The F-117

During the air war over Vietnam, the most lethal threat facing US air elements was radar directed surface to air missiles (SAMs) and anti-aircraft artillery (AAA). It was extremely disruptive, often result­ing in attack aircraft missing their targets in order to evade SAMs or dodge AAA. Latterly, during the 1973 Yom Kippur war, the Israeli Air Force lost 109 aircraft in just eighteen days, virtually all falling victim to radar guided SAM or AAA batteries. With the Soviet Union having developed a highly sophisticated, integrated defence network, US planners estimated that if the Israeli loss ratio were extrapolated into a NATO/War Pact scenario, NATO Air Forces would be decimated in just over two weeks. Clearly, what was needed was a funda­mental rethink on how to redress this imbalance.

In 1974, Ken Perko in the Tactical Technology Office (TTO) at the Defence Advanced Research Projects Agency (DARPA), requested submissions from Northrop,

McDonnell Douglas, General Dynamics, Fairchild and Grumman, addressing two considerations:-

1. What were the signature thresholds that an aircraft would need to achieve to become essentially undetectable at an operationally useful range?

2. Did those companies possess the capabilities to design and produce an aircraft with those necessary low signatures?

Fairchild and Grumman declined the invitation to partici­pate, while General Dynamics emphasised the continued need for electronic counter measures. Submissions from McDonnell Douglas and Northrop however demonstrated a grasp of the problem, and consequently, they were awarded contracts worth approximately SI00,000 each during the closing months of 1974 to conduct further studies.

On 17 January 1975, ‘Kelly’ Johnson’s protege, Ben Rich, became president of the Skunk Works. It was while Ben was still Kelly’s Deputy that the former became aware of the low observability study. Lockheed hadn’t
been one of the five original companies approached by the DARPA team, simply because it hadn’t produced a fighter for nearly ten years (the F-104 starfighter). Ben however, obtained a letter from the CIA, granting the Skunk Works permission to discuss with DARPA the low observable characteristics of the A-12 and D-21 drone. After much negotiating, Lockheed were allowed into the competition without a Government contract – a move that ultimately paid a handsome dividend.

In early 1975, the initial Skunk Works Project Team consisted of Ed Martin (Project Manager), Dick Scherrcr and Denys Overholser. Ovcrholser had joined the Skunk Works from Boeing in 1964 and recalls, “When Dick Scherrer asked me, ‘How do we shape something to make it invisible to radar?’ I said, ‘Well, it’s simple, you just make it out of flat surfaces, and you tilt those flat surfaces over, sweeping the edges away from the radar view angle, and that way you basically cause the energy to reflect aw’ay from the radar, thus limiting the magni­tude of the return.’” Such radical thinking had its origins in a discussion Overholser had had with his then boss,

Bill Schroedcr some years earlier, concerning the mathe-

Top left This early model of Have Blue already depicts several of the type’s characteristics: highly facetted, intakes above the wing, inboard cantered tails and highly swept leading edge.

(Lockheed Martin)

Left Covered in foil, this wooden model is undergoing RCS tests in Lockheed’s anechoic chamber at Rye Canyon.

(Lockheed Martin)

Above Phase I of the XST programme culminated in RCS eval­uations between the two contending designs at the Air Force’s Radar Target Scatter (RATSCAT), test range, located at White Sands, New Mexico. (Lockheed Martin)

matics and physics of optical scattering. The two had concluded that detectable signatures could be minimised utilising a shape composed of the smallest number of properly orientated flat panels. In addition, Schroeder believed that it was possible to develop and resolve a mathematical equation capable of calculating the reflection from a triangular flat panel; this in turn he hypothesised could be applied in a calculation relating to RCS. As a result, Overholser hired his ex-boss out of retirement and as Schroeder’s mathematical computations became avail­able, Overholser and his team of two engineers were able to use these to write the computer programme that could evaluate the RCS of prospective design submissions nominated by Dick Scherrer and his group of preliminary design engineers. Denys and his team worked night and day and in just five weeks produced an RCS prediction programme known as ‘Echo Г. As tests proceeded, it was determined that the edge contributions calculated by Echo 1 weren’t exactly correct, due to a phenomenon known as diffraction. However, shortly after developing Echo 1, Denys became aware of a publication entitled Method of Edge Waves in the Physical Theory of Diffraction, published in an unclassified technical paper in the Soviet Union in 1962 by Pyotr Ufimtsev, Chief Scientist at the Moscow Institute of Radio Engineering. The paper had been translated by Air Force Systems Command’s Foreign Technology Division in 1971, and Denys was able to incorporate elements of its theory into a refined version of the Echo 1 programme. The resultant model was a facetted delta wing design which drew a healthy share of scepticism from within the Skunk Works, some in aerodynamics referring to the shape as “The Hopeless Diamond”. However, with S25,000 procured from the Lockheed board, two, one-third scale, wooden models of the Hopeless Diamond were built, one was used by the aerodynamists, the other to measure RCS values in Lockheed’s anechoic chamber. The first series of tests, conducted in June 1975, demonstrated that its RCS ‘spikes’ matched precisely those predicted by Echo 1. The model was then moved outdoors to a radar test range near Palmdale, in the Mojave Desert. Yet again, test results conformed well with Echo 1 predictions, creating greater levels of confidence in both the computer programme and the facetted design concept.

Lockheed submitted two proposals to DARPA, one included the predicted and measured signature data for the Hopeless Diamond, the other provided the predicted data for an air vehicle of flyable configuration. This came about in response to DARPA issuing proposals to the three competitors for what was to become known as the Experimental Survivable Testbed (XST) programme, which was informally requested in the late summer of 1975.

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