IN THE NAME of national security, tens of billions of dollars in defense spending are being hidden from public scrutiny so that we can build war planes that are equally invisible to our enemies. This program is called Stealth.

Already concealed from most public accounting are at least 50 Lockheed strike fighters that probably cost about $40-$50 million apiece; a new General Dynamics cruise missile program that is expected to cost about $7 billion; and the most expensive warplane ever built, Northrop's Advanced Technology Bomber (ATB), with expected program costs between $35 billion and (according to hostile sources) $80 billion.

The Pentagon considers Stealth to be so sensitive that it will not say what the Northrop bomber or the General Dynamics cruise missile will look like or when they will enter service. So that nobody can guess these dates from the way money flows into the program, the Pentagon has classified all the cost figures as well. In the case of the Lockheed fighter, the Pentagon bluntly refuses to acknowledge that the plane exists at all.

The Pentagon has classified these weapons because they are based on a radical departure in warplane design. Instead of using height, speed, defensive maneuvers, weapons or electronic radar jammers to protect themselves from attack, Stealth airplanes and missiles are designed to avoid detection by radar or other detection devices.

Here is why the Pentagon believes we need Stealth. In order for, say, the new B1B bomber to have its best chance of surviving against the Soviet Union's elaborate air defense system, it would fly barely 250 feet off the ground. But the terrain which hides the bomber from the defenses also hides the targets from the bomber. To hunt for a target such as a mobile ICBM, the B1B must climb to a better vantage point, exposing itself to attack. In a few years, too, it will be vulnerable to new Soviet airborne radars that can pick out low-flying targets against the "clutter" from the ground below them.

These ground-hugging tactics might not be enough for fighter-bomber pilots in Europe if they had to attack targets such as airfields, command bunkers and surface-to-air missile sites. The Soviet military has built up so much firepower around these targets that no strike force could escape without massive losses.

Stealth may be the answer to these problems. It aims at making the attacker virtually invisible to radar and other electronic detection devices without which defensive fighters, surface-to-air missiles and guns are impotent.

Why does it all have to be so secret? It is partly a matter of tradition. The first truly Stealth aircraft were unmanned spy planes and were developed in secret because their intended missions were covert. When a practical manned Stealth fighter became a possibility, its development was entrusted to a section of Lockheed called the Skunk Works, where secrecy is basic to the management doctrine. By the late 1970s, Stealth was beginning to emerge from the shadows, and details of the technology might well have become public knowledge had Jimmy Carter been re-elected in 1980.

As it was, Stealth became the first test of the Reagan/Weinberger philosophy: If in doubt, classify; if doubt remains, upgrade the classification. It could also become the first test of that policy's ability to withstand congressional and public opposition. The Stealth bomber may be able to hide from the Soviet Union's air defenses, but it may be too big to hide from Capitol Hill.

While the secrecy surrounding Stealth has precluded most discussion of the subject, reports of progress with Stealth aircraft have appeared in the aerospace and defense press from time to time and in technical papers presented at open industry meetings, most of which took place before the Reagan information freeze. While the information is fragmentary, it can be pieced together with context gleaned from unclassified textbooks. The latter also help to screen the disinformation issued by unidentifiable sources in the past few years.

Stealth is not a single magic trick but a means of designing a warplane so that its "signature" or "observables" are drastically reduced. A plane's radar reflections are the most important, but emissions of light, heat or sound are significant, too.

Most conventional aircraft are ideal radar targets. They present large flat surfaces, such as the body sides and vertical fin, at right angles to the direction from which most radar waves are likely to arrive. They have large intakes and exhausts for their engines, which trap and re-reflect radar waves. They are festooned with bombs and fuel tanks, which tend to create "corner reflectors." (Sheet-metal devices using the same principle are attached to small boats to make them more visible to radar.)

There are a few basics to designing a Stealth aircraft. Bombs and fuel must be carried internally; the engines must be concealed behind long, curved inlet and exhaust ducts, and vertical flat surfaces should be eliminated. (The B1 bomber's sinuous shape reflects some of this thinking, although it is not a Stealth design.) This does not sound too complicated, but the snag lies in a formula called the "radar range equation." This states that cutting the radar image of a target in half knocks no more than an insignificant 15 percent off the detection range. The benefits of Stealth are not felt at all until the radar image is cut by at least 90 percent.

This calls for the use of special radar-absorbent materials ranging from plastic-based coatings to complex sandwiches of fiber glass and chemically treated foam. Most of them contain an electrically resistant "active ingredient" such as carbon or one of a family of iron products called ferrites. The objective is to draw the energy out of the radar wave as it penetrates the material, just as food in a microwave oven absorbs radar waves and converts them into heat.

Another problem is that gaps, sharp corners and sudden changes in the conductivity of the airplane's skin will produce radar echoes. Stealth airplanes must have a smooth and seamless finish; they are likely to make most conventional aircraft look like tractors.

