After a quarter of a century of dazzling space flight that thrilled the world, U.S. space officials began wondering today if they are living under some sort of cosmic curse.

An eerie mood hung over the sprawling Kennedy Space Center. The rapid succession of three straight disasters -- the explosion of the Challenger space shuttle, the blow-up of a Titan 34D rocket, and now Saturday's disintegration over the Atlantic of a Delta rocket, the country's most dependable launch vehicle -- seemed to many so unlikely as to defy all rules of logic, not to mention the laws of probability.

"I just sat there and said to myself, I can't believe I'm seeing this," said a glum George Diller, a veteran NASA public affairs official who watched the Delta accident from inside the agency's domed press compound, which is decorated with large color photographs of successful rocket launches. "We were so confident about this, I would have bet you my month's salary we were going to have a successful launch."

Playing out an all-too-familiar scene, Adm. Richard H. Truly, NASA associate administrator for space flight, named an eight-member investigating board today to find out why the three-stage Delta rocket's main engine abruptly shut off 71 seconds into its flight, causing the vehicle to veer off course and burst apart 11.3 miles over the Atlantic Ocean.

As it tumbled out of control, the rocket and the $57.5 million weather satellite that it was carrying were destroyed on command by Air Force range officers 20 seconds later.

NASA officials offered no explanation or speculation today on the cause of the accident.

NASA spokesman Hugh Harris said investigators are carefully examining all data, including "what radio signals might have been on what radio frequencies in the area." NASA officials have said that the engine shut down almost as if it had been commanded to do so, raising the possibility of interference from a stray radio signal or sabotage.

"Nothing has been ruled out," Harris said, including sabotage, but he added that sabotage is being given no more credence than any other theory.

"There's been no accident in which this has happened before," he said.

President Reagan said today that he is not disheartened by the explosion, but conceded that "it could have picked better timing."

For NASA, the timing couldn't have been worse. The agency has scheduled a reunion on Monday to celebrate the 25th anniversary of Mercury astronaut Alan Shepard's trip aboard Freedom 7, the country's first manned space flight.

Instead, acting NASA administrator William R. Graham and investigating board chairman Lawrence J. Ross, the director of space flight systems at NASA's Lewis Research Center, flew here today and began the task of reviewing telemetry and other data on the latest mishap.

There was no hard evidence to explain why the rocket shut down so abruptly, and officials noted that in the entire 26-year history of the Delta program, in which there have been only 11 failures, there was no previous example of a similarly abrupt engine failure.

Veteran rocket experts were equally baffled. There is no apparent connecting thread between the three accidents, no common hardware, no plausible "failure scenarios" that would explain why three normally reliable rocket systems would fail on three different occassions in three different ways, they noted.

The implausibility of it all caused normally cool engineers to talk fancifully of unknown saboteurs or unexplained "sinister forces." A visibly shaken William Russell, the Delta project manager, said that the rocket's main engine had cut off "as though it were a commanded shut-down."

"This is going to give rise to all sorts of crazy speculation," said Brad Meslin, executive director of the Center for Space Policy Inc., in Cambridge, Mass.

In 177 previous flights, the Delta rocket had chalked up an unparalled 95 percent success rate, and its last 43 launches had gone off without a hitch. Moreover, NASA officials had taken special safety precautions for this mission, ordering extra preflight reviews and, for the first time, requiring contractors to sign written certificates assuring that their systems were ready to go.

"It's crazy for a scientist to say it's a jinx; we're not supposed to talk that way," said Herman Krier, a University of Illinois rocket propulsion expert who has done advanced research for the Air Force. "But this is queer . . . . If you would have asked me, as an engineer, to calculate the probability of there being a major accident on any of these rockets, I would have put down $1,000 against it. That all three should fail like this, I would have said the odds are in the millions."

Krier said the main engine rocket, manufactured by the Rocketdyne division of Rockwell International, would have been equipped with sensors that measure tank pressure and other variables. These sensors are programmed to shut down the engine if readings are awry, a possible explanation for the engine's sudden failure.

But in the postflight press conference, project manager Russell and launch director Charles Gay said there were no signs of any "anamolies" aboard the spacecraft.

"It's puzzling," Krier said.

That such a run of bad luck should strike NASA, the federal agency that put men on the moon, seems ironic. That it should happen now, on the very weekend that NASA was preparing for its 25th Mercury anniversary party, seems perverse. Some NASA officials said that the accident will take the edge off the reunion, and one noted that "it's going to be a low-key affair."

Publicly, at least, NASA is keeping an official stiff upper lip.

"It would be easy to fall into the trap of thinking that nothing can go right," said Shirley Green, the agency's chief of public affairs. "Do I see people wringing their hands and down in the dumps? No. It's more, 'Let's get back to work and figure out what went wrong.' "