NASA IDs Cause Of Shuttle Fuel Tank Cracks

HOUSTON — Space shuttle officials have concluded that a combination of weaker-than-expected materials and subtle assembly issues led to the external tank (ET) cracks that have stalled the launches of Discovery and Endeavour on NASA’s last two scheduled shuttle missions.

The two flights will mark the end of International Space Station assembly, though the space agency hopes to win congressional funding for an additional flight of Atlantis in the late June to late August time frame.

Modifications to Discovery’s fuel tank should be complete in time to launch the 11-day mission on Feb. 24. Discovery’s scheduling depends on a Feb. 23 arrival at the station of the European Space Agency’s second Automated Transfer Vehicle, the Johannes Kepler. The ATV will attempt to lift off on Feb. 15 and dock three days earlier than planned to accommodate Discovery.

Discovery’s cargo includes a station storage compartment and an external platform to secure spare parts.

Endeavour, which had been tentatively scheduled to lift off on April 1, will likely move to April 18 to accommodate further ET troubleshooting. Currently, managers do not believe Endeavour’s older fuel tank shares the same materials issue.

NASA Program Manager John Shannon told a Houston news briefing Jan. 11 that NASA “absolutely” has gotten to the bottom of the ET cracks. “And we have a fix that we are completely confident will eliminate those root causes,” he says.

Discovery’s flight has been on hold since a Nov. 5 launch scrub due to an unrelated hydrogen leak. Further assessment of a separation in the ET’s foam insulation revealed cracks in two adjacent underlying 21-ft.-long aluminum lithium stringers — support beams that separate internal hydrogen and oxygen containers.

Through structural testing and a Dec. 17 tanking test, engineers determined that 78 of the ET’s 108 stringers were fabricated using materials with only 65% of the specified fracture resistance.

The materials were delivered to NASA’s Michoud Assembly Facility in New Orleans for tank production in 2002. The shock of the low temperature propellant flow on Nov. 5 caused a contraction and slight twisting of the tank, imparting enough force on the components to induce the cracks, Shannon says.

All 108 of Discovery’s stringers are receiving a “radius block” modification that will strengthen and secure the tips of the stringers to withstand future stresses during tanking and the climb to orbit (Aerospace DAILY, Jan. 11).

Although Endeavour’s fuel tank was assembled with stringers from an older production lot, the orbiter will likely undergo a launch pad tanking test and X-ray analysis for verification, Shannon says.

Meanwhile, NASA plans to forge ahead with preparations to launch Atlantis on a 14-day encore mission later this year.

Though unfunded, Congress and the White House have voiced support for the mission, which would prepare the station for a lengthy though unforeseen delay in the arrival of the first unmanned SpaceX Dragon supply craft late this year.

The mission has been designated STS-135. Atlantis’ fuel tank, which faces the same materials issue, will undergo the radius block modification as well.

Though tentatively on NASA’s flight manifest for a June 28 liftoff, NASA Space Station Program Manager Mike Suffredini would prefer a late August departure.