The failure Saturday night of a Delta rocket carrying a weather satellite has stunned a U.S. space program already reeling from the failures earlier this year of the shuttle Challenger and an Air Force Titan rocket.

Although all three launch systems now are grounded, leaving the United States at least temporarily unable to launch heavy satellites, aerospace and national security experts said yesterday that the failures do not immediately endanger American security or communications capability.

U.S. satellites generally perform for long periods once in orbit, experts said, and the satellites lost in the accidents, as well as those now facing delayed deployment, are not immediately crucial.

However, if the unnerving series of events that started in January reflects a deeper and longer-term flaw, either in technology or in the operation of the space program, the situation could become critical.

Several experts said that the rocket failures, considered in the context of malfunctions in several satellites and satellite booster rockets over the last two years, may indicate serious quality control problems in the aerospace industry.

"You can look at this three ways," said John Pike, associate director for space policy of the Federation of American Scientists. "One is to say complicated technologies are inherently unreliable. The second is to say we've got the worst infestation of gremlins anybody's ever heard of. The third is to say we have a problem with quality control and reliability in the aerospace industry.

"I know the first is true, I hope the second is true, but I fear it's the third one.

"We've obviously got a quality control problem in the industry," Pike said. "I think we need a congressional investigation -- not a finger-pointing witch hunt, but a collective self-examination . . . . I suspect the proximate cause is poor quality control."

But George Rathjens, a national security specialist at the Massachusetts Institute of Technology, said, "I don't think there are any very serious national security implications in the short term, and I presume it will not be too long before we can begin to replace things if we have to."

The problems appear to have begun last August, when a Titan rocket suffered a liquid-fuel engine shutdown about four minutes into flight from Vandenberg Air Force Base, blowing up and destroying a KH11 spy satellite. Then came the Challenger disaster Jan. 28, in which the shuttle was destroyed and seven crew members killed, followed by a second Titan failure on April 18 that destroyed another intelligence satellite, and then by Saturday's Delta accident.

According to some experts, the accidents underscore the shortsightedness of relying on a single vehicle -- the space shuttle -- to launch all principal military and civilian satellites. As a result of that decision, the manufacture of expendable launch vehicles, such as the Delta, was severely curtailed and the United States now has only a dozen heavy-duty rockets in stock, aside from the three remaining shuttles. All of the rockets have payloads booked. The rockets available are:

*Six Titan 34Ds, like the ones that exploded on the last two launches, in August and last month. The six Titans, although booked, cannot be launched until completion of the investigation into the April explosion. The Air Force, anticipating that the shuttle would not be able to meet its ambitious schedule, insisted two years ago on production of a fresh line of more powerful Titans, the 34D7, but they will not be ready until 1988.

*Three Atlas-Centaurs, all scheduled to launch heavy Navy navigation satellites, two this year and one next year. No Atlas-Centaurs are in production, and the rocket's manufacturer, General Dynamics, said it would take two years to produce the next one.

*Three Deltas, which do not have the heavy lifting capability of the Atlas and the Titan. Two Deltas were scheduled for launch later this year, one for a scientific satellite, the other for a Strategic Defense Initiative or "Star Wars" payload. The third Delta, also to lift a Star Wars payload, is scheduled for next year. McDonnell Douglas, the rocket's maker, said it would take 17 months to produce the next Delta.

The dwindling inventory and the indefinite grounding of the shuttle come as demand is rising for rockets that can put satellites into orbit.

The Reagan administration's Star Wars program, even in its earliest developmental phases, needs to launch test materials into space.

Arms-control verification treaties with the Soviet Union rely in part on U.S. ability to monitor compliance using spy satellites. The United States has only one major photographic intelligence satellite in orbit now, sources said. American intelligence agencies would prefer to have at least two aloft.

"The policy of giving the shuttle the exclusive rights to carry big cargoes has been a policy of tragedy and national folly," said Albert B. Wheelon, vice president of Hughes Aircraft Corp. and also a member of the Challenger commission and of the president's advisory committee on foreign intelligence. "I've been saying it for 14 years," Wheelon said, "and I'm very sorry to have it borne out."

Ultimately, the most severe casualty of the three dramatic explosions may be the public's faith in technology.

The U.S. space program has had failures before, and although the public perception has been shaped by the success of the manned space program, periodic failure is assumed in routine unmanned launches.

The Delta rocket is considered one of the most reliable launch vehicles in American space history, but earned that reputation while failing 12 times in 178 launches, a failure rate of roughly one in 15. The shuttle failed for the first time on its 25th launch