How the Skunk Works Fielded Stealth

Nov. 1, 1992

In recent months, a steady flow of new information about the F-117 has emerged, providing a detailed understanding of just how the first “stealth” fighter was transformed from a novel laboratory concept into the combat star of Desert Storm.

Newly declassified technical data and the recollections of key Lockheed Corp. participants show that what often has been portrayed as a smoothly run, top-secret development program actually encountered its share of problems. They were, however, resolved with unusual ingenuity and skill.

Despite the F-117’s public image as the most modern of warplanes, the radar-evading aircraft had its technical origins late in the administration of President Dwight D. Eisenhower. Hula-Hoops were in vogue when Lockheed’s Kelly Johnson and Ben Rich turned their attention to improving on the U-2 spyplane then flying reconnaissance patrols over the Soviet Union.

“Much to our amazement,” Mr. Rich recently recalled, “the Russians tracked us all the way. They couldn’t do much about it, but they tracked us.” Then the USSR shocked the world in 1960 when it shot down a U-2 and captured its pilot, Francis Gary Powers.

The Central Intelligence Agency asked Lockheed to investigate ways to reduce the radar cross section (RCS) of advanced aircraft. Together with Lincoln Laboratories, Rich and company went to work on the problem. The team achieved early success in its work on the A-12 and, later, the SR-71 Blackbird, reducing the RCS of the planes by two orders of magnitude. Two years later, the D-21 unmanned drone achieved a reduction of another order of magnitude.

The SR-71 was a breakthrough, according to Paul Martin, the program manager at Lockheed Advanced Development Co., known to the world as the “Skunk Works.” It was in the SR-71 program, he said, that Lockheed “first developed aircraft shaping methods, radar absorbing structural edges, radar absorbing coatings, and other design features that greatly reduced the SR-71’s radar signature.”

In 1974, however, when the Defense Advanced Research Projects Agency (DARPA) requested proposals for a stealthy small fighter, Lockheed was not even invited to submit one. At the time, company officials were forbidden to boast of their prowess in low-RCS aircraft because the high-altitude snooping of the U-2 and SR-71 was being conducted for the CIA. The corporation was permitted to bid only after Mr. Johnson obtained a letter from CIA officials authorizing Lockheed to disclose limited details.

The Importance of “Echo 1”

In 1975, DARPA was persuaded to accept an unsolicited Lockheed proposal. The heart of the proposal was a computer program dubbed “Echo 1,” which allowed its aircraft designers to predict a radar return. Skunk Works experts manipulated a set of well-established mathematical formulas for determining how various shapes would reflect electromagnetic radiation.

Lockheed’s early computer modeling, however, was limited to calculations in only two dimensions, meaning that the resulting aircraft would have an ungainly, faceted design rather than a smooth, seamless one.

In fact, Mr. Rich and his Lockheed team christened that first, sharply angled aircraft “the Hopeless Diamond.” The seven-foot-long model was brought to a range in Palmdale, Calif., for early radar testing. Initially, Pentagon officials regarded with deep skepticism Lockheed’s claims of having developed some revolutionary radar-evading properties. “The reaction from the Air Force and DARPA was, ‘That was all theory,’ ” said Alan Brown, the first F-117 program manager.

Mr. Rich recalls demonstrating the new approach for a visiting officer. With radar beams bombarding the test stand, the two men intently watched a display screen. Nothing appeared. “All of a sudden,” recalled Mr. Rich, “a black crow landed on the model. The officer said, ‘I see something.’ ”

That series of demonstrations indicated Lockheed was on the right path, but the company and the Air Force had a long way to go before they would be certain they weren’t chasing a mirage.

One early stumbling block was financing. The Pentagon wanted Lockheed to build two stealth prototypes with $20 million in government cash. Mr. Rich knew he needed another $10 million to pull off the assignment and had to approach Lockheed’s top executives for the money.

His timing could not have been worse. Lockheed was just emerging from its L-1011 commercial aircraft debacle, which had cost the company billions. When Mr. Rich pitched the $10 million request to Lockheed Chairman Roy Anderson and President Lawrence Kitchen, they blanched.

In 1976, however, Mr. Rich and Mr. Kitchen made a joint presentation to Lockheed’s board on the potential of stealth. When they were finished, Mr. Rich had his $10 million check.

