After reaching 4 seconds before liftoff in the launch countdown, the maiden flight of the space shuttle Discovery has again been postponed. This could be a major setback for the US shuttle program.
An on-board computer detected a questionable fuel valve in engine No. 3 and shut down the launch after two of the three main engines had been ignited. (The shutdown started a small fire that was quickly extinguished, with no damage to the craft, NASA said.)
The problem may be quickly fixed, though at this writing, officials were indicating privately that it would be several weeks before the next launch attempt. But whatever success the mission may eventually have, the incident has strengthened the argument of the US Department of Defense (DOD) that it cannot rely on the shuttle and should curtail its participation in the program.
Were this to happen - were the DOD to shift many of its satellite launches to unmanned rockets as it now is lobbying to do - the US shuttle program would be seriously crippled.
There has been public concern in recent years about a ''military takeover'' of the shuttle system. But military participation has been an essential element of shuttle planning right from the start. Indeed, the National Aeronautics and Space Administration (NASA) needs DOD's launch business if the shuttle system is to become an economically viable operation. Far from fearing a ''takeover,'' what NASA officials are more concerned about is a possible military pullout.
Thus, the shutdown of Discovery's main engines within seconds of its scheduled launch at 8:43 a.m. EDT Tuesday, has implications beyond the delay of this particular mission.
For veteran space observers, the scene at Cape Canaveral was reminiscent of the attempted launch of Gemini 6 in December 1965, nearly two decades ago. On that occasion, a launch inhibitor plug released prematurely. The engines shut down 1.5 seconds after nominal launch. Those present can still hear the perplexed tones of Paul Haney in the Mission Control Center at Houston which had just taken over control. He had started to announce ''We have liftoff,'' choked off the last word, and reported with obvious bewilderment ''We have . . . we have a shutdown!?''
This time, control had not yet passed to Houston. Mark Hess in the Launch Control Center confidently announced the automatic firing of Discovery's main engines. Then, like Paul Haney before him, he suddenly had to report ''a shutdown.''
It was still unclear whether it was a faulty four-inch fuel pump in engine No. 3 or a faulty computer signal that aborted the launch (another on-board instument had indicated the pump was functioning properly). Earlier, Discovery's No. 1 engine had been replaced with one from Challenger when the heat shield in the original engine had delaminated. This postponed Discovery's scheduled launch from June 22 to 25. The launch was further postponed a day when the space-craft's backup computer also had to be replaced. Again, the replacement came from Challenger.
As matters now stand, DOD has an agreement with NASA to let the shuttle supply its launch needs. But, concerned about spacecraft reliability and launch delays, DOD now wants to build a new stable of unmanned rockets to launch at least some satellites. Indeed, Air Force Undersecretary Edward C. Aldridge Jr. has told Congress that DOD would have the option of removing all payloads from the shuttle in 1988 if it were cheaper to use expendable rockets. In 1988, NASA is to begin charging customers the full cost of satellite launches.
The DOD already has bids for building the rockets submitted by Martin Marietta and General Dynamics companies. It is seeking White House and congressional approval to go ahead with the purchase.
Meanwhile, NASA officials are fighting the move. NASA administrator James M. Beggs calls the shuttle ''the most reliable space transportation system ever built.'' He says it ''can meet all presently projected foreign and commercial, DOD, and NASA requirements for years to come.''
Mr. Aldridge does not agree. He is quoted in the current issue of Science magazine as being convinced that ''right now, we do not have a reliable, responsive launch capability for the future.''
From the customer's point of view, the shuttle system has several problems. Two shuttle-launched communications satellites failed to reach proper orbit due to transfer rocket failures. This problem still has not been resolved. Costs of shuttle launches are heavily subsidized. There is no assurance that, in 1988, the shuttle won't be undercut commercially by such alternative launch systems as the European Ariane. Finally, the shuttle itself continues to have annoying equipment failures and launch delays.
Regardless of how quickly Discovery's problem may be fixed, this latest abort on the launch pad will not help Mr. Beggs make his case in the commercial marketplace.