Radar is not the only signature to worry about. Stealth aircraft use special exhausts to mix the hot carbon dioxide from their engines with cool outside air; hot carbon dioxide has an infrared signature that can betray an airplane at ranges of almost 100 miles. Chemical additives have been developed to discourage the formation of contrails. Also, a Stealth aircraft cannot give itself away with the electronics it uses to find targets -- they have to be disguised as well.

Stealth had its roots in the late 1950s, when the Central Intelligence Agency decided to replace the U2 with a faster, higher-flying aircraft. But such an aircraft would be much bigger than the U2, which would allow the Soviets to detect it at a greater distance, partly offsetting the better performance. The CIA asked Lockheed's renowned Skunk Works to produce an airplane with a disproportionately small radar image. This was the Blackbird, which flew in 1962, is still in service and is still virtually invulnerable.

The Blackbird's designers gave it an elegant, sinister shape, slender in profile and carefully blended from long straight lines and geometric curves. A radical autopilot was developed so that the airplane would fly properly with undersized, low-profile vertical stabilizers. Lockheed's chemists produced an exotic (and still classified) radar-absorbent plastic, used for much of the airplane's skin. Finally, the whole structure was hand-finished to an exquisite degree.

If a large airplane could be made hard to detect -- it was soon realized -- a very small one could be made almost invisible. The Ryan company modified some Firebee target drones with radar-absorbent skins as early as 1961. Three years later, Stealth-modified Firebees were being used for spy flights over China and North Vietnam, and both Lockheed and Ryan were working on bigger drones, designed from the ground up to reduce the chances of detection by radar. Tested in the late 1960s, they proved that an airplane the size of a small fighter could be launched through a dense and sophisticated air defense system in confidence that it would not be detected. But in 1970, Stealth was still a technical curiosity which aroused little interest outside the reconnaissance community. No Stealth aircraft had been developed that was large enough to carry bombs or maneuverable enough to fight other aircraft.

In June and December of 1972, the U.S. Air Force launched its Linebacker I and II offensives against North Vietnam. They found that fewer than half the aircraft in a 70-plane strike could be used to drop bombs on the target. The rest were needed to deal with enemy radar: listening, jamming, dropping clouds of metal-coated fiber glass strands to create false reflections, or firing missiles at the sites. In the October 1973 war in the Middle East, the Egyptian forces were using the new Soviet SA-6 missile, against which American jammers did not work, and the Israeli air force took heavy losses.

In 1974, Lockheed proposed the development of a manned Stealth fighter, arguing that Stealth, unlike electronic radar-spoofers, works as well against new and unknown radars as it does against familiar systems. The Defense Advanced Research Projects Agency (DARPA) provided the money to build and fly a handful of prototypes. The program was variously known as Project Harvey (after the invisible six-foot rabbit that haunted James Stewart in the movie of that name) and the XST (experimental, Stealth, tactical) but its official name was Have Blue.

Lockheed could add many improvements to the Blackbird's technology. New materials, such as carbon fibers, could be used to create strong radar-absorbent structures. New electronic control systems made it possible to shape the airplane for the smallest possible radar image. The result might be a quirky flying machine, but in time it could be tamed by reprogramming the control computers. Techniques were developed to reduce the plane's infrared emissions. Perhaps the biggest single technological step forward, however, was the use of powerful computers to model and predict the radar image before the expensive process of building prototypes started.

In early 1977, the first of the Have Blue airplanes flew from Groom Lake, in the middle of the Nellis weapons range in Nevada. It was, by all accounts, a bizarre-looking aircraft, with a small triangular wing blended into a bulky body in which the engines -- General Electric turbofans -- were buried. Two very small, sloping fins partly concealed the jet exhausts.

At least two of the Stealth fighters crashed during tests, but the pilots survived and the losses were more than balanced by the success of the most important trials, which pitted the aircraft against both simulated and real captured Soviet SAM radars. The Stealth fighter, as big as an F4 Phantom, had a radar image no bigger than a small bird's. If the radar operators saw the Stealth fighter at all, it was so close that, in a real engagement, there would have been no time to fire a missile.

Increased spending on Stealth was part of the last two budgets of the Carter administration and probably included a contract to Lockheed to develop an operational aircraft based on the Have Blue prototypes. This aircraft was ordered into production in 1981, and is being built at Lockheed's Burbank plant. It is sometimes known as the F19, although the USAF categorically denies that there is any such aircraft. It is probably intended to hit high-priority, heavily defended targets such as missile sites -- clearing the way for an attack by conventional aircraft -- and command bunkers.

In 1984, a new air base was completed in Nevada on the site of an old airstrip. It is unusual in that it has many small individual hangers; at Air Force bases in sunny climates, the aircraft are usually parked outdoors. The only other base to be built like that is Beale AFB in California, where the Strategic Air Command (SAC) keeps its SR71s. Another odd feature of the base is that, according to unclassified documents and maps, it does not exist. In fact, it is home to the new Lockheed fighters.

