NASA’s investigation into the shuttle Columbia disaster raised more questions Friday about what happened to the orbiter’s left wing, prompted by the release of a blurry Air Force photo that suggested possible damage to the wing’s front edge.
That prompted questions of whether the super-strong heat-resistant material coating the wing edges, called reinforced carbon-carbon, was damaged and possibly allowed superheated air to burn through Columbia’s aluminum skin.
The so-called RCC is a thin layer of light-gray carbon composite that covers the leading edge of the orbiter’s 60-foot wings, to protect them from the tremendous temperatures the shuttle encounters when re-entering the Earth’s atmosphere.
RCC is far more durable than the ceramic tiles that provide thermal protection to the underbelly and other parts of the shuttle, but it is also heavier. So it’s used sparingly and only on the sections of the shuttle battered by the greatest heat: the nose cone and leading edges of the wings.
In those areas, temperatures can reach more than 2,300 degrees. Failure of the molded RCC layer, only a quarter-inch to a half-inch thick over a U-shaped block of metal-wrapped ceramic, would be catastrophic.
The possibility of such a failure was raised by Aviation Week & Space Technology, which reported Thursday that a “high resolution” Air Force tracking photo showed “serious structural damage” to the leading edge of Columbia’s left wing.
But the photo, a blurry black-and-white shot that did seem to show something coming off the shuttle’s left wing, was hardly conclusive.
“It’s not clear to me it reveals anything significant at this point,” shuttle-program manager Ron Dittemore said Friday. “All by itself, I don’t think it’s very revealing.”
Still, the photograph raised questions about how the RCC material performed as the shuttle’s guidance system battled a mysterious aerodynamic drag on Columbia’s left wing.
“If chunks of the leading edge came off during re-entry, the wing is in serious, serious trouble,” said Chuck Eastlake, a professor at Embry-Riddle Aeronautical University who teaches aircraft and spacecraft structural design. The possibility of RCC damage was at least a partial concern from the time NASA learned that a chunk of insulating foam weighing an estimated 2.67 pounds and traveling about 500 mph struck the left wing 82 seconds into Columbia’s ascent Jan. 16.
The Boeing Co., asked by NASA to use computer modeling to test for damage to both the RCC and the more-fragile ceramic tiles that cover the rest of the shuttle’s wing, said the foam was much more likely to damage the tiles.
Boeing did conclude that a more-or-less head-on hit of the leading edge could chip some of the RCC off its underlying ceramic insulation. Ultimately, the report said, the damage was thought to be “no issue.”
Researchers and educators familiar with RCC’s properties agreed.
“I’d be very surprised if you could prove there was damage to the RCC caused by the foam,” said Col. John Keesee, a senior lecturer at the Massachusetts Institute of Technology’s Aeronautics and Astronautics department.
“This stuff is extremely durable,” Keesee said.
RCC starts out as graphite “whiskers,” which are combined with another carbon-based binding material. The composite is cooked at up to 3,200 degrees to remove all the hydrogen and oxygen atoms.
What’s left is a graphite-carbon “cloth” with a carbon-based residue holding it together. Besides the shuttle, it’s also used in nose cones for ICBMs — intercontinental ballistic missiles — and brake discs for commercial airlines.
“Carbon-carbon is really hard. It’s like a rock,” Eastlake said. “I can’t imagine a piece of foam doing any damage to carbon-carbon.”
Yet there has been damage to the leading edge of the space shuttle’s wing on at least four flights, according to NASA reports. And there have been a variety of reasons for the damage.
In 1992, the RCC coating on Discovery’s right wing was damaged in two places by tiny meteorites. An analysis raised concerns that the holes could lead to a burn-through in the shuttle’s wing; one of the holes was about 0.17 of an inch deep — in 0.25-inch RCC coating.
The previous year, in two different Department of Defense shuttle missions, inspection revealed cracks in the seals between some of the 22 U-shaped, RCC-wrapped ceramic blocks that make up the wing’s leading edge.
And in December 1997, after Columbia sustained more than 300 hits from launch debris, investigators found “very small holes, or damage sites” on the leading edge of the right wing. A post-landing assessment attributed the holes to damage from insulation foam.
A retired longtime shuttle engineer said a high-impact collision with a chunk of foam could have cracked an RCC wing panel.
“If it cracked an RCC panel or [seal], you can have a bad day,” he said. “That was always one of the Achilles’ heels of the shuttle. We worried about it, but it held up pretty well.”
