Spaceflight Now

Breach moves slightly inboard; key tests on tap
Posted: April 29, 2003

CAIB board member Roger Tetrault. Credit: CAIB Photo by Rick Stiles 2003
Ongoing analysis of recovered debris from the shuttle Columbia indicates the deadly breach in the ship's left wing may have been centered on a broken leading edge panel and not slightly outboard at a so-called "T-seal" as investigators were thinking last week, the Columbia Accident Investigation Board said today.

While a T-seal failure has not yet been ruled out, several lines of evidence now point to a failure at or very near the lower half of leading edge panel No. 8. That's where NASA investigators assumed the breach was located in their own failure scenario, which was briefed to the CAIB last week.

"I wouldn't say that T-seal No. 8 is totally out of the picture," said board member Roger Tetrault. "I've found that every time you think you have an answer ... something shows up that takes the wind out of your sails. But I'd say it was getting more probable that the breach is a little bit farther to the inboard side of the wing than where T-seal No. 8 is."

Columbia was destroyed Feb. 1 during re-entry when a breach in the leading edge of its left wing allowed a plume of super-heated air to eat its way inside, ultimately triggering a catastrophic structural failure. The damage is believed to have been caused, at least in part, by the impact of foam insulation that broke away from the shuttle's external fuel tank 81 seconds after liftoff Jan. 16. The foam slammed into the leading edge of the left wing at more than 450 mph.

The leading edge is made up of 22 U-shaped reinforced carbon carbon panels that handle the aerodynamic loads and extreme temperatures of re-entry. The panels are held in place at each end by U-shaped T-seals bolted to the front spar of the wing.

The day after Columbia's launching, military radar systems detected an object moving away from the shuttle. That mystery object, first reported by CBS News, is now believed to have separated from the breach area. CAIB investigators said last week a T-seal, or a substantial piece of a T-seal, was consistent with the "radar signature" and area-to-mass ratio of the mystery object.

That is still true. But today, Tetrault said additional evidence raises questions about the T-seal hypothesis. First of all, there is now some uncertainty as to whether recovered T-seal fragments came from the inboard side of RCC panel 9 or from father outboard on the wing. More telling, perhaps, X-ray analysis of slag-like deposits found on the inner surface of fragments from RCC panel 8 are consistent with a breach in the lower part of that panel. NASA has recovered three pieces that once made up the upper section of RCC panel 8 but nothing from the lower half.

"What you see on the X-ray you do not see visually," Tetrault said. "What you begin to see (are) deposits of heavy metals in certain patterns that we don't see when we look at it visually. ... There are small nodules of metal which are being deposited in a very uniform pattern.

"If you stood behind the RCC panel and took a paint brush and sprayed it at the back of the RCC panel, you would get deposits that look very similar to this. They are very uniform and very straight up and down. It begins to indicate that we're not getting a lot of side flow from, say, a T-seal position or even a panel further back."

Translation: Assume a hole in the lower half of RCC panel 8 and visualize the shuttle oriented in a 40-degree nose-up entry attitude. Hot air would enter such a breach at an angle, impinge on and ultimately burn through insulation on the front of the spar and eventually through the spar itself. Molten droplets of metal would have been blown backwards as this process proceeded, onto the inside surface of the upper part of RCC panel 8. The slag pattern seen in the X-rays Tetrault referred to is not consistent with the sort of "side flow" one would expect if the breach was located at the outboard T-seal or farther out the leading edge.

When the plume finally burned its way into the wing's interior, it hit the upper surface of the outboard side of the left main landing gear wheel well and burned through cable bundles mounted there in a top-to-bottom sequence.

Based on analysis of the known locations of sensor wires just behind the spar; the timing of sensor failures as the plume burned through various wire bundles; the observed heat damage to recovered debris from RCC panel 8; and heat damage seen in debris from areas adjacent to panel 8, it now appears the breach may have, in fact, occurred on the lower side of panel 8 as NASA investigators assumed when they developed their own "best fit" failure scenario.

The plume "probably hit the (insulation) material before it hit the spar and that was the first deposits that were made to the back of the RCC panels," Tetrault said today. "With regard to the cutting of the wires along the wheel well, actually by moving a little bit father inboard, you actually close the distance between those wires that were cut along the spar versus the wires that were probably cut along the wheel well and that makes it even more possible this sequence works properly."

Starting next week, investigators using a nitrogen gas canon at the Southwest Research Institute in San Antonio, Texas, will begin firing large pieces of foam insulation at selected wing targets to determine what sorts of damage might result from high-speed impacts.

Foam will be fired first at large areas of heat-shield tiles representative of the surface of the underside of Columbia's left wing. By early June, investigators hope to begin firing foam at a test rig that will simulate the left wing leading edge system between RCC panels 6 through 9.

"The testing starts this week," said board chairman Harold Gehman. "But the testing is limited by the target, not by the bullets. We have plenty of foam. But when we decided to do foam testing 30 days ago, we were talking about (landing gear) wheel well breaches and so the first test rig that we built was a wheel well. That's what we're ready to shoot at. Now we're talking about the leading edge system and it'll take us a couple of weeks to build a representative of the leading edge."

To make the test as accurate as possible, high-flight-time leading edge panels from other shuttles, as well as pristine panels from the prototype shuttle Enterprise, will be mounted on the test rig using T-seals.

"We're building up a leading edge section that's going to be essentially from panel No. 6 to panel No. 9 so that we can shoot at anything we want," Gehman said. "By the time we get ready to actually shoot at it, we'll know more about what angles we cover. We may want to shoot it at an angle that covers two T-seals and one panel or two panels and one T-seal. And we may shoot a couple of times."

He said the tests are being designed to indicate "whether or not a foam strike of the size and the velocity we saw in this particular event has the capacity to damage the leading edge system with sufficient damage to cause a hole that would initiate this event."

"Even if we do create damage on the test article that is of sufficient magnitude to initiate this event, that doesn't prove it did it," Gehman said. "It just proves it could have done it. And oh by the way, we have oversimplified to some degree what we're talking about here. I can pound on this table with a sledge hammer and not do any damage to the table, but (I might still) break a leg. The same thing could be true of the leading edge system. We could hit the outside of the leading edge system and not do any damage to it, but we could break the support structure, break a bolt, break a pin, in which case we would have something flapping in the breeze that might come off later.

"So we're going to carefully instrument the inside of this target so that we can, through analysis, better characterize all the ways that it could fail, not just a fracture through the RCC."

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