Shuttle Investigators Look at Possibility of Weakened Wing
Columbia accident investigators said Tuesday that they have found most of the leading edge of the shuttle’s left wing and have discovered extensive damage that may help explain how the orbiter broke up on Feb. 1.
Investigators are looking at the possibility that the leading edge, which sustains some of the hottest temperatures on the shuttle skin as it flies back to Earth, was weakened through years of wear and tear that left pinholes and voids, investigators said.
The damage was similar to what termites cause in wood, said Harold W. Gehman Jr., chairman of the Columbia Accident Investigation Board. In this case, the voids occurred in reinforced carbon material just a quarter-inch thick that shields the aluminum wing from temperatures of nearly 3,000 degrees.
The voids and pinholes by themselves probably did not cause a failure in the leading edge, but they could have made the wing more vulnerable to a collision with debris, Gehman said. The board is still trying to assess whether the shuttle was damaged on liftoff by foam debris falling from the shuttle’s external tank or by a collision with something in space.
“It is possible that the foam striking a healthy orbiter might not have done enough damage to cause the loss of the orbiter,” Gehman said. “But it is possible that foam striking an unhealthy orbiter that had problems ... could do some damage. Is it an event she could have survived at age 10 but that she couldn’t survive at 21?”
The Columbia was on its 28th flight when it broke apart, killing all seven astronauts aboard. It flew its first mission in 1981.
The concerns about the leading edge panels are not new. NASA has commissioned or conducted at least a dozen studies in the last decade looking at the panels. The studies examined damage by orbital debris, such as micrometeoroids, and pinholes that were first discovered on the Columbia in 1992.
The leading edge is constructed of a thin layer of material known as reinforced carbon carbon, sandwiched between two coatings of silicon material. The pinholes penetrate the exterior coating, allowing contaminants to eat away the interior carbon and leave voids, the investigators said.
The board, which disclosed the latest findings in a news conference in Houston on Tuesday, is inching ahead, still adding to its staff and saying it is not close to announcing any conclusions. It plans to hold public hearings for the next three weeks at locations across the country. Nonetheless, the board seems to be developing more elaborate theories, backed by more evidence, about what wrecked the Columbia as it crossed over Texas.
Debris from the Columbia is still being picked up by a massive workforce that on most days numbers more than 5,000. The cost so far has surpassed $138 million, according to the Federal Emergency Management Agency. More than 28,000 pieces of Columbia wreckage have arrived at the Kennedy Space Center, where key pieces of the shuttle’s underbelly are being laid out in a hangar. The investigators have some of the pieces from almost all of the 22 leading edge panels that line each wing. Notably, the panels from the left side show significantly more damage than the ones from the right side, Gehman said.
Investigators want to know how NASA and its contractors inspected and maintained those leading edges and whether the procedures were adequate, said Maj. Gen. John Barry, a board member. A team of investigators next week will visit the Lockheed Martin Corp. plant that provides the panels, he said. On six prior shuttle flights, orbiters returned with their leading edges damaged by debris, micrometeoroids or other causes, Barry said.
A Boeing official said Tuesday that “several of the wing leading edge RCC panels were reinforced to resist penetration or damage” when the Columbia went through a 17-month overhaul in Palmdale from 1999-2001. The official declined to provide details, saying the company needed approval from NASA to release the information.
The investigation is still trying to find out how hot plasma got into the Columbia’s left wing as it soared over the Southwest. If that hole was on the leading edge, it would help explain a great deal of the evidence that has been gathered so far.
Sheila Widnall, a board member from the Massachusetts Institute of Technology, said flight data show that the shuttle began experiencing unusual heating early in its return to Earth and only later a sudden aerodynamic problem. In its final minutes, the shuttle fought against aerodynamic forces trying to pull it left, known as a yaw. The yaw was created by problems on the left wing that increased the drag on that side of the craft.
Wind tunnel tests conducted at NASA’s Langley Research Center in Hampton, Va., assessed what would happen if a single leading edge panel were ripped off. Based on those studies, the aerodynamic forces acting on the Columbia would have required the loss of five or six leading edge panels. Photos taken during the launch appear to show the foam striking the orbiter’s leading edge between panels No. 6 and 8, an area that is the focus of wind tunnel tests.
NASA studied the leading edge problems as recently as March 2000. Donald M. Curry at Johnson Space Center, along with two Boeing engineers -- Ignacio Norman and Dennis C. Chao, who analyzed the foam insulation strike on the Columbia -- studied the potential for damage by debris hitting the panels at hypervelocity in orbit. The study concludes that such strikes pose the highest risk of critical failure on the shuttle.
Other studies during the late 1990s raised the possibility that hot gases during reentry could enlarge pinholes that were appearing on the panels but concluded that the problem was unlikely to endanger the spacecraft or its crew. The microscopic holes were discovered in 1992 as NASA officials inspected the Columbia shuttle after its 12th flight. Inspections found the pinholes in other orbiters and NASA officials concluded that the holes generally developed after 10 to 15 flights, although the Columbia seemed to have the worst experience with them.
The investigation board also said it was looking into new information that the Columbia underwent a period of unusually high structural stress about 62 seconds after liftoff when it encountered a large wind shear. It wants to know whether the Columbia’s maneuver to deal with the wind shear, programmed into its booster system before launch, may have contributed to the forces that caused foam to fall off the external tank.
Maj. Gen. Kenneth Hess, a board member, said investigators are also examining whether problems may have occurred during Columbia’s launch preparation, when the external tank was attached to the solid rocket boosters and then taken off sometime last fall. A problem report was generated involving the foam insulation on the external tank, though NASA ruled afterward that there were no concerns with the foam.
Hess also disclosed the board is looking at whether liquid hydrogen or oxygen may have seeped into cork-type insulation underneath the foam, known as ablative material. If so, the liquid gases would have boiled off during launch and possibly popped off the foam, Hess suggested.
The ablative is used in the bipod attachment area, where the external tank is hooked to the orbiter. The foam chunks are believed to have detached from that area during launch. Hess said the ablative material serves no purpose and that NASA is assessing whether to get rid of it.