Icons of Aviation History: F-117 Nighthawk

The first of the “stealth” aircraft to be deployed, the F-117 Nighthawk looks more like a spaceship than a military airplane.

F-117 Nighthawk on display at the US Air Force Museum

In 1973, war broke out between Israel and her Arab neighbors. The United States watched the Yom Kippur War intently: the Israelis were flying American-made fighters against Soviet MiGs and SAM anti-aircraft missiles, and the Pentagon wanted to see how the planes stacked up.

They got an unpleasant shock. Israeli losses totaled over 100 jets in two weeks, mostly to radar-guided missiles. Just a few years earlier, during the Vietnam War, the US itself had suffered heavy losses to Soviet-made radar-guided SAMs. The ability of the US Air Force to penetrate Soviet airspace in the event of a nuclear war was now in serious question.

One way to reduce American vulnerability to Russia’s air defenses would be to somehow make the attacking jets invisible to radar. The key lay in reducing the echo that an aircraft produced (known as the “radar cross-section”) to make it more difficult for an enemy radar to pick out the target, but nobody had any good idea how to go about doing that. And so, although nothing like that had been attempted before, the Pentagon’s Defense Advanced Research Projects Agency (DARPA) contacted several aerospace contractors and began brainstorming.

Ironically, the answer came from the USSR. In 1962, a Soviet mathematician named Pyotr Ufimtsev had published a technical article about acoustics which almost casually set out a mathematical formula that could be used to calculate the strength of the return echo from any flat object. Its key conclusion was that most of the strength of a flat surface’s echo was dependent upon its edge characteristics, not upon its overall size: a straight edge had a large acoustic return, while a jagged edge had much less. The USSR had understood the significance of this paper, but they also realized that any such structure would be unstable in flight, and dismissed the possibility as impractical. But engineers at Lockheed’s Skunk Works realized that, with their more advanced electronics than the Soviets, the US now had the technical ability to build computer-controlled “fly-by-wire” systems that could correct for this instability, and this formula was exactly what they needed to design a multi-faceted surface that would minimize the echo of a radar beam from all directions.

By plugging this formula into the relatively crude computers of the day, Lockheed determined that the best overall shape for minimizing a radar echo was a flattened pyramid in the shape of a diamond. (The designers had to work with flat facets because even the biggest super-computers at that time were not powerful enough to do the complicated calculations needed for curved surfaces.) Because each facet edge presented an inclined surface to any incoming radar beam, most of the radar energy would be reflected off it and travel away instead of being returned to the radar dish. It would make such a surface virtually undetectable by radar.

The next challenge was aerodynamic. A three-dimensional flat diamond may have been the best shape for reflecting radar waves, but it was a terrible shape for an airplane, and it wasn’t even clear if such an airframe could actually fly. Some wags dubbed it “The Hopeless Diamond”. So the engineers began a long process of adding or subtracting facets at various points, hoping to transform a diamond-shaped base into a flyable delta-winged airplane. Each addition or subtraction, in return, changed the number of geometrical facets—each of which then had to be rigorously calculated for the correct shape and angle to insure the minimum radar return. By 1976, the calculations had produced what seemed to be a workable aerodynamic design. It was inelegant and chunky looking, but, at least in theory, it would fly.

The next step was to build a flying prototype model. This project was buried in secrecy under the code name HAVE BLUE. In the end, two prototypes were built: one was used as a test plane to examine the flying characteristics, and the other would be flown against a variety of captured Soviet radar equipment to evaluate its “stealth” capabilities. The two HAVE BLUE test beds were ready in  1977.

Immediately, a major problem became apparent. Seen head-on, the primary source of radar reflection in a conventional airplane were the engine intakes, which were, in effect, two large megaphones that took in sound waves and bounced them back out. To counter this, the intakes on the HAVE BLUE planes were fitted with a set of fiberglass baffles and grates which would break up the radar waves as they entered, scattering them around so they would be dissipated inside the engine and not reflected back out. At the back of the engine, meanwhile, another system of baffles helped to disperse and cool the air leaving the flat slit-like exhaust ports, which reduced the plane’s thermal signature and made it harder to detect with infrared sensors (such as those used in heat-seeking missiles). Finally, Lockheed developed a highly-classified rubbery coating called Radar Absorbent Material (RAM) which acted like the walls of a soundproof room, absorbing acoustic energy and reducing the amount that was reflected away. The RAM was produced in sheets that could be cut to precise shapes and fitted together to cover the entire airplane. (The maintenance personnel who performed this task on the finished stealth fighter were jokingly known as “Martians”.) The entire plane was painted black for low visibility at night. And finally, since the pilot himself would reflect a detectable amount of radar energy, the cockpit windows were coated with a transparent material that deflected sound waves.

