NASA Revamps Foam-Safety Measures for Future Shuttle Flights

Feb. 1–When the next space shuttle roars aloft from Kennedy Space Center sometime in the next year or so, NASA engineers will make absolutely certain the orbiter won’t be hit by a chunk of insulating foam like the one that brought down Columbia.

They can say that with assurance because they’ve done away with the so-called “bipod ramp” that produced the 1.67-pound chunk that smashed a hole as big as 10 inches in diameter in the leading edge of Columbia’s left wing.

When the orbiter re-entered the atmosphere a year ago today, superheated gases flooded into the wing, melting it and dooming Columbia in the skies over Texas.

But what NASA engineers still can’t say for certain is why that foam broke off. So they’re revising spray-on techniques and developing a high-tech inspection device to try to make sure other thick areas of foam don’t fall off the tank.

“Foam is an inherently difficult material to work with,” said Columbia accident investigator Lt. Commander Johnny R. Wolfe, a deputy chief engineer for Navy Trident missiles. “The funny thing about foam is that you can actually have a section of foam fail and you can do a test right next to it and [it] passes.”

In the past year, NASA engineers have figured out how to eliminate the so-called “bipod ramp,” the wedge-shaped, 2.5-foot-long chunk of foam sprayed onto the 15-story fuel tank at the points where the shuttle’s nose attaches to the tank.

But they still haven’t figured out why the chunk broke away and struck Columbia’s wing 81.9 seconds after liftoff at a relative speed of about 500 mph. It struck the wing of the rapidly accelerating shuttle with more than a ton of force.

In fact, NASA had given up years ago trying to solve the mystery of falling foam. While the chunk that hit Columbia was the largest ever recorded, foam had dinged, dented and pelted shuttles during all 112 previous flights.

“There was obviously not the proper attention paid to the criticality of foam,” said Neil Otte, NASA’s deputy manager for the external tank.

Even for a short time after the accident, the space agency seemed willing to dismiss foam as the cause of the Columbia accident. While searchers were still mobilizing to recover shuttle debris in east Texas, NASA Administrator Sean O’Keefe told Congress that the foam strike was like “a Styrofoam cooler blowing off of a pickup truck ahead of you on a highway.”

But it quickly dawned on NASA engineers — and investigators for the Columbia Accident Investigation Board — that too little was known about external-tank foam to describe it in such simplistic terms.

Though often compared to the stuff a foam cooler is made of, shuttle foam is tougher. A person might be able to poke fingernail-deep into the foam, which weighs about 2.4 pounds per cubic foot, but would have to put effort into breaking a bipod ramp with a baseball bat.

“A person couldn’t possibly pull that ramp off a tank,” said Paul Munafo, who was the top materials expert at NASA’s Marshall Space Flight Center and now is the deputy director of the newly formed NASA Engineering and Safety Center. “We tried.”

Yet if not applied with precision, Munafo said, foam can simply fall apart.

The spray-on insulator is used by industry primarily in appliances, such as refrigerators, and as attic or roof insulation. On the shuttle’s fuel tank, it prevents icing when the tank is filled with a half-million gallons of super-cold liquid oxygen and hydrogen. “The foam was never intended as a structural material,” Munafo said.

In fact, though, the bipod ramp did serve a structural role; it deflected wind away from two joints where a “bipod” of shuttle-attachment arms is bolted down.

At the Michoud Assembly Facility in New Orleans, where the tank is made, most of a tank’s two tons of foam coating is applied by automated sprayers.

But each bipod ramp is hand-sprayed by a pair of technicians, suspended with their heavy spray gear 60 feet in the air alongside the tank. Once spraying starts, it cannot stop until the job is done.

After each spray pass, technician must wait 30 to 60 seconds before the next pass. Spray too soon, and the expanding foam will overheat; too late and the layers won’t stick together. The awkward shape of the attachment hardware also requires a skilled touch to avoid formation of lumps or bubbles.

The finished product is a mound 4 feet long by 2 feet high and 2 feet wide which is then machine-shaved to a wedge. To the experts brought to the Columbia investigation, it quickly became clear how little NASA knew about the insides of those ramps.

“The thing I found most interesting is that never at any point had they [NASA] cut up one of these things,” said Wolfe, the Trident engineer, who said a routine practice in high-risk industry is to take apart materials to “figure out what you are getting internally.”

While investigators watched, Michoud technicians used wood saws and fillet knives to remove bipod ramps from other external tanks in the shuttle inventory. In a laboratory, the pieces were sliced like cold cuts for microscopic analysis. No gaping flaws were found. But the accumulation of defects began to speak for themselves.

“As we were doing the dissections, there were some guys who were very upset because they had sprayed these things,” Wolfe said. “We found flaws, and they took it very personally.”

What they discovered were portions of the ramps riddled with weak seams and internal voids, the largest no bigger than a pencil or a golf ball.

Though investigators estimated that these flaws added up to no more than 1 percent of the overall ramp volume, Munafo said an “interaction of defects” probably weakened the ramp. Internal weak spots may have lined up like holes in perforated paper, he said, so that the pounding wind of launch tore the ramp free.

Another theory suggests that when the tank was filled with super-cold fuel, moisture condensed inside the bipod foam. During the simultaneous heating and the drop in atmospheric pressure that occurs while a shuttle thunders toward orbit, the liquid would have boiled into a vapor — with a volume much greater than the liquid — that essentially blew off the foam.

That theory had been dismissed by the investigation board. But Otte said recent experiments have revived the idea.

Still, despite dissections of bipod ramps and launch-simulation experiments in vacuum chambers and wind tunnels that involved bending, vibrating, battering, heating and chilling foam, the theories have remained only theories.

“We had pretty elaborate tests,” Otte said. “We never did blow a bipod ramp completely off.”

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