The MX -- a conclusion

In June of 1979 President Jimmy Carter settled an argument which had been raging for months inside the Pentagon.

There was agreement over in the big building on the south side of the Potomac that there should be a new land-based nuclear weapons system to update the American nuclear deterrent. It would be called MX. It would supposedly be able, by using the most modern and most accurate guidance system then becoming available, to deliver several warheads to almost any point on the Earth's surface within about an eighth of a mile of any given target.

But there was hard disagreement on the size of the new MX.

Some favored a model 85 inches in diameter which could have multiple uses. At that diameter it could fit into the Navy's new Trident submarines. It could be lifted by any large aircraft and fired from the air. It could be mounted on a truck or railway flat car and trundled at will around highways and railways. It would be mobile.

Because it would be mobile it would conform with the limitations of SALT II which prohibited any new fixed, land-based system. That limitation was written into the SALT treaty on United States insistence to prevent the Soviets from setting up a new land system with bigger missiles.

Because it would be mobile it would also escape from the vulnerability which had begun to diminish the value of the existing Minuteman missiles. No enemy could know just where a given MX would be at any given time.

The Navy was happy with the 85-inch model for MX. It is in fact being built for the Navy as Trident III. It will be fitted into the new Trident submarines as soon as ready.

But the Air Force did not want a ''commonality'' weapon.

The Air Force waged a campaign for a larger carrier for the new and superaccurate warheads. President Carter gave in to Air Force pressure and allowed them to design the present and controversial MX.

This Air Force model is 71 feet long, 92 inches in diameter, and weighs 190, 000 pounds. It is too big for a Trident submarine, too big to be carried and launched from existing aircraft, and too big to be trundled around by flatcar or truck. It has to have a hole in the ground for launching.

But because it has to have a hole in the ground it has lost mobility and thus regained the vulnerability of the Minuteman. The Air Force thought up the ''race track'' scheme for giving it some mobility. It could be moved by special vehicle around a track from one launch hole to another. But that fell down on political grounds. No state wanted to give up the space necessary for the race track.

President Reagan has been persuaded to accept a new system for the big MX, the ''dense pack.'' It is based on the theory that if 100 are packed together in a mere 20-mile long area the Soviets would have trouble knocking all of them out.

But there is so much doubt about this that there is now talk of protecting the MX in its ''dense pack'' field with batteries of anti-ballistic missiles - ABMs. But more ABMs are prohibited under SALT I. Besides, enough ABMs to protect the 100 MXs could nearly double the cost of the MX - an extra $20 billion for ABMs on top of probably about $30 billion for the 100 MXs.

Trident III missiles will be available for Trident submarines as soon as the MX could be built and deployed. One Trident III missile would be just as accurate as an MX. But it would not be able to carry as many warheads. However, the same number of warheads with the same blast power could be achieved with more Tridents, or by using the Trident III-type missile from large planes, surface ships, railway flat cars, or trucks.

But the Trident III, or the smaller MX at 85-inch diameter, lacks one feature of the big 92-inch MX. While the smaller is mobile and can be used in a submarine or trundled a-round on a truck - it is not the exclusive possession of any one military service. The MX, in its big version, is an exclusive Air Force property. It cannot be used by Army or Navy.

The conclusion from all of the above and from the other factors considered in the two previous columns on this page on the MX debate is that with one exception anything the big MX can do can be done as well and probably for less money by other means and without breaking the bounds of SALT I and II. The one exception is giving the Air Force a new weapon which is its exclusive property.

Considering that the fixed missile is immensely more vulnerable than any mobile missile it would seem that the US government could have extra counterforce strength to balance off Moscow's newly accurate weapons more quickly by scrapping the big MX and going instead for a mix of Trident IIIs, mobile 85-inch MXs, and cruise missiles. There would probably be some money saved.

Is it worth staying with a vulnerable and probably soon obsolete fixed land-based system just to give the Air Force an exclusive?