The need for the overflights to remain undetected by the Soviets was thought to be satisfied by designing an aircraft able to fly at altitudes of 70,000 feet or greater, something that could be done with existing aviation technology. This placed the aircraft above the reach of Soviet jet fighters, and, it was believed, would also make the plane difficult to detect by Soviet radar. U.S. radar had great difficulty spotting a target at an altitude of over 40,000 feet at a range of 200 miles. During World War II, American radar had been supplied to the Soviets, and, it was assumed, their radar would have similar problems tracking a high-flying target. It was on this basis that President Eisenhower gave approval for development of the U-2. The theory seemed to be confirmed when U-2 training flights began over the United States. Even with advanced notice, it was difficult to spot the U-2, much less track it.[850]
Yet, on the first U-2 overflight of the Soviet Union, the Soviets were not only able to spot the U-2, but to track the plane and send MiG fighters after it. What had not been realized was that Soviet radar design had advanced considerably over those of World War II. The detection of the U-2 adversely effected the overflight program. The assumption had been that the Soviets would only detect a few unknown radar targets, and would not understand what was occurring. The Soviet's ability to track the U-2 changed both the threat to the aircraft, and what was needed to counter it. For the first time, an aircraft's Radar Cross Section (RCS) had to be reduced.
Since the early days of radar, it had been known that not all aircraft echos were equal. Rather, they would vary according to the frequency of the radar, as well as the size, shape, and orientation of the plane. The problem was in the technology; the available means of analyzing a plane's RCS was no better than "cut and try."
The first attempt, Project Rainbow, with its radar absorbing coatings, and special wires extending from the nose and tail to the wingtips, was disappointing. The wires and coating were effective at some radar wavelengths, while at others they actually increased the plane's RCS. The need was understood, but the technology to meet it was lacking. The best that could then be done was to paint the U-2 with the sinister-looking black iron ferrite-based paint. This reduced its RCS to a limited extent, and made the plane harder to spot.
The various reconnaissance aircraft which followed the U-2 had to rely on a mixture of techniques to escape destruction. The A-12 Oxcart depended on altitude, speed, a low RCS and countermeasures. The A-12's altitude put it on the fringe of a Surface-to-Air Missile's (SAM) envelope, while its speed cut the time the SAM
had to engage the aircraft. This, along with the low RCS made the A-12 difficult to track and to gain a radar lock-on for firing. Should a lock-on be made, then the countermeasures would break it.
During the development program, the air force was worried that if an A-12 carrying existing countermeasures equipment was lost over hostile territory, this would compromise the systems used by U.S. fighters and bombers. Even if the countermeasures equipment was only similar, the Soviets would have an insight into U.S.
designs. The CIA started Project Kempster, which tried to develop ion guns that would be mounted on the A-12. These would ionize the thin air in front of the plane, which would reduce its RCS. (The phrase "Cloaking Device" comes to mind.) Kempster proved unsuccessful, and more conventional systems were developed for the A-12.[851]
In contrast, the Model 147 drone initially used radar-absorbing blankets, a contrail suppression device, and the now-standard black paint. The Lightning Bug's maximum altitude of between 50,000 and 65,000 feet, and a subsonic speed, placed it within reach of both MiGs and SAMs, but their small size made the drones difficult to "squash." The later model Lightning Bugs carried additional equipment to jam missile radar, and a device that caused the drone to take evasive maneuvers when illuminated by a MiG's radar. Despite the accomplishments of these aircraft, the inability of the available technology to meet the political need for an undetectable aircraft eliminated any possibility they would be used over the Soviet Union.
While for the Dark Eagles a reduced RCS was a critical design feature, this did not figure in the design of operational fighters and bombers. They, like the B-17s and Lancaster bombers of World War II, relied on conventional countermeasures packages to overcome air defenses, as well as such techniques as flying through gaps in radar coverage and attacks on air defense sites. With the start of the Vietnam War, the threat again changed. In addition to interceptor aircraft and anti-aircraft guns, U.S. aircraft now faced SAM missiles.
SAMs were initially a difficult challenge for tactical aircraft. What was needed was the means to defeat a weapon that was faster than a conventional jet fighter, and which was guided to the target aircraft by radar. The technology necessary existed, although it took time to develop. This involved improved countermeasures, Wild Weasel aircraft to suppress the SAMs, and, finally, maneuvering to avoid any incoming SAMs.
These proved effective in Vietnam, but following the war there was a split in the assessment of the future threat. Even with the proliferation of new Soviet SAMs, and improvements in radar technology, the opinion in the tactical units was that countermeasures, Wild Weasels, and maneuvering could still overcome the threat posed by any new enemy air defense. They were "Manly Men," who could out fly SAM's, given the proper warning. The need, as they saw it, was more of the same— improvements in the existing technology.
Others had a different opinion. Their threat assessment was not based on U.S.
success against SAMs in Vietnam, but, rather, on the Israeli failure in the 1973 Yom Kippur War. The threat they saw was that countermeasures were susceptible to technological surprise when faced with the multi-layered, interlocking network of SAMs, radars, and guns that the Soviets and Third-World countries were deploying. What was needed against this threat was not the kind of incremental approach that the tactical units wanted, but, rather, a whole new approach. What was needed was what had long been part of the earlier Dark Eagles, a reduced RCS. To be effective, however, a level of reduction was needed which was of a magnitude greater than that achieved with the U-2 and A-12.
This also required a change in outlook. Because of the A-12 and SR-71, the assumption had been that the key to survival was ever-greater speeds and altitudes.
The CIA had considered such possible high-speed replacements for the A-12. In mid-1964 General Dynamics completed a feasibility study called Project Isinglass, which proposed an aircraft able to reach Mach 4 to 5 at 100,000 feet. This was followed in 1965 by a McDonnell Aircraft design called Project Rheinberry. This was to be a rocket-powered aircraft launched from a B-52 that could reach Mach 20 at 200,000 feet. The cost of development would have been equally spectacular, and could not be justified given the on-going reconnaissance satellite programs.[852]
In the face of the threat of improved SAM systems, this faster and higher assumption had to be turned on its head. Such speeds were incompatible with stealth, due to the sonic boom and infrared energy the vehicle would produce. Rather than hypersonic, or even supersonic speeds, stealth required a subsonic vehicle. Its operating mode would be like that of a submarine. Just as a submarine relies on silence while cruising slowly in the depths of the sea, a stealth aircraft would have to use a similar philosophy to conceal itself in the sky.
The question was one of whether the technology existed to create a truly invisible aircraft. In the two decades since Kelly Johnson had begun Project Rainbow, the goal had proven to be as elusive as a rainbow's end. How it was finally accomplished illustrated the changes made in those intervening years, including who made the innovative discoveries, how they did it, and the tools they used.
850
851
"The U-2's Intended Successor: Project Oxcart, 1956–1968." This is actually Chapter 6 of the CIA's official history of the U-2 program. It was declassified separately, and is not included in the published version of the U-2 history.