Shuttle Discovery Returns To Launch Pad

CAPE CANAVERAL — Space shuttle Discovery was returned to Launch Pad 39A on Feb. 1 in preparation for a targeted Feb. 24 launch of the STS-133 mission to the International Space Station, the final planned flight for this orbiter.

Launch was delayed after cracks were discovered in the shuttle’s fuel tank following a Nov. 5 launch attempt that was scrubbed due to an unrelated hydrogen leak.

After months of analysis and testing, engineers determined that the cracking was caused by a weakness in the materials used to manufacture the tank’s aluminum-lithium support beams, known as “stringers,” plus assembly issues. The tanks have 108 21-ft.-long stringers to separate the hydrogen and oxygen containers. Tests showed that 78 stringers in Discovery’s tank were manufactured with materials that had only 65% of the specified fracture resistance.

Engineers determined that the shock of the low-temperature propellant flow during fueling on Nov. 5 caused a contraction and slight twisting of the tank, imparting enough force on the components to induce the cracks.

To fix the problem, the stringers were buttressed with a “radius block” to strengthen and secure the tips of the stringers so that they will withstand the stresses during tanking and the climb to orbit.

Five cracked stringers were repaired with a combination of doublers and radius blocks; 94 stringers were reinforced with radius blocks; eight stringers were not modified because they were manufactured with a different batch of aluminum-lithium alloy that is not susceptible to stress-relief fractures; and one stringer was not modified due to access issues. “I have full confidence we’ve done everything we need to do on that tank,” says Stephanie Stilson, Discovery’s flow director. Similar modifications are scheduled for the tank earmarked for the STS-135 mission, a contingency flight for the STS-134 crew. NASA intends to fly the STS-135 mission this summer with a load of supplies for the station, pending congressional funding.

The fuel tank for Endeavour, which is scheduled to fly the STS-134 mission to deliver the Alpha Magnetic Spectrometer in April, was assembled with stringers from an older production lot that is not believed to be susceptible to fractures, though NASA is likely to order a launch pad tanking test and X-ray analysis for verification.