Confusion after a shift change is to blame for the mistake that damaged a trial version of Morton Thiokol's redesigned space shuttle booster rocket, a NASA official said.

The accident occurred July 9 when a worker testing one of the rocket's three field joints inadvertently switched pressurization lines and damaged the bonded insulation that helps contain super-hot gases.The accident delayed a planned full-scale motor test until August.

The redesign project began when a presidential commission found that a faulty rocket joint was responsible for the January 1986 Challenger explosion that killed seven crew members and grounded the U.S. manned space flight program.

The test is the final of five required to qualify Morton Thiokol's redesigned rocket before the space program can resume with the flight of the Discovery, officially slated for early September.

However, a tiny gas leak discovered last week in the shuttle may force a delay of up to two months, NASA officials have said.

Royce Mitchell, director of NASA's shuttle rocket motor program, said Tuesday that a replacement shift had come on duty just before the accident.

He said the fresh workers incorrectly believed the first crew had pre-checked the equipment.

"That little, significant piece of data was lost between the two crews," Mitchell said in a telephone interview from the Marshall Space Flight Center in Huntsville, Ala.

He said the second crew would have discovered the hoses were improperly attached if they had checked them before the pressurization test, one of many to inspect and prepare for a test-firing.

He said that after the accident, Morton Thiokol workers and NASA officials inspected the motor and found that a seal had been deformed when the hose sent 950 lbs. per square inch of pressure into the joint instead of the intended 100 lbs. per square inch.

An O-ring was pushed out of its groove and the insulation wrinkled, breaking the seal at a spot that was deliberately weakened to test the joint's ability to withstand the pressures of launch and flight even in the event of some failures.