NASA has found 32 stringers on space shuttle Discovery's external fuel tank in need of modification and two that will be replaced entirely, the US space agency says.

Those repairs are in addition to fixes already made or in progress after 29 December scans revealed cracks on three stringers and the two that were repaired following the shuttles 5 November failed launch attempt.

Crews are fitting pieces of metal, called radius blocks, over both edges of the 21ft-long (67.4m) stringers where they attach to the external tank's thrust panel area, strengthening them at the point that receives the most stress on the shuttle's way to orbit. The method is the same used to repair cracks found earlier.

The work on about one-third of Discovery's 108 aluminium stringers is expected to be complete by the end of next week, NASA says. The team is continuing to run backscatter scans on all the tanks support beams, with various NASA centres analyzing the data. Programme managers will meet 6 January for an update and backscatter scans will wrap up over the weekend.

The root cause of the cracks has still not been determined, NASA says.

The next available launch date for Discovery's STS-133 mission to the International Space Station is 3 February, the beginning of a launch window that extends through 10 February. Should the repair work prevent from making that window, the next opens 27 February and runs through early March - though that time slot has already been promised to space shuttle Endavour.

Source: Flight International