NASA is marking a major step in assembling the space shuttle for its Return to Flight mission. Monday, workers successfully "mated," or attached, the redesigned external tank and twin solid rocket boosters (SRBs). The fuel tank and booster rockets will help launch space shuttle Discovery on its mission to the International Space Station, currently targeted for May 15-June 3.

An overhead crane moves the external tank inside the Vehicle Assembly Building for mating to the solid rocket boosters. Photo: NASA

The space shuttle's external tank was lifted by a giant crane and joined to the already assembled, or "stacked," boosters in the 52-story Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center, Fla. Mating the tank with the boosters is another major step in readying the space shuttle system for flight.

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

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

The external tank is moved into position for mating with the solid rocket boosters. Photo: NASA

During ET-SRB mating, the left and right boosters are bolted to the tank at both the forward, or top, and the aft, or tail, ends. At the forward end, a vertical bolt mechanism attaches each booster to the tank. On launch day, approximately two minutes after liftoff, 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 also be the first flight with a modified bolt catcher, which 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 design.

Prior to orbiter Discovery joining the stack, final closeouts on the external tank will include attaching the new bolt catcher and electrical cable connections, as well as installing an aerodynamic fairing and the bi-pod struts, which are the attach points for the nose of the orbiter to the tank.

The external tank 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 an 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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VIDEO: EXTERNAL TANK MATED TO THE SOLID ROCKET BOOSTERS QT
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