Others realized that a new set of assumptions was needed. Countermeasures had always been based on overpowering the radar. Even Black aircraft with reduced RCS — the A-12 and Model 147–154 drones — used ECM equipment for protection. The key was not more powerful ECM, but to make the RCS a primary design consideration. It would be eliminated, not simply reduced. With no echo, the radar would be blind. No radar would provide early warning as the aircraft approached; no radar would direct MiGs, antiaircraft guns, or SAMs. There would be no need for support aircraft. Air defenses would revert to the 1930s, against an enemy traveling at near supersonic speeds.
The problem was the amount of RCS reduction needed. A tenfold reduction would only shorten the range at which a plane could be detected. A hundredfold RCS reduction would merely degrade the effectiveness of radar. It would take a thousandfold reduction of a plane's RCS to make it undetectable to radar.[346]
Moreover, to be fully effective this reduction in RCS would have to be combined with other design features to reduce detectability. Just as the aircraft could not reflect any radar signals, it also could not emit any — no bombing radar or ECM transmissions. The infrared emissions from the engine would have to be hidden. The engine could not produce smoke. The airplane also would have to be quiet; the sound of a plane gives warning of its approach. The plane could not produce a contrail — this had been a major problem with the Model 147 drones. The final problem was visibility.
Although true optical invisibility was not possible, efforts had to be made to reduce the distance at which the plane could be seen. One problem was "glints" from the canopy. A plane could be seen at a distance of five to ten miles; the reflection of the sun could be seen at a distance several times that.
The effort to make this possible became known as "Project Harvey," after the invisible rabbit in the play and film of the same name.[347]
In 1974, the Defense Advanced Research Projects Agency (DARPA) issued requests to five aircraft manufacturers to study the potential for developing aircraft based on a minimal RCS. They were to design a small, low-cost test aircraft to demonstrate the possibilities. It was called the "XST," for "experimental survivable testbed." The companies were General Dynamics, Northrop, McDonnell Douglas, Grumman, and Boeing.[348] All had recent experience with fighter design and manufacturing. Lockheed, which had not built a fighter since the F-104 program of the early 1960s, was not included.
By early 1975, Ben Rich had learned of the program. He had been involved with the work Lockheed had done on the Dirty Bird U-2s, the A-12, SR-71, and D-21, and knew it gave Lockheed the experience needed for the DARPA project. Rich obtained a letter from the CIA granting permission to discuss the reduced RCS work of the earlier projects. This was part of the request to DARPA for Lockheed to be included in the program. The effort was successful, and Lockheed joined the design competition.
The keys to Lockheed's efforts were Lockheed mathematician Bill Schroeder and Skunk Works software engineer Denys Overholser. They produced the conceptual […] that allowed a stealth aircraft to be designed.
Schroeder went back to the basic equations derived by Scottish physicist James Clerk Maxwell a century before. These described how electromagnetic energy was reflected by a surface. Maxwell's equations were revised at the turn of the century by German electromagnetic expert Arnold Johannes Sommerfeld. For simple shapes, such as a cone, sphere, or flat plate, these formulas could predict how radar signals would be reflected. In the early 1960s, a Soviet scientist named Pyotr Ufimtsev developed a simplified approach which concentrated on electromagnetic currents set up in the edges of more complex shapes, such as disks.
The Maxwell, Sommerfeld, and Ufimtsev equations still could not predict the RCS for a complex shape like that of an airplane. Schroeder's conceptual breakthrough was to realize that the shape of an airplane could be reduced to a finite set of two-dimensional surfaces. This reduced the number of individual radar reflections that would have to be calculated to a manageable number. Rather than a surface made of smoothly curving surfaces, the whole airplane would be a collection of flat plates, which reflected the echo away from the radar. This system of flat, triangular panels became known as "faceting," because it resembled the shape of a diamond.
Schroeder asked Overholser to develop a computer program that could predict the RCS of a faceted aircraft shape. It took only five weeks for the Echo I program to be completed. Now, with the faceting concept and the Echo I program, it would be possible to predict the RCS of an aircraft. Possible designs could be tested and refined in the computer. The way was clear to build a truly invisible aircraft.[349]
The initial design was dubbed the "Hopeless Diamond." When Overholser presented a sketch of the design to Ben Rich on May 5, 1975, Rich did not quite grasp what had been achieved. Rich kept asking how big the radar return of a full-size aircraft would be — as large as a T-38, a Piper Cub, a condor, an eagle, an owl? Overholser gave him the unbelievable answer.
"Ben, try as big as an eagle's eyeball."
The Hopeless Diamond met with a frosty reception by Kelly Johnson, who was still working as a consultant to the Skunk Works. Having built some of the most graceful planes ever to take to the skies, he was not impressed with this alien design. Johnson's opinion was shared by many of the Skunk Work's senior engineers and aerodynamicists. They preferred a disk-shaped design — a real flying saucer.
A disk had the ultimate in low radar cross section. The convexed surface of the disk would scatter the radar signals away from the source. The problem with a disk-shaped aircraft was control. The WS-606 project of the mid-1950s was to have relied on a large spinning fan to provide gyroscopic stability, while directional control was to be provided by thrusters on the rim of the disk, fed by a complex network of ducts. A disk also has poor aerodynamic qualities, such as high subsonic drag. How to make a flying saucer fly was the problem, and, as Rich later noted, "The Martians wouldn't tell us."
Johnson thought the radar return from the Hopeless Diamond would be larger than that of the D-21. A ten-foot mock-up of the Hopeless Diamond was built. On September 14, 1975, it was tested against the original mock-up of the D-21. The Hopeless Diamond had a radar return one-one thousandth that of the D-21. This was exactly that predicted by the Echo I program.[350]
By October 1975, the DARPA competition had been reduced to the Lockheed and Northrop designs. The Northrop XST was a pure delta wing with a faceted fuselage. The rear of the wing was swept forward, giving it the appearance of a broad arrowhead. The single, large intake was located above the cockpit. To mask the inlet from radar, a fine mesh screen was used. Two tilted fins shielded the engine exhaust.[351]
The Lockheed design, in contrast, had sharply swept-back wings—72.5 degrees. The rear of the fuselage came to a point; with the swept-back wings, this gave it a W shape. The two intakes were placed on the sides of the aircraft and were covered with grills. This allowed a higher speed than the screen on the Northrop XST, which was not usable above Mach 0.65.
Twin inward-canted fins shielded the exhaust. These were slotlike and were called "platypus" nozzles.
The XST design philosophy was to have the lowest possible radar return from the front and bottom of the aircraft. As the plane would fly at high altitude, the top was not considered as important.[352]
In December 1975, Lockheed and Northrop built one-third-scale models of their XST designs. These were shipped to the Gray Butte Microwave Measurement Range in New Mexico and mounted on poles for radar signature testing. A second series of tests was run in January 1976 after minor modifications had been made. This was followed by a full-scale RCS model, which was tested at the air force measurement range at White Sands, New Mexico.[353]
346
Bill Sweetman and James Goodall,
350
Ben R. Rich and Leo Janos,
351
"Northrop's 1976 Stealth Fighter Proposal Featured Faceted Body with Overhead Inlet,"
352
"Declassified Photos Show 'Have Blue' F-117 Predecessor/Mvzafr'o/z
353
Jay Miller,