Foam Issue May Delay Resumption of Shuttle Flights

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 spaceflights, an official close to the investigation said.

The recommendation may put a chill on NASA’s plans. Agency officials have vowed to fly again by early next year, even though they have not figured out how to fix all of 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 struck the orbiter’s left wing during the Columbia launch, most likely causing grave damage that led the shuttle to break up as it returned to Earth.

Investigators have determined that the insulating foam applied to the space shuttle tank has contained defects in the past, but they have not been able to explain exactly why foam falls off during some launches and not during others.

Over the last week, top NASA engineers concluded a preliminary review 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 January.

Independent experts are trying to brainstorm a solution on their own. Engineers at the University of Southern California, for example, are sending NASA a proposal this week on using a 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 strong, lightweight carbon fibers.

Foam debris has dogged the shuttle program since the first flight in 1981, but NASA officials came to view the foam as a maintenance nuisance, not something that could cause catastrophic damage. Throughout the 22-year history of the program, NASA has made various attempts to reduce foam debris, but has never succeeded in eliminating the problem.

In October, a large foam piece broke off during the launch of the Atlantis and struck the shuttle’s solid rocket booster, though it did not prove catastrophic. NASA initiated an internal study of the problem, but it was not scheduled to be completed until mid-February, two weeks after Columbia was lost.

The accident now may force NASA to fix a problem in six months that it has been unable to solve in two decades.

The shuttle’s external tank, which holds the liquid oxygen and hydrogen propellents for the shuttle, is covered by 4,800 pounds of foam, mostly a sprayed-on polyurethane. Its purpose is to prevent ice from forming on the tank exterior and to keep the liquid propellents cold.

One problem with the foam involves 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 the falling foam.

A much bigger threat is the so-called bipod ramp, a wedge-shaped piece of foam that has fallen off on seven prior shuttle missions. It is believed a 1.6-pound piece 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, which boiled off so quickly during launch that they 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 results have revealed a new void in the understanding of the foam problem.

“It behooves NASA to understand these processes better,” Osheroff said.

All of the foam strikes violated the original safety rules for the shuttle, which said no debris should ever strike the thermal protection system. If investigators say NASA must now abide by that rule, the agency may 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 could ever eliminate all foam debris coming off the external tank.

“With the environment, the materials, the complexity, I don’t believe there’s a way to find an engineering solution,” Otte told Associated Press. He said that he would never agree to a mandate for zero foam loss. “I don’t think there is anyone in the system who will,” he added.

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 from 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 allow NASA to get a head start on making fixes to the shuttle rather than waiting for the final investigation report in late July.

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 sophisticated inspection processes that Boeing uses to ensure that the foam is properly bonded to the rockets. NASA inspections are not as thorough. Chong also noted that there are alternatives to the polyurethane foam that NASA uses on the external tank, including a variety made in Japan that NASA rejected because of its higher cost.

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 polyurethane foam on the shuttle is very cheap and it can be as soft as a pillow or as stiff as a board,” Nutt said. “We have been working on an approach to stiffen the foam using chopped fibers.”

The fibers multiply the fracture resistance of foam by several times, according to USC tests. Moreover, once a piece of foam cracks, the fibers can still hold separate pieces together.

Other independent engineering experts say the whole foam problem would go away if NASA would encase the external tank with a lightweight filament -- fine strands of high-strength glass or carbon that would be wound around 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 firms.

“Even the thinnest network of fibers would retain foam and keep it in place,” Weingart said.

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 covering 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 $34-million cost.

The filament plan is endorsed by Charles McMahon, a professor of materials science and engineering at the University of Pennsylvania.

“It seems to me that encapsulating this foam tank with a fiber-reinforced cover makes some sense,” McMahon said.

H. Thomas Hahn, an aerospace engineering professor at UCLA, agreed that a fiber enclosure would be promising. Hahn calculated that a single strand of carbon fiber can carry a load of nearly half a pound. Thousands of these fibers every square inch would be capable of retaining any foam debris, he said.