radar-absorbing blankets. This made the former target drones difficult to shoot down, as the Chinese and North Vietnamese soon learned. With the advanced Model 154, a reduced radar cross section was built in.
In all these cases, however, the reduced radar cross section was only one of the design considerations. The maximum possible altitude was the driving requirement in the design of the Black reconnaissance airplanes.
But by the early 1970s, a reduced radar cross section became the dominant consideration in the design of new aircraft. This became known as 'stealth.'
The first attempt to build an 'invisible' airplane was made in 1912.
Petrocz von Petroczy, an officer with the Austro-Hungarian air service, covered a Taube with clear sheets of a celluloid material called Emaillit.
The theory was that a transparent covering would make the plane harder to see and hit with ground fire compared to a fabric-covered plane silhouetted against the sky. The Taube was test flown in May and June 1912.
It was the Germans who soon took the lead. An engineer named Anton Knubel built two monoplanes with clear coverings in 1913-14. The second of the planes had its framework painted a blue gray color to make it even harder to see against the sky. In August 1914, World War I started. In 1915, Knubel built a biplane to test its military applications. Unfortunately, Knubel was killed in a crash of the plane on September 8, 1915.
The idea was seen as having promise, and three Fokker E III fighters were delivered in the summer of 1916, covered with Cellon. Unlike celluloid, it did not burn or shatter. Cellon had found wide use in the automotive industry as a glass substitute. Cellon was soaked in water to expand the sheets. It was then attached to the plane's framework and allowed to dry to a taut finish. The material was called D-Bespannung
The trio of E III fighters appear to have seen limited combat. On July 9, 1916, the No. 16 Squadron of the British Royal Flying Corps reported that 'a transparent German aeroplane marked with red crosses was pursued by French machines in the Somme area.' Several more German aircraft were tested with the Cellon coating. These included four observation-light bombers: an Albatros B II, an Aviatik B, an Aviatik C I, and a Rumpler C I. Two heavy bombers, a VGO I and R I, had their tails and rear fuselages covered with the material.
Very soon, however, it was apparent this first attempt at a stealth airplane was a failure. A report dated July 11, 1916, states: 'In clear weather, the aircraft is more difficult to spot, but in cloudy weather, it appears just as dark as other aircraft. In sunshine, the pilot and observer are unpleasantly blinded by the reflections.' The major problem was the Cellon itself: 'During longer periods of rain or damp weather… the covering becomes so loose that it would be better not to fly such aircraft… The covering itself is strong, but should a shrapnel go through the wing, the whole sheet would tear to pieces.'
It was far more effective simply to paint the aircraft in camouflage colors. This could not make the plane invisible, as the German planes attempted to be, but would make the plane less visible.[339]
With the invention of radar in the mid-1930s, a new approach was needed. A variety of countermeasures emerged during World War II. The simplest means was strips of aluminum. Called 'chaff' in the United States or 'window' in England, the strips would be released from a plane. They would reflect the radar signals and produce false echoes, which would hide the plane. A more active method was to interfere with the radar. Called 'noise jamming,' the target plane transmitted signals on the same frequency as the radar. As the echo from a plane was a tiny fraction of the radar's original signal strength, it was possible for the plane to drown out the echo, making it impossible to detect the target plane.
With development of jet bombers like the B-47 in the late 1940s, it was thought that they would fly too high and too fast to be detected. This soon proved false, and development of electronic countermeasures (ECM) continued.[340]
During the Cold War, both the ECM and the tactics of its use grew more sophisticated. The first step was to avoid the radar entirely. The Soviet Union was vast, and many areas had little or no radar coverage. The bomber's route would take it through these gaps in the radar. The plane would not transmit any jamming signals, as this would only advertise the plane's presence. As the bomber neared the target, the number of radars would increase, and it would no longer be possible to avoid them. The bomber would then start to drop chaff and jam the radars. A more subtle approach was to transmit carefully timed signals, which made the plane appear farther from the radars, or at a different bearing. This is referred to as 'deception jamming.' As a last resort, the air defense centers, radars, and SAM sites would be bombed.
All this was based on the idea of hiding a plane's echo. As long ago as the mid-1930s, Sir Robert Watson Watt, who designed the first British radar, realized that bombers could avoid the whole problem by having a reduced radar cross section.[341] The problem was in the details. The radar cross section of a plane depends on three factors: the shape of the plane, the frequency of the radar, and the 'aspect angle' between the plane and the radar.
The prime source of a large radar cross section is two or three surfaces, such as a wing and fuselage or the floor, sides, and back of a cockpit, which meet at a right angle. The radar signal strikes one surface, is reflected to the other, then is bounced directly back to the radar. Nor were tubular shapes immune — radar signals striking a round fuselage can actually 'creep' around the fuselage and back to the radar. Still other sources are sharp points on the wings or tails, wing fences, external weapons, intakes that allow the front of the engine to be 'seen' by the radar, gaps formed by access panels, and antennae.
Frequency has a similar effect. A feature that has a strong radar return at one frequency may not be detectable at another. This is quite independent of size — a small vent or grill may produce a major part of the plane's radar cross section.
The final factor, aspect angle, is the most complex. The interactions between the reflections from each part of the plane cause huge changes in the radar cross section. In some cases, a one-third-degree change in the aspect angle can result in a thirty-twofold change in the radar cross section. When all these factors are taken into account, a plane's radar cross section may vary by a factor of 1 million. A 1947 text on radar design noted:
Only for certain special cases can [radar cross-section] be calculated rigorously; for most targets [it] has to be inferred from the radar data… Only a rough estimate of the cross-section of such targets as aircraft or ships can be obtained by calculation. Even if one could carry through the calculation for the actual target (usually one has to be content with considering a simplified model) the comparison of calculated and observed cross-section would be extremely difficult because of the strong dependency of the cross-section on aspect.[342]
By the mid-1950s, basic research was underway in the United States on understanding the sources of a plane's radar cross section. A team headed by Bill Bahret at the Wright Air Development Center did much of this work.
A large anechoic chamber was built to test the radar return of different shapes.
By the late 1950s, Bahret and his team felt they understood the sources of large echoes. Once they knew this, the obvious next step was to reduce the echoes. This would have two advantages in terms of electronic countermeasures: the amount of power needed to hide the plane's echo would be reduced, and, for a given jammer, the effectiveness would be increased. As yet, there was no intent to build a plane invisible to radar.
A second part of this effort was development of radar-absorbing material (RAM). Since World War II, Dr. Rufus Wright and a team at the Naval Research Laboratory had been working on RAM. Together with Emerson and Cuming Incorporated, a plastics manufacturer, they had developed a practical RAM. The material was in the form of thin, tilelike sheets. It was pliable like rubber and could be cut and formed into any shape. The navy lost interest in the project, and Wright went to the air force.
The air force was very interested — the RAM was both thin and strong and, therefore, could be attached to the skin of an airplane. After tests with scale models, it was decided to cover a T-33 jet trainer with the RAM. This
