SAN ANTONIO — Columbia investigators obtained their first direct evidence that foam debris caused the space shuttle accident after a foam block fired at a wing panel left a 3-inch crack in a test Friday.
Engineers used a powerful gas gun to shoot a 1.6-pound block of foam at a replica of Columbia's left wing, damaging the delicate heat shield on the leading edge. Such damage may explain how a breach opened, allowing superheated gases to enter the wing and melt the aluminum structure.
The foam, shot at 768 feet per second, or about 530 miles per hour, also dislodged the leading-edge panel by about one-tenth of an inch and opened a slightly larger gap than is normal along one side of the panel and slightly smaller on the other side.
The board previously indicated that it believes foam debris fell off the shuttle's external tank 82 seconds after launch and struck the leading edge of the wing, which is constructed of a material called reinforced carbon carbon, or RCC.
Investigators for the Columbia Accident Investigation Board said the test results represent an important step in their efforts to solve the Feb. 1 accident, when Columbia broke up over East Texas.
"This is the first evidence that a piece of foam that approximates what was observed in the accident can crack and damage a piece of flight reinforced carbon carbon," said Scott Hubbard, a member of the investigation board.
The test culminated months of planning that had been put on hold Thursday by a Texas thunderstorm and again Friday morning when an electrical circuit malfunctioned. After a dramatic countdown to zero, nothing happened and a crew of workers had to partially disassemble the gun.
By early afternoon, the gun was back together and precisely aimed with a laser. After another countdown, the gun let loose with a loud whoosh followed by the thunderclap of the impact.
The foam slammed into the slate gray wing one-third of an inch thick with a force of 4,500 pounds -- enough to kill a person. It sent puffs of foam dust and sprayed larger chunks across the test site.
After the test, Hubbard said the foam shot had created a 3-inch crack that extended from the surface of the leading-edge panel, under a part known as a T-seal and through reinforcement called a rib.
The leading-edge panel used for the test Friday was taken from the shuttle Discovery and had flown into space on 30 missions, similar to the flight history of Columbia. It cost an estimated $775,000 to make in the 1980s.
Paul Fischbeck, an engineering professor at Carnegie Mellon University in Pittsburgh who has helped conduct independent studies of the shuttle's thermal protection system, said the results of the test are "very revealing."
"It is quite remarkable that foam could do that to an RCC panel," Fischbeck said. "To me, that is phenomenal. Six months ago, nobody would have believed that foam could crack an RCC panel."
Although the leading-edge panel sustained a serious crack that would disqualify it from any flight into space, the foam test did not provide absolute evidence that such a crack would have created a flight hazard, Hubbard said.
Until now, investigators were looking for a gap or breach in the leading edge that would have allowed superheated gases an entry into the wing's interior. It could take days of analysis to fully assess whether the cracks observed Friday could have led to a subsequent breach during reentry.
"We have some of the world's top reinforced carbon carbon experts here and they are scratching their heads and wondering what it means," Hubbard said. "They are not sure this crack would be sufficient to cause the entry of hot gases." The foam test was recorded by a dozen high-speed cameras and 199 channels of data from sensors embedded throughout the sophisticated test rig that held the wing replica in place.
The tests were conducted at Southwest Research Institute, a nonprofit research and testing center that has 2,800 employees and conducts government and private industry work.
More Damage Expected
A number of the investigators had been expecting more dramatic damage to the wing, perhaps a large hole or a disintegration of the panel, given the results of a similar test two weeks ago when foam was shot at a leading-edge panel made of fiberglass. The fiberglass was dislodged and permanently deformed.
Hubbard noted, however, that the foam strike did not crack the fiberglass, but did crack the reinforced carbon carbon.
The RCC panel is made of a cloth reinforced with resins, the only substance known that can withstand the temperatures of more than 3,000 degrees when the orbiter hits the atmosphere at more than 17,000 mph.
The block of foam was shot from a nitrogen gas-powered gun with a specially constructed rectangular barrel that custom-fit the foam block measuring 19 inches by 11.5 inches by 5.5 inches.