Science / Medicine : Stealth Technology : Military planners are gambling billions on the revolutionary technology known as stealth--ability of an aircraft to fly without detection.

By Ralph Vartabedian

May 16, 1988 12 AM PT

Times Staff Writer

The U.S. investment in stealth technology has become one of the greatest military expenditures in the history of armament, accounting for estimated spending of more than $200 billion by U.S. armed services alone through the end of this century.

Virtually every new aircraft in development or early production has stealth as a central design criteria. The technology remains among the most coveted of U.S. military secrets and an enormous financial commitment by taxpayers.

Stealth, the ability to fly undetected by the enemy, plays a key role in such programs as the Air Force’s B-2 stealth bomber in production at Northrop, a top secret stealth fighter in production at Lockheed, the Air Force’s new Advanced Tactical Fighter, the Navy’s Advanced Technology Aircraft and a variety of stealth tactical and nuclear cruise missiles.

Senior Pentagon officials have insisted that stealth is not a risky gamble on an unproven technology, as some critics have suggested, but rather a revolutionary concept that will force heavy Soviet spending to develop new defenses. Former Undersecretary of Defense Donald Hicks has called stealth a breakthrough that will “rival any in history.”

The concept of building military aircraft that can escape enemy detection began to take form in the mid-1960s in a series of highly secret programs financed by the Defense Advanced Research Projects Agency.

These programs were primarily paper studies, but they included some models and even some flying drones. These included such craft as the Lockheed D-21 drone using radar-absorbent paints and AQM-91A developed by Teledyne.

It was a confluence of diverse technologies, however, that convinced Air Force planners that they could design and fly a large, manned aircraft that could escape detection by most radars. Such technical areas as aircraft materials, aerodynamic forms, radar-absorbent materials and computerized flight controls all are necessary to achieve stealth.

Target Size Crucial

A key factor, of course, is the target size that an aircraft presents to an enemy radar; this is measured in terms of radar cross-section. In order to avoid detection, stealth requires an enormous reduction in radar cross-section because of the physical laws of radio wave energy.

A 90% reduction in radar cross-section, for example, actually results in only a 37% reduction in detection range--the furthest distance away from radar at which it can be detected.

The B-52 bomber has an estimated radar cross-section of about 1,000 square meters, the B-1A bomber about 100 square meters and the B-1B about 10 square meters,according to Bill Sweetman, author of the book, “Stealth Aircraft.” Stealth aircraft will certainly be under 1 square meter.

Aircraft designers always knew at least some things about how to draw an airplane that could be stealthy, but building the airplane and getting it to fly well--or fly at all--remains a challenge.

Resembles Flying Wing

The Northrop stealth bomber, revealed recently in an artist’s sketch released by the Air Force, is a flying wing resembling the original Northrop flying wings built in the 1940s.

The jet-powered YB-49 of 1940s vintage was cancelled by the Air Force after it encountered aerodynamic problems of excessive drag, instability and inadequate range.

Some aeronautics experts remain unconvinced that a flying wing configuration represents a sound design philosophy, but other experts are highly impressed by the new design. At least some of the early problems have been solved by new technologies.

The artist’s sketch of the stealth bomber indicates that the aircraft’s wings are swept back at a greater angle than observers had anticipated, according to Richard Kaplan, a professor of aerospace engineering at USC.

The additional wing sweep may have been driven by the necessity to move back the center of lift, to obtain better longitudinal stability, Kaplan said. The idea is to get the center of gravity in front of the center of lift to provide for good pitch control, something the original flying wing lacked.

Based on an analysis of the B-2 drawing, the aircraft probably cruises at about 0.8 Mach, or 550 miles per hour, somewhat faster than a commercial jetliner, Kaplan said. It has a fairly wide wing, compared to a commercial aircraft, but a lower drag ratio, he added.

That the aircraft flies at all is probably attributable to computerized flight controls. Only in the last decade have systems enabled a central computer to command an aircraft’s control surfaces through a set of flight rules embedded in its software. Such designs as the General Dynamics F-16 and the Grumman X-29 are inherently unstable and require computer control to allow the aircraft to fly.

“With new control technology, there is no reason why you can’t make a barn door fly. . . , “ said one stealth aircraft designer in Los Angeles recently.

Turn Attention to Design

Now that designers can rely on computers for stability, they can turn their attention to creating an aircraft shape that will deflect radar signals away from the originating source.

Radar operates by broadcasting high powered pulses of microwave radiation and then listens for the echoes that bounce back. If the aircraft is flying at any reasonable altitude, the open sky offers an empty background against which a metallic aircraft readily stands out.

In an effort to defeat radar, designers are working to create aircraft shapes and contours that reflect radar pulses at oblique angles that do not echo back to the originating source. Surfaces that meet at right angles permit radar pulses to carom off the two surfaces and return to the source. When it is considered that an infinite combination of aircraft and radar geometries are possible, the task becomes extremely complex, requiring statistical modeling attainable only by super computers.

In the artist’s model of the Northrop stealth bomber, the most notable design feature is a lack of vertical stabilizers. One of the largest radar reflectors on an airplane is the jointbetween a vertical stabilizer and a horizontal fuselage.

