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Technicians Use Scanners to Survey External Tank
Technicians apply foam insulation to the external tank.Technicians spray foam insulation on space shuttle Discovery's external tank to cover a pair of repaired stringers. Technicians are using scanners to see through the foam on other stringers to examine them for flaws. Photo credit: NASA
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A work platform gives access for technicians. A technician brings some tools into the environmental enclosure built at Launch Pad 39A so workers can get access to part of the external tank. Photo credit: NASA/Dimitri Gerondidakis
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A technician examines a cracked stringer.A technician takes pictures of a cracked stringer on space shuttle Discovery's external tank. Photo credit: NASA/Troy Cryder
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One of the doublers to fix a cracked stringer. A technician shows one of the "doublers" used to repair and reinforce the stringers that showed cracks. The stringers make up the ribbed intertank section of the shuttle's external tank. Photo credit: NASA/Dimitri Gerondidakis
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A couple kinds of high-tech devices and steady nerves by technicians have been called on to carefully survey the external fuel tank of space shuttle Discovery as it sits on Launch Pad 39A at NASA's Kennedy Space Center in Florida.

The scans are expected to be finished sometime Wednesday, giving Space Shuttle Program managers additional information they need to help decide whether Discovery is ready to launch on the STS-133 mission to the International Space Station. The mission is to launch no earlier than Dec. 3.

Specialists using two forms of imagers are examining the stringers that make up the ribbed intertank section of the external tank. There are 108 of the 21-foot-long metal stringers that connect the cone-shaped liquid oxygen tank on top with the oblong liquid hydrogen tank on the bottom.

Technicians are looking for cracks or other flaws in the aluminum alloy material that makes up the intertank. They have to use scanners because all of the tank's metal has been covered with a thick layer of foam insulation to help keep the super-cold propellants chilled and protect the structure from aerodynamic environments during ascent.

At their disposal are backscatter devices that bounce radiation off the tank and computing radiography scanners that work like an X-ray machine by transmitting radiation through the tank's foam and metal skin.

"It's pretty straightforward," said Alicia Mendoza, NASA's External Tank and Solid Rocket Booster vehicle manager at Kennedy. "It's been pretty efficient, actually."

Four cracks were found in two of the stringers after the tank was partially filled during a scrubbed countdown on Nov. 5. The cracks in the stringers cracked a section of foam on the outside of the tank. The cracked stringers have been replaced and reinforced with "doublers," which are shaped metal pieces twice as thick as the original stringer. Also, the latest scans show no signs of damage in the tank's other stringers.

Mendoza recalls the scrubbed launch when she saw the foam piece dislodged.

"The first thing that comes to mind is, what caused it, how do we fix it and how do we go forward," she said.

Since then, she and a team of NASA and contractors have been working around the clock at Kennedy and NASA's other spaceflight centers to find out about the cracks and repair them.

The information from the scans is being reviewed by officials at NASA's Marshall Space Flight Center in Huntsville, Ala., where the tank was designed, and at NASA's Michoud Assembly Facility outside New Orleans where the tank was built.

The stringers on the part of the tank facing Discovery are under the most scrutiny because foam breaking off in that area could pose a potential threat to the shuttle's heat shielding.

Getting to all the stringers facing Discovery is a challenge. Working some 15 stories above the launch platform, technicians venture out on platforms to set up the scanners carefully. Surveying the stringers directly between the tank and the shuttle means working on a platform that is 18-inches at its widest point.

For the computing radiography machine, the one that works like an X-ray, a technician also climbs inside the intertank area to place the film for the image. The backscatter machine picks up the reflected energy on its own.

Because the examination is so important, neither process is particularly quick. It takes the backscatter scanner about 90 minutes to survey a single stringer. The computing radiography scanner can do up to five stringers in an hour.

"They definitely have to be meticulous with the angle, with the shot," Mendoza said.

The external tank is the largest single part of the shuttle when it is stacked for launch. Although it has no engines of its own, it provides about 535,000 gallons of propellants to the shuttle’s three main engines. The tank's machinery includes a series of valves and sensors that have to function perfectly before a liftoff is allowed.

The tank also undergoes changes as it is loaded with its propellants. At minus-423 degrees and minus-297 degrees respectively, the liquid hydrogen and liquid oxygen that fill the tank are some of the coldest materials on Earth. The tank's diameter shrinks about an inch when the propellants are loaded. Likewise, if the tank has to be emptied for a scrub, as happened Nov. 5, the tank expands as it warms up to Florida's outdoor temperature.

Managers expect the two high-tech tank scanners will help provide them with information that will support Discovery’s next launch opportunity.

Steven Siceloff
NASA's John F. Kennedy Space Center