The Lockheed F-117 Nighthawk Stealth Fighter

The United States currently possesses a combat asset with capabilities not matched by known weapons of any other country: the F-117 Nighthawk Stealth Fighter. This aircraft can perform precision strikes on heavily-protected targets with little chance of being detected. In many cases, the defender's first alert of an attack is bombs going off. Along with this capability comes an interesting design and an exotic history. This document provides a history and description of the F-117 Stealth fighter.

The F-117 was a "black", ultra-secret program for much of its life, only emerging into public view in the late 1980s. The secret development of the F-117, the rumors and myths that surrounded it as it went into operation, and its emergence from the shadows make for an interesting story.

The Origins Of Stealth

The development of radar during World War II immediately led to the development of technologies to defeat it. Early countermeasures were simple and crude, including "chaff", strips of foil designed to provide a false radar echo, and radio-frequency (RF) jamming, in which intense radio signals were broadcast to drown out the radio signals being used by radar transmitters.

Such techniques became more sophisticated after the war, but many wondered if there might be a more elegant approach: make aircraft that were invisible to radar, or in more formal terms, to minimize the "radar signature" or "radar cross section (RCS)" of the aircraft.

Radar reflections tend to occur where aircraft structural elements meet at a right angle, such as the junction of the wing and fuselage. An incoming radar signal can bounce off the fuselage, strike the wing, and reflect back to the radar receiver. Other radar reflectors include sharp points on the aircraft; antennas; external stores such as bombs, fuel tanks or electronic pods; and the fan blades of turbojet engines exposed through the air intakes, causing radar "glint" as they spin around, similar to the reflections of light of a rotating disco ball.

Designing an aircraft minimize such reflections is complicated enough. Worse, the reflectivity of these different elements tends to change, drastically, as radar frequency increases or the viewing angle changes. For decades after the war, there was no realistic way to calculate all these effects.

Another approach to reducing RCS was to cover the aircraft with "radar-absorbing material (RAM)" that absorbed RF energy, preventing a signal from being reflected back to the radar receiver. In fact, in late 1958 a Lockheed T-33 trainer was covered with flexible RAM material and tested under a program named "Passport Visa". The results were disappointing. While the modified T-33 did have a lowered RCS, its flight handling was very poor, and the test program was discontinued.

The Lockheed SR-71 Blackbird reconnaissance aircraft, developed in the early 1960s, did incorporate "stealth" features that gave it a lower RCS, but it also carried "electronic countermeasures (ECM)" and flew at very high altitude and great speed, reducing its chances of detection accordingly. High-altitude Ryan Firebee reconnaissance drones developed in the early 1960s had stealth features, including a screen over the air intake and blankets of RAM material on the sides of the fuselage.

However, strike aircraft didn't have the luxury of avoiding air defenses, and during the Vietnam war the US Air Force (USAF) found that improved surface-to-air missiles (SAMs), along with better radars that punched through countermeasures, made air defenses tougher to crack. Formations of fighter-bombers had to be accompanied by of ECM and "defense-suppression" attack aircraft to keep losses down. The growing effectiveness of SAMs was emphasized by the 1973 Yom Kippur War in the Mideast, when the Israelis suffered large air losses to the Soviet-made mobile SA-6 missile launchers operated by their Arab adversaries. It became clear to a faction in US military aviation community that something had to be done to counteract improved air defenses.

In 1974, the US Defense Advanced Research Projects Agency (DARPA) initiated a program known as PROJECT HARVEY, after a well-known comedy about an invisible giant rabbit, that requested designs of an "experimental survivable testbed (XST)" aircraft with a low RCS. Lockheed was not among the companies contacted by DARPA with this request, but in 1975 Ben Rich, an engineer who had worked on the secret Lockheed U-2 and SR-71 reconnaissance aircraft, got wind of the project and lobbied the government successfully to have Lockheed included.

Rich had the services of two Lockheed employees, mathematician Bill Schroeder and computer scientist Denys Overholser, to work on the XST program. Schroeder realized that it would be much easier to compute RCS if the shape of an aircraft could be reduced to a set of flat surfaces, or "facets". Schroeder approached Overholser with the idea, and within five weeks Overholser had written a computer program named "Echo I" that could determine the RCS of a "faceted" aircraft. Armed with Echo I, Schroeder came up with an initial XST design that he called the "Hopeless Diamond", and handed Ben Rich a sketch of it in May 1975.