The Lockheed fighter's early success attracted the notice of the Strategic Air Command, still reeling from President Carter's 1977 decision to scrap production of its B1 bomber. In 1979 SAC asked the industry to examine ways in which a strategic bomber -- well over ten times the size of Lockheed's machine -- could be made stealthy enough to defeat the new generation of defensive weapons that the Soviets were fielding to counter the B1. One company, Northrop, concluded that the best approach was to revive an old idea -- the Flying Wing, a batlike shape with a large, thick wing, no body and no tail surfaces.

The design was not without risk. Because the bomber would be so big, reducing its radar image might have to take priority over the normal conventions of aerodynamics. A sophisticated computer control system would be needed, like that on the Lockheed fighter, but nobody had ever tried such a system on a large aircraft, let alone a massive bomber with no tail.

Soviet defenses include very-long-wave-length radars -- new "over-the-horizon" systems and old long-range devices from the 1950s. When a radar illuminates a body that is about the same size as its wavelength, the body can act like an antenna and may emit a strong return regardless of its shape. Special coatings and materials would be needed to overcome this problem. (Some of these challenges, identified in 1980-81, have recently been reported as "new" problems with the Stealth bomber.)

The Air Force and industry concluded in 1981 that a Stealth bomber could be built, but it was considered more prudent to start with a "risk-reduction" program, in which the most ambitious features of the Stealth bomber could be tested separately. If no major snags emerged, the seven-to-eight-year job of building the bomber and getting it into service in 1991 or 1992 could start in earnest.

With the support of the Reagan administration, the Air Force devised a two-bomber program, carefully planned to provide SAC with the aircraft it needed at the right time. The B1 was updated and launched straight into production, under contracts signed in October 1981, with the sole aim of getting a 100-plane force into service as soon as possible. On the same day, a team led by Northrop was selected to develop the Stealth airplane, renamed the Advanced Technology Bomber, of which 132 were to be built.

Under the plan, Northrop had about 3 1/2 years to show that the ATB could be developed without too much risk of failures or cost overruns. If the ATB was in technical trouble in early 1985, the Air Force could order another batch of B1s and allow the Stealth program to mark time until the problems were solved. As the deadline passed, however, Northrop started recruiting aggressively and built a massive new assembly hangar at Palmdale, while Rockwell's efforts to sell the Air Force more B1s went unrewarded despite the aid of superlobbyist Michael Deaver.

More Stealth programs have followed the Lockheed fighter and the ATB. General Dynamics is developing a Stealth cruise missile, which will be the main armament of the B1B when the ATB replaces it in the "penetrator" mission. With its small size and the latest Stealth technology, the new missile should have a radar image about the same size as a large insect.

The Navy is planning a carrier-based Stealth strike plane for the mid-1990s. Called the Advanced Tactical Aircraft, it is designed to deliver a heavy load of conventional weapons to Soviet coastal targets. The Navy is also working on a cruise missile of its own, possibly intended to attack heavily defended Soviet warships.

The Air Force's new Advanced Tactical Fighter (ATF), to enter service in 1995, may be almost as hard to track on radar as the Lockheed Stealth fighter, but will be faster and far more maneuverable, thanks to advances in design techniques and radar-absorbent materials. The Air Force has relentlessly driven the seven companies competing for the massive ATF contract to put more Stealth in the design, because Stealth is an enormous advantage in fighter-versus-fighter combat; a Stealth fighter will nearly always see the enemy before the enemy can see it.

There can be no doubt that the Soviets are working on countering Stealth by improving their own defenses. In principle, the best way of detecting a small target by radar is to increase the size of the radar dish -- just as the aperture of a camera lens defines its ability to take pictures in poor light. In practice, such radars are likely to be extremely large. If they are based on the ground, they will be immobile and easy to spot by satellite; if the Stealth bombers cannot maneuver around or between them, they will simply use their short-range attack missiles like a gunslinger shooting out the bar-room lights.

The Navy, which believes that the Soviets may deploy Stealth versions of their anti-ship missiles by the early 1990s, is working on a way of putting a wide-aperture radar in the air aboard an airship -- a dirigible or lighter-than-air craft. Although it may seem an anachronism, the airship is still unique in its ability to stay airborne for days on end, and its envelope can accommodate a far larger radar antenna than any conceivable airplane.

In principle, too, the Soviets could develop Stealth aircraft. But Stealth leans heavily on some of the technologies in which the West retains a clear lead: supercomputers to manage the complex design task, and high-speed electronics for the Stealth's control system and sensors. Also, the Soviet system is geared to producing cheaper, less well-built aircraft than those of the United States, and quality is critical to Stealth.

Meanwhile, Stealth remains a huge secret devouring billions in funds whose accounting itself is classified. At the very least, all the secrecy may help to keep the program small enough in the public eye that it becomes a reality before interceptors in the form of congressional opponents or defense critics are able to shoot it down.