NASA wants to do three Shuttle flights this year

But that's a big "if," said space shuttle program manager Wayne Hale.

Engineers are still are working out problems with the external fuel tank and other details.

"I remain optimistic that if we fly by this summer, we will be able to get three flights off," Hale said at a news conference. "But time will tell. This is not like the railroad."

In a best case scenario, shuttles would fly in May, July and November. Sticking to a May launch schedule for Discovery, though, doesn't give NASA much time to fix any major technical problems that might pop up, said NASA launch director Mike Leinbach.