The Air Force is pressing ahead with preparations for the first launch of a space shuttle from Vandenberg Air Force Base, Calif., now scheduled for mid-July, after receiving "informal" advice from NASA officials that the cause of the Jan. 28 Challenger explosion has been located, Defense Department sources said yesterday.

National Aeronautics and Space Administration officials think that efforts to prevent a repeat of the Challenger catastrophe, in which seven astronauts were killed, "will not require a redesign or retesting" of the orbiter, the sources added.

"It is not as serious as we once thought it would be," a Pentagon official said, although he was unable to provide specifics on what triggered the catastrophe. "The shuttle may be back flying in six months."

NASA spokesmen refused to comment on this report, but Kennedy Space Center director Richard G. Smith told employes at that facility yesterday that the delay in shuttle flights may be only for "a couple or three months."

"I'm absolutely certain we'll be up flying again sooner than we believe today," Smith said.

After the accident on Jan. 28, NASA grounded the three remaining shuttles pending an investigation and correction of any deficiencies. The second shuttle launch facility at Vandenberg has been planned since the 1970s and was built at the request of the Pentagon to launch satellites over the poles -- and thus over the Soviet Union; launches from Cape Canaveral generally follow an equatorial orbit.

"They don't know for sure," what caused the explosion, a Defense Department official said of NASA officials, but "they have infrared film" that they think pinponts the location of the problem in the solid rocket booster, now widely considered responsible for the explosion.

If the solid rocket booster turns out to be the sole problem, it could push the West Coast flight forward in the NASA schedule, sources said.

That is because on the Vandenberg flight the shuttle Discovery will be powered by two new solid-fuel booster rockets rather than the model used on Challenger and other shuttles launched from Kennedy Space Center at Cape Canaveral.

The casing of the new booster is made from tightly wound filament material rather than steel as found in the existing solid rocket boosters. The Air Force and NASA went to the more costly filament casings to enclose the solid-rocket fuel for Vandenberg launches because they weigh 4,600 pounds less than the steel ones and thus allow the shuttle to carry heavier, military payloads.

The two filament-wound booster rockets that are to be used in the Vandenberg launch were scheduled to have a qualification test firing next week at the Utah plant of Morton Thiokol Corp., the manufacturer. That test was delayed yesterday, a congressional source said, because most of the team involved was participating in the review of the Challenger explosion.

NASA planned to fly the shuttle Discovery from Florida to Vandenberg within the next 10 days, according to congressional sources. Pentagon officials said the orbiter had to be at Vandenberg at least four months before its initial launch in order for the shuttle, its crew and the launch team to carry out a flight readiness firing.

This test, in which the main engines are fired up to 85 percent of capacity, is intended to determine whether the new launch facility is ready for use.

The $2.8 billion Vandenberg launch complex has been plagued with delays and cost overruns that have characterized the space program. The first launch originally was scheduled for late 1984 but subsequently slipped to October 1985, then March 1986 and finally to mid-July.

The delays, Pentagon officials said yesterday, involved problems both in the launch complex and the defense payload to be taken aloft. The Air Force has announced that the first Vandenberg launch would carry into space Teal Ruby, an experimental infrared sensor that is supposed to look down into the Earth's atmosphere and detect aircraft by the heat of their jet engine exhausts.

One official said yesterday that he would be surprised if the launch date at Vandenberg isn't delayed again because of the Challenger accident.

In a related matter, the Air Force yesterday delivered to Assistant Defense Secretary Donald C. Latham the service's recommendations on how to handle future defense space needs in the wake of the Challenger explosion, according to a Pentagon official. Latham was asked last week by Defense Secretary Caspar W. Weinberger to evaluate the accident's impact on the Defense Department's space program.

The service reportedly proposed that the Pentagon seek additional funds to purchase new unmanned rockets for launching satellites, according to Defense Department sources.

The Air Force did not suggest building of a new shuttle on the grounds it would take at least three years and thus would not be ready to handle the big increase in Pentagon satellite launches scheduled to begin in 1988, sources said.