LAST DECEMBER, THE EXTERNAL TANK THAT WILL SOON THUNDER OFF THE LAUNCH PAD bolted to the space shuttle Discovery lay on its side in NASA's Michoud Assembly Facility, near New Orleans. It was a pale, pretty yellow. The insulating foam applied to its metal skin would later darken with exposure to ultraviolet light, eventually turning the familiar reddish orange color of the tanks we see on the launch pad.
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A dozen technicians were working at various levels of a multi-story scaffolding erected around the tank, which they refer to as ET-120-the 120th external tank built at Michoud. Off to the side of the broad scaffolding, several clear plastic garbage bags held pieces of foam that had been cut away. I reached into one and pulled out a hand-size piece. I was shocked at how little it weighed; I could have been holding a Kleenex. But this is the stuff that cracked a hole in the leading edge of the space shuttle Columbia's wing, resulting in the vehicle's disintegration and the death of all seven of its crew on February 1, 2003. When the culprit was discovered, the people at NASA and tank builder Lockheed Martin Space Systems-people who had worked with the foam for twenty-odd years-were just as shocked as I was.
Neil Otte [pronounced "OTT-ee"] is NASA's chief engineer for the External Tank Project at the Marshall Space Flight Center in Huntsville, Alabama, though he has spent much of his time at Michoud over the past two years. "The morning of the Columbia accident," Otte said, "I know personally-when I got called and went into Marshall because I was on the investigation team-[the foam] truly didn't cross my mind as being a potential candidate. When [the investigation] led to the left wing, when it led to the foam strike, yeah, it hit us hard. Because we didn't know that our foam could do that type of damage. And we felt responsible. So, yeah. It hit us hard."
At the direction of the Columbia Accident Investigation Board (CAIB), technicians at Michoud cut away foam from other tanks, the first time in shuttle history that the inside of the material had been examined after being applied. What they found, said Otte, "is that the structure of the foam that we were putting on in certain areas was not the structure that we thought we had in there."
Meanwhile, at the Johnson Space Center in Houston, engineers testing the large, curved panels of insulation that form the leading edge of the shuttle's wing were also finding surprises. This reinforced carbon-carbon (RCC) material turned out to be more vulnerable than anyone knew, and the problem of repairing it in orbit more challenging than anyone thought. The early hope that the next mission, STS-114, could launch in the spring of 2004 quickly faded. The date was postponed to fall, then to March 2005, then May.
At Michoud, Marshall, Johnson, and other NASA centers, and at government labs and contractor facilities around the country, thousands of people have been trying for two years, at an estimated cost of $2 billion, to better understand a machine NASA has been flying for nearly a quarter of a century. The work has been guided by 29 specific recommendations from the CAIB, 15 of which need to be addressed before the shuttle flies again. One requirement, though, has stood out from the rest. As NASA head of space operations Bill Readdy told a group of reporters in January 2004, about six months after the CAIB report was released, "Return to Flight has always been driven by fixing the tank."
The reason the surface of the bullet-shaped container-all 12,620 square feet of it-is covered with insulating foam is to keep the supercold propellants within from boiling away in the Florida sun. To stay liquid, hydrogen must remain at -423 degrees Fahrenheit; oxygen at -298 degrees. The insulation also helps to keep ice from forming on the tank.
Computer-controlled machines spray the smooth areas of the tank with a polyurethane foam, while technicians use spray guns to apply another foam called BX-265 to irregularly shaped areas-around propellant feedlines, for example. BX-265 is similar to foam used in refrigerators and roof insulation, but has more exotic thermal and cryogenic properties, and was formulated especially for NASA.
"You spray it down and it goes on about like water," said Otte. "Then you build it up in layers as you spray it on. When you spray it around [protuberances], the foam can lap over itself and create voids. It can cause pressure as it's expanding. If the geometry is right, it can pull on the foam that you just sprayed underneath it. Those are the types of features that we saw inside the foam structure at the bipod ramp."
Until 2003, each tank had two such ramps-wedges of foam covering the feet of a V-shape (hence bipod) fitting that connects the orbiter to the tank. Oddly, the right one had never been known to shed foam; only the left was troublesome, and it was the instrument of Columbia's destruction. The irony is that the foam ramps were applied to prevent ice from forming on the bipod attachment post; NASA feared that during launch, ice on the post could break away and damage the orbiter.