Defense Secretary Harold Brown has ordered the Air Force to reexamine the safety of the entire Titan II missile system, less than six months after a congressionally mandated study declared the missile safe.

The Air Force concluded in that study, released last May, that the system was better than when it was originally deployed 18 years ago.

Brown, who said he ordered the rexamination after talking with President Carter, said his office would review the Air Force findings this time.

Brown also announced yesterday that the Titans are scheduled to be replaced by the solid-fueled MX missile. The MX is not expected to start being deployed until 1986. At that point, the Titans, which originally were designed to last 10 years, will have been operational for 25 years.

Friday's accident at the Titan site near Damascus, Ark., presented a new problem for Air Force officers trying to keep the aging, liquid-fueled missile system operational.

According to sources familiar with the Titan, it was the first time there had been a major leak of its volatile fuel, Aerozine-50.

Two major Titan accidents in 1978 -- which caused the death of two airmen and injured more than 30 others -- stemmed from leaks of nitrogen tetroxide, the oxidizer that is mixed with the fuel to provide the missile's propellant force.

Sources said yesterday that once the skin of the missile had been punctured by a dropped wrench socket late Thursday afternoon, no one could figure out how to stop the substantial Aerozine-50 leak.

Investigation of the leak also may raise anew the question of the adequacy of the safety equipment used by technicians called on during accidents such as Friday's, which killed one airman and injured 21 others. The dead airman, Sgt. David Livingston, died of pulmonary complications as a result of breathing toxic fumes, according to doctors who treated him.

One doctor involved in treating Livingston and Sgt. Jeff Kennedy, who also was exposed to fuel and oxidizer vapors, said yesterday he believed the deadly gases passed right through their protective suits.

That possibility was partly confirmed by the vice commander of the Strategic Air Command, Lt. Gen. Lloyd R Leavitt Jr., at his news conference Saturday in Little Rock.

Leavitt said Livingston and Kennedy had been told to leave the silo when their vapor detectors showed a vapor level at the highest calibration. It "wasn't safe even in their suits," Leavitt told reporters.

According to an Air Force report delivered to members of Congress after the explosion Friday, technicians evacuated the silo after noticing "vapors rising from Level 7," the site of the puncture.

That first indication of the vapors was noted at 6:47 p.m.

Slightly more than one hour later, according to the Air Force report to Congress, the missile crew -- located in an underground, concrete-protected control room off to the side of the silo -- reported "hazard indicators" due to the fuel leak.

At that point, the missile was sprayed with water from tanks at the top of the silo. Two hours later, the leak had become worse and, according to the Air Force report to Congress, "It was decided to evacuate the control center" at the missile site.

At the same time, between 9:30 and 10 p.m., according to the report, local officials were told to evacuate the civilian population "within a two-mile area."

The Air Force people on the scene, one source said yesterday, "believed it was touch and go as to whether or when it would explode."

Around midnight, however, the Air Force decided to look inside the silo to see what was happening.

"We decided we needed to know more," was the way SAC Vice Commander Leavitt put it Saturday. "We decided to send a crew in," he said, recognizing it was "extremely risky."

Leavitt said the men sent down to the silo were told that if the readings on their vapor detectors rose to the highest point, they were to return immediately. The first group, according to Leavitt, had no high vapor readings but ran into difficulty opening the sealed doors between the control center and the silo. They returned to the surface when the oxygen in their safety suits ran out.

Livingston and Kennedy then went down. Their vapor detectors, however, went off scale at the first set of doors. They immediately returned to the surface and had just emerged when the fuel exploded.

Livingston survived an operation on his stomach for a wound caused later, his lungs "became like a sieve" and in another two hours he died, sources said, Kennedy showed the same to type of lung decline Friday evening, according to sources, but his condition, unlike Livingston's, did not get worse. He was in stable condition last night.

Sen. David Pryor (D-Ark.), who has pressed for immediate installation of a civilian warning system at all Titan sites as a result of the 1978 accidents, said yesterday that he was reviewing data on the Tatan IIs before deciding his next step. Pryor's original proposal in 1979 for such a system was opposed by the Air Force as costly and unnecessary, and last May's safety report repeated that contention.

The Senate, however, passed Pryor's proposal last Tuesday as part of the fiscal 1981 defense construction authorization bill. To equip all 54 sites -- 18 each in Arkansas, Kansas and Arizona -- would cost about $540,000.

During an appearance on "Face the Nation" (CBS, WDVM), Brown refused to discuss the specifics of the missile's nuclear warhead. Nonetheless he made it clear that despite the violence of the propellant explosion the warhead had remained intact.

News reports from the Arkansas site yesterday said heavy trucks and a crane had been moved onto the site.

On Saturday a government source said the warhead could be trucked to Little Rock Air Force Base.

Sam Tatom, Arkansas director of public safety, said yesterday he felt the 10-megaton warhead had been moved, "Unofficially, I don't think there's a warhead" in the silo area any longer, he said.

He based his conclusion on Air Force officials' attitudes toward security in the area and their agreement to allow him and Arkansas Gov. Bill Clinton to make a personal inspection of the grounds today.

Brown also told reporters that he does not know of the 1972 note then President Richard Nixon gave Soviet Premier Leonid Brezhnev assuring the Soviets that the United States had no intention of taking down the Titan IIs and replacing them with submarine-launched missiles.

Participants in the 1972 Moscow summit, as which the exchange took place, said recently that the Soviets had demanded such an assurance because U.S. sub-launched missiles than they were of the then nine-year-old Titans. u