THE ADMINISTRATIONS design for strategic weapons -- at least, in its key elements -- seems sensible, more so than its predecessors'. But nothing will be clear until the broadly stated intentions of last Friday evolve into precise decisions about basing new missile systems. And we may not see those decisions before 1984.

In the main, Reagan's people were trying to solve what in recent years has been the fashionable strategic problem, the so-called window of vulnerability. The phrase describes a time just ahead when improved Soviet missile forces will, in theory, be able to destroy most of America's silo-based Minuteman ICBMs in a literal bolt from the blue. The theory goes on to suggest that our leadership might be unwilling to retaliate against Soviet society with America's alert submarine and bomber forces, because to do so would invite the destruction of our cities.

This administration didn't create the fashion -- its root is deep -- or even the phrase, which crept into the Senate's hearings on the SALT II treaty in the summer of 1979. But Reagan has treated the question with far greater concern than any predecessor; his defense policy and, to a degree, his foreign policy arise from his emphasis on the need to remove the threatening window.

It is rather ironic to have a president so firmly tied to the fashion just when its credibility is starting to decline. There was always skepticism, but muted. Moreover, the numerous schemes advanced in recent years for deploying land-based missile launchers in a less vulnerable way seemed to be adrift from reality and lacking in feasibility.

When the Ford administration, for example, seemed on the verge of a decision to conceal ICBMs in a continuing 3,000-mile-long trench, most people assumed correctly that the idea would go nowhere and was not, therefore, a threat to sensible policy. Then the Carter people elected to scatter 200 MX launchers in 4,600 shelters located on special roadways in remote parts of Utah and Nevada. This scheme seemed slightly more serious than Ford's, mainly because the Republicans were then hurting Carter with accusations of stalling the MX after having cancelled the B1 bomber. Still, most people sensed the Carter's scheme would fare no better than Ford's. The ensuing litigation alone would have submerged the legal resources of Utah and Nevada.

Reality in the nuclear era becomes the stuff of trendy perceptions and often their victim. Now, the pendulum is swinging away from the window of vulnerability. Predictably, serious people are questioning and even denying the threat, probably because this administration gave it so exalted a priority. The proposition that nuclear war can be limited is going to be seen more clearly for what it is -- a notion that would be merely frivolous if it were not so dangerous.

In time, it will be even clearer that the sea is the best and most secure environment for nuclear weapons, and the United States, with its long coastlines and easy access to deep water, has obvious advantages in operating at sea. Gradually, we should put an even larger proportion of our forces out to sea, probably in submarines. That would be a partial answer to the vulnerability problem. Another would be to bound the threat with limits and restrictions of the kind contained in the SALT II treaty; these could be made progressively tighter if the process were resumed and sustained.

Still another partial answer would bve deciding that land-based ICBMs are as secure in silos as anywhere. A sensible policy would combine each step: some redeployment to sea; a commitment to serious bargaining on realistic terms in SALT, and a judgment that the threat to silo forces is more apparent than real.

Fortunately, the president, in the decisions he announced on Friday, left himself in a position to move in all three directions. Unfortunately, his decisions may not be interpreted that way. In a broad sense, he's said that the 100 MX missiles to be deployed will be in silos, but may be protected by antiballistic missiles (ABMs). (The other announced alternative of deploying MXs in airplanes is probably not feasible.)

Leaving the MX launchers unprotected, if that is the eventual decision, would amount to a possibly embarrassing concession that the window of vulnerability wasn't there after all. But defending them could provoke another debate over the ABMs and reawaken anxieties of a rampant nuclear arms race. (A small deployment of ABMs would be seen as foreshadowing a major deployment.) It would probably be judged a violation of the SALT agreement. Perhaps the administration understands all this and intends nothing more than to sell the ABM yet again to the Soviet Union in SALT. If so, all the more reason to get on with SALT.

ABMs aside, the decision to put 100 MX launchers in silos makes much more sense than the Carter plan or any of its numerous antecedents. We can take comfort from the burial of the multiple protective shelter, or shell-game, approach to ICBM vulnerability.

