MX missile: why 'dense pack' idea rockets ahead

With two audiences in mind, the Reagan administration has narrowed its options and accelerated its plans for basing the as yet homeless MX missile.

Of the various plans for housing this heavy intercontinental ballistic missile (ICBM), the front-runner now is ''dense pack.'' The opposite of the hide-and-seek race-track system spread over vast areas, this plan would crowd the missiles close together.

With US missiles bunched together, the reasoning goes, incoming enemy missiles would have to be fired with extreme accuracy and precise timing in order to keep from blowing each other out of the sky -- an effect strategic planners call ''fratricide.''

The administration's first goal in settling the MX question is to pacify congressional critics. Even the normally friendly Senate Armed Services Committee and its hawkish chairman, John Tower (R) of Texas, have blocked funds for building the first nine missiles and temporarily placing them in existing silos.

The administration wanted to put off a final decision until 1984. But in its defense authorization bill, passed last Friday, the Senate insists that a decision be made by Dec. 1 of this year. In effect, the Senate is telling the administration it will have to stop changing its mind and settle on a final basing plan before it can begin producing and deploying the MX.

The administration's second audience in this case is the Soviet Union.

Although President Reagan has called for significant reductions in ICBMs as part of his new arms control proposal, he has indicated that the USSR may not take him seriously unless he also proceeds with his $180 billion, six-year program to modernize and expand US strategic nuclear forces.

In essence, he is on a two-track system of negotiate and rearm. But without congressional approval -- and for the moment this means approval of the MX -- he may not be able to progress along the buildup track.

The decision to favor the dense pack system is primarily political. The plan's key technical feature answers critics of the race-track plan proposed by the Carter administration. It also maintains some degree of deceptive basing, favored by many in Congress.

Under dense pack, each MX missile would occasionally be moved among capsules spaced about 2,000 feet apart. Twenty such packs would be grouped into arrays, most of which would be placed on existing military sites such as Nellis Air Force Base in Nevada. Each array would take up no more than 20 square miles. Capsules would be superhardened to withstand much greater shock and have sufficient ''rattle space'' to survive less than direct hits.

Planners assume that with this system, Soviet missiles would have only limited first-strike effectiveness. Shock, dust, and radiation would destroy or throw off course many enemy warheads racing toward a relatively small area. The time for the dust cloud to settle (up to one hour) would allow the many surviving MX missiles to be launched before a second batch of Soviet warheads could be aimed at the arrays.

Since Reagan has taken office, the White House has put forth three interim MX basing schemes: existing Titan missile silos, superhardened silos, and existing Minuteman missile silos. Each was rejected by Congress.

The plan reportedly favored by Defense Secretary Caspar W. Weinberger -- placing the MX aboard slow-flying ''Big Bird'' aircraft -- also had more critics than supporters.

The dense pack plan may also include placing MX missiles in tunnels 2,000 feet underground. But this is thought to be technologically more difficult. Some form of ballistic missile defense also may be part of the dense-pack solution to housing the MX.

The plan now favored by the administration may run into some treaty problems, however. SALT II (which was never ratified, but which both the US and the USSR say they are abiding by) forbids the construction of new missile silos. The Anti-Ballistic Missile (ABM) treaty does not allow new missile defense sites, but is up for review this year.

The MX missile -- scheduled to begin deployment in late 1986 -- weighs 190, 000 pounds and carries 10 warheads. It will be much more accurate than the Minuteman III, which weighs 78,000 pounds and carries just three warheads.