The rocket booster that is chilling in a Morton Thiokol test bay may mark the end of the redesign program that has preoccupied NASA, the aerospace industry and space watchers for the past three years.

The booster rocket is set for firing Thursday. In a sense, the test is the final verification of design changes ordered in the wake of the shuttle Challenger accident three years ago.But J.R. Thompson, who is head of NASA's Marshall Space Flight Center, which oversees propulsion systems, repeatedly has said the testing will not end with successful flights.

"It's the end of what we call the qualification program," Royce Mitchell, solid rocket booster program manager at Marshall, said in a phone interview from Huntsville, Ala.

"The biggest challenge we face right now is to keep people alert, keep people as conscious as possible," fight complacency and continue putting a "tender, loving feeling" into the boosters, Mitchell said.

"Eventually we will turn it over to a subroutine program," he said. "We are careful in this business - we may turn too much over to the routine."

Motors of the design that flew - and were destroyed - on Challenger still are hanging around. And those motors will be used for testing new gadgets and improvements NASA and Thiokol engineers develop, Mitchell said.

"The pre-Challenger motors give us an opportunity to do a certain amount of testing - take advantage of trying a new tire on an old car."

The National Aeronautics and Space Administration also requested in its budget two flight support motor tests each year, Mitchell said. "It's like Zenith pulling a TV off the line and really wringing out the thing."

NASA and the aerospace industry hailed the redesigned solid-fuel rocket boosters that successfully carried two shuttles into space in 1988 as the safest ever flown. A third flight is set for next month.

But the presidential commission that investigated Challenger ordered six tests be conducted to verify design changes would prevent a repeat of the gas leak that doomed the Challenger flight.

The cold-firing test this week will show whether the propellant in the booster will safely fire after being subjected to a month of temperatures as low as 20 degrees Fahrenheit.

Challenger was destroyed Jan. 28, 1986, when hot gases leaked through seals on a booster rocket. Subfreezing temperatures and the flawed design of booster O-rings led to the disaster that put shuttle flights on hold until the Discovery launch last Sept. 29.

Morton Thiokol postponed the test one day to allow the rocket fuel to cool to 40 degrees in the refrigerated Bay T-97, at the aerospace firm's Wasatch Operations plant west of Brigham City.

The rubbery, eraserlike rocket fuel cools at a slower rate than its outer shell, so it could be zero degrees outside while the propellant could be just a few shades cooler than when it was first exposed to the elements.

The 40-degree temperature of the rocket fuel is lower than considered acceptable for a shuttle launch under stringent guidelines adopted after Challenger.

"By chilling this whole motor, we've shown a great margin," Mitchell said, "that cooling the motor down is not a problem. Cooling down this steel robs it of some strength. It tells us something about the cool strength of this steel."

The two-minute test is scheduled at 1 p.m. Wednesday.

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Firing up under chilly conditions

More than 600 instruments will measure acceleration, pressure, thrust, strain, temperature and other conditions during the two-minute firing.

More than 1 million pounds of propellant has been chilled to 40 degrees for the test.

Design changes incorporated to enhance safety include:

- An additional O-ring seal, a liplike "capture feature" and a J-shaped deflection slit at each joint reduce stresses and increase sealing action.

- Joint heaters, mounted around the motor case at each field joint location, maintain joints at 75 degrees Fahrenheit.

- Heaters maintain a 66-degree temperature at the igniter. The igniter sparks a flame sent down the rocket nozzle to light the propellant.

- 100 radial bolts were added in the joint that connects the nozzle to the case, along with beefed-up insulation and an additional O-ring.