The final qualifying test of a redesigned solid rocket booster is being hailed as a success, but NASA and Morton Thiokol engineers are planning another full-scale firing later this month to see just how much they can fine-tune the booster.

The Jan. 20 test at Morton Thiokol's Wasatch Operations plant west of Brigham City was the final hurdle required to make the boosters fully certifiable for flight.Utah-made boosters successfully flew on two shuttle missions. But the test, in which the rocket fuel was chilled to 40 degrees Fahrenheit, was required by the presidential commission that investigated the Challenger disaster.

"So far everything looks great," Royce Mitchell, NASA solid rocket motor project manager, said Monday. "We got the first joint apart late Friday night. We didn't get any gas anywhere. Everything is cool."

The 125-second test verified the more than 100 design changes on the booster would hold up under extreme cold conditions, either at Cape Canaveral, Fla., or Vandenberg Air Force Base, Calif.

The test tentatively scheduled Feb. 23 will use a leftover motor that is the same design that flew, and was destroyed, on Challenger. It allows engineers to tinker with design changes and improvements to the rockets.

"It's a good evaluation tool for us try these kinds of design improvements," said Mitchell.

The shuttle exploded three years ago because a faulty O-ring seal on a ride-side booster rocket allowed gases to leak through and ignite the huge external fuel tank.

National Aeronautics and Space Administration and industry officials hailed the boosters as the safest ever flown following the $450 million redesign project.

"We're making some modifications of the nozzle" on the test motor, Mitchell said. Specifically, engineers are toying with the vent holes on the nozzle to equalize pressure endured during ignition.

"Another trick we're going to try is putting a Teflon kind of liner that tends to erode away and keep slag from building up" in the holes of the rocket nozzle.