Air Force officials explained in detail for the first time yesterday their plan for the controversial "dense pack" basing of the new MX nuclear warhead missile, expressing confidence that it will work and that the Soviet Union would not be able to knock it out.

Congress, which has complained about the Pentagon's inability thus far to figure out a safe and permanent way to base the new land-based intercontinental missile, has demanded that the Reagan administration present it a plan by Dec. 1. The House yesterday narrowly voted to begin building MX missiles at a cost of $2.6 billion next year, despite continuing skepticism about how they will be deployed.

The Air Force is now analyzing the extremely complex dense pack system, also known as "closely spaced basing," which its officials have said is the "most promising" answer to the MX quandary.

As explained to reporters yesterday by officials who asked not to be identified, a combination of two factors is supposed to make a successful attack against dense pack either very difficult or impossible. Those factors are the ability to super-harden underground silos, which means strengthening them against attack by other missiles, and to put those silos very close together.

The officials claimed that work by the Defense Nuclear Agency a year ago showed that silos could be strengthened to withstand pressures of about 10,000 pounds per square inch (psi) from atomic bombs exploded in the air overhead, or about 5,000 psi from bombs that exploded when they hit the ground.

Current U.S. Minuteman silos, by comparison, can withstand pressures up to about 2,000 psi, an official said.

The basic theory behind dense pack is that by putting silos very close together, the blast and radiation effects set off by the first Russian warhead to arrive will, in effect, either destroy or blow off course the ones that would follow within seconds in any attack meant to wipe out all 100 MX missiles in a single blow. This effect is called "fratricide."

The importance of the additional hardening, the Air Force claims, is to make it impossible for the Soviets to knock out the MX silos with an airburst because the new silos would be able to withstand that. It theoretically would force the Soviets to use ground bursts, which require even bigger warheads and kick up huge amounts of rocks and debris. This would cause the effect of the blast to last even longer, making it harder for incoming missiles to penetrate. The Air Force claims that ground bursts will increase the fratricide effect.

Other defense officials, including former defense secretary Harold Brown, have cast doubt on the ability to super-harden underground silos, and other Air Force officers say privately that it is a very uncertain area of technology. But they add that uncertainty itself causes the Soviets to lose confidence that an attack could succeed. They say they do not necessarily disagree with Brown but that super-hardening becomes more effective when used in a dense pack scheme.

As explained yesterday, the system would involve 100 MX missiles in silos about 1,800 to 2,000 feet apart. They would be placed in "shaped arrays" such as a triangular pattern four-and-a-half miles on each side or a single row nine miles long and one mile wide. In both cases, the array would be angled toward the north, the expected direction of attack, to make it harder for the Soviets to rain down a pattern of warheads hitting one silo after another without destroying themselves in the process.

Since the Soviet missiles would arrive at about a 30-degree angle, the Air Force officials contended those missiles would have to fly through a considerable amount of debris. The surviving MX missiles could escape the debris relatively quickly in a counterattack, they said.

Critics have argued that the Soviets, by simply continuing to lob atomic bombs over the MX base, could keep the United States from firing back by essentially "pinning down" the MX force. The Air Force contends this is also hard to do and could be disrupted by counterattacking American submarine-based missiles or bombers.

One of the officials said the dense pack system, if chosen, could be deployed beginning in late 1986. He stressed that whatever the continuing argument over the survivability of land-based missiles, the administration wants to deploy MX as a deterrent to threaten the Soviets, just as large Soviet missiles now threaten the United States.