On board billion-dollar ship with a computer-run defense

Three high-speed hydrofoils hide over the horizon. Eight to 20 miles out, they are poised to ''launch'' ship-to-ship missiles against the guided-missile cruiser USS Ticonderoga.

To try to evade the cruiser's radar, they bob behind large metal channel markers or cruise close to one of the many oil tankers steaming north.

''Speed 8, 16, 35 knots,'' calls out the radar systems specialist to Capt. R.G. Guilbault on the bridge of the Ticonderoga, pinpointing a hydrofoil speeding to attack.

''Hit it!'' the captain answers instantly. Five seconds later, a Harpoon missile would be bearing down at supersonic speed on the ''enemy'' ship. The same response presumably awaits the other two hydrofoils.

Fast reaction? Incredibly slow! The vessel's weapon systems can be controlled entirely by computer. So the human eye and spoken commands represent the slowest possible use of this, the Navy's newest, most potent surface warship.

The primary mission of this Aegis missile cruiser, which cost $1.2 billion ( 17 are planned), is to defend United States aircraft carrier task forces against saturation attacks by high-speed antiship missiles.

Initial plans for Aegis began in 1957, when a team of scientists tried to figure out what naval battles might be like in the 1980s. But it wasn't until 1963 that the program got under way. Today, it is the US Navy's highest-priority ship and weapons program. The Aegis will be the air-defense mainstay for carrier task forces through the remainder of this century.

The key to fulfilling this role is the ship's fast reaction time, which is needed to pinpoint and destroy air-, surface-, or submarine-launched missiles that travel at twice the speed of sound.

In this regard, the Falklands war between Britain and Argentina last year proved instructive to naval tacticians. The British had been trained in NATO exercises to expect 20 minutes between detection of an incoming Soviet plane and the impact of any missile fired. It took 21/2 minutes for the British destroyer Sheffield to determine that its radar contact on May 4, 1982, was a missile. Only 5 seconds later, ''a short, sharp, unimpressive bang,'' was reported by the ship's captain.

Traveling at 680 miles per hour, a French-built Exocet high-explosive missile hit the Sheffield amidships and sent it to the bottom. The lesson wasn't lost on the US Navy, which pointed to the sinking of the British ship to help justify its $32 billion, 20-year program to launch a new generation of missile cruisers.

In battle, where the time between seeing and hitting an incoming missile might be seconds, Ticonderoga commanders can resort to computer-programmed defense plans. The ship's electronic combat system, called Aegis (from the Greek word for ''shield of the gods''), automatically tracks any incoming object over a certain speed within a distance of 20 nautical miles. When the computers take over, no sailors are needed to man the ship's combat information center.

The heart of the Aegis combat system employs four fixed-array radars. Located on the ship's port, bow, starboard, and stern superstructure, each gives 90 degree-plus coverage, allowing the radar continuously to keep watch over a huge volume of air space.

Each phased array radar has more than 4,400 radiating elements and can, the Navy claims in tests it has conducted, ''burn through'' any electronic warfare interference encountered at sea.

The ship's radar can simultaneously track more than 200 incoming objects up to 500 miles out from a carrier task force, the Navy says. It adds that the radar also can aim weapons at and hit as many as 20 missiles at the same time, using computers and preprogrammed tactical doctrine to switch the radar-guided missiles rapidly from one target to the next.

Should a missile penetrate the protective envelope of air defense supporting a carrier task force, from F-14 fighters 500 miles out to close-in defense platforms such as the Ticonderoga, a rapid-fire antimissile Phalanx weapons system automatically takes over. The Phalanx is capable of firing 3,000 radar-directed rounds per minute. It not only deflects an incoming missile, but the nylon-jacketed, uranium-charged shells can penetrate and detonate a nuclear device should the incoming missile carry one.

The Aegis program has not lacked critics. Congress's General Accounting Office has expressed concern about one aspect of the ship's capabilities it claims the Navy never fully tested. A report states inadequate evaluation was done against one of the greatest threats to the Aegis, a supersonic missile coming in just over the wave tops, or a ''pop up.''

The Navy responded by having the Ticonderoga ''attacked'' 13 times by drones that popped up over the horizon and dived out of the upper atmosphere. Active radar jamming was also employed. All 13 were shot down, according to the Navy.

There also are critics of the entire concept and cost of a Ticonderoga-class ship. They argue that surface ''ships of the line'' were written off as primary combatants following World War II. The naval warfare torch had passed to nuclear submarines and aircraft armed with missiles. Any surface ship would be a sitting duck.

But the Navy argues that the US has become dependent upon sea commerce to fuel its economy and ensure its national survival. Control of the sea lanes is a vital role for Ticonderoga-class ships.