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Post by Enterprise on Aug 2, 2005 22:30:22 GMT -4
Ever since the space shuttle first launched there has been problems with the heat resistant tiles. I believe the Apollo CM never had such problems as I believe, correct me if I'm wrong, it had a solid moulded heat shield. Would it not this have been better to provide solid heat shield's for the space shuttles?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Aug 2, 2005 22:48:32 GMT -4
Apollo's heat shield consisted of a phenolic resin injected into honeycomb cells. It was an ablative heat shield, meaning it burned away as the spacecraft reentered. The Space Shuttle was built to be reusable, thus an ablative heat shield wouldn't work. I think the Shuttle's tiles are lighter as well, though I'm not sure about that. (The command module's heat shield made up about 15% of its total mass.)
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Post by Enterprise on Aug 3, 2005 0:34:41 GMT -4
I had wondered why the same technology was not used on the shuttles. Cheers for that Bob.
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Post by JayUtah on Aug 4, 2005 10:17:40 GMT -4
Yes, phenolic resin heat shields are pretty heavy. Since the orbiter heat shield has to be reusable, a whole different strategy for heat rejection comes into play. The entire problem has to be rethought from its first principles. The ablative shield is consumed; its strategy for heat rejection is to give the heat something to do -- a physical and chemical reaction that consumes heat.
The material on the orbiter takes a completely different approach. It uses the phenomenon of latent heat. That is, you put a lot of heat into something but it doesn't immediately start coming back out. The material "stores" the heat and releases it very, very slowly. After landing, the orbiter continues to radiate its re-entry heat for a very long time afterward. Other, more common materials, would transmit the re-entry heat to the orbiter skin too fast and cause its temperature to rise. In more understandable terms, the shuttle tiles are highly efficient heat sinks.
The drawback to this material is that while it has those magical thermal properties, it doesn't have the mechanical properties to survive being applied in large expanses on the surface of a machine that flexes. It's not an elastic material. And so they can't apply it in one solid sheet or block. If they did that, it would crack under the forces of atmospheric flight. The structures we build for flying in atmosphere have to flex and change shape in order to absorb the loads. This heat shield material can't do that. It's like glass; it pretty much has to stay in its original shape, and if you try to bend it it breaks instead. So if you apply the material as tiles instead of one sheet, one tile can shift ever so slightly against its neighbors and thus deal with the flexing of the underlying aluminum sheet.
That same principle is why we put lines on concrete sidewalks every four feet or so. You can't keep the concrete from cracking as the ground shifts underneath it. But if you provide many stress points to dictate where the crack will occur, you'll have many small cracks and many small differences in height instead of a few ugly, heinous cracks.
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Post by scooter on Aug 13, 2005 15:54:30 GMT -4
Very neatly put, Jay.
Did you all happen to see the testing film shown on Nasa TV as they were doing the ground testing prepping for the gap filler removal? They were doing the hacksaw blade thing, and were actually scraping the blade intentionally against the tiles, the surface, at least, is a bit more durable than I realized, nary a mark... I do understand that there is a new tile formulation that is available, more durable (though with slightly different thermal expansion properties). I suspect some tiles have been replaced by some of these, but not many.
Dave
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