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External fuel tank cracks become focus of more Discovery troubleshooting. Photo Credit/NASA
Baffled by small fuel tank cracks, NASA announced Friday that shuttle Discovery’s final mission, an 11-day assembly flight to the International Space Station, will be delayed from Dec. 17 to no earlier than Feb. 3.
The cracks in the “intertank” region of Discovery’s external fuel tank were found following a Nov. 5 launch scrub caused by an unrelated hydrogen leak.
During the launch bid, a section of insulating foam on the intertank cracked. When the damaged foam was removed for repairs, technicians found four cracks on the underlying aluminum alloy surface of the tank.
An around-the-clock effort by shuttle managers to establish a “root cause” for the cracks with analysis alone reached an impasse this week. Shuttle engineers are seeking assurances the damage is not a sign of a greater underlying risk.
“It’s time to pursue a different path,” Bill Gerstenmaier, NASA’s associate administrator for space operations, told a news briefing in which Shuttle Program Manager John Shannon outlined a strategy for a new launch pad test and other engineering evaluations.
“Basically what we’re going to do with these tests is make sure we didn’t overlook anything,” said Gerstenmaier. “We will see if these tests can reveal any new information for us, and it will also help us sort out what the real problems are that we need to be working on versus ones that we just think theoretically may be there.”
Initially, shuttle managers suspected a manufacturing flaw or an assembly error contributed to the cracks in two neighboring 21-foot-long aluminum-lithium stringers. They believed the flaws led to cracks as the fuel tank was loaded with super cold liquid hydrogen and oxygen propellants during the Nov. 5 countdown. In all, 108 of the stringers surround a region of the fuel tank that separates internal hydrogen and oxygen containers.
Oxygen and hydrogen must be chilled to temperatures of more than 300 and 400 degrees below zero to condense into liquids. The “cyrogenic” temperatures cause the tank metal to shrink and bend slightly.
One new test is likely to unfold late this month, as shuttle engineers re-fuel Discovery. First, though, they will attach strain gauges and thermocouples to measures the stresses on the tank as temperatures drop with the flow of the two propellants.
Another set of tests at NASA’s Michoud Assembly Facility, the New Orleans external tank production plant, will assess how intentionally flawed or mis-assembled parts respond to the stresses of the low temperatures.
If the new testing is productive, Discovery could be ready to launch on Feb. 3 at 1:34 a.m., EST.
The shuttle’s six astronauts have trained to deliver and equip the station with a new storage module and an external platform for spare parts. Discovery is also loaded with 6,500 pounds of scientific equipment, life support gear and other supplies.
The mission is the 39th and final flight for NASA’s fleeting shuttle orbiter.
The new trouble shooting will also delay the launching of NASA’s final scheduled shuttle flight using Endeavour.
Plans to send Endeavour with a crew of six astronauts to the station with the $2 billion Alpha Magnetic Spectrometer were also delayed Friday from Feb. 27 to no earlier than April 1.
The AMS is an external observatory designed to study dark matter and anti-matter.
Meanwhile, NASA remains hopeful of an encore mission for the shuttle Atlantis in mid-2011. Congress, however, has yet to fund the flight that would deliver supplies.