NASA is preparing to resume Space Shuttle flights on 28 September with the launch of Atlantis on mission STS 112 to the International Space Station (ISS).

The planned return to service will follow welding repairs to cracks in the liners of the fuel lines in the Space Shuttle's main engines. The cracks resulted in the fleet being grounded since late June.

NASA says the launch depends on "satisfactory welding and polishing processes to restore flow-liner integrity in the pipes carrying fuel to the Space Shuttle main engines". The liners are designed to prevent liquid hydrogen and oxygen turbulent flow into the engines during launch and climb to orbit.

Three small cracks on the iconel alloy flow-liners on Atlantis and two on Endeavour - scheduled for the STS 113 mission to the ISS on 2 November - will be welded, and the microscopic rough edges on the liner holes will be polished to avoid more cracks.

The much-delayed STS 107 Columbia FreeStar science mission has been tentatively scheduled for 29 November pending review.

Owing to the difference in material used and the number of defects on Columbia's steel flow-liners, "more engineering investigation is needed before a final repair option is selected" for that orbiter, says NASA. Three cracks have been found on Columbia and on sister-ship Discovery. Flow-liners on Discovery will be repaired during the already planned Orbiter Major Modification programme, due to begin later this summer.

The welding repair was chosen after engineers determined the most likely cause of liner cracks was "high-cycle fatigue, a phenomenon attributed to combined environments, such as vibration, thermal and acoustics".

Source: Flight International