DESIGN AND DEVELOPMENT

Lockheed A‑12: The CIA's Blackbird and other variants

DESIGN AND DEVELOPMENT

During the early‑to‑mid Cold War years, western leaders had to rely primarily upon reconnaissance aircraft to gather photographic intelligence (PHOTINT), electronic intelligence (ELINT), and communications intelligence (COMMINT) about the activities of the Soviet Union, her allies in the Warsaw Pact, and Communist China. This was in part due to the closed nature of these societies, but also to the size of the land mass and the level of technical ability prevailing at that time. The violation of a sovereign state’s airspace is contrary to international law. As a result, such planned incursions required prior clearance from the very top; and in the case of the United States, that meant the President. Routes and the number of sorties were highly regulated and the flights themselves were conducted under conditions of great secrecy. The name of the game was to acquire the requisite raw data, but avoid at all costs getting caught in the act.

^{Article 130 (60‑6933) seen taxiing from its hangar at Area 51. Early operational configuration of the A‑12’s paint scheme saw black paint applied only to the chine, nose and cockpit areas. (Lockheed Martin)}

Lockheed’s custom‑built strategic reconnaissance‑gathering aircraft, the U‑2, undertook the first in a series of vital but hazardous overflights of “denied territory” on June 20, 1956. Designed by Clarence L. “Kelly” Johnson, President of Lockheed’s legendary Skunk Works, and operated by the Central Intelligence Agency (CIA), the program went under the classified cryptonym of Aquatone (later redesignated Chalice). Photographic results gained by the project were sensational. Flying at altitudes in excess of 70,000ft, the frail, subsonic, glider‑looking aircraft was immune from fighter interception. But President Eisenhower was concerned that the U‑2 should also escape detection and tracking by Soviet radars.

On July 10, 1956, after just five incursions, the Soviets delivered their first protest note about the flights and Eisenhower ordered their immediate suspension. However, those missions had generated a vast amount of intelligence, revealing for the first time many aspects of Soviet military and industrial capability. During a meeting on July 19, and despite pleas to the President from the CIA’s Director of Central Intelligence (DCI), Allen Dulles, that Aquatone should be allowed to continue such overflights on the basis that the quality of intelligence gained far outweighed any potential damage to international relations, Eisenhower remained resolute and the ban stood – at least for the immediate future.

On August 16, 1956, the DCI’s Special Assistant for Planning and Coordination, Dr Richard Bissell, convened a meeting with Kelly Johnson and a number of prominent scientists – the meeting’s objective was to agree on a plan to develop some form of “electronic camouflage” that would render the U‑2 invisible to Soviet radars, thereby addressing Eisenhower’s concerns. The meeting lasted into the early hours and resumed again at 0700hrs the next day, and by midday they had devised a program to apply radar‑canceling devices to a U‑2, codename Project Rainbow. It was the first attempt to make an operational aircraft “stealthy.” Dr Edwin Land, of the Massachusetts Institute of Technology (MIT), chaired the project and via Marshall Holloway, Director of the MIT Lincoln Laboratory, in Lexington, Massachusetts, recruited a small number of radar experts into the program. They were based in a secure building on the roof of the Lincoln Laboratory and included Frank Rodgers, associate head of the Radar Division.

A major problem facing the team was that during the course of a U‑2 overflight, the aircraft would first need to remain invisible to Soviet long‑range, low‑frequency, early warning radars operating in the 65/86 MHz range. Then, upon penetrating deeper into the USSR, high‑ frequency S‑band and X‑band target acquisition radars operating in the 2–4 GHz and 8–12 GHz bands respectively, would need to be dealt with. This required the development of different methods and materials to defeat the different radar systems illuminating the target.

^{Lockheed Article 341, the U‑2 prototype, completed its first flight from “Watertown Strip” – Area 51 – on August 1, 1955. Its contribution to US intelligence gathering continues to this day. (Lockheed Martin)}

By the summer of 1957, the team had developed a number of innovative solutions. To reduce low‑frequency returns, the U‑2 utilized a system of wires and ferrite beads mounted on the aircraft’s vertical tail surface, which were also arranged to frame the aircraft’s planform. Known as “Trapeze” and “Wires,” this arrangement reduced the U‑2’s radar return by about 12dB, which in practical terms meant that long‑range detection was halved. To reduce returns in the high‑frequency bands, radar‑absorbent material (RAM) was applied to the underside of the aircraft’s fuselage. Operating on the principle of a Salisbury screen, the coating consisted – from the inside out – of fiberglass, a honeycomb spacer, a graphite‑impregnated layer, a protective layer for durability, and finally a layer of paint. This treatment was nicknamed “Wallpaper” and aircraft equipped with these devices were known as “Dirty Birds.” However, one of the problems resulting from these measures was that they added both weight and drag to the U‑2, reducing its maximum altitude by some 5,000ft and its range by about 20 percent.

During a meeting at the White House in early May 1957, preliminary approval was granted for more Soviet overflights, despite the fact that phase one of Project Rainbow hadn’t yet been completed. So in June one Dirty Bird was deployed to each of the three U‑2 Operating Locations (OLs). Two penetration flights of the Soviet Union were made from Turkey on July 21 and 30 to evaluate the effectiveness of the treatment. But despite “Trapeze,” “Wires,” and “Wallpaper,” subsequent analysis of the U‑2’s System 5 (a multi‑band radar recorder) revealed that Soviet radars had been alerted to the aircraft’s presence when it was flying directly toward or directly away from the radar head. This led to the conclusion that the source of the radar returns had emanated from the U‑2’s inlets, cockpit, and exhaust – none of which could be treated with what had been developed thus far. Clearly, a more radical approach to solving the problem was required.

