US seeks ways to thwart a missile shield. Air Force wants high-tech decoys and warheads that dodge defenses

The Air Force is quietly working to ensure that its nuclear missiles will be able to penetrate a Soviet version of a ``star wars'' defense. Toward this end engineers are dreaming of whole new types of missiles and warheads, all far more evasive than those atop today's strategic nuclear missiles. Among their ideas, as revealed in little-noticed Pentagon contract proposals:

Slow rocket boosters, which would not rise high enough to be vulnerable to lasers or other directed energy weapons beamed from space.

Special warheads for the modern MX missile, capable of homing in on and destroying defensive radars.

Decoys that would imitate the distinctive ``wake'' of nuclear weapons entering the atmosphere.

An aerosol spray to envelop warheads in a radar-wave-scattering cloud.

In addition, the Pentagon is close to perfecting long-range warheads able to maneuver as they approach their targets. The Pershing 2 intermediate-range missile, now being deployed in Western Europe, has a relatively crude version of such a maneuverable reentry vehicle.

To critics, there is irony in this effort to build more-crafty intercontinental missiles. They point out that it has a goal exactly opposite to that of the United States Strategic Defense Initiative (SDI) -- which is spending billions of dollars on research into methods of shooting missiles out of the sky.

President Reagan and other SDI backers envision a world where strategic defenses have made nuclear weapons unusable. But Air Force work on missile ``penetration aids'' shows that the more likely scenario is a continuing race between offensive and defensive weapons technology, says SDI critic John Pike of the Federation of American Scientists.

To the Pentagon, the missile work is a way for the US to hedge its bets. If the Soviets achieve a breakthrough in defense technology before the US and quickly erect a nationwide antimissile system, better missiles could keep the US from falling into a dangerous strategic spot.

Defense Secretary Caspar W. Weinberger, in his presummit report to the President on Soviet arms treaty violations, reportedly said: ``Even a probable territorial defense would require us to increase the number of our offensive forces and their ability to penetrate Soviet defenses to assure that our operational plans could be executed.''

Accessories to help missiles evade defenses are not a new idea. All current US long-range missiles, from the land-based Minuteman to the submarine-based Trident, can carry them.

``They range from foil strips to electronic means,'' says Maj. Rick ObornCQ of the Pentagon's ICBM Modernization Office, declining to elaborate.

The foil strips are intended to confuse radars searching for incoming warheads. ``Electronic means'' likely refers to devices that beam a radar-jamming signal.

Most Air Force thinking about new ways of sneaking warheads through defenses is done under the auspices of the Advanced Strategic Missile Systems (ASMS) program at Norton Air Force Base in California. The program's budget has risen from $74 million in 1984 to a projected $169 million in fiscal 1986. Current administration projections are to raise the program's funds further, to $215 million in 1987.

The ASMS is among the most secret of government programs, but some idea of its work can be gained by examining unclassified contracts and bid solicitations.

The slow-missile booster is one of the program's unusual ideas. Depending on your point of view, it would either be a low-flying ICBM or a fast high-flying cruise missile.

According to a bid solicitation sent to small businesses in October, such a missile might coast between stages, and would never rise higher than 80 kilometers (about 48 miles). At that relatively low altitude, it could avoid ``beam weapons,'' such as lasers, the Pentagon says. Its warhead is seen as a miniplane capable of evasive flying on its way to a target.

Fast-burn missiles, which would rise quickly through their vulnerable boost phase, are a possibility as well. The Air Force is now working on fuels that might be used for such rockets, says the newsletter Military Space.

One of the most vulnerable parts of any strategic defense would be its ground-based radars. Large and fragile, radars are like searchlights -- their beams reveal their location. The Air Force has for some years worked on ``defense suppression vehicles,'' a euphemism for radar-homing warheads. Many experts think they aren't yet perfected. They may be coming close. Last year, the Air Force awarded a $7 million contract to Avco Systems Division for a defense suppression vehicle to fit on the MX missile. Th e first production MXs are scheduled to enter service next December.

To avoid defenses altogether, missiles might adopt disguises. Decoys -- cheap warhead imitations intended to distract defenses -- are becoming increasingly sophisticated. They have progressed from mere warhead-shaped balloons to devices that can tell when they are being scanned by enemy radar and send out spoofing countersignals, says MIT researcher Matthew Bunn.

Decoys might soon imitate the distinctive wake of friction-heated air that nuclear warheads make on entering the atmosphere. They could accomplish this by leaving a trail of some material, such as salt, that is itself quickly destroyed by friction. In its October solicitation, the Air Force asked small businesses to submit ideas for such ``wake augmentation.''

The Air Force also sought ideas on the radar-scattering properties of aerosol sprays. It asked for innovative thoughts on ways to spoof satellite sensors and for methods of basing radar-jamming equipment in space.

Perhaps the ultimate in evasive warheads would be one that could actually maneuver. The US has been developing two such warheads since the 1960s. One, the Navy Mark 500 ``Evader'' for submarine-launched missiles, is steered by shifting internal weights. The second, an Air Force model, depends on movable flaps to change direction.

Both are basically ready to go, awaiting final engineering development if the need arises.