CAPE CANAVERAL, Fla. (AP) -- NASA decided Friday to conduct a second fueling test on Discovery at the launch pad before returning the space shuttle to the hangar and replacing its tank with a safer, updated model.

Despite the extra work, shuttle managers still hope to launch Discovery in mid-July on the first mission since the 2003 Columbia disaster.

Discovery's first fueling test, on April 14, uncovered sensor and valve problems that still puzzle engineers. NASA hopes to better understand the trouble by filling Discovery's fuel tank sometime the week of May 15, said spokeswoman Jessica Rye.

Rye said Discovery will then be moved off the launch pad and back into the Vehicle Assembly Building in late May for a tank swap.

Shuttle managers decided to remove Discovery's fuel tank, which is attached to a pair of booster rockets, and install a brand new set that had been meant for the second post-Columbia flight, by Atlantis.

A heater that arrived at Kennedy Space Center on Thursday will be inserted on the new tank to prevent the buildup of ice once super-cold fuel is pumped in right before liftoff. Just last week, engineering tests found ice to be as dangerous as flying foam.

A large chunk of foam insulation broke off Columbia's fuel tank during launch and gouged a hole in the left wing, dooming the spacecraft and its crew during re-entry on Feb. 1, 2003. All seven astronauts were killed.

NASA modified its fuel tanks to prevent big pieces of foam from coming loose, but decided a week ago that Discovery's fuel tank needed to be repaired because of the ice threat. As a result, the launch was bumped from late May to mid-July.

It should be easier and quicker to replace Discovery's fuel tank than to try to fix it, Rye said.

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