The first three launches of next year are now scheduled for March 16 using space shuttle Atlantis, June 28 using Endeavour and September 7 using Atlantis again.
Those three launches on missions to construct the international space station originally had been set for February 22, June 11 and August 9.
NASA also plans shuttle missions next year in October and December to deliver space station parts constructed by the European and Japanese space agencies.
The space agency is considering further design changes to the external tank.
Its problems with shedding foam were blamed for 2003's Columbia disaster, which killed seven astronauts.
Managers plan to hold a critical meeting in two weeks, at which they will decide whether to make changes to the tank.