President Reagan's Dense Pack deployment scheme for the MX missile is so complex and futuristic it requires a new vocabulary for Pentagon briefers to explain it.

In rooms darkened to make slides show up better, they have been using terms like "threat tube" for the path of incoming missiles, "ejecta" for the boulders and dirt that would be thrown up if Soviet warheads exploded on the Wyoming prairie and "sequential dumping" for unloading tons of rock on top of the MX silos to make "low grazing" Soviet missiles skip off target like flat stones skittering across a pond.

Because 100 MX missiles, each containing 10 nuclear warheads, would be grouped north to south in a rectangle 14 miles long and 1 1/2 miles wide, attacking Soviet missiles would have to fly through a narrow corridor of space to hit the formation Reagan wants to build outside Warren Air Force Base near Cheyenne, Wyo.

"The threat tube is very narrow," said Richard D. DeLauer, Pentagon research director, in explaining one of the advertised advantages of Dense Pack. If the Soviet warheads "go out to the side," he said, "they don't get anything."

If one Soviet warhead explodes in the air over the MX field to try to crack the silos so the missiles inside could not fly out, according to the Dense Pack doomsday scenario, the X-rays and other radiation from the first blast would destroy the innards of other Soviet warheads in the first attack wave, making them duds or throwing them off course.

The Soviets, to avoid this nuclear fratricide, might well go back to loading up their big missiles with single, 25-megaton warheads to hurl at the Dense Pack field, setting them to explode on the ground to dig out the MX missiles buried deep below.

"For ground bursts," Air Force Brig. Gen. J. P. McCarthy, MX special assistant, told the House Appropriations subcommittee on defense at a secret briefing, "the detonation raises ejecta, which consists of large boulders, some of them larger than a Volkswagen, that are thrown up and ejected out of the crater for some significant distances." The explosion also creates a dust storm, which grinds off the noses of the warheads following the first one, making them miss the MX silos.

But, McCarthy was asked, would not that same debris get in the way of MX missiles as they tried to fly out of their silos in a retaliatory strike?

"The MX missile is starting close to the ground at near zero speed," the general answered in the subcommittee briefing recently made public in censored form. The MX "gains speed as it gains altitude, flies at that point virtually straight up. It is protected by a shroud. Now, it is not going to withstand Volkswagen-size boulders, but the dust and debris that would be following."

To help it determine if surviving MX missiles could get through the ejecta after an attack, the Air Force said it might deploy a "pathfinder" missile in the Dense Pack field and launch it first to determine "if a safe dust and debris environment existed after the MX field has been attacked. This missile would transmit information to a control center during its flight. "If it reached a safe altitude, a safe path would have been found for MX to launch."

Besides hurling boulders and dirt high into the sky, a Soviet nuclear bomb exploding in or near the Dense Pack field would cover the lids on the MX silos with tons of debris.

But George A. Keyworth II, President Reagan's science adviser, told a recent Pentagon news briefing that "it's really a straightforward thing when you think of how extremely strong this [MX] structure is to simply push up through up to 50 feet of debris and rock. It does not require a complicated auger or boring mechanism. One simply pushes up through this debris that is already loose. It would be done with a strong push . . . . "

MX briefers have acknowledged that the Soviets might attempt to "pin down" MX missiles by exploding one warhead after another over the missile field, spreading an umbrella of radiation to destroy any American missiles trying to fly through it. Air Force planners contend this reverse fratricide would cost the Soviets too many warheads to be tempting, given all the other U.S. land missiles and bombers they also have to keep targeted.

To avoid fratricide and ejecta, briefers say that the Soviets might set their warheads to explode after every one of them in the attacking wave had burrowed into the earth around the MX silos. Such "earth penetrators," flying in at 5,000 mph, could be "very, very readily and very inexpensively countered" by spreading layers of rocks over the MX field before the attack, according to Keyworth.

An Air Force general, elaborating on this "sequential dumping" tactic, said the Soviet warheads would fly into the MX field at such a shallow angle that they would bounce off the rock and land somewhere else.

The Soviets could avoid this skipping effect by packing parachutes into their warheads, setting them to open over the MX field so the nuclear warheads would land softly near the silos and explode all at once at the pre-set time. Administration MX briefers said they have already thought of this. "We would need to deploy a number of Phalanx radar-controlled anti-aircraft guns" to shoot holes in these "soft landers" if the Soviets moved in this direction, Keyworth said.

If the Soviets went for maneuvering warheads to avoid ejecta and fratricide, Keyworth said, the United States could counter with electronic countermeasures to ruin their guidance. And if more insurance against Dense Pack were needed, say the MX briefers, anti-ballistic missiles could be deployed to cover the narrow "threat tube."

Keyworth told the Senate Armed Services Committee last week that there was no room for disagreement "among those who have studied it carefully" that Dense Pack would be an effective deployment scheme. "I must tell you, doctor," responded Sen. Carl Levin (D-Mich.), "I find that an arrogant comment."