A full-scale shuttle booster riddled with more than a dozen major flaws and loaded with 1.1 million pounds of rocket fuel is scheduled for test firing Thursday in Utah in a make-or-break hurdle for the first post-Challenger flight.

The rocket, called production verification motor No. 1, or PVM-1 for short, is the fifth and final redesigned booster required to be test fired before the shuttle Discovery can be cleared for blastoff this fall.Discovery's three liquid-fueled main engines were successfully test fired last Wednesday at the Kennedy Space Center, setting the stage for this week's firing of PVM-1. And if PVM-1 goes well, Discovery will be solidly on track for blastoff in late September or early October.

PVM-1 is scheduled to be fired at 1 p.m. MDT Thursday at Morton Thiokol Inc.'s rocket plant near Brigham City.

The 126-foot rocket, anchored on its side in a massive test stand, will fire for a full two minutes, burning up 1.1 million pounds of solid propellant in the process.

For the first time, major, deliberate defects have been built into a full-scale solid-fuel booster to prove redesigned O-ring seals will work as advertised in the presence of major failures.

Challenger was destroyed Jan. 28, 1986, by an O-ring failure in its right-side booster and since then, the joints have been thoroughly redesigned.

Instead of two O-rings, each joint now has three, along with improved insulation to keep hot gas from reaching the seals in the first place. In addition, a metal lip called a "capture feature" has been added to firmly lock joint members together, thus preventing possible leak paths from developing.

But with PVM-1, NASA has built in deliberate defects that effectively circumvent most of the post-Challenger design improvements, allowing hot gas to reach the primary O-rings in two joints.

The test was approved as the only way to demonstrate the ability of the primary and secondary O-rings to seal a joint if all else fails.

Critics charge the risk of failure is too high in that if the rocket suffers a catastrophic "burn through," for example, the overall joint design will be in serious jeopardy, even though the defects were deliberately built in.

But NASA managers believe the new joint design is fail safe and that PVM-1 will prove it once and for all.

"It is our major flaw verification unit," astronaut Robert Crippen said last week. "There are a significant number of flaws in it.

"Yes, there are some people who would say there is a certain amount of risk to that, but we felt that particular test was necessary to certify the design will do what we have stated it will do."

Shuttle boosters are made up of four fuel segments bolted together at three O-ring "field" joints, so named because they are assembled at the Kennedy Space Center - in the field, so to speak. In addition, the rockets feature a case-to-nozzle joint where a steerable nozzle is attached to the booster's metal casing.

Solid-fuel rockets are hollow and the propellant burns from the inside out toward the walls of the booster.