The model also shows jet engines embedded in the body of the wing with inlets on the top of the wing, all features that shield key radar targets from a ground-based radar searching toward the aircraft.

Engine compressor blades are also known to be one of the key radar targets on an aircraft. Some advanced jet fighter radars can even count the number of compressor blades on enemy aircraft and identify the aircraft type. Baffled ducts, such as those used on the Rockwell International B-1 bomber, shield the compressor blades from an oncoming radar pulse.

Other important aerodynamic shaping concerns include carrying weapons, fuel tanks and instruments internally, since they all are large radar targets. The Northrop B-2, for example, is thought to have a radar antenna embedded in the leading edge of the wing, using new phased-array technology that does not need a moving dish to scan for targets.

Materials have played a key in stealth aircraft for several reasons. First, it is not always possible to create the complex contours necessary for stealth with aluminum skin. This has depended on the evolution of composite materials--fiber reinforced resins in the form of thermoplastics and thermosets. Second, composites may help eliminate “surface nonconformities”-- such as a rough rivet head--that can show up on radar.

Much has been made of the radar transparency of plastic in stealth technology. Anyone who has ever cooked with a microwave oven knows that microwave radiation passes right through most plastic.

Transparency to radar, which operates in the microwave region, would be helpful in some areas of an airframe and a disadvantage in others. If radar waves pass through the plastic skin of a stealth aircraft, they could end up reflecting off in ternal metal structure or the engines themselves, creating a larger echo than even a metal skin would create.

In many cases, these radar transparent plastics can be made to attenuate radar energy, so that the resulting echo that has twice passed through the material is substantially weakened.

Radars operate at a variety of frequencies and each one must be dealt with in a stealth aircraft. Generally, short-range track ing radars have higher frequencies and very rapid pulse repetition rates, while long-range search radars have lower frequencies and slower pulse repetition rates.

Another materials technology that has been critical is radar-absorbent materials or RAM. The B-1 bomber uses RAM in a number of critical areas, such as mating surfaces on the fuselage, around antennas and along the trailing edges of the horizontal stabilizer and the wings.

Like Audio Tape Coatings

RAM materials are primarily ferrite or iron-based materials, similar to the coatings on audio tapes that absorb electromagnetic radiation from the tape recording head. The formulation of the ferrite will determine which wavelengths of radar energy it absorbs.

Although the central feature of stealth is to prevent identification and location of an aircraft by radar, stealth covers five other important identification modes as well, according to senior aircraft designers in Los Angeles. By defeating only radar, stealth aircraft would remain highly vulnerable in a battle.

The other five key areas of deception are infrared emissions, engine smoke, contrails, noise and visual appearance. Although these may seem like rather low-technology areas, they are critical.

The F-4 Phantom jet was notorious in the Vietnam War for its smoke trails that allowed enemy missile and gun batteries to easily target aircraft by visual means. Newer jet engines are virtually smokeless.

Containing infrared emissions is becoming far more critical, because infrared-imaging sensors are more capable than ever in detecting and identifying aircraft. The range of such sensors is now measured in dozens of miles.

Engine Exhaust Baffles

The key to defeating such infrared sensors has been to create jet engine exhaust baffles to substantially lower the temperature of the engine outlets and the exhaust gases. It is notable that--for security reasons--the B-2 photo released by the Air Force deletes any representation of this part of the aircraft, though some aeronautic experts reason that the engine outlets are almost certainly on top of rather than underneath the wing.

The best fighter pilots today are often ones able to rely on their own body sensors, namely keen vision. The frontal profile of a stealth aircraft that minimizes radar reflectivity also presents the smallest visual target to another plane approaching it.

Stealth technology is unlikely to permit warplanes to operate with impunity. A mission over highly defended targets will almost certainly allow enemy search radars to detect the aircraft’s presence and make an effort to counter the aircraft. But even though an aircraft may be detected, stealth technology is intended to make the aircraft survivable.

For example, even if an enemy air-defense system could detect the vague presence of aircraft in an area the size of Los Angeles County, about 4,070 square miles, precisely tracking those aircraft would be almost impossible.

Stealth would also help an aircraft survive an attack, such as defeating the tracking radar of an air-to-air missile. Some experts believe that stealth may even defeat proximity fuses on missiles, which use a crude radar.

Of course, nothing on a stealth aircraft would defeat the high-powered cannon fire from an adept fighter pilot at 6 o’clock. FEATURES OF AN ADVANCED TECHNOLOGY BOMBER

ENGINES EMBEDDED IN BODY OF WING

WEAPONS, FUEL AND INSTRUMENTS CARRIED INTERNALLY TO AVOID DETECTION

RADAR TRANSPARENT / ABSORBING MATERIALS

PLANE’S OWN RADAR PROBABLY EMBEDDED IN EDGE OF WING

AIR INTAKES MOUNTED ABOVE WING TO SHIELD FROM GROUND-BASED RADAR

FLYING WING BLENDS WITH FUSELAGE, CREATING CONTOUR THAT CONFUSES RADAR

THIN FRONTAL PROFILE MINIMIZES RADAR REFLECTIVITY