The fuel tank and booster rockets will help launch Space Shuttle Discovery on its mission to the International Space Station. Launch is targeted for sometime between May 15 and June 3.

The ET was lifted Monday by a giant crane and joined to the already stacked boosters in the 52-story Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center, Fla.

Following integration and final checkout of the ET with the SRBs, Discovery will join its propulsion components in the VAB. Discovery will roll over from the Orbiter Processing Facility later this month, which will mark the completion of Return to Flight processing. The orbiter will be attached to the stack in the VAB.

The ET will fly with several modifications. They include two new forward bipod heaters at the forward attach fittings that connect the tank to the orbiter. NASA and Lockheed Martin Space Systems spent nearly two years modifying the tank to make it safer.

During ET-SRB mating, the left and right boosters are bolted to the tank at both the top and tail ends. At the forward end, a vertical bolt mechanism attaches each booster to the tank.

On launch day, approximately two minutes after lift-off, the SRBs will separate from the ET, when pyrotechnic devices fire to break the 25-inch, 62- pound steel bolts.

One half of the bolt is caught in canister-like bolt catchers located on the tank; the other half remains with the boosters. Discovery will be the first flight with a modified bolt catcher.

It was upgraded from a two-piece welded design to a one-piece, machine-made design. By eliminating the weld, the modified bolt catcher is structurally stronger than the original.

Prior to Discovery joining the stack, final closeouts on the ET will include attaching the new bolt catcher and electrical cable connections. An aerodynamic fairing and the bi-pod struts, the attach points for the nose of the orbiter to the tank, will also be installed.

The ET is the largest element of the Space Shuttle system, which also includes the orbiter, main engines and SRBs. It measures 27.6 feet wide and 154 feet tall.

Despite the tank's size, the aluminum skin covering it is only one eighth of an inch thick in most areas. Yet it still withstands more than

6. 5 million pounds of thrust during liftoff and ascent. The tank is the only Shuttle component that cannot be reused.

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