Shuttle Cracks Baffle NASA

HOUSTON — Though nearing a 2011 retirement, NASA’s shuttle program has not lost a penchant for throwing a curve ball — this time a collection of cracks on a region of Discovery’s external fuel tank that separates internal oxygen and hydrogen propellant containers. The mystery over the underlying cause of the four cracks in adjacent “intertank” stringers has had Discovery’s final flight, an 11-day assembly mission to the International Space Station, on hold since early November. A Nov. 5 launch attempt was scrubbed by an unrelated hydrogen leak at the fuel tank’s Ground Umbilical Carrier Plate, a vent line fixture. At the time, the external tank was undergoing cryogenic shock as it was filled with liquid oxygen chilled to -300F and hydrogen propellant at -423F. The launch has since slipped from Dec. 3 to no earlier than Dec. 17-20 and perhaps January or late February. Meanwhile, NASA is performing an intense fault-tree analysis, with a strong suspicion the cracks were the result of defects or damage to the two 21-ft.-long aluminum-lithium stringers during manufacture or tank assembly. The fissures were found on Nov. 5 below a worrisome 20-in.-long crack that formed in the insulating foam that jackets the tank. NASA may have more to say this week, perhaps after the Shuttle Program Requirements Control Board meets on Dec. 2. “It’s not a design issue,” Shuttle Program Manager John Shannon said on the eve of a Thanksgiving break in the analysis. “These stringers, how they are designed and how they are attached, the loads they would take during this initial cryo loading and also during launch, they have plenty of design margin. A properly assembled stringer in the expected flight environment will not crack.” A ring connecting the top of the 108 fuel tank stringers is designed to shrink slightly in response to the low temperature of the liquid oxygen. Nonetheless, there was unexpected damage. And if there were defects in or damage to the stringers that escaped NASA’s attention before Discovery reached the launch pad, shuttle managers cannot be assured there is not more that could crack and unleash a chunk of foam in flight. A similar scenario unfolded in 2003 as Columbia sustained heat shield damage during liftoff from flying fuel tank foam. The undetected breach of the left wing led to its destruction during re-entry. The external tank was modified more than a decade ago, with the more brittle aluminum lithium alloy replacing aluminum on the tank. The change increased the shuttle’s payload capacity to support the space station’s assembly. Forty-three tanks have been constructed with the lighter alloy, requiring just more than 4,600 stringers. So far, 31 cracks have been found, including those on Discovery. “All of those have been known assembly issues,” Shannon said of the previous cracks, which were traced to misalignments of the stringers as they were fastened to the tank or to mishandling in which the fragile stringers struck or were struck by other hardware. Discovery’s cracks were the first found and repaired at the launch pad using techniques previously employed only at the production plant. The ongoing detective work is immune to schedule and budget pressure, according to Bill Gerstenmaier, NASA’s associate administrator for space operations. “We would like to have found the most probable cause by now,” Gerstenmaier said. “This is turning out to be a lot more complicated from an analysis standpoint, and it does not lend itself to a very easy answer.”