From this point forward, stealth development went deep into “the black,” or the classified world. The vehicle was the Have Blue program, which produced stealth technology demonstrators. To enhance security, program management was shifted from DARPA to the Air Force Special Projects Office.

“That’ll Never Fly”

Under Have Blue, Lockheed built two single-engine stealth demonstrators, which began test flights in 1977. Each was about forty percent smaller than the F-117. Each was distinguished by tail surfaces that pointed inward. Even some Lockheed partisans were initially unimpressed with the Have Blue’s unusual appearance.

“Kelly [Johnson] came in, kicked me in the ass, and said ‘That’ll never fly,’ ” Mr. Rich recalled.

The eighteen-month Have Blue test series, however, was a striking success and convinced the Air Force to go ahead with development of a stealth fighter.

On November 16, 1978, Lockheed received the contract for full-scale engineering development of what became the F-117, but the Have Blue flights foreshadowed a number of critical problems that would not be resolved until F-117 testing. Among them: how to mask the aircraft’s exhaust and how to integrate a host of off-the-shelf avionics packages with the new airframe. In addition, the designers had to grapple with 1,001 details that could have derailed the program.

“It was very good that we did Have Blue prototypes first,” said Mr. Brown. “That gave us a real head start.”

F-117 development commenced under a program called “Senior Trend.” By the time the Skunk Works team was done, it had produced fifty-nine copies of the unique charcoal-black fighters for a unit flyaway cost of $42.6 million. The program moved from design go-ahead to first flight in a mere thirty-one months and to initial operational capability in sixty months.

Once the F-117’s existence was confirmed publicly in November 1988, Air Force and Pentagon officers depicted the jet’s procurement as a model program whose success was due largely to the streamlined procurement and oversight of the black world. In the past year, however, a somewhat different picture has emerged.

“The F-117 program faced several fiscal, performance, and schedule challenges,” the Air Force reported not long ago to a Senate subcommittee. Those challenges included “a shortage of aerospace workers, rapidly increasing inflation, growth in material lead time, and delays in security clearances.”

It appears that none of the problems was ever viewed as a show-stopper, but the schedule for first flight and initial operational capability slipped by almost one year after two test aircraft crashed. Over the life of the program, full-scale development costs increased by fifty-three percent, the Air Force told the Senate subcommittee.

The new information makes it clear that fielding the first operational stealth airplane did not stem from one technological solution or breakthrough. Rather, the work of the designers and engineers was all-encompassing, and close attention to seemingly minor details was critical. In the words of Mr. Brown, “We paid lots of attention to details—door openings and hinges, that sort of thing.”

Seven Deadly Signatures

Lockheed officials say that, to meet their goal of building a warplane that was all but impossible to detect at useful ranges, F-117 designers had to address seven types of observable signatures: radar, infrared, visual, contrails, engine smoke, acoustic, and electromagnetic.

At a top speed of 646 miles per hour, the F-117’s survival depends on its being impossible to detect in time for an enemy to take action. While the public understands the plane to be “invisible,” stealth really is a combination of factors that makes the F-117 difficult for an enemy to detect, track, and target.

“You’re not making these things invisible,” said Benjamin Lambeth of RAND Corp. Stealth “reduces substantially the distance at which the plane can be detected. By the time detection occurs, it won’t be tactically useful.”

Relatively prosaic parts of the aircraft posed stiff design challenges. Concerns about the exhaust system had surfaced in the Have Blue flights. Normally, a jet’s red-hot exhaust pours from the tailpipe in a cylindrical boil, leaving a beer-can-shaped hot spot in its wake. To make the F-117 difficult to see with infrared sensors, Lockheed needed a tailpipe that would flatten and cool the exhaust while simultaneously shrouding radar-reflecting portions of the plane.

In July 1979, the Skunk Works turned to a small, Santa Ana, Calif., firm to design the fighter’s distinctive exhaust system. Astech/MCI, one of Lockheed’s 500 or so F-117 suppliers, took only ten months to design a nickel alloy tailpipe of a special honeycomb design that flattened the exhaust. The design also hid the pinwheeling turbine blades of the twin General Electric F404 engines buried inside the airframe.

