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.
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.
Project Suntan
In the mid-1950s and in parallel with Rainbow and Gusto, Johnson also began looking at a high-altitude, Mach 2.5, non-stealthy replacement for the U-2, funded by the Air Force. Codenamed Project Suntan, it was proposed that the engines, built by Garrett, would be fueled by liquid hydrogen. On February 15, 1956, two Skunk Works design proposals, designated CL-325-1 and CL-325-2 and powered by the supersonic Rex III liquid hydrogen engine, were presented to the Air Force at Wright Field. But after extensive studies the Air Force became convinced that Garrett was incapable of building an engine as complex as that proposed. Consequently, on October 18, 1956, it issued a directive demanding that all work on both projects be stopped. However, as the result of an earlier meeting at the Pentagon with Lt Gen Donald Putt, head of Air Research and Development Command, Kelly offered to build two prototype hydrogen-fueled aircraft powered by more conventional engines and have them delivered within 18 months of contract signing. Based upon the CL-325, they would be capable of cruising at an altitude of over 99,000ft at a speed of Mach 2.5 and have a range of 2,500 miles.
Whilst the Air Force funded studies to verify Kelly’s proposal, they also invited both Pratt & Whitney and General Electric to submit proposals to build a hydrogen-fueled engine. On May 1, 1956, two six-month study contracts were signed, one with Pratt & Whitney for the engine and the other with the Skunk Works to evaluate airframe configuration and material options. As a result, contracts were awarded by the Air Force to the Skunk Works to produce four production aircraft and a single static test specimen; the design was designated CL-400-10.