It should be noted, however, that we don't need the MX at all. Soon the Minuteman III improvement program will be finished. And the president is going forward with a new submarine-launched missile system, the Trident II. There is no need to have all three. Any two of them, in theory, will be able to destroy Soviet ICBMs, a prospect that our protracted discussion of vulnerability has probably taught the Soviets to worry about, especially since so much of their strength is at risk. We should be trying to maneuver the Soviets away from dependence on hair-trigger ICBMs with first-strike accuracies. That can only be done through example and the negotiating process. If the MX makes any sense at all, it is as a bargaining chip in SALT.

The disturbing element in the Reagan package was the least expected -- the decision to deploy nuclear-armed cruise missiles on attack submarines. There is no need for such weapons. They may very well give the arms competition another dimension. Once deployed, there will be no reliable way for the other side to count them. The administration says it will deploy several hundred of these cruise missiles. If the Soviets said the same thing, we would assume eventual deployment of thousands.

The Navy also plans to deploy hundreds, possibly thousands, of conventionally armed cruise missiles. These will not be distinguishable from the nuclear-armed weapons. Deploying nuclear-armed cruise missiles at sea, if it is actually done, will collide with the principle of verifiable arms control agreements; just announcing such a deployment may be interpreted as another step away from the SALT process.

The decision to build the B1 bomber is trhe most controversial part of the package. It's far from clear that the administration will get the money it needs to fund the project, which may even serve to penalize other programs. Actually, the B1 makes considerably more sense than any of the other new programs, except for the scheduled improvements in command, control and communications. The president would do all of us a favor if he bargained away the MX and the sea-based cruise missiles but sustained the B1. Heavy bombers are slow, recallable and manifestly second-strike weapons. They set an excellent example. They also constitute military power that is actually usable in contingencies less remote than nuclear warfare; hence, they have inherent political attributes.

The aging B52s cannot be maintained very much longer. Why not, it may reasonably be asked, leapfrog the B1 and await the arrival of the higher technology Stealth bomber? There are at least two reasons to support the administration's plan, apart from the desirability of having a new heavy bomber sooner rather than later. First, the Stealth bomber may not be available in this millennium. It probably will be possible to build an airplane around a radar-resistant technology. It may not, however, be possible to create a modern bomber around it. Second, by the time a Stealth bomber finally appeared, improved radar technology might very well have nullified its advantage.

There is also some question as to whether the B1 will be able to penetrate Soviet air defenses. It will never be proved that it can. And it won't be proved -- not to my satisfaction -- that it cannot. Not so long ago, in the spring of 1978, a South Korean airliner lost its way and accidentally penetrated Soviet air space; it traveled hundreds of miles before being picked up and forced to land. The airplane was flying at its cruising altitude and cruising speed. The point is that any system, however sophisticated, is really no better than the people operating it.

The B1 should be judged in terms of this country's need to replace useful weapon systems when they become obsolete. Not doing so would create difficulties of all kinds. It is the nature of armed forces that they modernize. However, those who say, as most of the Reagan people do, that we've been standing still and have fallen behind are quite wrong. Although Soviet forces have been very substantially improved in recent years -- more so than our own -- it is because they have had to come from far behind to catch up. Meanwhile, we have created a new ballistic missile system, the Trident; we are upgrading the major part of our Minuteman force, and we will soon begin equipping heavy bombers with long-range cruise missiles.

In his fiscal year 1980 posture statement, Gen. David C. Jones, the chairman of the Joint Chiefs of Staff, said: "I would not swap our present military capability with that of the Soviet Union, nor would I want to trade the broader problems each country faces." Nothing has changed, except that Soviet difficulties have worsened, a development that should be judged soberly, not smugly.

The only sensible strategic policy will require investing in new systems and negotiating limits on them all. There is no conflict here. The new systems are needed in order to assure one's own society that its defenses will not slip unacceptably. They are also essential to successful negotiations with the other side. In diplomacy, as in life, there is no free lunch. Each side must feel that it is negotiating from a reasonably strong position. The dangers lie in creating progressively more destructive weapons which, like the MX and similar Soviet systems, are judged threatening to each other.

With sensible and moderate efforts, this country can comfortably maintain a stable strategic balance based on a parity -- real and perceived -- with the Soviet Union. We should do neither less nor more. Our more urgent and obvious military requirements lie in the area of conventional forces.