The Boeing Co., concerned that the giant MX missile system may be in trouble, has begun briefing Capitol Hill and the Pentagon on an alternative system; a small intercontinental ballistic missile that the company says it could build and begin deploying by the end of 1986.

Opposition to the ambitious and expensive MX intercontinental ballistic missile system continues to build, and its future is far from certain. "It looks like the king might die,' a key Senate aide on defense matters said recently in describing the situation, "so it's not surprising that companies are pushing other contenders forward."

But while Boeing is promoting what its own briefing book, obtained by The Washington Post, calls the "SICM" -- for small "ICBM, it is not eager to discuss the MX alternative publicly. "There is no way company officials will discuss the small ICBM," a Boeing spokesman said last week.

The company's hesitancy may stem from questions about the propriety of Boeing, a major MX contractor, pushing an alternative to MX. Another possible reason for its public reticence is that the Boeing official most responsible for developing its SICM concept. Thomas K. Jones, has just joined the Reagan administration as deputy undersecretary of defense for research and engineering for strategic and theater nuclear forces.

Jones was not available for comment, but colleagues said he had to be pressured into leaving Seattle and will not participate in an Defense Department discussions about whether the Pentagon picks up the Boeing proposal.

Meanwhile, Boeing's effort, a classic example of how companies in the weapons industry go about developing business, has angered its competitors. Officials of the Martin Marietta Corp., perhaps the biggest MX contractor, reportedly are "livid" about Boeing's promotion of its samll ICBMconcept, according to one source.

The idea of building several thousand small, single-warhead intercontinental missiles -- rather than hundreds each -- has been around for a long time. The real father of the concept is Paul Nitze, a deputy secretary of defense in the Johnson administration and a negotiator of the SALT I arms limitation treaty.

The idea never had much support, "but suddenly the Boeing small-millile program is being discussed with interest at the highest government levels," one Capitol Hill defense expert said last week. "That's because the Pentagon and the White House are looking for something other than the MX." p

A commission appointed by Defense Secretary Caspar W. Weinberger to search for an alternative basing plan for the MX has been throughly briefed on the Boeing SICM, and informed sources said several of its members vere very impressed. One member of the panel -- called the Townes commission for its chairman, physicrist Dr. Charles Townes -- is David Packard, a former deputy secretary of defense and now a director of the Boeing Co.

Boeing, as the manufacturer of the 1,000 Minuteman missiles that are now the backbone of the U.S. land-based strategic force, has always wanted to develop the follow-on system. Its role on the MX, however, has been limited to developing the transportable launcher associated with the mobil basing system of the giant missile -- not the missile itself.

While the Carter Pentagon and the Air Force worked, beginning in 1977, to develop the MX and its controversial multiple-shelter plan for the Great Basin of Utah and Nevada, Boeing's strategic milliles organization worked at designing concepts associated with a smaller missile that could be based on existing military facilities.

What Boeing has come up with is a missile that is 38 feet long and weighs about 22,000 pounds -- as compared to the MX, which is to be 71 feet long and weigh 192,000 pounds.

To meet the current Soviet threat, Boeing proposes to build 3,350 of the smaller missiles, each armed with one warhead. The Carter administration planned 200 MX missiles, each of which would have 10 warheads for a total of 2,000.

The Boeing briefing says that its missiles could carry either of the two warheads now being proposed for the MX -- the Mark 12A with its 340-kiloton explosive power or the newer, larger 500-kiolton design being proposed by Strategic Air Command.

As more and more opposition emerged to the MX basing plan, Boeing fashioned several alternative ways of deploying the small missile, each of which would overcome problems encountered by the MX.

In its SICM briefing, Boeing emphasizes that the new missile could be placed "in very hard silos to provide the desired number of surviving [warheads] after experiencing a Soviet counterforce attack."

Unlike each MX, which would be moved among 23 different shelters to confuse Soviet targeters, the SICM would be placed in cement-hardened silos that would withstand 3,000 or more pounds of pressure per square inch -- a force that would require a Soviet warhead to hit it right on the nose to destroy the missile. That amount of hardening also would allow the silos to be packed closer together than other missiles, therefore taking up less space.

The SICM would be in a dormant state in its silo, thereby lessening the need for equipment within the silo. It would not require "environmental control," according to the Boeing paper, "except for 20 watts of power to the guidance heaters."

It would be started up during a time of threat by an air-launched control center and take "five to 15 minutes after wake up," to prepare it to be fired at preset targets.

Boeing also has worked out two mobil basing schemes. One called the "road mobile" calls for the missile to be on a truck, parked during non-criss periods on military bases. In a crisis, the mobile launcher would drive to a "pre-surveyed roadside sites" from which it could fire its missile.

Another version of that, called the "beehive concept," has the truck/launchers parked on a base and the warheads stored together at another hardened site.At crisis time, the launchers and the warheads are mated and driven to pre-surveyed launch sites.

Like other missils proposals, the SICM has its drawbacks. Its dormant characteristic means it can't be rapidly launched in the face of the highly improbable Soviet first strike. Its mobile modes require movements in a crisis period before a Soviet launch that could be considered threatening to the Russians and, according to some critics, could precipitate an attack.

Beyond those points, it would take the United States above the ICBM limits set by the SALT II agreement, which is in limbo.For a Reagan administration looking for ways to show its differences with the previous Carter administration, that might be a plus.

The other political plus is that it would counter the criticism of the MS basing scheme that is growing in the Great Basin area.

"There's a certain trendiness" to the SICM idea, one Pentagon official said last week. And Boeing is pushing it "because there is a corporate interest in getting to do the missile as well as the basing mode," something the company doesn't have with MX.