In response, Rich asked how big the RCS of a practical version of the Hopeless Diamond would be: As big as a T-33? A Piper Cub? A condor? An eagle? An owl? Schroeder shot back: "Ben, try as big as an eagle's eyeball."

Not everyone was impressed by the Hopeless Diamond. Lockheed's prestigious Kelly Johnson disliked it, likely on the traditional aircraft designer's belief that "no aircraft that looked that ugly could possibly be any good." Johnson favored a flying-saucer-like design, but nobody could figure out how to make a saucer shape fly very well, and Rich commented later: "The Martians wouldn't tell us."

To strengthen his case, Rich had a three meter (ten foot) sized mockup of the Hopeless Diamond built and tested against radar in September 1975. The RCS proved to be as small as predicted.

By October 1975, the DARPA XST competition had been reduced to two finalists: one from Northrop, and a refined version of the Lockheed Hopeless Diamond. The Northrop entry was a delta with a faceted fuselage, with the jet engine mounted on the back and the intake above the cockpit. A fine mesh screen sealed the air intake from radar, while a pair of tilted fins hid the exhaust.

The Lockheed design had two wings with a sweep of 72.5 degrees and a pointed rear fuselage, giving the rear of the aircraft a "W" outline. Twin intakes were placed along the cockpit and covered with grilles, while the exhausts were arranged as long slots along the rear fuselage. Such "platypus nozzles" reduced the "infrared signature" of the exhaust, which was also concealed by twin inward-canted fins.

Since RCS is a function of viewing angle, both designs were optimized to have the lowest RCS from the bottom and front, the most probable viewing angle of adversary ground defenses. Northrop and Lockheed built one-third scale models in December 1975. The models were put through preliminary RCS tests, and full-scale models were tested the next year.

The Lockheed design had a tenth of the RCS of the Northrop design. It was so invisible to radar that a radar operator performing tests on the model at White Sands, New Mexico, thought it had fallen off the pole. Bird droppings increased the RCS by 50 percent, and so the model had to be regularly cleaned. Lockheed won the competition in April 1976. The Northrop team was heartbroken, even though their engineers admitted Lockheed had the better design. They returned to their calculations and would eventually catch up with Lockheed at the stealth game, with the B-2 Spirit bomber.

The Lockheed contract specified construction of a pair of XST aircraft for flight and RCS tests, with $32.6 million USD provided by DARPA and the USAF. Lockheed had to furnish $10.4 million USD more out of its own pocket, which was a big risk at the time, since Lockheed was in difficult financial circumstances.

The Have Blue Demonstrators

Interestingly, up to this time PROJECT HARVEY was not a secret program, and in fact had been mentioned in the aerospace press. When the Carter Administration took office in early 1977, Bill Perry, an influential defense undersecretary for research and engineering and later defense secretary in the Clinton Administration, learned of how dramatic the results of the model tests had been. Perry immediately saw to it that program became secret. Responsibility was transferred from the mostly-civilian DARPA to the USAF Special Projects Office, and funding was increased. Orders went out stating that the word "stealth" was not be used in unclassified documents, and the program was assigned a meaningless two-word codename: HAVE BLUE.

The first of the two HAVE BLUE demonstrators was intended for aerodynamic tests. Faceting had an interesting consequence: unlike almost every other aircraft ever built, HAVE BLUE's wings did not have a curved cross-section, being composed instead of flat planes. Its aerodynamics were suspicious, and in fact the machine was so unstable that it had to be controlled by a computerized fly-by-wire system. The first prototype was needed to ensure that the design could fly at all. The second would be a more finished product that would be used for stealth demonstrations.

The HAVE BLUE prototypes were 17.25 meters (38 feet) long, with a wingspan of 10.2 meters (22.5 feet) and a weight of 5.67 tonnes (12,500 pounds). Each was powered by a pair of General Electric J85-GE-4A engines with 13.1 kN (1,340 kgp / 2,950 lbf) each, obtained from Navy T-2B Buckeye trainers. Other scavenged equipment included the fly-by-wire system, modified from the F-16A fighter; and ejection seat, landing gear, and cockpit instrumentation taken from an F-5 fighter.

