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 was to verify the echo reduction predicted by the scale tests. The project was code-named "Passport Visa," although the white-painted T-33 was better known as "Bahret's White Elephant."
The Passport Visa T-33 was completely covered with the RAM. This included the skin, wing tanks, and control surfaces. The plane was only an experiment, with no operational applications in mind. The air force test pilot selected for the project was Capt. Virgil "Gus" Grissom. (The following year he was selected as a member of the first group of astronauts; he would later die in the 1967 Apollo 1 launchpad fire.) Test flights began in late 1958. The results were mixed — many of the echo reductions were confirmed, but the T-33's flight characteristics were degraded by the added thickness of material. Grissom found the plane was hard to control; it slid in turns, overdived, and coming in for a landing it behaved like a roller coaster.[343]
Clearly, a plane's radar cross section could not be reduced simply by covering it with RAM. It would have to be designed in. Despite all these efforts, there was no simple way to calculate the radar cross section of a plane. With the computers and theoretical models of the time, too many factors entered into the calculation for it to be a practical possibility.
This meant designers would have to take a crude cut-and-try approach.
When Kelly Johnson wanted to test the radar cross sections of the A-12 and D-21, he first used small models. Then full-scale mock-ups were built and tested. From this data, the final designs were developed. Still, it was not until the planes actually took flight that the true radar cross section could be determined.
Such efforts could be made for Black airplanes. Reduced radar cross section had little impact on the design of operational aircraft. Until Vietnam.
The air defenses of North Vietnam required a fundamental change in tactics. A typical Rolling Thunder strike was composed of sixteen F-105D bombers. The force needed to protect them was made up of eight EF-105F "Wild Weasels," which attacked SAM sites, and six F-4D escorts against MiGs. Even though each F-105D carried individual ECM pods, two EB-66 jamming aircraft would also accompany the strike force. The EB-66s, in turn, each required two F-4Ds as protection against MiGs. Thus, to protect sixteen bombers, a total of twenty jamming and support aircraft were needed since the support aircraft themselves needed protection.[344] The net result was that most of the available aircraft were diverted from attack missions to defensive roles.
The revolution in air defense caused by SAMs would be underlined in the October 1973 Yom Kippur War. The Egyptian and Syrian armies that attacked Israel were equipped with the new SA-6 Gainful SAM. Mounted on a tanklike transporter, it could move with the frontline troops. The Israeli air force did not have the ECM pods needed to counter the SA-6 and suffered heavy initial losses. During a single strike against a Syrian SA-6 battery, six Israeli F-4Es were lost. The air defenses also prevented the Israeli air force from providing close air support to ground troops.[345]
Although the Israelis overcame the early setbacks, the SA-6 was a clear warning. As long as U.S. countermeasures and tactics were specifically tailored to enemy radars and SAMs, they would be vulnerable to technological surprise. The Soviets were then in the process of deploying a new generation of SAMs. In the event of a war in Europe, NATO forces could suffer the same huge losses as the Israelis had. Many academics theorized the end of manned aircraft was at hand. Technical advances in radar design, such as the traveling wave tube and computers, had increased power and the ability to defeat ECM. Any new technological advances in ECM would be countered by improved radars.
342
Doug Richardson,
343
Alfred Price,
345
Steven J. Zaloga,