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Discovery's booster rockets redesigned after their failure caused the Challenger explosion - did their job flawlessly Thursday, powering the shuttle toward orbit for 2 minutes, 4 seconds before dropping off into the Atlantic Ocean.

Two ships were waiting 150 miles offshore to locate the spent casings and tow them back to shore, where they will be examined and then refitted for a future launch.Stuart Shadbolt, a spokesman for Lockheed Space Operations Co., which is in charge of the recovery, said the boosters should reach Port Canaveral, Fla., by Friday night.

Thursday's launch represented a high-stakes gamble that the fragile, flawed rockets which destroyed Challenger could be remodeled into tough, durable engines able to return Americans safely to space.

Pressed by time and too short of money to start from scratch, the National Aeronautics and Space Administration spent $800 million to make 140 changes in the Challenger rocket design and then to conduct the most demanding tests ever.

Experts say the results of all the changes are rockets just short of the best design possible.

"If you started with a clean sheet of paper, there are things you could do" to improve the rockets, said Russ Bardos, NASA's director of shuttle propulsion.

J.R. Thompson, director of the Marshall Space Flight Center, said NASA can be confident of the booster rockets because every part has been analyzed, retested, and, where needed, redesigned. The booster has been tested five times on the ground, including one test with deliberate flaws built in - a test he said had never before been attempted.

NASA plans to use the redesigned rockets only until a new one is built. Development of a new booster, said Bardos, will start with "a clean sheet of paper," but the booster won't be ready for four or five years and will cost even more money.

The Rogers Commission, which investigated the Jan. 28, 1986, destruction of Challenger, blamed the disaster on a flawed rocket seal that allowed flame to escape and ignite a fuel tank. NASA engineers knew as early as 1984 that the boosters leaked during earlier launches and that disaster stalked each time the rockets fired. At the time Challenger exploded, engineers at Marshall and at rocket manufacturer Morton Thiokol Inc. in Utah already had started a low-key effort to redesign the flawed rocket.

Shuttle boosters, the largest solid rockets ever built, are 149.16 feet long and 12.17 feet in diameter. Loaded with 1.1 million pounds of propellant, a shuttle booster weighs 1.3 million pounds and produces 3.3 million pounds of thrust.

Two boosters are used for each shuttle launch. They provide 80 percent of the power needed to lift the winged spacecraft from the pad and send it streaking toward orbit. The boosters burn for just over 2 minutes and then are released to parachute to the ocean.

Because the rocket engines are so massive, they are manufactured in four segments. The parts are shipped from the Thiokol plant in Utah to the Kennedy Space Center here, where workers assemble them.

Each segment is joined at a seam called a field joint. It was a field joint that failed on the Challenger.

The joints used for 25 shuttle flights had a metal rim, called a tang, that fit into a deep metal groove called a clevis. On one side of the clevis were two O-ring seals made of a rubberized material. The clevis and tang were held together by bolts.

Experience showed that when the rocket booster fired, generating about 900 pounds of internal pressure, the thick steel booster casing ballooned out slightly, an action called "rotation." This caused the clevis and tang to separate slightly, preventing the O-rings from sealing.

The O-rings also are less likely to seal if the rubberized material is made hard by cold temperatures. Challenger was launched after soaking in freezing temperatures overnight.

The rotation problem was corrected by a new joint that added a metal lip, called a capture feature, to the tang. This joint resembles a handshake, with the new "thumb" on the tang giving extra strength.

The redesign team also added a third O-ring and installed heaters to keep the joint a constant 75 degrees. Inside the rocket, insulation was redesigned to include a J-shaped slit. Under pressure, the slit causes insulation to shift, forming still another seal.