Whether America's manned space flight program finally takes off or remains indefinitely grounded will hinge on a brief, albeit critical, test that Morton Thiokol will conduct of its redesigned shuttle booster rocket later this week.

A 126-foot long, 1.2 million-pound motor will undergo a full-duration two-minute horizontal test firing at the company's remote West Desert test facility 25 miles west of Brigham City Thursday. The motor will be deliberately flawed to test fail-safe measures that have been incorporated in the redesign.The full-scale test, the fifth in a series, will culminate more than 30 months of testing of the redesigned booster and represents the final test hurdle left to be cleared before the Space Shuttle Discovery blasts off in late September or early October - marking the first post-Challenger shuttle flight.

The test was originally scheduled for early July but had to be delayed when a worker accidently damaged the booster during test preparations.

"It was an embarrassment to screw up like we did last month, but now we're on target . . . ., said John Thirkill, vice president of Morton Thiokol's Space Operations. "We're essentially done. I think we're out of the woods."

Meanwhile, NASA engineers at Cape Canaveral, Fla., have run into a steady stream of problems on the launch pad - the biggest being a nagging fuel leak - while readying Discovery for liftoff.