Board to suggest repairs for shuttle insulation
Monday, June 23, 2003
The Columbia Accident Investigation Board is preparing to recommend as early as this week that NASA fix foam insulation problems linked to the shuttle tragedy before resuming space flights, an official close to the investigation said.
The recommendation might put a chill on the National Aeronautics and Space Administration's plans. Space agency officials have vowed to fly again by early next year, even though they have not figured out how to fix all the problems related to the foam debris.
Despite the intense investigation since the Columbia accident Feb. 1, the foam problem remains poorly understood and a significant challenge to the space program. A piece of foam became a ballistic weapon during the Columbia launch, striking the orbiter's left wing and most likely causing grave damage.
Investigators have determined that the insulating foam applied to the space shuttle tank has contained voids and other defects in the past, but they have not been able to explain the precise mechanics of the why foam falls off during some launches and not during others.
Last week, top NASA engineers concluded a preliminary review at a Lockheed Martin plant near New Orleans on proposals to fix at least part of the problem, but have not decided on a course of action, an agency spokesman said. At the same time, senior NASA officials have made optimistic projections about resuming flights as early as next January.
Independent experts are trying to brainstorm the problem on their own. Engineers at the University of Southern California, for example, are sending a proposal to NASA this week to use a special fiber-reinforced foam that the university's Composite Center has developed. And independent engineers have proposed to the Columbia board that the foam could be encapsulated in high strength, low weight carbon fibers.
Foam debris has dogged the shuttle program since the first flight in 1981, but NASA officials gradually came to view the foam as a maintenance nuisance, not something that could cause catastrophic damage. Throughout the 20-year history of the program, NASA has made various attempts to reduce foam debris, but has never succeeded in eliminating the problem, according to senior NASA officials who have testified before the safety board.
Only last October, a large foam piece broke off during the launch of the Atlantis and struck the shuttle's solid rocket booster, although it was not catastrophic. NASA initiated an internal study of the problem, although it was delayed and not scheduled to be completed until mid-February, two weeks after Columbia was lost.
The accident might force NASA to fix a problem in six months that has eluded it for two decades.
The shuttle's external tank, which holds the liquid oxygen and hydrogen propellants for the shuttle, is covered by 4,800 pounds of foam, mostly a common type of sprayed on polyurethane. Its purpose is to prevent ice from forming on the tank exterior and to keep the liquid propellants cold.
One problem with the foam is smaller pieces that fall and damage heat-resistant tiles. NASA has come to expect an average of 30 debris hits on the shuttle's delicate thermal protection system on every mission, mostly from falling foam.
A much bigger threat is the so-called bipod ramp - a wedge-shaped piece of foam that has fallen off on seven previous shuttle missions. It is believed a 1.6 pound of bipod ramp foam rammed the Columbia's left wing moments after liftoff.
Until recently, Columbia investigators and NASA engineers surmised that voids and cracks in the foam allowed the formation of liquid nitrogen pockets. During launch, the liquid nitrogen boiled off so quickly that it popped foam off.
But shuttle investigator Douglas Osheroff, a Nobel prize-winning physicist from Stanford University, cast doubt on that theory last week when he disclosed the results of an experiment he did in his kitchen sink. The experiment examined whether internal pressures would simply crack or break away foam, and the result has left a new void in the understanding of the foam problem.
"It behooves NASA to understand these processes better," Osheroff said.
All the foam strikes violated the original safety rules for the shuttle, which said no debris ever should strike the thermal protection system. If Columbia accident investigators say NASA must now abide by that rule, the agency might have a tough time complying.
Neil Otte, deputy manager for the tank at NASA's Marshall Space Flight Center in Huntsville, Ala., said last month that he doubted NASA ever could eliminate all foam debris coming off the external tank.
Other NASA officials have concurred, according to Columbia accident board sources, but the board largely has dismissed them.
As for the bipod ramp, the Marshall experts are working on three possible solutions: covering the ramp with a metal shield; eliminating the ramp foam and using a heater to prevent ice form forming; and simply reducing the size of the bipod ramp.
But the board is preparing to issue a number of interim recommendations this week to give NASA a head start on making fixes to the shuttle rather than waiting for the final investigation report in late July.
Among those recommendations will be a call for NASA to fix the foam problem, according to the official close to the investigation who asked not to be identified.
Outside experts say the entire foam problem can be solved with a high level of confidence.
Boeing foam expert Keith Chong has told Columbia investigators that he is not aware of any foam losses during launches of Boeing's Delta IV launch system. Moreover, Chong has described fairly sophisticated inspection processes with lasers that Boeing uses to ensure that 100 percent of the foam is bonded properly to the rockets. NASA inspections are not as intensive or complete. Chong also noted that there are alternatives to the polyurethane foam that NASA uses on the external tank, including one variety made in Japan. NASA rejected that because of its higher cost, Chong said.
USC's composite center, which has a team of 22 researchers, has developed a revolutionary type of high strength foam that uses glass or aramid fibers, according to director Steve Nutt.
The fibers increase the fracture resistance of foam by several fold, USC tests have shown. Moreover, once a piece of foam cracks, the fibers still can hold separate pieces together.
Other independent engineering experts in the aerospace industry and at major universities say the whole foam problem would go away if NASA would encase the external tank in a lightweight filament winding - fine strands of high strength glass or carbon that would be wound around the the tank and embedded in the foam.
Columbia investigators received a proposal along those lines from consulting engineer Oscar Weingart, who holds five patents in the area of filament winding systems and pioneered the technology at a number of major aerospace companies.
A proposal sent to Columbia investigators suggests that the entire surface area of the external tank could be covered with carbon filaments at a weight of just 800 pounds. But Weingart suggested that a looser network, forming a net that would cover one-fourth of the surface area of the tank, would be effective at a weight of just 200 pounds. Weingart said the process would cost less than $1 million per tank, a small percentage of the tank's current $34 million cost.
The validity of the filament plan is endorsed by Charles McMahon, a professor of materials science and engineering at the University of Pennsylvania who first suggested the idea in the days following the Feb. 1 accident.