A long string of mechanical and human failures and confusion preceded the explosion last Friday morning at the Titan II missile silo near Damascus, Ark., the House Armed Services Committee was told yesterday.

Among other examples, Air Force Secretary Hans Mark described how, three hours after the missile's fuel tank was accidentally punctured, an Air Force hazard team at the site and consultants from Martin Marietta, the contractor who built the Titan, were giving conflicting advice on how to handle the increasingly dangerous situation.

Strategic Air Command officers decided to wait for the situation to take care of itself -- as the contractors suggested -- rather than following the suggestion of their own officers who wanted to open the 740-ton cement cover over the missile and allow the explosive fuel and oxidizer vapors to escape before they exploded.

Despite the series of mechanical problems he described, Mark told the committee that the age of the 18-year-old missile was not a factor in the accident, as some critics have suggested. He said the immediate cause was the dropping, 70 feet inside the silo, of an 8-to-10 pound wrench socket that punctured the fuel tank.

"It could have happened on the first day of the deployment of the missile," Mark said.

The first safety feature of the Titan that failed, according to Mark's presentation, was a rubber matting placed between the spot where an airman was standing using the wrench and the side of the missile. It was supposed to catch and hold any dropped item.

The wrench, Mark said, was too heavy for the rubber and fell through.

The missile's skin is one-eighth inch thick, according to the Air Force secretary.

Sources in Arkansas said yesterday that the initial puncture was three to five inches wide.

The warning systems inside the silo worked, Mark said, and there an almost immediate indication of dangerous vapors and loss of fuel.

Two other safety systems, however, apparently failed to do the job.

A spray system with nozzles above and alongside the missile sent 100,000 gallons of water washing down its side. It was supposed to carry off the highly-volatile fuel, which is supposedly soluble in water.

The Titan system, however, was not designed to take care of leaks as large as this.

The water and the fuel went to the cavity below the missile and remained there. But as additional fuel leaked, the water became completely saturated and fuel vapors began to build up again within the enclosed silo.

At that point, sources said, an exhaust fan system normally would have come on to clear the air. It did not turn on at this site, Mark said, but gave no further explanation.

Shortly after 8 p.m. a special Air Fore maintenance team entered the silo and found the toxic vapor level very high, Mark said, and found that the water spray had not solved the problem.

The four-man Titan missile crew works out of an underground concrete control center that is separate from the silo but conneted by a passageway. Two heavy blast doors separate the silo from the control center.

At 8:23 p.m., the crew, which is only supposed to leave its post under extreme danger, moved out to the surface.

At 8:30 p.m., after 15 minutes of inspection, the maintenance team also abandoned the silo, according to Mark.

For the next hour, Mark said, the Air Force concentrated on evacuating civilians within a 2,500-foot radius of the site.

SAC headquarters used that time to try to decide what to do next. A two-man team re-entered the control complex near 10 p.m. and saw indications that the silo was still dangerously filled with fuel vapor.

SAC chose to take the advice of the contractor and wait things out. "The technical experts did not expect any explosion at all," Mark said.

At about 1 a.m. according to Mark, there was "a decision of people at the site to go back in to assess the situation."

That step was approved by SAC and at 2:13 a.m., Mark said, the two-man team got as far as the second door and their vapor meters went "off scale." They were "told to get out" because of the dangerous readings, and just as they were emerging the site blew up.

One of the airmen later died and the other is still in serious condition in a Little Rock hospital.

"Some people made some errors," Mark said, but added that until the accident report is concluded in about a month, he did not "know what they were."