that the tiles would all fall
-J> off during reentry. (The fact that Enterprise lost many of its tiles during
-J> transport, and that Columbia lost some off its engine covers during launch
-J> added to the concern.) 

-Enterprise, not being a _real_ shuttle, didn't have real tiles.  It
-had a fairly fine-textured plastic foam, painted black.  Not foamed
-ceramic with a ceramic coating, like real shuttles.  I have a small
-chunk of the Enterprise that I picked up off the hangar floor when
-they were doing the structural test evaluations.  These fake tiles
-stayed on very well, both in transport and in flight.

Thanks for the correction - I hadn't realized that the Enterprise tiles
were fake.

-On the other hand, the tiles falling off Columbia during its first 
-takeoff on the 747 were thicker than the the leaves falling off the
-fruitless mulberry in my front yard after the first hard freeze last
-month.  I think that the Air Force had a FOD walkdown before they'd
-open the runway after the takeoff.

Maybe it's just as well that Columbia wasn't assembled at KSC. :-)

I believe the solution was dipping the bases of the tiles in something
to make them stronger (and more dense), possibly better glue, and pulling
on every tile to verify adhesion.

I think the Shuttles have actually reentered with a few tiles missing from
the bottom - apparently the heat distribution is such that a few tiles
missing doesn't cause too much damage.

-J> Now it seems to be treated as a solved problem (except
-J> for the weight, of course, and the inability to fly the Shuttle through
-J> rain during landing). 

-The various fabrics have replaced the tiles most prone to departure in
-midair.

Is the outer surface of the fabric flexible?

-J> I got to play with some Shuttle tile replicas
-J> at the Technology 2002 conference - they're being promoted as a solution
-J> to some Earth-based problems.

-Did you get to hold it in your bare hand while they played the butane
-torch over it?  It took a lot of faith for me to keep my hand out there
-when the top of the tile started glowing orange.

They had a torch there, but they weren't using it when I walked by.
I got to see that demo once in high school.

Is the method by which Shuttle tile material is made now known to the
public? Does it start out as an aerogel?

John Roberts
roberts@cmr.ncsl.nist.gov