“We were trying to reduce the infrared signature and the radar signature,” said Astech/MCI program manager Don Cunningham. “This [design] contributes to that at the back end of the aircraft.”

Another sticky problem was development of the four pitot tubes that extend from the aircraft nose to gather air data. The four faceted and heated probes contain multiple openings that gather differential pressure readings. The data are compared and analyzed by on-board computers, providing the F-117’s flight-control computers with all the information they need to fly the airplane. Designers quickly decided it was impossible to coat the tips of these four tubes with radar-absorbent material. Coming up with another process to preserve the plane’s low observability “took about three years of very careful design,” Mr. Brown said.

A host of problems, some very minor and several fairly serious, were encountered in development of the F-117’s cockpit avionics. Most of the on-board systems were lifted from the Navy F/A-18 strike fighter and modified for the stealth fighter.

The Skunk Works’ Richard Silz said that initially even the fighter’s intercom suffered “severe background noise.” Getting a UHF radio antenna to retract properly at high speeds and “stay together structurally” was another problem.

Other Pilots, Other Planes

Many avionics problems surfaced only in flight testing. As off-the-shelf avionics packages were mated to the F-117, engineers found it difficult to determine if a problem had actually been solved. Six test pilots were flying three different planes, and what was good enough for one pilot was subpar for another.

The differentiation problem would reappear on weapons-release testing as each airplane demonstrated “its own personality due to equipment installation tolerances,” Mr. Silz said.

Adapting the Infrared Acquisition and Detection System (IRADS) built by Texas Instruments was a major problem. From the start, Lockheed encountered half a dozen serious problems with the dual turret system, including picture quality and stability, acquisition and designation, video noise, and laser noise.

“Flight test got into trouble trying to solve too many problems at the same time,” Mr. Silz told a meeting of the American Institute of Aeronautics and Astronautics in Irvine, Calif., earlier this year.

With the program stumbling early, Lockheed formed a “tiger team” under the direction of the Skunk Works chief scientist. The team, comprising experts from several different divisions, began by designating a single aircraft and pilot to conduct the IRADS tests, thus ensuring consistent test appraisals. The tiger team solved the problem in about one year and 100 test runs, according to Mr. Silz.

Making the turret openings on the aircraft invisible to radar proved more of a problem and is illustrative of the fine detail required for stealth to work in an operational environment. The challenge for Lockheed was to find a material for the turret coverings that would allow the gear’s own laser and infrared emissions to penetrate freely while remaining opaque to enemy radar.

The first choice, Mr. Brown said, was a gallium arsenide window. After a technician accidentally fractured one of the $500,000 sheets in a test, however, Lockheed engineers decided it probably wasn’t right for the tough acoustic environment of the turret housings.

Lockheed ultimately opted for a far less expensive and more effective solution, creating a stainless steel “tennis racket” screen to cover the turrets. The wire spacing was tight enough to keep radar out, while loose enough for IR and laser to penetrate, even though F-117 designers were concerned about the aerodynamics of allowing air into the housings. “I had to be damn sure I didn’t make a harmonica,” Mr. Rich said.

As weapon certification testing began, the F-117 team began to see more of the details that needed to be ironed out for stealth to work in practice.

Lower the Trapeze

The F-117’s weapons bays, for example, are fitted with a trapeze sling for loading and unloading bombs. At first, engineers feared that bombs like the GBU-10 would damage the plane and ordered that they be released from the aircraft with the trapeze sling down.

“This was a major detectability problem for the aircraft,” said Mr. Silz, because the sling would create a significant radar signature. Eventually, Lockheed’s aerodynamics specialists reworked their calculations and found that the weapons could be dropped with the trapeze safely stored inside the bays. That cut the plane’s radar exposure window during attacks by a factor of five, according to Mr. Silz.

F-117 weapon testing ended up taking about twice as long as it could have, however. Test managers had decided early in the program to fill one of the two weapons bays with test instrumentation. Forty-eight individual test plans were originally drawn up for the F-117. Eight to nine months into the flight-test effort, Lockheed concluded it would be impossible to carry out each of the written test plans and still achieve an early IOC. The Air Force and the test team decided to shelve the original plan in favor of a streamlined approach to certifying initial proficiency in day training, refueling, weapons, instruments, practice weapons, and initial guided weapons tests.

