The Pentagon is studying still another way to base its homeless MX missile, one that relies in part on a far-out nuclear war game concept known as "fratricide."

The Air Force has labeled this latest deployment idea the "MX Deceptive Dense Pack (D2P) Basing Concept." A Pentagon executive said yesterday that Dense Pack picked up steam after the Senate Armed Services Committee recently refused to let the Air Force put the first MX missiles in existing Minuteman silos until some permanent basing plan is found.

Dense Pack, as the name implies, involves bunching MX missiles in fairly tight formations. That is quite different from earlier basing plans, in which it was argued that, to make them less vulnerable, the missiles ought to be spread far apart.

In fact, Dense Pack backers figure, it may be better to keep the MX silos fairly close together. The reasoning goes this way: the first Soviet warhead to come down on a pack will not only destroy its target, but in a kind of sympathetic detonation, will blow up the warheads flying right behind it on the way to other U.S. missiles in the cluster. These other U.S. missiles may then survive.

That is fratricide: incoming warheads destroying each other. The only trouble is, no one is sure whether fratricide would really occur. The Soviets might figure out a way to make all their warheads flying into an MX missile field explode at once, wiping out everything.

Air Force and civilian weapons experts currently are exploring the fratricide question before giving a recommendation on Dense Pack to Defense Secretary Caspar W. Weinberger.

Finding a home for the MX has been one of the great defense problems for the Reagan administration. The president charged in the 1980 campaign that preceding administrations had let U.S. strategic forces--particularly land-based missiles--lag to the point where they were vulnerable to attack.

He promised to rectify this in proposing his own basing plan for the MX. Last year, however, he found this as hard to do as his predecessors had, and proposed an interim basing system while he pondered the question further of where to put the MX.

Air Force leaders, after years of analysis, still favor basing MX on the ground in some kind of deceptive deployment rather than the alternatives, which include trying to keep the missile airborne inside a giant plane called "Big Bird." Dense Pack may be easier to sell than the earlier Air Force schemes, partly because the emplacements would be spread around the nation in relatively small areas rather than taking up vast tracts in one or two states.

The Air Force MX deployment that President Carter had approved called for hauling 200 MX missiles covertly from one cement garage to another across the valley floors of Nevada and Utah. There would have been 23 widely separated garages for each MX, which would theoretically have forced Soviet gunners to guess which one held the missile or target two warheads on each garage to be sure.

President Reagan ridiculed the scheme as too extensive and complicated, while Weinberger has argued that the Soviets would build enough warheads to cover all the MX shelters. Under Dense Pack, each missile would have 10 vertical capsules it could go into, spaced 2,000 feet apart. The combination of one missile and 10 capsules is called a pack. Only when a missile was taken out of a capsule for maintenance "once or twice a year" would it be slipped into another one covertly. There would be no more hauling MXs from shelter to distant shelter as in the old race track and drag strip schemes.

Twenty packs, meaning a total of 20 missiles and 200 capsules, is what the Air Force calls an "array." An array "would require less than 20 square miles," according to the Air Force Dense Pack point paper obtained by The Washington Post.

And best of all from the standpoint of minimizing protests, the Air Force notes, the arrays are so small that they could be located mainly on existing military bases.

No army of construction workers would have to invade an existing community to build a missile base, as was the case under earlier plans for the MX. Ten arrays would get the Air Force up to its goal of 200 deceptively based MX land missiles.

Because the 10 capsules in each pack would be fairly close together, the Air Force reasons, Soviet warheads would have to fly in tight formation to destroy them all.

"Some Soviet warheads will destroy each other," states the Air Force paper, "significantly reducing Soviet confidence in the attack . . . . Therefore, there is inherent survivability in the deceptive Dense Pack concept, with an inherent limit on the effectiveness of a Soviet attack."

Adding an anti-ballistic-missile defense to the Dense Pack system, states the Air Force, "would create even further uncertainty for Soviet planners considering an attack on D2P."

The same point paper acknowledges that Dense Pack may present verification problems for future arms control agreements, particularly if the Soviets adopt it for their own land missiles. Then again, states the paper, Dense Pack "could create uncertainty in Soviet planning to lead the Soviets to arms control concessions."