After the STS-87 flight in December 1997 -- on which a new kind of foam, including a more environmentally friendly "blowing agent" (inflation gas) than Freon, was used for the first time -- the Shuttle's tiles were peppered with small scars caused by bits of foam coming off in the external tank.

It turned out that the new gas filled the foam's cells at higher pressure than the Freon had and that the new foam was also a bit less tough, so that as the Shuttle rose and the outside air pressure decreased, the buildup of pressure inside the foam cells made bits of it "popcorn" off.

This problem was solved by puncturing the foam, in any areas where it had a chance of hitting the Shuttle after popping loose, with thousands of tiny needle punctures to let out some of the internal gas -- but this in turn raises the possibility that these same punctures might have destroyed the waterproof nature of the foam.

Rainwater might have soaked into the punctures and then frozen into ice when the external tank was filled with its cryogenic fuels -- or simple air humidity might have diffused into the punctures during the few hours between fueling and launch and frozen into ice from the bottom of each puncture almost up to the top.

It has been revealed during the hearings that NASA actually began making these vent punctures in some specialized areas on the external tank -- including the bipod ramps -- as far back as the fall of 1985, after some bits of foam had popcorned off one of those areas for a different reason and hit the Shuttle on a flight that August.

After some bits popcorned off one of the bipod ramps themselves and hit the Shuttle during a January 1990 launch, they deepened those punctures, and they increased them in number after another incident in June 1992 in which a chunk of foam as big as the one that later hit Columbia came off a bipod ramp.

What caused those earlier foam separations was that fact that air had seeped into small voids between two layers of different kinds of foam applied in those regions. In 1993, this difference in foam types was eliminated -- but nevertheless, on STS-112 last October, a large chunk of foam came off one of the bipod ramps and struck the lower part of one of the shuttle Atlantis' sold boosters without doing any significant harm.

The dissection of an external tank's foam bipod ramps after the latest tragedy has now revealed the existence of dozens of small gaps produced by the complex way in which the foam must be laid down in such oddly shaped areas. A piece of duct tape was even found embedded in the foam.

The Board thinks there is a good chance that the bipod foam separation on both STS-112 and STS-107 may be due to "cryopumping" -- the possibility that, after the Shuttle has been fueled, either air from outside or some of the nitrogen gas used to purge the "intertank" area (between the upper liquid oxygen tank and lower liquid hydrogen tank within the External Tank) seeps into some of these gaps and is actually chilled enough to become liquid or even solid where the gaps in the foam are close to the supercold metal wall of the external tank.

Then, during the Shuttle's ascent, as the supercold propellants are drained out of the inner tanks, the liquified gas warms back up and boils back into vapor too swiftly to escape back through the cracks by which it originally entered, and so the pressure of the trapped gas pops off a big chunk of the overlying foam.

If so, then these same gaps may also be partly filled with frozen atmospheric water vapor or rainwater, increasing the weight and hardness of the foam.

However, the Board revealed on April 1 that the estimate that the STS-107 fragment weighed only a kilogram was not based on an arbitrary assumption that the foam was dry, with no ice infused into it or attached to it that could have massively increased its weight.

Instead that weight estimate comes directly from analyses of the rate at which the fragment was seen dropping -- if it had been heavier, it would actually have had more ability to resist being slowed down by the air it was plowing through, and its inertia after coming loose would have kept it moving upward along with the Shuttle so that it would actually have dropped behind and hit the Shuttle at a lower relative speed.

(What DOES hinge on the assumption that the foam was dry is NASA's estimate that the fragment was 12 cm thick -- it turns out that this can't be judged from the actual fuzzy photos; and so, if the fragment was made denser by ice, the estimate of its total weight would remain only a kilogram but the estimate of its thickness would be reduced.)

That assumption that the foam definitely was lightweight was what led Boeing engineers, during the flight itself, to conclude that the foam strike had almost certainly not caused serious damage. So why did it do so?

First, it's possible that the weight estimate is wrong after all, and that the fragment was a good deal heavier. Two witnesses have testified that the estimate is probably pretty accurate; but given the fact that the foam fragment was tumbling and that the air turbulence next to the fast-rising Shuttle was complex, this may not be certain.

Second, it's become increasingly clear that the confidence of the engineers during Columbia's flight that such a lightweight fragment couldn't possibly do serious harm was itself misplaced.

The "Crater" computer program used by them to reach that conclusion has now been thoroughly discredited during the Board's investigation -- it was based on ground tests in which bits of foam only 50 cubic cm in size were fired at Shuttle tiles with gas guns, and any attempt to extrapolate from that to the effects of a fragment 600 times bigger and more massive is likely to be seriously inaccurate.

Moreover, it has been pointed out that even the Crater program predicted that such an impact had a good chance of knocking a hole clear through several tiles at once that were almost twice as thick as those actually on the Shuttle; and a Feb. 17 study by Aviation Week revealed that the Crater program, contrary to what the Boeing in-flight assessment stated, was NOT wildly biased toward overestimating likely damage.

Indeed, when the Crater program was applied to the STS-50 incident in June 1992 -- in which a big chunk of foam broke off a bipod ramp and struck Columbia's underbelly at a very shallow angle, digging a 12-mm deep gouge -- its calculations of the gouge's depth agreed well with what had actually happened.

The problem seems to be not so much that Crater misled the Boeing engineers, but that they blithely ignored the serious dangers which it did predict. As yet there is no good explanation of why they did so.

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