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NASA's Latest Mission: Fix Fuel Tank's Shedding Foam

Posted on: Tuesday, 21 August 2007, 09:15 CDT

By Kurt Loft, Tampa Tribune, Fla.

TAMPA -- NASA can put men on the moon, people say, but can't prevent the space shuttle from damaging itself during a launch.

Agency engineers bristle at the comment because the track record speaks otherwise. Yet, their work was tested again Aug. 8 when debris peeled off the shuttle's giant fuel tank and struck the underbelly of Endeavour. The material, a combination of insulation and ice, knocked a small hole in two protective tiles, but officials say the shuttle is safe to fly home.

So today, seven astronauts, including former schoolteacher Barbara Morgan, will slip into the atmosphere and ride out the forces of re-entry, with a planned touchdown at 12:32 p.m. Once craft and crew are safely on the ground at Kennedy Space Center, engineers will turn their attention to a bigger issue: the design of the fuel tank.

Small debris tearing off its upper portion may be a given on any launch, but the size and potential danger of other pieces are not, said Norman E. Thagard, a former astronaut who logged 140 days in orbit. Now at the engineering department at Florida State University, Thagard said frost and insulation are no match for the forces of a launch.

"I launched four times on the shuttle and it always looked like a snowstorm coming off the tank," he said of his ascents into orbit. "It's basically the foam shedding that's the problem."

After nearly 120 shuttle missions since 1981, the National Aeronautics and Space Administration continues to grapple with the 154-foot-tall tank, the largest segment of the shuttle "stack." It proved to be an Achilles heel in January 2003, when debris knocked a hole in the leading edge of Columbia's left wing. Unaware of the fatal flaw, the crew was minutes from landing when the orbiter broke apart, killing all aboard.

Since then, the $50 million tank has been redesigned -- for the fourth time. When debris came off the tank during the launch of Discovery in the summer of 2005, NASA again grounded the fleet to study the problem and modify the fleet for safety.

No matter what changes are made to the tank, it still must endure unbelievable loads and stresses, said John Chapman, manager of the External Tank Project Office at NASA's Marshall Space Flight Center in Huntsville, Ala.

"The tank does a lot of different things, and it has to do them well," Chapman said by telephone. "It looks like a big soda can out there, but it's the structural backbone of the shuttle because everything's attached to it."

Pressures, Velocities, Temperatures

Critics accuse NASA of a poor tank design and failure to fix its problems, but the tank is an imperfect technology, Chapman said, because it must function in an environment both extreme and fluctuating, yet be inexpensive enough to throw away after each launch.

The tank is designed to be lightweight and strong, carrying 525,000 gallons of fuel off the pad and surviving speeds up to 14,000 mph. Essentially a giant thermos, it keeps liquid hydrogen chilled to 423 degrees below zero and oxygen to nearly 300 degrees below zero. Without its one-inch jacket of insulation, the tank's aluminum skin would freeze in seconds.

"So next time you look at an ice chest, think of the challenges we have to face," Chapman said.

NASA workers spray the foam evenly over the metal skin. However, pipe connectors and fasteners can't be completely covered. They expand and contract, making them areas of critical concern, Chapman said. Debris broke free from one of those points during Endeavour's launch.

"All these little brackets have to swing back and forth to compensate for all these changes in temperature, so you can't insulate them completely or they couldn't move," he said. "You're really in a Catch-22, because we have to have some bare metal on the tank."

The foam also must survive the ride high into the sky. After the shuttle jettisons the tank 70 miles above the Earth, the near-empty canister begins to fall and burn up. By precisely timing its destruction, engineers can avoid large pieces hitting ships in the ocean.

"It's very important that the tank breaks up at a known altitude, and the way you do that is with the insulation," Chapman said. "We calculate out breakup altitude based on the amount of foam on the tank."

A Recurring Problem

The big orange fuel tanks have been an issue with both of this year's shuttle missions. In February, a hailstorm damaged insulation as Atlantis sat on the pad. Workers wheeled the shuttle back to the Vehicle Assembly Building and repaired 2,000 holes and dings. Before another mission began, a woodpecker picked holes in the foam.

"It's hard for me to believe that they couldn't find some sort of insulation that's more durable," said Alex Roland, professor of history at Duke University in North Carolina and an expert on the space program.

"Given that NASA lost one spacecraft and with all these nagging problems, you would think they might have opted for a different solution," he added. "The insulation has a great weight advantage but a fatal operational disadvantage."

Others disagree. The design is the best possible considering the tank's extreme workload and low cost, said Corin Segal, an associate professor in the department of Mechanical and Aerospace Engineering at the University of Florida in Gainesville.

"This is not a flaw," he said of the insulation problem. "It's actually a great idea because the foam is porous and this is what makes it a good insulator. Unfortunately, when the shuttle takes off there is substantial vibration, and a piece can fall off, but I would not call it a flawed design."

Flawed or not, the tank as it stands today will be mothballed along with the shuttle fleet in three years. Then, the agency will focus its energies on a new spaceship design to carry crews to the moon and back. That design puts astronauts above -- not below or alongside -- the fuel and any falling foam or ice.

To end the shuttle era, however, NASA must fly at least a dozen more shuttle missions with its current tank. If foam and ice continue to strike orbiters during launch, Thagard said, the odds of a major malfunction will increase.

"I don't know what the alternative to the foam is, but something that can cause the loss of a shuttle is not a solution," he said. "It should be resolved."

Reporter Kurt Loft can be reached at kloft@tampatrib.com or (813) 259-7570.

Source: Tampa Tribune

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