A United Space Alliance shuttle external tank technician sands off the residue from a liquid oxygen feed line bracket after the removal of cracked ablator material and exterior foam. The repair will help prevent foam debris during the October 2007 launch of shuttle Discovery.
Credit: Jim Grossmann.
NASA engineers will patch up the last of four brackets with cracked insulation on the next space shuttle fuel tank to fly Friday to help prevent the same type of foam debris that dinged the Endeavour orbiter earlier this month.
Fuel tank workers were a bit ahead of their nine-day schedule to strip off cracked layers of ablative cork from the tank at the agency's Kennedy Space Center (KSC) in Florida, NASA spokesperson George Diller told SPACE.com.
The fuel tank repair must be complete before the 15-story vessel can be attached to the twin solid rocket boosters that will help launch the shuttle Discovery on NASA's planned Oct. 23 construction flight to the International Space Station (ISS).
NASA discovered the cracks in five brackets on Discovery's fuel tank during X-ray scans after a piece of foam insulation popped free from Endeavour's own tank and dinged the orbiter's underbelly during an Aug. 8 launch. Only four of the five brackets on the new tank are in need of repair, Diller said.
"We've definitely found where our problem is," Diller told SPACE.com, adding that Discovery's fuel tank could be attached to its rocket boosters by Sept. 5 depending on the success of today's work.
The cracks run through a layer of ablative cork and silicone, now found to be unnecessary, sandwiched between the tank's aluminum brackets and their outer layers of lightweight foam insulation. The brackets themselves help secure a liquid oxygen feed line to the outer hull of the tank.
During Endeavour's launch, a piece of foam - with the denser ablative material and possibly ice attached - fell from the orbiter's fuel tank, then unexpectedly ricocheted off a metal strut to etch a small gouge into the fragile heat-resistant tiles along its underbelly. The damage posed no risk to the spacecraft or its seven-astronaut crew, but NASA double checked its next three fuel tanks to fly and ultimately found cracks in need of repair.
NASA has worked to minimize the amount of shuttle fuel tank launch debris since a chunk of foam damaged the Columbia orbiter in 2003 at liftoff, leading to the loss of the spacecraft and its crew during reentry.
On Monday, engineers began the delicate task of cutting out the ablative layer on the brackets, then recoating each area solely with foam insulation. The last two of the four brackets under repair will receive new foam insulation today, Diller said.
Once engineers cut down to the cracked ablative material, the cork-like substance simply popped out, supporting suspicions that similar subsurface cracks led to the foam debris that damaged Endeavour, Diller added. NASA officials have speculated that the cracks may have formed during the fuel tank manufacturing process.
"Why it was cracking, I think, is still a subject of some discussion," Diller said. "But we've removed the source of the problem."
NASA was already working on a permanent fix for its shuttle fuel tank brackets, which involves replacing the aluminum fittings with new ones made of titanium, when the cracks were discovered. The first tank with the new brackets is set to fly on NASA's planned April 2008 shuttle flight.
Commanded by veteran shuttle flyer Pamela Melroy, Discovery's STS-120 mission is slated to deliver the new Harmony connecting node to the ISS during a planned 13-day mission. When finally installed, Harmony will serve as the foundation for the addition of new international laboratories to the ISS.
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