After two successful space shuttle launches, intensive tests and examinations of Morton Thiokol's redesigned booster rockets have revealed only one minor mystery, a company spokesman says.

For some reason, the metal "capture features" added to the booster's field joints in the wake of the 1986 Challenger disaster have small scratches, said Morton Thiokol spokesman Rocky Raab.However, Raab said in an interview from Cape Canaveral, Fla., that the scratches, also called "fretting," pose no threat to flight safety and the aerospace manufacturer is convinced its booster "really is the best rocket motor ever made."

As in the past two flights, the twin solid-fuel boosters that were scheduled to hurtle Discovery skyward on Monday carry special instruments to track every nuance of their two-minute performance.

The instruments, modified versions of the kind used on six full-scale booster test-firings at Morton Thiokol's Wasatch Operations in northern Utah, are intended to "record the actual flight environment to compare with the test environment," Raab said.

"That's to ensure that the tests are accurate for predicting flights and that no unexpected phenomena occur," he said.

Raab said that except for the scratches, which did not occur during the test-firings, and the harmless loss of small pieces of cork insulation on the earlier flights, "there appear to be none."

The capture feature consists of a metal-to-metal lip that helps hold the joint together under the intense stress of firing.

Raab said engineers have speculated the scratches may have been caused by wave action in the ocean, where the boosters fall after spending their propellant and are towed to land by ships.

He said that while safety isn't an issue if the scratches occurred at that point, the reusability of the rocket casings may be. The cases are designed to be used repeatedly.

"It's not clear yet," Raab said. "The bad news is that because we don't understand it, we've proposed to NASA that we put instruments on the towback to record what happens, but they haven't funded that yet."

The capture feature, the only metal-to-metal part of the redesigned field joint, was one of scores of changes to the booster's three field joints to ensure that no superheated gas escapes, as it did in the Challenger explosion.

A presidential commission found that a faulty joint allowed a plume of gas to escape, triggering the blast that destroyed Challenger, killed its seven astronauts and grounded the nation's manned space flight program for more than 2 1/2 years.

Investigators determined that subfreezing temperatures before the doomed launch stiffened an O-ring seal, causing it to lose pliability and contributing to the failure.

The redesign project was estimated to have cost well over $500 million, and equipping the first three flight rockets with instruments to measure temperatures, pressures, stresses, motion and vibration has cost at least $3 million, Raab said.

He said that over the next several months, Morton Thiokol analysts will compile the information gathered on the boosters, then turn it over to the National Aeronautics and Space Administration.

NASA, in turn, will "make the decisions on what, if any, changes should be made," Raab said.

Morton Thiokol is betting that the redesigned motor will retain NASA's favor, despite the agency's call for industry proposals for a new generation of advanced shuttle rocket motors.