The RCC panels -- about 5 to 10 mm thick -- are made of a "carbon-carbon" composite material (tough carbon fibers embedded in a carbon resin binder), which is splendid insulation against the high temperatures of reentry.

But during reentry, when this material is exposed to a high-speed blast of atmospheric oxygen, then -- being carbon -- it will tend to "oxidize", which is simply a fancy way of saying that it will slowly burn into carbon dioxide! Any such hole might enlarge during reentry to the point where it might burn completely through the RCC panel, allowing exactly the sort of intrusive blast of superhot air that wrecked Columbia.

So the carbon-carbon panels must also be shielded against exposure to the air during reentry. They are coated with a layer of silicon carbide glaze 1 to 2 mm thick, which has a slight tendency to form microscopic cracks while hardening, so that in turn it is then covered with another thin coat of glassy sealant.

This system seemed to work perfectly until after that same June 1992 flight of Columbia, when inspection revaled a rash of tiny "pinholes" in the surface of the silicon carbide glaze. These were then found on the other Shuttles -- today, the average RCC panel on each Shuttle has about 20 to 40 of them.

Apparently tiny bits of some substance are being deposited on the surface of the silicon carbide glaze while each Shuttle sits on the pad, and are then chemically reacting with it under the high temperatures of reentry to corrode it away. There are two suspects. One is simply flecks of sodium carbonate from the salt spray of the nearby ocean itself.

The other is the Shuttle's launch gantries, which -- as part of NASA's general tendency to neglect renovating its ground facilities -- have not been repainted in years, so that their overcoating of paint has come off in many places and exposed the zinc oxide primer undercoating. Tiny flakes of this primer falling from the towers onto the Shuttles may be the corrosive chemical during reentry.

Whatever the cause, they were not regarded as any kind of serious danger -- the very process that creates the pinholes gives them a glassy lining, and they never seemed to actually cause any of the underlying carbon-fiber substrate material in the RCCs to start oxidizing during reentry. (The latter happens only slowly in any case.) Those less than 1 mm across weren't even bothered with, and larger ones were sealed with the same glassy sealant used on the tiny craze cracks in the silicon carbide glaze layer.

However, after Discovery's March 2001 flight, inspectors decided to use thermography to double-check its RCC panels for possible underlying damage. (This involves heating the panel and then doing IR scans of it to locate spots where it is cooling unusually quickly.)

They were disturbed to find a spot underneath a particularly large glaze crack in the silicon carbide where the carbon-carbon material HAD oxidized away during reentry, producing a pit fully 10 cm long and 1 mm deep. This required immediate repair work on the RCC panel (although NASA didn't actually replace the panel because of a lack of spare parts in the Shuttle program).

After Atlantis' November 2002 flight, a similar hole was found in one of its RCC panels. In both cases, the outer hole in the silicon carbide was considerably smaller than the pocket beneath it where the carbon-carbon material had burned away during reentry. "Think of termites", says CAIB Chairman Adm. Hal Gaiman. The actual damage in such an event can be a lot bigger than external visual evidence suggests.

Moreover, despite these events NASA's inspections of the RCC panels for such internal damage were limited. Three techniques can be used to do "Non-Destructive Evaluation" (NDE) of the panels' interiors without dissecting them: thermography; regular X-ray tomography (the "CAT scan"); and shearography (in which a panel is mechanically tapped and its vibration rate is carefully measured for signs of changes indicating internal physical changes).

But, even after the recent discoveries, no NDE was used on the other panels; their examination still consisted of simple visual inspection for pinholes, and "tap tests" on such holes to try to detect underlying voids in the carbon-carbon by their sound. CAIB member Major Gen. John Barry said that what is needed is "certainly getting a little better discipline on the RCC Non-Destructive Evaluations".

As things stand, the possibility can't be ruled out that a Columbia RCC panel broke when hit by that lightweight foam fragment because it had been internally weakened by a spot of such internal corrosion. Indeed, most of the RCC's recovered so far by Columbia search teams have apparently broken along their midlines -- although this may be due simply to the pattern of mechanical stress on them from the overheated, distorted left wing rather than to any internal oxidation.

But the remaining question with which the Board is concerned is why NASA showed such relatively small concern over a repeated pattern of physical phenomena that might damage the Shuttle.

Even after the big chunk of foam came off Atlantis last October, it was not treated as an "In-Flight Anomaly", which might have required postponement of the next flight until the problem was understood and resolved. It was, instead, regarded as a random problem, which was unlikely to repeat, and was in any case not any significant danger.

And yet the design of the Shuttle's Thermal Protection System officially requires that no debris whatsoever must fall onto it from the rest of the Shuttle.

However, ever since the Shuttle program began, an average of about 20 scars more than 2.5 cm wide have been found on the Shuttle's tiles after each flight due to debris falling onto them during launch, whether bits of foam, bits of ice, or occasional objects from the solid boosters (one of which knocked half a tile off a Shuttle in 1989). Nor has this overall figure diminished over the years, despite NASA's occasional attempts at measures to make the foam somewhat safer.

The CAIB is expressing great interest into how NASA came not only to accept this phenomenon but to regard it with a near-total lack of concern. Over the past week, the Board has made it clear that it is coming to agree with the belief some outside observers developed almost immediately after the accident: NASA had applied exactly the same disastrously fallacious reasoning to foam impacts that it applied to O-ring erosion before the Challenger disaster.

That is, it took each flight that didn't end in disaster from a foam impact as actual positive evidence that such impacts weren't really as dangerous as it had initially assumed -- rather than just assuming that it had had another lucky escape, and that it could not count on thus escaping forever.

"I think I'm hearing an echo here", board member Sally Ride said on April 8 -- an echo of the past. Admiral Gehman agreed that there once again seems to be evidence that NASA once again engaged in "the normalization of deviation": the acceptance of anomalies as actually being safer than originally thought, because they had not led to disaster -- up to now. This is like playing Russian roulette and assuming that the fact that the gun keeps failing to go off is evidence that it actually isn't loaded at all.

But there may be another deeper analogy to the Challenger disaster.

Thanks for being here; We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain. With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords. Our news coverage takes time and effort to publish 365 days a year. If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor $5 Billed Once credit card or paypal		SpaceDaily Monthly Supporter $5 Billed Monthly paypal only