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Engineers visually "pinpointed" an elusive fuel leak aboard the shuttle Discovery Saturday but its hard-to-reach location could force NASA to roll the ship back to the hangar for repairs in a major delay for the first post-Challenger flight.

NASA managers decided late in the day to give determined engineers more time to come up with a plan to fix the leak, discovered Friday, before making any decisions about rolling off the launch pad, and they hoped to have a better idea of their prospects by Sunday afternoon.But sources said later that the location of the leak would make it extremely difficult to reach for repairs at the launch pad. The leak must be repaired before launch, currently scheduled for early September, because it represents a potential for major damage if the problem worsens.

While NASA was getting bad news on the Florida coast, engineers in Utah said only minor repairs would be needed before reassembling a giant booster rocket.

Morton Thiokol Inc. technicians last week accidently overpressurized an O-ring joint.

"What repairs are necessary are very minor and they can be done in place with existing hardware," Thiokol spokesman Rocky Raab said. "Basically, we have to replace the one capture feature O-ring, clean up the joint and put it back together."

Technicians at pad 39B worked through the night Friday and into the day Saturday to locate the leak in Discovery's left-side "orbital maneuvering system" (OMS) rocket pod, one of two that houses rocket engines used to steer the shuttle in space.

Late in the afternoon, they concluded the leak was in an area around a propellant tank that feeds nitrogen tetroxide, an "oxidizer" that facilitates combustion, to the pod's low-power "RCS" rockets.

An engineer familiar with the work said a high-tech, remotely controlled device called a "cobra," equipped with a miniature television camera and instruments capable of sniffing rocket propellant, was snaked into the pod and videotaped the leak.

"It's gas vapor," the engineer said. "They looked and sure enough, they were getting vapor. It's in a fitting in a vent line."

He said the fitting was on one of several fuel lines that enter the RCS oxidizer tank and that it will be difficult to gain access to make repairs. If the decision is made to roll Discovery off the launch pad, he said it would take more than a week to complete preparations.

NASA spokesman Charles Hollinshead said engineers planned to "continue working . . . tonight and tomorrow to further pinpoint it and try to get the exact spot of the leak to make sure we're dealing with only one leak."

He said once the leak is isolated, "we'll determine where to go from there." He agreed the leak is in a difficult place to reach and said, "I don't think they've given up on fixing it at the pad. But I don't think that's encouraging."

If the leak cannot be repaired at the pad, NASA managers would be forced to roll Discovery back to its hangar for repairs because the OMS pods cannot be removed from the shuttle when it is in a vertical position, engineers said.

The problem is one of access. Because the removable pods are so packed with tanks, fuel lines, helium bottles and other equipment, access to the interior is severely limited when they are bolted to the shuttle. Even if the leak was found, it might be in a location that could not be reached at the pad.

A NASA source who asked not to be named said a final decision on what to do had to be made by Monday, but Hollinshead said a decision might not have to be made until early next week.

Rollback to the hangar would mark a major setback for Discovery's long-awaited launching _ some engineers speculated liftoff could slip six weeks or more _ and would threaten NASA's flight schedule for the rest of this year and early 1989 as well.

It was not clear what impact the leak might have on plans to test fire Discovery's three liquid-fueled main engines July 26 because the leak did not involve the primary engine system.

If the shuttle has to be hauled off the pad for repairs, engineers could elect to complete the engine firing first to save time later.

* Deseret News staff writer Joyce Cutler contributed to this report.