By 1977, the two HAVE BLUE testers were undergoing test flights at Area 51 in Nevada, under strict secrecy. Flown only at night, it has been speculated that test flights of the stealth aircraft may have been the source of many “UFO sightings.”

Although it flew, the clunky faceted design was inherently unstable, and the only way it could be controlled at all was with a system in which the plane’s onboard computer was constantly making automatic flight corrections, many times per second, to keep the jet stable. In spite of this, both of the HAVE BLUE prototypes crashed during testing.

But enough had been learned to go ahead with production. The stealth plane had never been intended as a combat dogfighter and was not designed for maneuverability or speed. Instead, it was intended to fly between enemy radar sensors, undetected and invisible, allowing it to approach high-value targets and hit them with its precision-guided bombs. It carried no defensive weapons and had no radar capability of its own—its security depended solely upon not being detected. Navigating by GPS (which was at that time used exclusively by the military), the aircraft would fly on autopilot through a series of waypoints that took it to the target, at which point the pilot would control the aiming and release of the bombs. The plane could carry laser-guided weapons or anti-radiation missiles which home in on enemy radar beams. The final production version was about 33% bigger than the HAVE BLUE test versions.

In 1979, Lockheed received the go-ahead to produce a production prototype, code-named SENIOR TREND. They were given just 18 months. To save time, many of the standard avionics systems would  be lifted wholesale from other  planes like the F-16, F-18, and even the venerable B-52. After successful testing in 1981, an order was placed for 59 frontline jets. The first combat-ready planes were delivered in 1982.

Although it was designed and built as a light attack bomber, the new aircraft received the designation F-117, officially classifying it as a fighter. The new “stealth fighter” was at first known by a variety of nicknames, including “Cockroach” (because it avoided daylight),  “Wobblin Goblin” (because it was notoriously unstable at low landing speeds) and “Frisbee” (because of its uncontrollable fall when it loses power). But the name that eventually stuck was “Nighthawk”.

The first combat test of the stealth fighter came in December 1989, after the collapse of the Soviet Union and the end of the Cold War, when the United States invaded Panama to capture strongman Manuel Noriega. In the opening minutes of the attack, two F-117s delivered precision-guided bombs to strike a field next to the Panamanian military barracks, to stun and disorient the troops inside and allow an assault by US special forces. Although the strike was carried out successfully, many of those who had worked on the project were disappointed to see the once-super-secret plane, developed at great expense to penetrate a heavily-defended superpower, being used merely to bomb an empty field in a Third World country that had virtually no air defense system.

Then in August 1990, Iraqi strongman Saddam Hussein invaded Kuwait. The action was condemned by the UN, and the US led a coalition of military forces to drive the Iraqis out. In “Operation Desert Storm”, a series of airstrikes decimated Iraq’s air force and destroyed most of its air defense capability. Key to this was the stealthy F-117, which was the only strike airplane capable of penetrating the radar and hitting important targets in Baghdad. Stealth fighters flew almost 1300 missions and dropped some 2000 bombs on target, with no losses.

In 1999, the F-117 was called into combat again, this time in Kosovo. Here, Serbian forces were carrying on a genocidal policy of “ethnic cleansing” against Bosnians, and NATO forces were deployed to end the fighting. Nighthawks were sent on strike missions, and on March 27, the Pentagon was shocked to learn that Serbian forces had shot one of its F-117s down. The next day, Serbian aircraft showered NATO troops with propaganda leaflets: “We’re sorry”, it read, “we didn’t know it was supposed to be invisible.”

Worried that the Serbians had found a way to detect the plane, the US launched an intensive and highly classified investigation, which has still not been released publicly. From the bullet holes visible in films of the wreckage, some aviation experts have concluded that the Serbs simply got lucky: they had merely fired a lot of guns into the air, and one of them made a one-in-a-thousand hit. By some reports, though, the Serbians had specially modified one of their SA-3 missiles to work at an unusually long radar wavelength, which was able to “paint” the F-117 for short periods of time—especially when it briefly opened its bomb-bay doors. After the shoot-down, the US seemed to downplay the role of its once-premiere attack aircraft: a trend that was reinforced after an F-117 crashed during an air show in Baltimore.

When the  US launched  its invasion  of Iraq in the  wake of the 9-11 terrorist attacks, the F-117s once again flew penetration missions into Baghdad, but in 2006 the remaining aircraft began to enter retirement, intended to be replaced by the new F-35 Lightning II ground-attack fighter. By 2009 the Nighthawks were all out of service, although a number of them are still maintained in such a way that they can be returned to readiness if necessary.

The US Air Force Museum in Dayton OH has one of the early production F-117s on display. Two more Nighthawks can be seen at Nellis and Holloman Air Force Bases. The wreckage of the shot-down F-117 is on view at a museum in Serbia.