Further Discovery Launch Delay Possible

By Mark Carreau
HOUSTON — Space shuttle program managers are assessing the need for radius block modification to each of the 108 support stringers on the shuttle Discovery’s external tank (ET), an upgrade that would likely require NASA to push the senior orbiter’s final mission out of an early February launch window.
Managers will consider the additional work during a Program Requirements Control Board meeting on Jan. 6.

Discovery’s 11-day assembly mission to the International Space Station has been on hold since a Nov. 5 launch scrub, while managers have been trying to establish the root cause for eight cracks found so far in five 21-ft.-long aluminum lithium alloy support stringers on the ET’s “intertank.”

On Jan. 4, technicians began the installation of radius blocks on the top of 34 of the stringers to prevent further cracks, a modification called for by shuttle managers a day earlier.
The modification will be applied to a total of three dozen stringers that flank the ET’s two thrust panels, when two previous crack repairs are included.

The thrust panels face the shuttle’s solid rocket boosters and shoulder most of the forces as the 4.5-million-lb. shuttle lifts off.
The limited radial block modifications ordered on Jan. 3 can likely be carried out in time for Discovery’s mission to get underway during the next launch period, Feb. 3-10, NASA shuttle program spokesman Kyle Herring says. If all 108 stringers are modified, Discovery’s flight will likely be moved to the next launch period, Feb. 27-March 6, according to Herring.

Discovery’s ET stringer section also is undergoing a backscatter X-ray analysis this week to check for the possibility of more cracks masked by the insulating foam that jackets the tank.
Discovery was returned to the Vehicle Assembly Building at Kennedy Space Center following a Dec. 17 launch pad tanking test.

In addition to repairs, shuttle managers are overseeing a fault tree analysis as well as materials and subcomponent testing at several NASA installations in an effort to establish a root cause for the cracks.
The initial cracking coincided with the flow of chilled hydrogen and oxygen propellants into Discovery’s ET on Nov. 5.

The underlying damage was discovered when cracks at the tops of two adjoining stringers caused a separation in the ET insulating foam that was long enough to qualify as a violation of Launch Commit Criteria.

Engineers believe the cold temperatures may have exacerbated a materials defect or a slight misassembly on the ET support stringers.

During their mission, Discovery’s six astronauts will install on the International Space Station an equipment module and an external platform to stow spare parts.
Discovery photo: NASA TV

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