The new armor, Dr. Epstein said, appeared to be quite effective against missiles fired from ground level but less so against missiles that attacked the relatively thin top armor of tanks. The explosive armor was not effective against mines or high-velocity projectiles, he said.

''Right now, too many questions remain unanswered by the Army,'' Dr. Epstein said. ''For instance, if our antitank guided missiles are ineffective against Soviet armor, why are we continuing to buy more of them? For that matter, why haven't we put reactive armor on our own tanks - a step that would probably be prudent right now?

''In view of this seemingly collosal intelligence failure, shouldn't we be impeaching Weinberger? We've spent $2 trillion over the past five years to modernize our defense establishment, and now it's alleged that the fruits of this effort have been nullified by a simple, mass-produced Soviet weapon - reactive armor.

''On the other hand, maybe this reactive armor is not as revolutionary a development as the Army is giving us to believe. These scares seem to come up every 10 years when the Army needs more money for something. This problem may indeed be serious, but it would be a mistake to succumb to technological hysteria.'' 'Slow to React'

Gen. Donn A. Starry, who recently retired from the Army, has deplored what he said was the armed forces' slow response to the new armor technology.

''The fact is, we've been terribly slow to react to it,'' General Starry said. ''It's not that we lack the laboratory technology to match the Soviets. The problem is that we are notoriously slow to field innovations. We tinker with them for years, while the Russians know when to call a halt to their fine-tuning, and they get new systems out to their troops quickly. You also must realize that we have an enormous investment in an inventory of guided missiles, and it takes time to shift gears.''

The antitank rocket that military officials say is becoming obsolete has a long history.

Since the German Panzerfaust and American bazooka antitank rockets carried by infantrymen in World War II, all antitank rockets have depended on hollow-charge warheads. Today's missiles, launched from the ground,vehicles, airplanes and helicopters, fly to their targets under the guidance of wires, laser beams and other systems. But they still depend on explosives. Judging Soviet Progress According to General Starry, who developed much of the doctrine governing American armored warfare, the Soviet Union has reacted swiftly and effectively to every American innovation in tank weaponry.