NASA takes a major step toward returning astronauts to space when engineers this week ship an improved rocket fuel tank that has been refitted to avoid the falling debris that...

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WASHINGTON — NASA takes a major step toward returning astronauts to space when engineers this week ship an improved rocket fuel tank that has been refitted to avoid the falling debris that caused the destruction of Columbia and the deaths of seven astronauts.

National Aeronautics and Space Administration officials said that the redesigned fuel tank, a massive vessel that supplies propellant for the launch of the space shuttle, will start a barge trip on Friday from a Mississippi assembly plant to Kennedy Space Center on Florida’s east coast. The trip will take five to six days.

Sandy Coleman, NASA’s external-tank project manager, said improvements made on the fuel tank “give us confidence that problems like what happened on Columbia will not happen again.”

“This is the safest, most reliable tank NASA has ever produced,” Coleman said yesterday at the Marshall Space Flight Center in Huntsville, Ala.

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The changes in the external tank add less than 150 pounds in weight. The total cost of the new tank, including tests and redesign, is still being calculated, but it will be more expensive than the $40 million cost of the old-style tank, she said.

NASA plans a May or June launch of space shuttle Discovery. The shuttle fleet has been grounded since the Columbia accident as NASA scrambled to make changes in hardware, procedures and personnel to comply with recommendations from the Columbia Accident Investigation Board.

Fixing the external tank was a key part of the NASA’s recovery, officials said.

The tank holds the liquid hydrogen and oxygen that are the propellants for the shuttle’s main rocket engines during launch. The supercooled chemicals cause the formation of ice on the outside of the tank as the shuttle is prepared for launch.

Insulation, applied as a foam, reduces the amount of ice. But investigators believe it was chunks of foam insulation that peeled off the external tank during launch that ripped a hole in Columbia’s left wing, leading to the shuttle’s destruction.

To correct the problem, engineers from NASA and Lockheed Martin Space Systems, manufacturer of the fuel tanks, conducted extensive tests to find out why the foam insulation broke loose during launch.

This led to several changes, including new ways of applying the foam insulation, the addition of heaters at key points to prevent the formation of ice before launch, and adding cameras that can monitor the outside of the tank during launch.

“We can never completely eliminate foam coming off the tank,” Coleman said. But she said tests suggest that any debris that does fly free will not cause damage like that which destroyed Columbia.

Redesign of the external tank was considered to be a key and critical part of NASA’s effort to return the shuttle to space, but is only one of a long list of recommendations from the Columbia Accident Investigation Board.

NASA also is designing ways to check for launch damage to the space shuttle after the vehicle is in orbit. The agency is also developing ways for spacewalking astronauts to fix damage to wing panels like that which destroyed Columbia.