Black Tiles Turned White

This exchange took place on the Orbiter Forum, regarding the
differences in color of the Shuttle's black tiles...
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garyw: "I think the blackend tiles are the new ones. i.e. ones that
have not yet experienced a re entry."
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Quick_Nick: "yep. When the launch was origanaly supposed to occur, they
had a news guy show a tile. They are supposed to be black, but easily
get the cover scraped down to a white inner part, even by rainfall
since the shuttle hits rain so hard. Thats one reason not to launch in
rainy weather."
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Somehow, I don't think the black glazing actually wears down to the
white silica fibers, but rather gets a layer of oxidization
(oxidation?) on top of it.

I couldn't find anything on the Net more detailed than the Wikipedia
entry, and it doesn't cover that.

http://en.wikipedia.org/wiki/Thermal_Protection_System

Anybody got a good explanation?

Thanks in advance.

Chris

wrote in message
oups.com...
Somehow, I don't think the black glazing actually wears down to the
white silica fibers, but rather gets a layer of oxidization
(oxidation?) on top of it.

I couldn't find anything on the Net more detailed than the Wikipedia
entry, and it doesn't cover that.

http://en.wikipedia.org/wiki/Thermal_Protection_System

Anybody got a good explanation?

The tiles take on a gray appearance as a result of reentry heating. The
heat causes the gap fillers between the tiles to outgas, coating the tiles.