Early Stealth Aircraft
The survival techniques of aircraft in combat situations has undergone dramatic changes throughout the years. In the earlier days of aviation, survival of a combat aircraft was based upon speed. As avionics progressed, survival techniques progressed from speed to electronic capabilities. During World War I, visual detection in daylight did not exceed 15 miles. Experiments with stealth capabilities occurred as far back as the early part of the 1900s when Germany tested an aircraft with a transparent wing, designed to make it difficult to spot by observers on the ground.
Even in the late 1930s, defenders were expected to listen and watch for attacking aircraft. By 1940, however, radar could spot incoming aircraft at a distance of more than 100 miles. Early detection gave defenders much more time to organize their air defenses and to intercept attacking planes, such as radar height-finding assisted anti-aircraft gunners. Primitive airborne radar sets were installed in night fighters in the later years of the war. Now, however, avionics have evolved to the point where one of the big keys to survival of an aircraft is in the stealthiness.
Camouflage is necessary for deception and is often used by both animals and humans for disguise and protection. Camouflage techniques for the military have been pursued for well over a century but have primarily taken the form of surface colors and textures chosen for the particular milieu. In addition to personnel and land-based forces using these techniques, naval and aviation applications have been used since WWI. Coatings have ranged from neutral colors to razzle-dazzle schemes that break up the outline of large surfaces making it difficult to see the shape of the object. A variety of coloring schemes have been used aboard aircraft for years to provide delay of observation during daylight sorties. The Compass Ghost program during the Vietnam War is one such example.
Beginning in WWII however, a new technique was developed that is now generally termed active camouflage. The addition of energized lighting or display surfaces has been tested but rarely deployed even though shown to be successful in principle. This has the benefit of making the object not appearing to simply be a shadow. Through the use of surface illumination, an object can be made to substantially integrate with its surroundings, making it difficult to see with the eye.
During WWII, The US Navy's Project Yehudi used lights mounted on the leading edges of the wings of a torpedo bomber to successfully hide the plane in broad daylight when attacking a submarine. Visual detection range in the tests dropped substantially from 12 to 2 miles. As the plane approached a target, the lights, which pointed forward, were coupled with a photocell such that the output intensity (not color) of the light was set to match the intensity of the sky behind the approaching plane. This effect takes advantage of a physiological phenomenon termed isoluminance where objects of similar intensity can be indistinguishable from one another under certain conditions.
Yehudi, kept secret for many years, was never used because the advent of airborne radar systems in WWII rendered it moot. During the Vietnam War, however, a program called Compass Ghost revived advanced paint schemes and an attempt to try the Yehudi technique again on an F-4 Phantom. More recently in the mid 1990's were reports of a Project Ivy done by the Air Force that considered or used color panels.
The rapid development and deployment of radar systems combined with the end of the war eliminated the need for such techniques. The electromagnetic techniques of radio ranging through radar meant that eyes were trained upon radar displays and not the sky, and made pointless the need for such developments.
In World War I1, the snorkel was developed by Germany to allow its diesel powered U-boats to operate submerged when recharging their batteries. This enabled them to avoid detection by long range maritime patrol aircraft. When the Allies developed air-borne radars to detect the snorkels. the Germans countered by putting a rubber coating over the snorkels which degraded the radar's effectiveness. 2 Thus, an early forerunner of radar absorbent material (RAM) was used on submarines.
According to Alan Brown, who retired as Lockheed Corporation's director of engineering in 1991 and a man regarded as one of the founders of stealth, technologies to reduce radar cross section began almost as soon as radar was invented. The predominantly wooden de Havilland Mosquito was one of the first aircraft to be designed with a focus on low radar cross section.
Since World War II, the radar game between attackers and defenders has determined who will control the skies. Radar domination allows firepower of air forces to bear against a foe or to deprive an enemy of this most valuable asset. Highly survivable aircraft contribute directly to achieving joint force objectives, and thus the ability to project power with efficient and effective air operations depends on controlling the radar contest.
By far the most difficult problem is the passive radar signature, that is, the signal a search radar would detect. In the 1950s a Wright Field electrical engineer, William F. Bahret began an investigation to understand the radar echo from an aircraft. Bahret had to design new experimental facilities and new testing techniques to provide the tools necessary to discover the behavior of this very complex electro-magnetic phenomenon. His goal was not only to understand the radar echo, but also to use the insight to devise mathematical algorithms to predict the echo. From there it was possible that engineers could develop design tools for creating aircraft configurations exhibiting extremely low radar signatures.
