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A recent change in the assembly of solid rocket booster nozzles is probably the reason for the O-ring problem that has delayed the next space shuttle launch, NASA says.

Utah-based Thiokol Corp. changed the way a liquid-rubber insulation is injected into the nozzles to protect the pair of back-to-back O-ring seals from exhaust gases, shuttle operations director Brewster Shaw said Friday.On July 28, the launch of shuttle Endeavour was postponed indefinitely because of the discovery of burn damage on the rubber O-ring seals on booster nozzles used by Discovery and Atlantis this summer.

Before the Atlantis mission, technicians began injecting more of the liquid rubber than the procedure called for. The extra injections may have caused small air pockets in the liquid insulation, creating an undesired path for exhaust rushing through the nozzles to reach the O-rings.

The interior of the steel and aluminum nozzle is fashioned from a carbon composite designed to burn away as the shuttle's two solid rockets propel the spacecraft from its launch pad.

When erosion meets an air pocket in the liquid rubber, the heat can singe an O-ring.

Officials have said Endeavour may be launched before the end of August. Shaw said engineers are about a week away from devising a strategy to ensure its O-rings are protected.

The strategy probably will be executed on the launch pad and involve the removal and re-injection of the liquid-rubber insulation in the nozzles.