from Dr. Osheroff":1n3yxrgp said:
Dear Roger:
I presume that you meant to write Columbia rather than Challenger, since the Challenger accident involved leaking rubber O-rings on the SRB's (the 1,000,000+ pound solid rocket boosters, each of which provides 3,000,000 pounds of thrust). The Columbia Shutte accident did indeed involve the urethane foam that insulates the external fuel tank (ET) containing the liquid hydrogen and oxygen used by the main rocket engines.
NASA was given waiver which allowed it to continue using freon as a blowing agent after the ban was put in place, but it decided that since freon would soon become unavailable, it would convert to a new blowing agent for the 'acreage' foam that covers the smooth surfaces of the ET. Their reamining supply of freon would be used to blow foam on the irregular surfaces which must be blown by hand. The foam that actually fell off the -Y shuttle bipod foam ramp was thus the original foam with the freon blowing agent.
The foam has fallen off in large and small pieces ever since STS-1. It is very fragile, having a density of only 1/30th that of water, and physical properties that vary widely with temperature from the 21K of liquid hydrogen to the 400+ K of supersonic flight. NASA never kept track of inciences where large pieces of foam had fallen off, but Columbia was the 7th instance, and this was the 5th time a large piece had fallen off Columbia. For all the other shuttles combined, this had happened only twice. In all cases, it was the left (-Y) foam ramp which had shed, and never the right (+Y) foam ramp. No one knows why, but it is likely to be associated with air turbulence, as there is a large liquid oxygen line that passes close to the +Y ramp which might protect it from air turbulence. Why Columbia more than any other will never be known.
Best wishes,
Doug
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Douglas D. Osheroff
Department of Physics
Stanford University
Stanford, CA 94305-4060