The aging Titan II missile system "is safe and supportable" but "is also potentially hazardous," according to a 300-page Air Force study to be released this afternoon.

The study was ordered by Air Force Secretary Hans Mark after an explosion last September at a Titan II site near Damascus, Ark., killed one airman and injured 21 others.

"As with any technologically complex system," the study says, "improvements can be suggested." Then it lists hundreds of recommendations to improve the operations, maintenance and safety of the 18-year-old intercontinential ballistic missiles.

The study points out several deficiencies in the missile system that previously had not been made public.

For example, it reports that 40 of the 52 Titan II missiles in operation "have never been through" the two reviews that "assess reliability, determine aging characteristics and permit extension of the engine service life." That is because only "two missiles are evaluated each year," the report said in calling for a new programmed depot maintenance system.

The study also admits that "the fixed [in silo vapor detection] system is currently out of commission 40 percent of the time." This is the primary device to warn missile crews and maintenance personnel that there may be dangerous vapors around.

The study then attempts to give reassurance by noting that while the vapor system is not working, the Air Force does not allow liquid fuel propellant transfers to take place. No mention is made of what this does to the alert time of the missiles.

Among the proposals are to:

Develop a new in silo fixed vapor detector system.

Develop "hardware and procedures" that would permit the burning, neutralizing or removal of oxidizer or uel spills before they can be released into the atmosphere and endanger nearby civilians.

Ensure that the missile crew can remain in its underground capsule, safe from spilled oxidizer or fuel vapors.

Develop a means to remove oxidizer or spilled fuel by remote control from the crew capsule.

"Select and deploy" warning systems that "are clearly required around Titan II complexes." Less than six months ago, the Air Force was arguing against such systems, saying thay would be too costly and would create unneeded concern among those who live near the missile silos.

Nowhere in the study, however, do the Air Force experts who prepared it estimate how much the proposed changes would cost.

The reports reiterates Air Force statements to the effect that the Damascus, Ark., accident, and one in 1978 at Rock, Kan., that killed two airmen and injured 29, were caused by human error. Both sites are now closed.

"Since 1976," the report says, "mishaps caused by personnel error have shown a sharp rise." The study then notes that reenlistments among personnel working on the missiles have ben low and recommends hazardous duty pay for those handling the volatile fuel for the missiles and higher reenlistment cash bonuses for technicians who work on the system.

Despite its consulsion about the safety of the system, the report makes numerous references to the increasin number of accidents in the past two years and the difficulties of finding replacement parts for the missiles.

The Damascus accident occurred at a time when the need to retain the Titan system was being questioned on Capitol Hill, particularly by congressional delegations from the three states where the remaining 52 missiles are located.

At the urging of these delegations, the House and Senate Armed Services committees have begun investigating the Titan II system.

The study raised the prospect that a Titan II site near Phoenix may have to be closed because the surrounding area has become heavily populated with retired people.

A prototype exhaust shaft burner or scrubber to remove dangerous fumes is to be tested at this site, along with a new warning and evacuation scheme.

"If this is not practicable," the study says, "this site should be considered for conversion to training status. . . ."