Mr. Silz said essential testing was completed by IOC in October 1983. For several years after that, Lockheed and the Air Force continued to fill in the missing parts of the test equation. “While this approach to testing worked and is probably in the best traditions of the Skunk Works, flight test is just this year finishing the final reports on the last of the original test plans written over ten years ago,” Mr. Silz said.

Initial worries about the maintenance sensitivity of the F-117’s radar-absorbent materials appear to have been overstated. However, Lockheed went to great lengths to simplify the necessary maintenance and reduce the frequency with which radar absorbent material (RAM) coatings would need to be removed.

Designers placed the access panels for servicing aircraft subsystems within the landing gear and weapons bays and also used a single avionics bay to minimize “the need to remove and replace RAM coatings around access areas,” according to Martin.

Lockheed engineers made all of the panels on the F-117’s underside hexagonal in shape. Every line on the plane’s underside thus was parallel to the trailing edge of the wing, which ensured that any return from a door would be swamped by the minimal signature of the wing, Mr. Brown said.

No Swiss Watchmakers

“It had to be built by conventional airplane technicians,” he added. “You don’t want to design it so it had to be built by Swiss watchmakers.”

Obtaining the necessary security clearances for each of the workers on the program was a daunting task. Each worker had to be cleared at the “secret” level, a process requiring several months. Mr. Rich said he spoke personally with every member of the F-117 work force, which started with a core of about seventy-five and later grew to a few thousand. “We had a tremendous clearance problem,” said he. “It held us back.”

Top union officials were cleared first so that Mr. Rich could work with them to smooth the labor flow. Meanwhile, workers awaiting clearance were assigned to “the icebox,” a large shop where they worked on parts that did not give away the real focus of their work.

Perhaps the key element in Lockheed’s approach to moving stealth from concept to reality was a conscious decision to rely on off-the-shelf hardware as much as possible. In this way, engineers were able to focus on the breakthrough of reducing RCS without having to “invent” new avionics and engines. “We utilized proven systems from existing aircraft to reduce the overall system development risk,” said Mr. Martin.

The F-117’s “borrowed” components included the GE F404 engines from the F/A-18, cockpit gear from the Air Force F-16 and Navy F/A-18, the B-52’s navigation system, environmental control systems from the C-130, and critical fly-by-wire flight-control computers from the F-16.

A corollary to the initial reliance on off-the-shelf hardware was a continuous improvement program after deployment. That effort was illustrated by Lockheed’s response to the crash of three F-117s early in the program. The first crash, Mr. Rich said, occurred because of a mix-up in the pitch and yaw controls. The pilot called for a change in pitch, and the airplane yawed into a crash. The pilot survived with a broken leg.

Two of his colleagues were not so lucky when their operational fighters crashed. After finding that spatial disorientation had played a role in the two crashes, the Air Force commissioned the development of the Pilot Activated Automatic Recovery System. Mr. Silz described PAARS as operating at all altitudes and speeds whether the landing gear is up or down. Even if it is not engaged, the autopilot on command orders the flight-control system and autothrottle to execute a series of preplanned maneuvers that will recover a tumbling aircraft to straight and level flight.

“To our knowledge, no one has had to resort to use of this system for real, or at least they haven’t admitted to it,” said Mr. Silz.

PAARS, which was part of a broader improvement program, was delivered to Tactical Air Command in late 1990. It is the first automatic recovery system installed in Air Force fighters for general use, according to Lockheed.

The F-117’s makers believe their success demonstrates the benefit of avoiding the traditional procurement approach. Working in secrecy and enjoying relative immunity from congressional and press scrutiny, the Skunk Works team worked with a small Air Force office to bring stealth from the laboratory to operational use. Mr. Brown, who was the first F-117 program manager, said he was relieved of normal contracting and paperwork chores, which went to another Lockheed manager. “That left me more time to do real work,” he said. The Air Force monitoring team, he added, consisted of six officers, “which was delightful.”

David J. Lynch covers defense and aerospace matters for the Orange County Register in California. He is a former editor of Defense Week in Washington. His most recent article for Air Force Magazine was “The Risen Sun?” in the October 1992 issue.