The HAVE BLUE prototypes had no provisions for weapons or inflight refueling. They were strictly demonstrators, with a flight endurance of only an hour. The two aircraft were assembled in a secure section of the Lockheed Burbank, California, plant. They were numbered "1001" and "1002".

HAVE BLUE 1001 was completed in November 1977, and put through initial static tests at the Burbank plant. Its wings were removed and the aircraft was sent by a C-5 Galaxy transport to the secret test facility at Groom Lake, Nevada, where it was reassembled.

HAVE BLUE 1001 was painted in a broken camouflage scheme of gray, black, and tan to confuse anyone who might be able to steal a glimpse of it. Extreme security measures accompanied the first flight of the aircraft on 1 December 1977. The ugly-looking little aircraft handled surprisingly well, and most of the deficiencies in handling were corrected by tweaking the fly-by-wire system software. Unfortunately, it was not so easy to correct the fact that the aircraft landed like a flatiron, and on its 37th flight, on 4 May 1978, HAVE BLUE 1001 hit the runway a little too hard and had to lift off for a another pass instead of going into a skid.

The hard landing had bent the right main gear strut. The landing gear had been retracted after the "touch and go", and now the right main gear leg wouldn't go back down again. Despite many attempts, there was no way to get the gear down. Critically low on fuel, Lockheed test pilot Bill Park decided to eject and let the aircraft crash into the desert. Park took a serious back injury in the attempt, ending his career as a test pilot. The smashed-up HAVE BLUE 1001 prototype was bulldozed under the desert, and it is said that now nobody knows exactly where it is. News of the crash leaked to the press, and some vague comments were made about the possible existence of "stealth" aircraft.

HAVE BLUE 1002 was delivered in July 1978, making its first flight on 20 July. Unlike the first prototype, the second was painted light gray and was actually covered with RAM. Tests against friendly and adversary radar systems demonstrated phenomenal results: While HAVE BLUE was by no means completely invisible to radar, its RCS was far lower than that of a conventional aircraft. Most Soviet SAM radar systems could not detect the aircraft until it was right on top of them.

Achieving such results wasn't trivial. Before takeoff, doors and access panels were sealed off with metallic tape, and once the pilot was in the cockpit the edges of the canopy were sealed with RAM applied as paint. On one test, the plane proved to be far more visible to radar than it had been before. A close inspection revealed three screws that hadn't been tightened and were sticking up a millimeter or two.

HAVE BLUE 1002 was lost on its 52nd flight, on 11 July 1979, when a hydraulic leak set the aircraft on fire. The pilot, Lieutenant Colonel Ken Dyson, ejected safely, but the prototype was destroyed. Like its predecessor, it was buried under the desert.

The loss did not delay the stealth effort since the test program was almost over, with only one more flight scheduled. The secret of HAVE BLUE would be kept for over a decade. Although the code name would be published in AVIATION WEEK magazine in 1981 and the program would be acknowledged by the government in 1988, pictures of HAVE BLUE 1001 were only released in 1991. However, by the time the second HAVE BLUE prototype was lost, speculations on stealth were beginning to appear in the press, and in 1980 fuzzy reports on the technology appeared in publications such as AVIATION WEEK and THE WASHINGTON POST.

The reports were generally inaccurate, but they presented the Carter Administration with the choice of ignoring the reports; denying or obscuring them; or confirming them. Partly influenced by the damage to American prestige by the Iranian hostage crisis and the botched attempt to rescue the hostages, on 22 August 1980, the Carter Administration publicly acknowledged the existence of the stealth program, though no details were released. The announcement led to wild speculations in the press, and fingerpointing and accusations of "grandstanding" in the political arena. If the Carter Administration had hoped to enhance their credibility with the announcement, it didn't work, and when Ronald Reagan was elected and took office early the next year, the stealth program went totally black and invisible.

Senior Trend - almost an F-117

In the meantime, stealth was moving towards operational status. The Air Force was impressed by the flight tests of HAVE BLUE 1001, and in mid-1978 Lockheed suggested two designs for an actual weapons system: a medium bomber with four engines and a two-man crew, and a single-seat twin-engine strike fighter. The Air Force preferred the strike fighter concept, and issued a design contract to Lockheed for such an aircraft in November 1978. The aircraft was given the code name SENIOR TREND.