Frank Rodgers at the Lincoln Laboratory was certainly up for a radical approach. He returned to basic research in a bid to understand the relationship between a radar return and the physical shape of the target without regard to the aerodynamic practicality of such shapes. To his surprise he discovered that if a metal saucer shape was treated by layering circular sheets of Teledeltos paper (a paper with a constant resistance) on top of the shape, with the first sheet of greatest diameter and each successive sheet smaller, by the time the sixth sheet was positioned, resistance was down to 300 ohms – the same as in free space through which the radar wave was moving. This prevented reflections completely, effectively rendering the shape invisible to radar! Rodgers had produced a “broad‑band” treatment that was effective against any radar operating at any frequency within a very broad range of frequencies. This was in contrast to the “narrow‑band” techniques that had been developed and applied to the Dirty Birds, whereby if a radar was encountered operating at a frequency different to that for which the treatment was designed, its effectiveness was significantly reduced. Unfortunately the shape of the vehicle was completely aerodynamically unstable; but Bissell was extremely impressed and both Rodgers and Norm Taylor were instructed to present the findings to Johnson. But the Skunk Works boss sent the two hapless scientists away with a flea in their ear. Rodgers and Taylor decided that it wasn’t the idea that had been wrong, but their presentation. In future, they wisely decided, they would leave the final design of the aircraft to Johnson and instead feed him guidelines that he could incorporate to reduce its radar cross‑section (RCS).

Back at the Lincoln Laboratory, concern was being expressed in some influential circles that work being conducted by Rainbow for the CIA was inappropriate. So, in October 1957, it was moved out to a building in Cambridge, Massachusetts, where it was incorporated into the Scientific Engineering Institute (SEI), a Boston‑based CIA proprietary. The move also prompted some changes within the team, but Frank Rodgers stayed.

Johnson of course had his own team of electronic engineers back at the Skunk Works. Headed up by Luther Duncan “LD” MacDonald, the team also included Perry Reedy and physicist Ed Lovick. Ed recalls, “Kelly thought that an aircraft made of plastic materials might have sufficiently small low‑frequency radar backscatter to defeat the 70 MHz early warning radars. I warned him that it would not and that you’d see the internal structure, the square corners it formed and the fuel. But Kelly wanted to test it anyway. Despite the fact that it was known that practical fiberglass structure would be dense enough to scatter 4in S‑band radar waves, he still hoped that an all‑plastic airframe might not backscatter 14ft wavelengths significantly. Engine, landing gear parts, and any other metallic items were to be hidden by an, at that time, unspecified means:

Several all‑fiberglass models that incorporated appropriate internal plastic structures were built and tested. One model was a one‑eighth scale and was too large for indoor testing, so it was tested at Indian Springs AFB.

Backscattering measurements showed that the thick plastic sections required for strength, and especially corners, were very reflective. When kerosene fuel was added to the interior of the wings, the reflections increased and became characteristic of a solid piece of plastic. When the fuel in partially filled tanks was vibrated and standing waves occurred, the backscatter increased even more.

After attempts to hide the structure, fuel and simulated engines yielded poor results, Kelly agreed to abandon that idea.

^{Lockheed Skunk Works boss, Clarence L. “Kelly” Johnson, was the powerhouse behind such designs as the F‑104 Starfighter, U‑2, A‑12, and SR‑71. (Lockheed Martin)}

More gloomy news followed in January 1958 when two intelligence assessments of Soviet interception capabilities had been compiled. They indicated that the only areas where the U‑2 could fly without certain detection were central Siberia and east of Tashkent to China. Two new types of radar had been detected; in addition, it was considered likely that limited numbers of a specially designed fighter, having the ability to operate above 70,000ft, would soon be available and that SAMs were expected to become a serious threat to Aquatone from 1959 onward. The pressure was now on to rapidly develop and deploy not a stealthy U‑2, but a U‑2 replacement.

January 1958 saw “the Agency” (CIA) assign the cryptonym Gusto to phase two of Rainbow; and at the end of the month, Johnson wrote to Bissell proposing a four‑point work statement for Gusto which was approved on February 11. It was also at about this time that Lockheed built their first anechoic chamber in which to measure the RCS of various design models.

Johnson and some of the key members of his team, including Dick Boehme, Ed Baldwin, and Harry Combs, now began working on a number of high‑risk subsonic designs – low RCS being their top priority. They would formulate a shape with minimum RCS values and then work on ways to make it fly within the specified performance envelope. This series of designs were known to the Agency as Gusto Model 2, and over the following months Lockheed studied numerous design permutations under this overarching codename. One rejected design featured cutting notches out of the leading and trailing edge of the wing and inserting triangular wedges of graded dielectric material. This technique, called “softening,” avoided generating an abrupt change in resistance of the incoming radar beam when it first met the aircraft (it is these abrupt changes that generate reflections). Utilizing this technique, when the incident beam strikes the baseline of the triangle (located at the outer edge of the aircraft), it is reflected inside the wedge, generating electrical currents that turn the radio frequency energy into heat. The resistance progressively reduces to zero by the time the energy reaches the tip of the triangular wedge, at which point it matches that of the adjacent metal structure. Invented by Ed Lovick, the technique would play an important role in reducing the RCS of Johnson’s ultimate design.

^A‑12

^{During the missions over North Vietnam, the bogus five‑digit red serial number was the only detail applied to the overall black paint scheme.}