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NASA and Morton Thiokol engineers are trying to determine whether an older-model part that was inadvertently placed on a rocket test fired last month in Utah is the same model on the boosters that will lift the shuttle Discovery into space.

"It's pesky and it's an embarrassment. I don't think it's going to be a problem at all," said Royce Mitchell, solid rocket motor program manager for NASA's Marshall Space Flight Center in Huntsville, Ala.An X-ray analysis of a booster test fired Aug. 18 at Morton Thiokol's Wasatch Operations facility west of Brigham City shows workers apparently installed pre-redesign hardware in the rocket.

The redesign of the nose ring, although not required for safety considerations, was part of a rocket-booster overhaul required in the wake of the Challenger disaster.

"The question is, `Well, if you did that there, how do you know the flight (boosters) are OK?' And we're chasing that down," Mitchell said.

The nozzle inlet ring, inside the nose of the 126-foot booster, was redesigned while rocket maker Morton Thiokol was overhauling the booster after the Challenger accident.

"Even though the (old) part was performing satisfactorily, the part was redesigned to increase performance," Morton Thiokol spokesman Rocky Raab said Wednesday.

"At this point, we're not sure how it happened," Raab said. "We do know it is not a safety-of-flight issue, that both parts are equally reliable and have performed satisfactorily."

But engineers are pretty sure the new design is in the rockets on Discovery. A paper search accounted for all new rings, Mitchell said.

"As a double check, we're going back and trying to see if we can account for all the old ones. And even if we don't, it is an

mbarrassment, but it is not a safety-of-flight issue," said Mitchell.

The presidential commission that investigated the January 1986 Challenger disaster found a faulty O-ring seal in a joint connecting two segments of the right-side rocket booster allowed hot gases to leak and ignite Challenger's huge external fuel tank. Seven astronauts were killed in the explosion that has grounded the shuttle fleet and America's participation in space.

The nose ring was redesigned while Morton Thiokol engineers remodeled the booster rockets.

"The difference between the old part and the new part is what is called a ply angle - it's the angle at which those plys or layers are applied to the mold when the part is being made," Raab said. "After the part is cured, there's no visible difference between the parts."