The U-2 spyplane, which was started in late 1954 by Lockheed Aircraft under a contract with the Central Intelligence Agency (CIA), was intended to be stealthy largely by flying at a very high altitude. Its designers expected that Soviet air defense radar would not be capable of detecting aircraft that high, although U.S. radar certainly could. The designers were wrong about Soviet radar, however, and the first U-2s to fly over Soviet territory were immediately detected. This prompted U.S. radar and aircraft experts to evaluate a number of ways to reduce the radar signature of the airplane. Because the U-2's shape was already established, they focused on adding things to the airplane that would absorb or scatter the radar energy that reached the plane. These included a fine wire mesh that was molded over the plane and covered with a paint that contained iron, and wires strung from the nose to the tail. However, none of these efforts reduced the airplane's radar signature very much, some of them significantly reduced its performance, and all were abandoned.
It wasn't until Lockheed's"Skunk Works" produced the A-12/SR-71 aircraft that any success was gained in reducing an aircraft's radar cross section (RCS). In 1958, the CIA began studying a replacement for the U-2 that could fly at speeds above Mach 3. This aircraft, soon named OXCART (possibly an inside joke because it implied a vehicle that moved very slowly), was intended to fly very fast and very high. It would also have a small radar signature, meaning that it would appear as a very small object on a radar screen. Its designers hoped that its small size and high speed, so that it would move a great distance between each pass of the radar beam, would cause radar operators to think the radar blip was only "noise" in the radar signal. The single-pilot OXCART, which was also designated the A-12 and built by Lockheed, had a number of radar-reducing features. It was coated with special materials that absorbed radar energy. Designers also developed parts of its structure to "trap" radar energy and prevent it from traveling back to its source. In addition, they added a chemical to the aircraft's special fuel to reduce its radar signature. Overall, the OXCART had a relatively small radar signature, but it was still visible on radar. The Air Force soon developed the two-seat Lockheed SR-71 Blackbird based on the OXCART design, and the Lockheed D-21 TAGBOARD reconnaissance drone. Both aircraft incorporated stealthy features. The A-12/SR-71 employed radar-absorbing coatings and RCS reduction in the structural edges of the airframe, but a decade would pass before signature management was again attempted on an aircraft.
During this time-the late 1950s and early 1960s-aircraft designers and defense planners in the United States were extremely aware of the importance of an aircraft's radar signature to its survivability. North American Aviation's Mach 3 B-70 Valkyrie bomber was canceled in 1961 because, among its other many problems, it had an enormous radar signature and could be spotted on radar a great distance away. The U.S. Army and CIA developed what could be considered a stealthy helicopter during the Vietnam War. There, they were primarily interested in reducing the amount of noise that the helicopter generated, and they named the helicopter The Quiet One. Reducing the heat an aircraft generates is also important, and most battlefield helicopters include systems like mufflers to reduce the heat coming from the engine exhaust. Stealthy characteristics were incorporated into some small planes, but they were not heavily applied to aircraft during the 1960s. This was primarily because significantly reducing radar reflections was very difficult to model mathematically.
The Arab-Israeli war of 1973 startled many U.S. Air Force leaders because a large number of Israeli aircraft were shot down by Russian-built surface-to-air missiles in a very short period of time. The experience in Vietnam had earlier also prompted Defense Department leaders to seek new aircraft that were not so susceptible to attack from surface-to-air missiles. They realized that any conflict with the Soviet Union could result in a large portion of the U.S. Air Force being shot down in the early days of the war. This prompted them to begin looking for ways to avoid this.
In 1974 the Defense Advanced Research Project Agency (DARPA) and the Air Force jointly initiated a program to build an experimental stealth aircraft. About the same time a radar signature specialist with the Lockheed Corp. discovered an analytical method that could be used to design a stealth aircraft external configuration. Ironically, the method was contained in a Soviet technical journal, which had be translated by the USAF Foreign Technology Division. This discovery in electro-magnetic analysis made it possible to achieve a very low level of RCS (Radar Cross Section) for the complex geometry of an aircraft. It also marked the first time in aircraft design that electro-magnetic analysis, not aerodynamic theory, determined the external configuration.
The initial application of this technology created the distinctive facetted configuration of the experimental stealth aircraft, the Have Blue vehicle. The success of the Have Blue in evading air defense radar units led to a decision to develop an operational stealth aircraft, which would receive the designation F-117.
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