Solid rocket boosters blamed in the Challenger accident have undergone intense analysis in the nearly three years the shuttle fleet remained grounded - resulting in a greater understanding of the safer, more reliable system, say engineers.

To put it another way: The redesigned booster that will carry Discovery into space later this month is simply the most scrutinized motor in the history of the shuttle program."We've done a tremendous amount of testing. And we've done like five times the testing that was done on the first shuttle flight," said Royce Mitchell, solid rocket motor program manager for NASA's Marshall Space Flight Center in Huntsville, Ala.

When Challenger exploded Jan. 28, 1986, killing its seven-member crew, a faulty O-ring seal in a joint connecting two right-side segments of the 126-foot booster rocket was blamed. The joint was unable to perform in cold launch-day temperatures, allowing hot gases to leak and ignite Challenger's external fuel tank.

In the 32 months that have ensued, NASA and Morton Thiokol engineers have incorporated more than 100 changes to the boosters and developed instrumentation to verify the adjustments were on target with predictions.

"We have literally scrubbed, probed, X-rayed, scanned, tested and analyzed to the nth degree," Mitchell said.

"We've got an excellent motor here. And I'm able to look the crew in the face and say, `This motor is a safe motor.' And that's a good feeling to be able to do that. No reservations whatsoever; it's just a good feeling."

Major changes in the design include:

-Eliminating vacuum putty in the joint, which engineers believe allowed tiny "blow holes," or leak paths for gas.

-Adding a third O-ring seal and a lip-like metal latch (capture feature) that closes with pressure, to prevent gas leak paths.

-Including 100 radial bolts in the nozzle that attack the rocket to the dome.

-Increasing insulation inside the rocket.

-Adding a heater to maintain a 75 degree Fahrenheit temperature at the joints.

"I think we designed the program that went far beyond what anybody anticipated it would do as a result of that failure 21/2 years ago. And we built in a joint that is clearly in my mind the safest thing on the space shuttle vehicle," said Allan McDonald, the Thiokol engineer in charge of the redesign project.

Five full-scale, 2-minute tests and 20 sub-scale tests were performed on the booster to evaluate the modifications and the rocket's ability to withstand whatever nature could throw at it.

The culmination of all that testing appeared in an Aug. 18 firing of the redesigned booster at Morton Thiokol's remote Wasatch Operations, 25 miles west of Brigham City.

The 126-foot rocket, anchored horizontally at the test stand, leaked no gases despite 14 deliberate flaws built into the booster.

"We did indeed have some tough testing. But the testing always is proof. That alone makes me feel very pleased," Mitchell said.

"We've learned a lot. There have been surprises along the way. We've learned how this joint was made; we learned how the joints pressurized; we learned just how to build a better motor," he said. "And it's been a very exhaustive and revealing process that I feel very good about."