As originally conceived, the Army's AH64 Apache helicopter was to hover over or near a battlefield, fasten a laser beam on an enemy tank, then knock it out with a Hellfire missile guided by the beam.

But the Apache, now $11 million apiece, became so expensive that the Army decided to build a cheaper and less vulnerable scout helicopter to work in tandem with the Apache.

The scout would hide behind trees and use a novel electronic periscope perched just above its rotors to spot tanks and to fasten a laser beam on one of them. The Apache then would pop up from behind a tree or hill, fire a missile locked on to the scout's beam, then drop down again out of danger.

Now the scout helicopter, originally designed to cost $700,000 without the laser device, is up to $4.3 million a copy as it approaches October flight testing with its electronic periscope.

The new helicopter, nicknamed AHIP (for Army helicopter improvement program), is the latest expensive element in the already costly family of laser-guided weapons. And, like many other first-generation, high-technology weapons systems, the cost of the scout helicopter has grown steadily as it acquired its enlarged role with the Apache and perhaps other laser-guided systems.

When first designed, the modified scout AHIP was solely to act as aeroscout for an air cavalry team and perform command and control and range-finding missions.

Now each of them, with the added job as laser designator for the Apache, will cost six times as much, or $2.5 billion for the 578 now planned.

In fact, the AHIP cost could go higher. The Army is considering a plan to add an air-to-air missile to the helicopter, giving it the added job of protecting the Apache from an attack from enemy helicopters. Adding the necessary equipment and testing it would cost an additional $283 million, according to the Army.

Originally, the Apache helicopter was to do its own designating for the Hellfire missiles it carries. It still has that capability with its expensive target acquisition and designation system, which is designed to work both day and night and in bad weather.

However, as the price of the Apache rose to over $10 million apiece, Army planners decided that they could not afford to let it hover exposed over the battlefield.

The less-costly AHIP was given that job. But it soon became obvious that the AHIP, too, would be exposed to enemy ground fire as it kept its laser beam on enemy tanks until the Hellfire missiles arrived, fired from Apaches hidden in the rear.

The planners then came up with the notion of building a so-called mast-mounted sight and placing it above the rotating blades of the helicopter.

"There is a payoff in reduced detectability," Col. Ivar W. Rundgren Jr., AHIP project manager for the Army, told a House Appropriations subcommittee last year.

The new sight would contain a day TV sight, an infrared sight for night operations, a laser designator and a laser range finder. Its viewing eye could swing in a 180-degree horizontal arc and be elevated or lowered 30 degrees from the horizon.

Just three years ago, the Army proposed to Congress buying 720 AHIPs for $1.2 billion, or $1.7 million each. Last year, the Army told Congress it planned to buy only 578 for $2.5 billion, or $4.3 million each.

The increased cost changes, according to Rundgren, came from such things as "hot day performance" so that it would have "worldwide employment capability," a more powerful commercial engine and a four-bladed rotor system.

But the "pacing" item in the program, according to Undersecretary of the Army James R. Ambrose, was the mast-mounted sight, which alone will cost $55 million to develop.

Ambrose told a House subcommittee last year that he forced the service to cut the number of AHIPs because the bids from prospective contractors ran "$500 or $600 million over budget." Army officials, he said, were told they could have no more money and "although it hurt, they cut out in a rational way this many helicopters in order to stay with the budget . . . ."

The Army has been testing the "theory" of a mast-mounted sight since 1976, Rundgren told the subcommittee.

Ambrose, however, noted that the sight would contain "an awful lot of stuff," and added, "The packaging and environmental maintenance in the proper environments is an ongoing kind of problem [and requires] lots of engineering work."