Mum's the word on likely US strategic arms changes. US negotiators hope a strategic arms agreement will force the Soviet Union to restructure its forces and enhance world stability. But such an arms deal will also require the US to do some restructuring of its own - a subject Washington doesn't even want to discuss publicly.

There is a catch in effecting the West's desired restructuring of Soviet strategic forces in the direction of stability through strategic arms control. The catch is that the United States also must restructure its forces in the direction of stability. This sets off alarm bells in the military services, since rejigging procurement programs is always expensive and sucks money away from existing pet projects.

So sensitive is this issue that no American official is talking publicly about what changes would actually take place under a strategic arms reduction treaty (START). And some senior officials are suggesting in private that the biggest barrier to concluding a treaty at this point is not Soviet-US differences over sea-launched cruise missiles (SLCMs) or even President Reagan's Strategic Defense Initiative (SDI or ``star wars''), but American-American gridlock over developing a US mobile missile.

On the face of it, such a hang-up is curious. Few question the need to get away from the hair-trigger combination of enormous threat but enormous vulnerability inherent in multiple-warhead intercontinental ballistic missiles (ICBMs) in fixed silos. The US, unlike the Soviet Union, recognized the coming vulnerability of stationary missiles years ago, and has dispersed five-sixths of its strategic warheads to far more survivable submarines and airplanes, according to the International Institute for Strategic Studies.

What is at issue, however, is the specific treatment of the 16 percent of US strategic warheads deployed on fixed ICBMs - and whose ox gets gored in the service responsible for America's land-based strategic missiles, the Air Force. In recent years - once the Reagan administration overcame its earlier scare about a ``window of vulnerability'' - the administration has inclined to leave the ICBMs as they are without worrying about them. The reasoning has been that the ``deterrence'' of any Soviet nuclear attack remains strong, since the other 84 percent of American warheads on submarines and bombers more than suffice to assure a devastating retaliation should Moscow launch an attack and wipe out the bulk of American ICBMs.

The Air Force in the past five years has been far from united in this view, though. Some officers have been pinning their hopes on the panacea of a space shield. A very few have been arguing that the US should move to less vulnerable mobile basing of its ICBMs. And some traditionalists still begrudge any further diversion of appropriations from good old-fashioned planes to new-fangled missiles.

In addition, some hardliners inside and outside the Air Force have regarded Washington's official insistence in the START negotiations on a total ban on mobile missiles as the ideal spoiler to jinx arms control altogether, since the Soviets have already begun to deploy two mobile missiles and would never accept such a ban.

The result has been what one think-tank analyist with high-ranking Air Force contacts calls a decade-old ``history of confusion, chaos, and disagreement [on ICBMs] that is awesome.'' Only in the past year - under the leadership of Air Force Chief of Staff Gen. Larry Welch, Gen. John Chain, commander in chief of the Strategic Air Command, and Defense Secretary Frank Carlucci - has the thinking of key Air Force generals begun to coalesce around the idea of developing a rail-mobile system, the analyst said.

No broad political consensus on this system has yet emerged in Washington, however. The House of Representatives favors a road-mobile system, while the Senate favors one that is rail-mobile one. The Reagan administration and both Presidential hopefuls George Bush and Michael Dukakis all formally oppose development of any mobile missile at all, but are hedging their bets. The adminsistration, for example, has been negotiating contingency verification measures ``in case'' mobiles are permitted under START.

Comparable behind-the-scenes maneuvering is taking place in the US Navy. There the issue is not the basing mode, but how many of the limited number of warheads allocated to SLCMs should go onto each strategic nuclear submarine - and therefore how many submarines the US would have at sea at any one time under START ceilings. The higher the concentration of warheads - the newest Tridents carry 192 - the fewer the boats and the fewer the targets the Soviets would need to find.

Ex-Secretary of State Henry Kissinger, warning the administration not to ``rush'' into an arms control agreement, objects to the START numbers that are shaping up both on land- and sea-based missiles. In a column in the Washington Post last April, Kissinger argued that under START the ratio of accurate Soviet warheads to American ICBMs would rise dangerously. He also maintained that US strategic submarines would probably go down to 18, allowing only six to be on station in the Atlantic at any one time. He suggested that those six could become sitting ducks for the Soviets.

Administration officials treat Kissinger's worry as a non-issue, arguing that the ratio of Soviet warheads to American ICBM targets under START depends entirely on how the US itself deploys its forces. If the US makes rational decisions, they say, the ratio will actually go below the crucial 2:1 ratio.

Without mentioning Kissinger by name, presidental negotiatiating adviser Paul Nitze rebutted him in a commentary in the Post a week ago. In it, Mr. Nitze stated categorically that ``The emerging START treaty would reduce the Soviet threat to our retaliator forces and put us in a stronger position to ensure the continued survivability and effectiveness of our nuclear deterrent.... START would...reduce and bound the problem, making it easier for unilateral measures to redress it.''

On the submarines, Nitze also said flatly, ``In fact, the increased stealth of the Trident and its ability to operate in a vastly larger ocean area will make even a smaller force more survivable.''

Moreover, Nitze contended, the alternative to a START regime is not the unfavorable status quo, but a much more unfavorable buildup of Moscow's forces from ready Soviet missile assembly lines.

Administration officials differ on the next logical question: whether or not service, executive-branch, and Congressional consensus on rational deployments must precede signing of a START agreement or could be wrestled out when a treaty came up for a ratification debate in the Senate.

For his part, ex-National Security Adviser Robert McFarlane calls for an early basing decision to avoid ending up ``with the worst of both worlds: first, a vulnerable intercontinental ballistic missile force that could provide the incentive for a Soviet nuclear attack during a crisis; second, no treaty restraints on the growth of Soviet strategic forces.'' In a column this month in the New York Times, he decries ``parochialism from the Air Force and hypocritical cheap shots from former Ford and Carter Administration officials.''

(Second of two articles)