The crippled U.S. space program suffered a major setback yesterday morning when an unmanned Titan 34D launch vehicle carrying a secret electronic communications satellite exploded five seconds after liftoff at Vandenberg Air Force Base in southern California.

Officials said they had no clue to the cause.

Following destruction of the space shuttle Challenger on Jan. 28, the accident jeopardizes the Defense Department's ambitious program to place U.S. military intelligence gathering, communications and navigation gear into space.

It was the second Titan 34D catastrophe in a row, following seven successes, Air Force officials said, while expressing some bafflement. Last Aug. 28, one of the workhorse rockets exploded after launch, destroying an $800 million KH11 photo reconnaissance satellite it was to carry into orbit.

The postponement in the shuttle program and now the failure of two Titans apparently leaves the Air Force with no capability to launch heavier satellites.

Last year, two-thirds of the Pentagon's most critical payloads went on the shuttle or the Titan.

Titans were scheduled this year to carry two of the Pentagon's Defense Satellite Communications System III (DSCS), the newest satellites that provide super-high-frequency scrambled voice and high-speed data transmissions.

The Titan explosion, at 250 to 300 feet, formed a huge orange cloud that wafted out to sea, witnesses said. "It looked, as it exploded, like a large mushroom cloud capped with a bright orange layer of oxidizers," said Dan Bolton, editor of the Lompoc Record, who watched the launch from the town adjoining the base.

Fragments from the blast fell in the launch area, a portion of which was burned, apparently by ground fires started by spilled fuel. The fires were put out by Air Force ground crews.

Some 58 people on the base were treated for skin and eye irritations, the Air Force reported. Three were hospitalized for eye burns. Many of the injured had watched the launch from a parking lot at least three miles away from the pad, a witness who works on the base said.

The orange cloud contained highly toxic vapors from the Titan's main liquid-fueled engines, which use nitrogen tetroxide and hydrazine propellants. However, it rose to about 8,000 feet and was drifting offshore toward uninhabited islands late yesterday, according to military and community officials.

"The red cloud looked like blood -- a deep, deep red," said Pete Marquez of Santa Maria, a building contractor who was entering the base at the time of the launch. The pad, he said, "looked like someone had dropped a bomb," with multiple explosions.

Dave Johnson, building supervisor at the Diablo Canyon nuclear power plant, about 70 miles north, said, "We weren't within sight of Vandenberg, but I could feel the pressure on my ear drums. It startled me."

In testimony to the House Appropriations defense subcommittee two years ago, Pentagon officials said losses of the new communications satellites "would be extremely serious." They were referring at the time to the perceived risk of launching them on the shuttle, which was to carry the first two.

Congress and a series of administrations since the early 1970s had forced the Pentagon, over its objections, to rely heavily on the shuttle to carry its heaviest payloads. It was not until last year that Air Force officials finally persuaded Congress and the administration to permit it to buy a new generation of large rocket boosters, 10 of which are now being built for $2 billion.

The first of this new generation -- the Titan 34D7 -- is not expected to be available until 1988. The inventory of Titan 34Ds, now six, was supposed to last through the spring of 1988. The White House is considering a new proposal to add 13 more in the wake of the Challenger accident.

"We won't launch a 34D again now until we've found out what was wrong and fixed it," said an Air Force spokesman.

An investigation of the August Titan explosion found that there had been two "catastrophic failures, either one of which would have doomed the mission, and each happening independent of the other," the spokesman said. "It's weird."

Investigators determined that there had been a massive leak of one of the liquid chemicals that fuel the Titan and also a fuel pump failure. "But they couldn't identify why or identify a single chain of events" that led to the double failure, the spokesman said. "It's beginning to look like gremlins."

The Titans use two large solid-fuel booster rockets with jointed segments similar in design to the booster joint that caused destruction of Challenger, although there is no indication the joints had anything to do with yesterday's accident