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The Space Shuttle's first super lightweight external tank is lifted in KSC's Vehicle Assembly Building for STS-91 pre-flight processing. STS-91 is targeted for launch in late May. The improved tank is 7,500 pounds lighter than its predecessors and was developed to increase the Shuttle payload capacity on International Space Station assembly flights. Major changes to the lighter tank include the use of new materials and a revised internal design. The new liquid oxygen and liquid hydrogen tanks are constructed of aluminum lithium -- a lighter, stronger material than the metal alloy currently used. The redesigned walls of the liquid hydrogen tank were machined to provide additional strength and stability as well.