On December 1, 1977, a truly strange bird took flight for the first time in the skies over a desolate corner of Nevada. Looking more like a giant faceted gemstone than something designed to lift-off, the aircraft (nicknamed the "Hopeless Diamond") had been flown out to Groom Lake in parts aboard a Lockheed C-5 Galaxy cargo plane.

While much of the Hopeless Diamond was a conglomeration of spare parts from other existing aircraft, it was the first of a new breed—the progenitor of Stealth. Hopeless Diamond was the first of two technology demonstrators built for a program called "Have Blue," an initiative program spawned from a Defense Advanced Research Projects Agency effort to create an aircraft that could evade the Soviet Union's increasingly sophisticated integrated air defense systems.

Forty years have passed since the Have Blue project's two demonstrator aircraft—built on a relative shoestring budget by Lockheed's Skunk Works—flew over the Nevada desert and ushered in a new era. Over time, the engineering, physics, and mathematics that created the Have Blue prototypes would be refined to create the F-117 Nighthawk stealth fighter and serve as the basis for the designs of the F-22 Raptor and F-35 Lightning II.

This miltech evolution began because Lockheed was willing to internally fund an effort to win a program from which it had been essentially excluded by DARPA. Using its engineering talent, some sophisticated mathematics, and the best computing technology of the day, Lockheed's Skunk Works rapidly created a prototype on the cheap. That prototype demonstrated what Lockheed Martin Skunk Works Senior Fellow Edward Burnett described to Ars as "our one miracle"—an aircraft shape that had a radar cross section smaller than a bird's.

The invisible rabbit

The story of Have Blue begins with a DARPA effort called Project Harvey—named for the invisible six-foot-three-and-a-half-inch rabbit from the play and film of the same name. The future of integrated air defense systems had already proved effective in the last years of the Vietnam War. It combined long-range radar that could detect high-flying attack aircraft from hundreds of miles away, electronic warfare sensors that could detect the ground-following radar of low-flying aircraft passively, and radar-guided surface-to-air missiles and anti-aircraft guns. And a Defense Science Board study in 1974 concluded through war-gaming out an air war with the Soviet Union in a conventional invasion scenario—specifically, the Fulda Gap scenario at the center of most Cold War military strategy at the time—that the US had to develop some technology to counter those defenses.

So in 1975, DARPA kicked off Project Harvey. The challenge would have seemed like an ideal fit for Lockheed's Skunk Works, given that the organization had been producing "low observable" aircraft for the CIA and Air Force for years. The previous U-2 surveillance plane wasn't technically a "stealth" aircraft, but it was coated in radar absorbent material. The same was true of the A-12 "Oxcart"/SR-71 "Blackbird" and the D-21 supersonic reconnaissance drone, which were intentionally designed to have a reduced radar cross section and painted with radar absorbing "iron ball" paint. Despite its size, the design of the SR-71 reduced its radar cross section to that of a Piper Cub, making it difficult for long-range radars to detect (at least until it was too late for someone to shoot at it).

US Air Force

USAF / Judson Brohmer

Lockheed Martin

But this was a stealth fighter project, and Lockheed had not built a fighter jet for over a decade. While Lockheed had experience with low radar cross-section aircraft, its work was so classified that the DARPA project team didn't know about it. As such, DARPA didn’t initially invite Lockheed to the dance. General Dynamics, Fairchild, Grumman, McDonnell Douglas and Northrop were instead asked—but only McDonnell Douglas and Northrop RSVP'd for $100,000 each to craft initial entries.

Ironically, at about the same time, Denys Overholser—a Skunk Works mathematician and radar expert—discovered equations in a nine-year old research paper from Russian scientist Pyotr Ufimtsev. Recently translated by the Air Force's Foreign Technology Division, the paper reworked some of Maxwell's Equations to predict the radar reflectivity of a geometric shape. In his memoir, then-Skunk Works chief Ben Rich called the equations the "Rosetta Stone breakthrough for stealth technology."

The equations were eventually used as the basis for a computer program called Echo 1, which would allow engineers to break down the design of an aircraft into a series of triangles to calculate their radar cross section for any particular angle of attack. From there, this allowed engineers to optimize the shape of an aircraft for the smallest possible radar return.

Rich, who was fighting to keep the Skunk Works afloat during a turbulent period in Lockheed's business history, was already trying to convince DARPA to let his team join the competition. "Ben went around and made sure that the people who were in control of Project Harvey were actually briefed in on some of the things that had been done before," Burnett said. "That really helped to get DARPA to say, 'We'll let you compete on your own dime.'"

That decision ended up being in Lockheed's favor. According to Rich, DARPA actually offered to let the Skunk Works work on Harvey for a symbolic one dollar payment. Lockheed refused it—and as a result, all the work Lockheed did would remain proprietary to the company. (Accordingly, in 1993, Lockheed was ultimately granted a patent for the Have Blue concept.)

Overholser had already suggested a "faceted" design to reduce radar signature for the initial design submission for the Harvey project, but Echo 1 showed that there were issues with the first attempt because of diffraction. Using the software, the design team was able to sort through the 20 design candidates quickly to find the one with the most optimized radar cross section.

The diamond-like look of the design was largely dictated by the limits of the computing hardware of the day. "Some of the mathematics were being done on slide rules still, and a PDP-8 and other like computers, so yeah, computer limitations really kept the shaping down," Burnett told Ars. "We were just really beginning to understand the mathematics of the physics that govern this technology."