The space shuttle Discovery and its six-member crew appear headed for a launch as soon as Feb. 24 now that engineers have solved a mystery behind cracks that cropped up on the orbiter's external fuel tank.
At a briefing Tuesday, NASA officials outlined a repair plan for the tank that they say will prevent cracks from appearing in critical structural supports on the outside of the tank – a problem whose cause took two months to identify and verify.
"This was a tough problem," acknowledged John Shannon, the shuttle-program manager. But after "two months of very vigorous activity, I'm very confident that we've finally gotten it figured out and we have a fix that is easy to implement."
Discovery had been slated to launch on its last mission Nov. 5. But as technicians loaded liquid helium and liquid oxygen into their separate storage tanks inside the external tank's orange shell, sensors detected a hydrogen leak that forced controllers to scrub the launch.
Ground crews returned to the pad and inspected the orbiter, which led to the discovery of cracks in two of 108 vertical aluminum ribs that reinforce a key section of the external tank's shell.
Subsequent inspections turned up more cracked ribs, called "stringers."
Technicians repaired them. But mission managers decided not to launch the craft until engineers understood what caused the cracks and whether that cause would require additional modifications to the tank.
Engineers figured that the stress of loading extremely cold liquid oxygen and liquid hydrogen into the tank triggered the cracking. Indeed, the investigation team found that the stringers weren't as fracture-resistant as earlier batches had been, Mr. Shannon explained.
But that alone wouldn't have triggered the cracking, he continued. Testing showed that the stringers still had more than enough resistance to cope with the stress imparted as the external tank filled with its super-cold fuels.
Further testing, however, revealed that when the tank was built, the assembly process added still more stress to the stringers. The combination of the two stresses pushed the affected stringers to the cracking point.
The cracks appeared in the upper foot of the 22-foot-long ribs, and engineers found they could prevent the cracking by inserting what in effect is a thin aluminum shim between that final foot of stringer and the tank.
Officials have decided to place these shims under all the stringers, work that should wrap up by Jan. 23.
Discovery is loaded with hardware for the International Space Station, including a new pressurized module that will add storage space and Robonaut 2, the upper torso of a humanoid robot that will become a permanent fixture on the station.
The final shuttle mission, in which Endeavour and its six-member crew will also carry equipment to the space station, is currently scheduled to launch in April. The fiscal 2011 authorization bill Congress passed at the end of last year contains money for yet one more mission, but it remains to be seen whether the funds for that mission will be appropriated. If it is, planners envision launching it toward the end of the fiscal year, perhaps in late August.