The Air Force and the Pentagon research office have approved conditionally yet another basing scheme for the MX missile: the so-called Dense Pack deployment, which would bunch the missiles so close that incoming warheads would disable each other in what is called "nuclear fratricide."

Pentagon officials said yesterday that Air Force four-star generals and civilian research directors recently agreed that pursuing Dense Pack, although troublesome from several standpoints, was better than giving up on finding a home for the MX land missile.

Earlier ideas, ranging from rotating 200 MXs among 4,600 cement garages spread around the valleys of Nevada and Utah to stuffing the first missiles in existing Minuteman holes, were derailed by political opposition.

"It's our last time at bat," Defense Secretary Caspar W. Weinberger reportedly told Air Force leaders in urging them to get behind the Dense Pack scheme. Weinberger has not formally made an MX recommendation to President Reagan, Pentagon officials said, but he has an option paper in which Dense Pack is featured.

In closed-door debates over Dense Pack, some Air Force leaders emerged as champions of the second strategic arms limitation treaty (SALT II), negotiated by the Carter administration and assailed by Reagan and Weinberger. Air Force backers of SALT II said it has helped keep the lid on the number of deployed Soviet warheads; opting for Dense Pack might pry off that lid, they warned.

Article IV of SALT II, which was signed by the United States and Soviet Union but was not ratified by the U.S. Senate, states that "each party undertakes not to start construction of additional fixed ICBM intercontinental ballistic missile launchers" or "relocate fixed ICBM launchers."

Under Dense Pack, the United States would build new silos for the MX with enough concrete and steel to withstand a nuclear blast exerting up to 5,000 pounds per square inch. One Pentagon idea is to rotate 40 to 50 MX missiles among 100 such silos. Whether such rotation would convince the Soviets that the silos were temporary shelters, not additional launchers forbidden by SALT II, is one of the questions bothering military and civilian specialists who see value in adhering to the limits established by SALT II.

William E. Jackson Jr., director of the arms control advisory committee during the Carter administration and now a guest scholar at the Brookings Institution, said yesterday that Dense Pack would violate Article IV of SALT II.

The Soviets are likely to view Dense Pack "as constituting a SALT breakout," he said. "A super-hardened hole in the ground will be considered as a missile launcher."

Although Reagan may not care whether SALT II is violated, the strategic program he has unveiled to date does not violate its provisions. The administration may decide to tell Congress it will pursue the Dense Pack basing scheme but will not begin deployment until more technical and political studies are conducted.

On the technical front, Dense Pack advocates in the Pentagon believe that such a deployment would make a surprise attack on the United States look more futile than ever to Soviet planners. To be sure of destroying tightly bunched MX missiles, say Dense Pack advocates, the Soviets would have to find a way to make their warheads targeted at one location explode simultaneously to avoid fratricide.

The first of several warheads to explode, specialists said, would kick up such a thick cloud of cutting dust and send out so many X-rays and shock waves that the warheads flying in behind it would be destroyed or thrown off-course.

The warheads would be flying in so fast that the dust would cut into their nose cones, throwing them off-course, and perhaps slice through the casing, according to Pentagon leaders working on Dense Pack.

In addition to relying on fratricide to save MX missiles that would be clustered around the country on government land, Pentagon research chiefs would like to station anti-ballistic missiles on the sites.

The existing anti-ballistic missile treaty, updated by a protocol in 1974, limits the United States and the Soviet Union to deploying no more than 100 anti-ballistic missiles and launchers in "a single area," either its capital or an ICBM site. The ABM treaty comes up for formal review in October.

Under Reagan's strategic blueprint, 100 MX missiles would be produced, with the first ready for deployment in 1986. Putting the first batch in Dense Pack silos, spread about 2,000 feet apart, could be done about a year later, according to the Pentagon.

The Senate Armed Services Committee this year refused to go along with basing the first MX missiles in existing Minuteman silos as the administration recommended, prompting the Pentagon to find an alternative quickly for fear Congress would declare the 10-warhead blockbuster hopelessly homeless and cancel it.