OGDEN, UTAH, AUG. 26 -- In a moment of truth for NASA, the first full-scale test of a redesigned shuttle booster is set for Thursday in the foothills of the Wasatch Mountains.

The 14-story test rocket, loaded with 1.1 million pounds of propellant and locked in a horizontal position, is scheduled to be ignited at 3 p.m. EST, roaring and blasting flames several hundred yards up the hill behind it. The test, which will cost NASA about $15 million, is the first of five or six planned to verify that a new O-ring joint and other redesigned parts of the giant booster will work.

"We want to get on with it, we want to show the world we've done a good job on the redesign," said Royce Mitchell, National Aeronautics and Space Administration project manager for the shuttle's solid-rocket motor program.

It was the failure of a poorly designed seal in a joint of Challenger's right solid-rocket booster that allowed escape of hot gases and triggered destruction of the shuttle and seven crew members in a ball of fire Jan. 26, 1986, according to findings of a presidential commission.

The shuttle fleet, and most unmanned U.S. rockets, have been grounded. The next shuttle flight is scheduled no earlier than next June.

Late last year, in order to expedite the redesign effort, Mitchell and about two dozen engineers moved from NASA's Marshall Space Flight Center in Huntsville, Ala., to temporary residences near a desolate plant an hour north of here, where contractor Morton Thiokol Inc. makes the boosters. Hard-pressed to meet the shuttle launch schedule, he said, the NASA-Thiokol redesign teams have been working 12 hour days, six or seven days a week.

The engineers are betting heavily that their redesign will work. Based on that assumption, hardware for the next shuttle flight has been ordered and is beginning to arrive at the Morton Thiokol plant to be prepared for shipment to Cape Canaveral, Fla., he said.

If Thursday's test yields any major surprises, the next shuttle launch will be delayed weeks or months.

Although later full-scale booster tests will have even more stringent technical requirements, "psychologically, this is probably the most important," said Myron Uman, staff director of a National Research Council panel of experts that has been monitoring the booster redesign.

The test rocket, known as "DM-8" (development motor number 8) will, if successful, fire two minutes. More than 500 instruments will measure temperature, pressure, thrust, strain, electrical properties and many of the other forces that a real launch will inflict on the new design.

In initial small-scale tests Aug. 4 and 17, the redesign performed well, by all accounts.

Testing solid-fuel rockets presents special problems. It is expensive and time-consuming and yet cannot duplicate all the conditions of actual flight. This makes the relatively small number of tests that are conducted important.

Each 12-foot-diameter shuttle booster -- the largest solid-fuel rocket made -- consists of four segments bolted together at three "field joints," so-called because they are assembled not at the factory but in the field, at Kennedy Space Center. It was one of these field joints that failed on Challenger.

Engineers have added a third O-ring seal to the joint design and a "capture feature," or metal lip, to lock joint members together and minimize their ability to flex open under the pressure of ignition. They have also changed design for the interior insulation that serves as the front-line pressure seal against hot gas leaks. And they have added heaters to prevent hardening of the O-ring seal material, caused by cold temperatures, that contributed to the Challenger accident.

Engineers said they feel confident about field-joint redesign. They are more concerned, they said, about the factory joint where the rocket casing meets the aft rocket nozzle, the cone-shaped skirt where the flames come out. This joint had a history of major problems before the Challenger accident. A hundred radial bolts have been added to lock joint members together.

At test time, "the nail-biting will come in the case-to-nozzle joint," Mitchell said.

There are other areas of uncertainty, he noted, such as a leaky backup O-ring seal discovered recently in a joint in the test rocket's nozzle exit cone. But the problem has been thoroughly reviewed, he said, and the main seal is expected to work. "We wouldn't be firing if we thought it would be a problem."

While booster redesign is a major element in meeting the shuttle flight date target, officials said, there are other potential hitches in the continuing overhaul of shuttle components, such as problems connected with the shuttle orbiter main engines.

{NASA said a solid rocket booster engine being tested for the Delta project skidded out of control at the Redstone Arsenal in Huntsville yesterday, Reuter reported.

{The rocket, being developed by Morton Thiokol, experienced a failure in the steel forward dome five seconds into a 58-second test, causing the steel restraints to break, and it skidded out of control along the ground before stopping about 100 yards from the test site, Reuter reported.}