The Air Force successfully launched a Titan 34D rocket yesterday, ending an 18-month grounding of the nation's most powerful unmanned launch vehicle.

It carried a classified military payload believed to be an "eye-in-the-sky" satellite crucial to monitoring international crises and verifying Soviet arms control compliance.

The launch, delayed several times in recent weeks, appears to end a period in which major U.S. launch systems have been grounded following spectacular launch failures, including the January 1986 destruction of the shuttle Challenger. Among other things, the grounding left the nation dependent for a variety of national security functions on military satellites already in orbit, including some old ones.

"This launch has tremendous significance for the nation's space program," said Air Force Secretary Edward C. Aldridge Jr. in a post-launch statement.

Not only does it restore the Air Force's ability to place large national security satellites in orbit, he said, "it also reopens all of our current expendable launch vehicle paths to space . . . . Our nation's fleet of expendable boosters -- Scout, Atlas, Delta and Titan 34D -- are now operational again."

After a secret countdown, the Titan thundered skyward at 1:32 p.m. PST from a pad at Vandenberg Air Force Base on the Pacific coast northwest of Los Angeles.

The Titan 34D had been grounded indefinitely following back-to-back failures in late 1985 and in April 1986. The first was caused by a massive pump failure and leak. The second was the result of a faulty bond between insulation and the skin of the Titan's solid-fuel booster, officials concluded.

After the twin failures and the shuttle disaster, the Air Force began an aggressive program to expand its fleet of expendable launch vehicles and reduce its dependency on the shuttle.

For the Titans, it has developed testing and inspection methods -- using X-rays and ultrasonic tests -- to detect flaws in the rocket case. Air Force officials said the production process has been improved.

Officials do not comment on classified payloads. However, space experts said yesterday's payload most likely was the last model of an Air Force-Central Intelligence Agency KH11 strategic reconnaissance satellite costing $500 million to $700 million. It can take finely detailed photographs of Earth.

If yesterday's payload was a KH11, "it will reduce the pressure on the shuttle to resume launches," said John Pike of the Federation of American Scientists.

There are supposed to be two such satellites in orbit around the Earth's poles. But because a Titan launch failure destroyed one, experts said, there is only one aloft, and that one reaches the end of its normal three-year life expectancy early next year.

U.S. satellites tend to outlive their expected lives. However, if the orbiting KH11 deteriorated, and yesterday's launch had failed, the country would have faced a gap of perhaps a year with no "eyes" on Soviet activities, Pike said. "We wouldn't be deaf, because we have 'ears' {eavesdropping satellites} in orbit, but we would be blind."

The shuttle is scheduled to launch a more sophisticated replacement for the KH11, the KH12, in December 1988 at the earliest. Too heavy for the Titan 34D, the KH12 could be carried by its successor, the Titan IV, which might begin flying late next year.

The KH11, which weighs almost 30,000 pounds and is the size of a school bus, has engines that can shift the satellite around in orbit and is equipped with powerful sensors, including a giant telescope, that can zoom in on selected areas of the Earth, experts say. The images are converted into computer data and beamed electronically to analysts on the ground.

The one presumably launched yesterday is said to be a rebuilt unit that was used on the ground by manufacturer TRW Inc. to test new sensors.

The Titan 34D rocket, built by Martin Marietta Corp., can carry payloads of up to 34,900 pounds into low-Earth orbit. It consists of a liquid-fueled core vehicle and two solid-fuel boosters. Another 34D is awaiting launch on a pad at Cape Canaveral Air Force Station in Florida. It also carries a classified payload.

Aldridge has credited the "outstanding performance" of orbiting U.S. satellites with preventing a national security crisis while the U.S. rocket fleet has been grounded. The Air Force and contractors reportedly have modified orbits and routines of certain intelligence satellites to extend their operating lives.