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Morton Thiokol and NASA are investigating an apparent short circuit inside the solid rocket booster motor that sent sparks flying and workers scrambling.

The incident occurred Dec. 8, while employees were preparing the booster rocket for a crucial test recently at Morton Thiokol's Wasatch Operations plant in Brigham City, spokesman Rocky Raab said.The 1.1 million pounds of rubbery propellant inside the 14-story boosters is ignited by a charge inside the rocket nozzle that sends a flame down to the rocket fuel.

"There's a heater that fits around the igniter, and the crew had installed and tested the motor when there was an arc inside the igniter," Raab said. Electricity to the test stand was shut off and workers were evacuated.

The igniter, which is made by a subcontractor for the Utah aerospace firm, was redesigned along with an overhaul of the rockets after the shuttle Challenger disaster nearly three years ago.

"It was not a serious problem. It was simply a defective heater. The heater may have to be redesigned," Raab said.

No one was injured and no damage reported to the booster, which will be fired next month in the last test required by the presidential commission that investigated the Challenger explosion.

"No damage was done to the igniter and no damage to the motor" in the mishap, Raab said. The incident does not appear to have an effect on the scheduled firing of Qualifying Motor 8, or QM8, Raab said.

Nor does the short circuit affect the next space shuttle launch in February, he added. "We've already tested those (heaters) before they were put on."

United Press International, quoting anonymous sources, Tuesday reported the problem may have been caused because the heating element had to be bent into its proper position during installation, which may have brought electrical elements into contact.

The redesigned solid rocket boosters were hailed by NASA engineers as the safest ever flown when the shuttle Discovery lifted off from Cape Canaveral, Fla., three months ago.