NASA considered three redesigns to replace a long piece of foam insulation on the space shuttle's external tank where a chunk broke off during the launch of Discovery, according to NASA officials and records, but the space agency decided against all of them because the old design was deemed "safe to fly."
The long ridge of foam running down the side of the tank was one of several areas that got special attention after the Columbia disaster 2 1/2 years ago because it was in a critical area where dislodged foam could slam against the orbiter's vital heat shielding.
NASA grounded the shuttle fleet Wednesday after analysts discovered that Discovery's tank lost pieces of foam insulation similar to those that fatally damaged Columbia. The foam ripped off in an area known as a protuberance air load (PAL) ramp, a 36.5-foot-long triangular wedge that acts as a windbreak to protect the tank's cables and pressurization lines from being ripped off by air pressure in flight.
The new foam mishap, which NASA officials do not believe caused serious damage, illustrates the difficulty of eliminating every potential problem with the tank design, which has had a long history of losing foam chunks. Records show that after spending millions of dollars making repairs, some officials at the space agency had lingering doubts that they had done enough and continued to investigate potential fixes for the foam covering.
"After 2 1/2 years of testing, the foam shouldn't have come off. They should have done something to get that foam to stick," said Paul Czysz, a professor emeritus of aerospace and mechanical engineering at Saint Louis University. "I would have thought after 2 1/2 years it would have been solved."
Chunks of foam from the same part of the external tank had fallen off during two shuttle flights in 1982 and 1983, said NASA spokeswoman June Malone at the Marshall Space Flight Center in Huntsville, Ala. But NASA officials decided then that those incidents, coupled with another detailed analysis, were not enough to warrant changing the design, Malone said.
Since the Columbia accident, however, records on NASA's Web site and interviews show that redesign of that area had been under careful consideration. A fact sheet dated August 2004 said "three redesign options were under consideration for future flights" and one would be selected. But an April 2005 fact sheet said the old design of the foam wedge, used on Columbia, was acceptable.
"Based on material analysis, testing and flight data, NASA is satisfied that the current design configuration of the PAL ramp is safe to fly," the April fact sheet said.
The earlier instances of trouble with the PAL wedges occurred during a Columbia flight, launched June 27, 1982, and on a Challenger flight, launched June 18, 1983. In those cases, NASA determined the foam came off because of the configuration and repairs that had been made to the foam on that area of the tank.
Joseph W. Cuzzupoli of Kistler Aerospace Corp., who served on a task force that helped oversee NASA's redesign, said his group agreed with the agency's decision not to change the PAL ramp area design because that section was not prone to foam breakage and because NASA planned to perform more detailed inspections for tiny air pockets that might have formed when the foam was installed. Pockets, or voids, were believed to have caused the foam breakage that doomed Columbia, Cuzzupoli said, but had never been detected in the PAL ramp area.
"I think what they have learned from this incident is that there are other failure modes they should look at," he said.
In 2003 and 2004, NASA officials conducted extensive wind-tunnel tests on three possible redesigns at NASA's Glenn Research Center in Cleveland, said G. Scott Williamson, the engineer who conducted the tests. They considered eliminating the foam wedges, reducing them or building a kind of "fence" on the back edge of the rectangular tray that holds cables to redirect the wind.