The SENIOR TREND aircraft was a direct outgrowth of the HAVE BLUE prototypes, with many changes to turn the design into a practical combat aircraft. HAVE BLUE's wings had a sharp sweep of 72.5 degrees, which gave it the flatiron flight characteristics that had led to the loss of the first prototype. As a result, the sweep of SENIOR TREND's wings was reduced to 67.5 degrees, and the wings were extended as far back as possible.

Another visible change was in the positioning of the tailfins. HAVE BLUE's tailfins had tilted inward in order to conceal the exhaust, but this had proved self-defeating, since the tailfins reflected heat back to the ground. SENIOR TREND's tailfins were tilted outward in a vee, or "butterfly", configuration. This improved the aircraft's controllability and did not compromise stealth.

SENIOR TREND was fitted with the systems required by an operational aircraft, including proper avionics and cockpit instrumentation; formation and anticollision lights; brake parachute and runway arresting hook; additional fuel tanks; an inflight refueling receptacle; and a long list of other items. To reduce the maintenance required to keep the aircraft stealthy, service access to aircraft subsystems was established through wheel wells and weapons bays, while all the avionics were consolidated in a single bay.

SENIOR TREND's teeth consisted of a pair of 900 kilogram (2,000-pound) laser guided bombs (LGBs), each in its own weapons bay. The bombs were loaded on a trapeze system that extended them out of the weapons bay for release. Matching a stealth strike aircraft to a precision munition like the LGB implied a new approach to bombing: instead of sending a formation of bombers to plaster a target, a single aircraft would attack a single target with a precision weapon and destroy the target in a single pass: "first shot first kill".

Since active sensors like radar advertised an aircraft's presence, the SENIOR TREND aircraft navigated with an internal inertial navigation system (INS). Long-range targeting was through a forward-looking infrared (FLIR) camera mounted in a turret in the nose, set in a well just in front of the cockpit and covered with a fine mesh. The mesh was heated to keep it from icing up in cold weather. A second downward-looking infrared (DLIR) camera, mated to a laser target designator, was mounted under the nose, also in a well covered with a mesh. The DLIR camera was used for close-range targeting. The FLIR and DLIR were "ganged" so they tracked the same scene, displaying their images in a head-up display (HUD) that the pilot could in principle switch from the FLIR to the DLIR as he approached the target.

SENIOR TREND was about twice as big as HAVE BLUE. It was 20 meters (65.9 feet) long, with a wingspan of 13.2 meters (43.25 feet), and an empty weight of 13.6 tonnes (30,000 pounds). The canopy was heavily framed and had poor visibility. The mid-air refueling receptacle was positioned behind the cockpit. The door over the receptacle had serrated edges to reduce radar reflection, as did the landing gear doors and canopy leading edge.

SENIOR TREND was mostly built out of aluminum, though titanium was used around the engines. It was powered by twin General Electric F404-GE-F1D2 turbofans, like those of the F/A-18 Hornet fighter but without afterburners, providing 48.1 kN (4,900 kgp / 10,800 lbf) thrust each. The intakes were covered by grilles, which were electrically heated to prevent them from icing up. The pilot could also activate lights on either side of the cockpit to allow him to check the intake grilles for icing.

Cockpit equipment, including HUD, multifunction & sensor displays, and hands-on-throttle-and-stick (HOTAS) controls, also came from the F/A-18. Sensor displays were borrowed from the OV-10D Bronco observation aircraft and the P-3C Orion ocean-patrol aircraft; the navigation system was obtained from the B-52; with other systems taken from the SR-71, C-130, L-1011, and F-104. The pilot sat in a standard ACES ejection seat, with the heavy canopy blown off by explosive rams before ejection.

The end result of all this work was an ugly aircraft. Among the nicknames assigned to it, "Cockroach" was one of the most descriptive. It was angular and flattened, resembling a chipped obsidian arrowhead. Four probes on the nose provided airspeed and other data, enhancing the aircraft's "buglike" appearance.

