Scientists at NASA's Ames Research Center in California prepared yesterday to retest the materials they helped develop in the 1970s for protecting the space shuttle from burning up on reentry, amid possible new evidence of damage to Columbia's left wing.
Ames labs that simulate the pressures and heat of reentry were gearing up to reexamine what effect those forces have on broken pieces of insulating material, especially the reinforced carbon-carbon (RCC) that lines the leading edges of space shuttle wings.
Researchers also planned to study what aerodynamic stresses would have buffeted the craft if tiles or panels were seriously damaged.
"We're planning to do a variety of tests," said Howard Goldstein, who is retired as chief of the Ames space technology division and now consults at the center.
He said materials experts at Ames were intrigued by the shadowy Air Force image released yesterday of Columbia with what appears to be a ragged leading edge on the left wing, though he and NASA officials cautioned that the image was too imprecise to suggest conclusions. Ames is one of NASA's leading facilities for materials testing and helped what is now Lockheed Martin Corp. come up with substances to protect the shuttle from heat.
A spokesman for Lockheed Martin said he didn't know whether NASA consulted company scientists yesterday on the reinforced carbon-carbon that they developed to protect the leading edges. "But we are supporting NASA with any and all resources that this corporation has with this investigation," corporate spokesman Jeffery A. Adams said.
The company's Missiles and Fire Control unit in Dallas made the leading edges for Columbia and completed delivery to NASA by 1978, Adams said. Lockheed Martin also refurbished the surface sealant on 12 sets of Columbia's leading edge panels in 2000, he said. Each wing has 44 sections of the material on the leading edge.
RCC is designed to be more rugged than the brittle ceramic tiles that lined the orbiter's underside. It protects the areas of greatest stress, including the tip of the nose cone and the "chin" between the nose and the front wheel well, which can heat to nearly 3,000 degrees during reentry.
NASA officials said they are looking at a range of possible factors that could have led to the Columbia's disintegration a week ago, including the possibility that the ceramic tiles were damaged by foam insulation or ice shearing off the external fuel tank during liftoff. They are also considering whether the tougher leading edge of the wing may have been damaged, which raises the possibility of more serious impact from something such as space debris.
"It sounds like it had to have hit something," said Francis Schwind, president of Carbon-Carbon Advanced Technologies Inc. in Fort Worth, which makes the material for other NASA programs but not the shuttle.
Schwind had not seen the new images released yesterday nor consulted with NASA. But he said the properties of reinforced carbon-carbon make it unlikely that anything less than a significant impact would cause the leading edge panels to fail.
The substance starts out as a bolt of black carbon cloth about the consistency of heavy bedsheets. Like dressmakers, workers use patterns to cut shapes from the cloth, which are then layered onto forms in the configuration of the shuttle component they will ultimately become.
The form is put into a vacuum bag so it holds its shape and baked at high temperature in an autoclave, which causes phenolic resin in the fibers to make the cloth rigid. After another bake at roughly 1,500 degrees, the phenolic resin breaks down and becomes carbon, creating carbon fibers held in a carbon matrix. A few more bakings make the substance even harder.
The material is packed with a silica coating and heated again, converting the top few layers into silicon carbide, which keeps oxygen from penetrating the piece at high temperatures. Finally, workers apply glass sealers to the outer surface. When the part heats up during the shuttle reentry, the silicon carbide expands at a different rate than the carbon beneath it, forming microcracks. The glass sealers fill those cracks.
The U-shaped components that form the wings' leading edges attach to a spar inside the wings with metal bolts. The reinforced carbon-carbon is made thicker around the bolt hole for more strength.
Despite all the hardening and coatings, RCC remains a brittle substance, though not as fragile as the ceramic tiles on the shuttle's underside. "You could hit one with a ball-peen hammer and you could probably chip the coating," Schwind said.
If Columbia had such superficial damage, he said, the only result would have been "a local area of oxidation," with a few layers of reinforced carbon-carbon burning away during reentry. For the total failure that apparently destroyed the craft, he said, the panels would have had to have been completely smashed.
There also is insulation packed behind the wings' leading edges, he said, so that probably was damaged as well.