Work began 3 January to repair four cracks in three support beams on Space Shuttle Discovery's external fuel tank.
NASA is using a methodology similar to the repairs made on cracks found on two of the 21ft-long (67.4m) stringers after Discovery's Nov. 5 launch attempt, the US space agency says.
The additional cracks were found after an extensive scan of all of Discovery's 108 aluminium stringers using X-ray image scans after the shuttle was moved back inside the Vehicle Assembly Building at NASA's Kennedy Space Center in Florida after a series of aborted launch attempts late last year.
The repair work is expected to take two to three days, NASA says.
Programme managers also began another round of stringer imaging on 3 January, this time using a backscatter methodology. Any further work will be evaluated thoroughly early next week after additional data is reviewed, NASA says.
The next launch window for STS-133 - which was originally planned for 1 November - begins 3 February and extends through 10 February. The flight, to be Discovery's last, will deliver a pressurised logistics module called Leonardo and a 136kg (300lb) humanoid robot to the International Space Station.