Washington — Is Uncle Sam about to put his shiny new MX missile into a home that provides less than adequate shelter?
Many experts (including some who favor the as-yet homeless intercontinental ballistic weapon) see big problems for the ''dense pack'' basing plan that is now the Air Force's No. 1 choice. Even though billions of dollars would be spent on this plan, critics warn, American ICBMs still would be vulnerable to a Soviet first strike.
Among other things, it is argued, the Soviet Union can (or shortly will be able to) ''pin down'' the missiles by exploding a series of relatively small nuclear weapons above the MX groupings; it could use its hefty ICBMs to drop very large nuclear warheads on the MXs, destroying or disabling several at a time even in their ''superhardened'' silos; or it could spread nuclear land mines over the MX field.
''In no way does dense pack provide enduring survival,'' says Richard Garwin, a physicist who helped develop the hydrogen bomb and has been advising the government on nuclear technology and strategy for 30 years.
Says Colin Gray, president of the National Institute for Public Policy and an MX advocate who has been advising the Pentagon on the missile, ''What I don't like so much about closely spaced basing (the official designation of dense pack), and why this system alone is not enough for ICBM basing, is that it does not provide an enduring ICBM capability.''
''You're talking at most about a matter of hours of survivability, and that's being optimistic,'' adds Dr. Gray.
Among others who have also recently raised questions about dense-pack basing for MX are: William Perry, former defense undersecretary for research and engineering; retired Adm. Noel Gayler, who was director of the National Security Agency and a strategic planner; retired Air Force Lt. Gen. Brent Scowcroft, national security adviser to former President Gerald Ford; and retired Army Lt. Gen. Daniel Graham, former head of the Pentagon's Defense Intelligence Agency.
Among the ways the MX dense pack might be countered:
* After an initial attack against the MX missiles, the Soviet Union could pin down the surviving missiles by exploding at high altitude a series of relatively low-yield weapons that would blanket the area with radiation and electromagnetic pulses through which the MX missiles could not be launched. As the dust and debris from the initial Soviet attack subsided, a second Soviet strike could be launched before the MXs could be fired in retaliation.
* According to some reports, the Soviet Union has tested a nuclear weapon of 60 megatons. Dr. Garwin estimates a weapon of no more than five megatons could destroy one MX and inflict damage on other launch sites. This attacking procedure (called the ''spike'') could mean the destruction of a group of MX missiles with relatively few warheads, large and overlapping in effect.
* The Soviet Union also might counter MX missiles by attacking with nuclear landmines scattered over the dense pack site and penetrating the earth. These could either be fused to detonate all at once or could be triggered by the launching of an MX. ''Against missiles emerging from their silos, such a mine of one-megaton yield would have an effective radius of two miles or so,'' says Dr. Garwin.
The Air Force's response to the mounting criticism of its MX basing plan is that critics assume too much about Soviet capabilities; that planners have other schemes up their sleeves for protecting MX in its dense-pack fields; and that uncertainty about exactly how the plan would perform in an nuclear exchange works in favor of the United States.
''It's a very complex system,'' says a senior Air Force official who works in strategic planning. ''If you change one parameter, you change eight others.''
By crowding groups of MX missiles no more than 2,000 feet apart and increasing the hardness of their silos by as much as five times, the Air Force argues, the Soviet Union would have to successfully launch one warhead at each US missile with precision and timing.
Air Force officials say such an attack would result in ''fratricide,'' in which most attacking missiles would be thrown off course or disabled by their own blast or debris.
''We don't believe that the Soviets can confidently or successfully pin down our CSB (closely spaced basing),'' says one official.
Because of budget considerations here and possible treaty implications, the Pentagon is not playing up its development of ballistic missile defense systems that many MX advocates say must accompany dense-pack basing.
There have been hints of such development in Pentagon budget documents and statements of top officials. The authoritative journal Aviation Week & Space Technology reports that Defense Secretary Caspar W. Weinberger has endorsed a US space-based ballistic missile defense system.
Some in Congress are pushing the Pentagon to accelerate its space-based weapons programs. But others raise questions about the need to further protect the MX dense-pack holes and about arms-control implications of pursuing ballistic-missile defenses.