Shuttle's balky motor causes major flight delay

Abort of Discovery's maiden voyage at virtually the last second is turning into a major delay for the entire space shuttle program. As of this writing Friday, technicians had not identified the cause of the fuel-valve failure in engine No. 3 that led to the shutdown four seconds before launch on June 26.

While the faulty engine has been replaced on Discovery, the failure must be understood before another launch can be attempted.

Thus, the National Aeronautics and Space Administration (NASA) had not yet even tentatively rescheduled Discovery's flight.

That could be set as early as July 17. But NASA officials also were considering scrubbing the flight entirely as a separate operation and combining its tasks with those of Discovery's second mission, nominally scheduled to start Aug. 29. Such a joint mission may get under way in August. But it might have to be bumped into September.

Meanwhile, Discovery's main payloads must be serviced as they wait on board the shuttle on the launch pad. These are the Hughes Leasat communications satellite in the shuttle payload bay and the McDonnell Douglas electrophoresis experiment for separating biochemicals on orbit. This is mounted within the crew cabin. The communications satellite batteries are trickle-charged 16 hours a day until launch. The electrophoresis equipment must also be periodically checked.

All of this is a highly visible embarrassment for an agency that has been trying to build customer confidence in the shuttle as a reliable, on-time satellite delivery system. The agency still plans to launch 13 missions next year. These are to be followed by 15 missions in 1986 and 24 in 1987.

Such a schedule requires the service of all four orbiters and of additional launch pads at the Kennedy Space Center and Vandenberg Air Force base in California.

Now there is considerable skepticism that NASA can maintain such an ambitious schedule. For example, the Defense Department, which would account for about a quarter of the shuttle's launch business, wants to rely more on throw-away rockets. Some potential industrial customers, such as GTE Corporation and Intelsat, are making reservations with the European Space Agency, which uses the Ariane rocket.

Such skepticism reflects the fact that NASA had previously cut its 1984 launch schedule from 10 to seven flights due to the failure of booster rockets on shuttle-launched satellites.

If that schedule is further reduced from seven launches to six by combining two Discovery flights, customer skepticism is ilkely be reinforced.

Meanwhile, NASA officials note that the June 26 launch abort demonstrated the proper action of systems designed to ensure safe operation of the complex shuttle spacecraft. The acting associate administrator for space flight, Jesse Moore, has said he hopes shuttle customers will be ''sophisticated enough'' to recognize that the procedures and mechanisms that aborted Discovery's launch are ''built in to minimize the probability of greater failures.''