Like HAVE BLUE, SENIOR TREND was aerodynamically unstable. It would be referred to on occasion as the "Wobblin' Goblin", and as Lockheed test pilot Hal Farley later put it: "The only thing it doesn't do is tip back on its tail when parked." As with HAVE BLUE, it used a fly-by-wire system to maintain stability in flight. The fly-by-wire system was derived from that used on the F-16A, and had quadruple redundancy to ensure operational reliability.

A wooden mock-up of SENIOR TREND was completed in late 1979. In December of that year, Lockheed was awarded a contract to build five full scale development (FSD) test aircraft and fifteen production aircraft. First flight was set for July 1980. Development proved anything but easy, however, with technical problems compounded by the death of Ben Rich's wife from a heart attack. After much trouble, the first FSD aircraft, designated number 780, was secretly rolled out of the Burbank plant in the spring of 1981 and shipped to Groom Lake by a Lockheed C-5 Galaxy transport. The designation numbers, by the way, were assigned almost at random as a security measure.

Number 780 took to the air on 18 June 1981, and demonstrated unfriendly flight characteristics that led to changes in the fly-by-wire system and a 50% scale-up of the tailfins. In the meantime, the other four FSD systems were shipped out to Groom Lake, with the last arriving in early 1982. The first two of the five, numbers 780 and 781, were aerodynamic test aircraft, while the last three, numbers 782, 783, and 784, were fully-fitted production test aircraft. They were named the "Baja Scorpions".

Number 780 had originally been painted in a camouflage scheme somewhat like that used with HAVE BLUE 1001, while the other four aircraft were originally painted gray. Some were said to have had the Lockheed Skunk Works logo on the tail, while number 784 had a picture of "Pete's Dragon", a cartoon dragon from the Disney film of the same name. This was in honor of Colonel Pete Winter, the commanding officer at Groom Lake, with the inside joke that in the movie the hero Pete was the only one who could see the dragon. Ultimately all the FSD aircraft were painted black and given inconspicuous USAF markings.

Further flight tests of the FSD aircraft demonstrated more aerodynamic problems, as well as de-icing troubles. The dual FLIR-DLIR targeting system proved a particular nuisance, since it proved tricky to align the two turrets to boresight together, and the hand-off between the two imagers on the HUD suffered from a number of glitches. Lockheed engineers devised a new scheme for aligning the two turrets by rolling the aircraft up a ground ramp so the two turrets could be focused on the same target.

Other technical glitches were hunted down until the aircraft worked reliably. RCS tests demonstrated the aircraft's invisibility to radar. It had about the same RCS as a seagull. Despite the basic aerodynamic nastiness of the design, it flew well, though as it was a delta-wing design it had a long take-off run and landed "hot". The drag chute was used as a standard practice to reduce landing roll.

While the test pilots and engineers were working the bugs out of the FSD aircraft, the USAF was preparing an operational unit to make use of the new weapon. The 4450th Tactical Group was established in October 1979, but training activities did not start until June 1981, about the time the first FSD aircraft took to the sky. The 4450th initially consisted of a few dozen pilots and ground crew. At first, training was conducted on cockpit mock-ups. The pilots were presently given Vought A-7D "SLUF" strike aircraft to provide a cover story, allow them to keep their flight skills from rusting, and for use as chase aircraft. The 4450th, officially, was a technical evaluation unit.

In the meantime, the Air Force was building a base for the black aircraft, the Tonopah Test Range (TTR), in Nevada north of Groom Lake. Security was tight from the start. Living facilities were at first very austere, but later better accommodations were provided. Rooms had black-out curtains to let pilots and crew sleep during the day, since the SENIOR TREND aircraft were to operate at night to help keep the secret.

The first production aircraft, number 785, was delivered to Groom Lake in the spring of 1982. It crashed and was destroyed on take-off on 20 April, badly injuring the pilot, Bob Ridenauer of Lockheed, who never flew again. The accident was traced to reversed wiring in the flight control system. Number 786 was delivered to Groom Lake in June and used for flight testing.

SENIOR TREND 787 was the first of the black aircraft to be flown by the 4450th, making its first operational flight on 15 October 1982. By Christmas, several more SENIOR TRENDs had been delivered to TTR, and the F-117A Nighthawk, as the aircraft had been formally named, was in business.

Author: Greg Goebel