By Kevin Mcgill
The Associated Press

NEW ORLEANS - The external fuel tank that launched the space shuttle Columbia - a focus of the investigation into Saturday's disaster - was a type that was being phased out and used insulation that had flaked off on a prior mission. But Lockheed Martin, which operates the facility that has made the external fuel tanks for every shuttle flight, said the tank used on Columbia's fatal flight was "perfect for this mission."

NASA chief Sean O'Keefe said Sunday that investigators will examine whether insulation that broke from the tank during its Jan. 16 launch damaged heat-protecting tiles and ultimately doomed the shuttle and its seven astronauts.

Before the shuttle broke apart, NASA had determined that Columbia was not damaged when the insulation broke off and struck the shuttle's left wing.

The Lockheed Martin Michoud Assembly Center in New Orleans produces the shuttle program's 154-foot-high external tanks, which are used only once.

Columbia's final mission used a "lightweight tank," a type first used in April 1983 by the space shuttle Challenger.

Since 1998, "super lightweight" tanks - 7,500 pounds lighter and made with an aluminum alloy - have been used to handle the heavier payloads and steeper rates of incline required for missions to the international space station, said Harry Wadsworth, Lockheed spokesman.

A summary of the shuttle program by Lockheed shows that the facility made three more "lightweight" tanks for non-space-station missions. NASA used one on the Endeavor in 2000, one was used on the ill-fated Columbia mission and remains stored in New Orleans.

Asked if cost was a factor in the use of the older-model tank, Wadsworth said, "I really don't think that plays into it at all," but referred specific cost questions to NASA.

"This tank was perfect for this mission," Wadsworth said.

The assembly center began using a new, lighter version of the inch-thick, spray-on insulation used on the external tanks in the mid-1990s. The switch was made to comply with an EPA mandate to limit ozone-depleting chemicals, according to a 1999 news release from NASA's Dryden Space Flight Research Center.

Small particles of the insulation had flaked off the lightweight tank used to launch Columbia in November 1997, to no apparent ill effect.

Tests were conducted at Dryden to try to determine the cause. To imitate launch conditions, panels covered with the insulation were mounted beneath an F-15B aircraft, but the insulation remained intact, according to the 1999 Dryden report.