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Parts Replacement to Delay Shuttle Flight : NASA Says Atlantis May Not Be Launched Until Late This Week

Times Science Writer

NASA officials said Saturday that the shuttle orbiter Atlantis will not be launched on its mission to send a planetary probe to Venus until late this week.

Engineers have decided they will have to replace the external hydrogen line that developed a leak, and they suspect they will have to replace a softball-sized hydrogen recirculation pump that shut down 31 seconds before liftoff Friday. The engineers will not know about the pump for sure, however, until they have physically examined it, a process that could not begin until Saturday evening.

The three mission specialists, Norman E. Thagard, Mary L. Cleave and Mark C. Lee, returned to Houston on Saturday, indicating that NASA does not expect the launch to occur soon. Commander David M. Walker and pilot Ronald J. Grabe were to fly there today after using Saturday to conduct practice landings in a jet that has been modified to handle like the shuttle orbiter.

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Expects to Meet Deadline

NASA officials do not expect to select a new launch date before Monday at the earliest. But Rear Admiral Richard H. Truly, the newly nominated head of the National Aeronautics and Space Administration, said in a written statement that he fully expected to be able to launch the planetary probe Magellan to Venus during the period when the Earth and Venus are in the proper orientations for a minimum-energy orbit.

“I’m convinced we’re still in a very favorable position to launch Atlantis during the Magellan window,” Truly said. The last available day for launch would be May 24. If Magellan is not launched by then, the mission will have to be postponed until 1991.

NASA officials were forced to scrub Friday’s attempted launching when they discovered a failure of the pump, which circulates supercold liquid hydrogen fuel through engine No. 1--one of the three main engines--to cool it before ignition. If the engine ignites without being precooled, it could crack or melt.

But NASA officials said the problem does not endanger the shuttle because interlocks on the vehicle and on the ground prevent ignition if the pump were not working.

Pump Believed Defective

Technicians spent Friday night checking the electrical circuitry associated with the recirculation pump and could find no problem, which suggests that the pump itself is defective. But they could not take off an aft panel of Atlantis and reach the three engine pumps until all the liquid hydrogen and liquid oxygen was removed from the external fuel tank, a safety precaution that was not completed until Saturday afternoon.

NASA has spare pumps on hand, and they could be replaced on the launch pad if necessary, officials said.

A second problem on Atlantis was not discovered until after the launch had been scrubbed. Reviewing videotapes of the attempted launch, technicians observed a plume of gas escaping from a four-inch-diameter liquid hydrogen line that carries the fuel back to the external tank after it has cooled the engines. About 15 minutes after the scrub, they observed a larger escape of gas from the same point, indicating that a safety valve had released.

Theory on Vapor Leak

Officials speculated that a pinhole leak developed in the insulation jacket on the hydrogen line, allowing air and water vapor to enter and mix with the inert liquid argon that is used for insulation. The cold temperature in the line condensed the air into a liquid.

When the recirculation pump stopped operating, hydrogen in the line began to warm up, vaporizing the air and water, which streamed back out through the leak. As pressure continued to build, a safety valve eventually ruptured as it was designed to do, relieving the pressure.

This problem also did not endanger the shuttle because no hazardous gases were involved. Officials decided to replace the line, but could not begin doing so until the rotating service structure that envelopes the rocket for maintenance was returned to its normal position around the rocket. That also could not be done until the hydrogen tank was emptied.


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