The Advent, Evolution, and New Horizons of United States Stealth Aircraft

USAF photo

Bartholomew Hott

George E. Pollock

Lockheed Advanced Development Projects division director Ben Rich rolled a small ball bearing across the desk of a U.S. Air Force four-star general. Rich accompanied this demonstration with the bold statement, “Here’s the observability of your airplane on radar.” Utterly amazed, the Air Force commanders could not wait to make use of an airplane with such a diminutive radar signature—by comparison most fighters of the time (mid-1970s) had radar signatures the size of a bus (Rich and Janos 63). The airplane Rich was referring to is the F-117—the now world famous “stealth fighter” of the U.S. Air Force. Although the F-117 was the first operational military aircraft to be designed entirely with the goal of minimizing its detection by radar, the roots of stealth technology date back to World War II—following the development of radar. Since the 1950s, the United States has pioneered the development of stealth technology and with a steady progression of military aviation innovations, retains its status as a modern world leader in this field.

Radio detection and ranging, an electromagnetic device commonly referred to as radar, is used to locate objects, and determine their speed and direction of motion. It functions by transmitting electromagnetic waves and receiving waves which have been reflected off an object. These received waves, or echoes, are processed and presented to interested personnel for analysis. The largest benefit of radar is its operating range advantage over the human senses. Before the invention of radar, the primary technique of locating airplanes was by sight or sound. This method had obvious limitations during the chaos of war, dark of night, and adverse weather conditions. As airplanes were used with increasing effectiveness in tactical bombing raids, it was evident something needed to be done to track the movement of enemy aircraft (Radar 458-459).

The historical roots of radar extend to the mid 19^th Century, when British physicist James Clerk Maxwell predicted the existence of radio waves before the means to authenticate his findings became available. This prediction was not experimentally validated until the 1880s when Heinrich Hertz confirmed the existence of radio waves and showed they travel at or near the speed of light (“Radar” 882). Advances continued in radar, but the first successful military application is credited to Sir Robert Alexander Watson-Watt, a British physicist. Watson-Watt began his career in 1915 as a meteorologist for the Royal Aircraft Factory, where he hoped to develop a thunderstorm warning system for pilots, with the aid of radio waves. As he continued through his career, Watson-Watt concentrated on the idea of detecting aircraft by means of radar, and in 1935, with the aid of his assistant Arnold Wilkins, published a report titled “The Detection of Aircraft by Radio Methods.” On 26 February 1935, Watson-Watt successfully demonstrated his method by locating a Heyford bomber in Daventry, Great Britain, at distances in excess of 70 miles (Barrett).

Sir Robert Watson-Watt’s timing was perfect—it gave Britain the time needed to implement radar technology before fighting broke out in World War II. By September 1938, installation of the British early-warning radar network, known as “Chain Home,” was completed along Britain’s coastline (Barrett). These early radar stations soon proved their value and operated nonstop throughout World War II. While later criticized for operating at low frequencies, in practical terms, Chain Home was facilitated by experience gained from short-wave radios (Richardson 27). This network demonstrated its effectiveness during the Battle of Britain (1940-1941) in which it provided the early warning necessary to allow the limited resources of the Royal Air Force to fend off relentless Luftwaffe bomber onslaughts. The Germans became aware of the advantage their British counterparts had acquired when RAF interceptors consistently greeted their formations of bombers. Great Britain was not the only country that had radar capabilities during World War II. In fact, Germany, the United States, and Soviet Union had been working on developing radar since the 1930s. Although, while the U.S. and Britain accelerated their efforts, Germany halted their labors in late 1940, believing they would be winning the war soon. The Soviet Union’s radar efforts were significantly hindered by the German invasion in June 1941 (“Radar” 461-462).

In the realm of defense systems, it is often noted that for every weapon, a new counter to that weapon will soon follow. Such is the case with radar air defenses and stealthy, or low observable, aircraft. As radar emerged as a critical defense mechanism in the Battle of Britain, German engineers began working feverishly in an effort to obscure airplanes and ships from the prying eyes of Allied radar stations. The Germans succeeded in developing the first, primitive forms of radar-absorbent material (RAM). They found that certain carbon compounds could be utilized to absorb radar waves rather than reflecting them and began to use this method to conceal the snorkel tubes of their fleet of U-boats (Sweetman Lockheed Stealth 11).

German aircraft engineers Walter and Reimar Horten first envisioned the use of RAM on aircraft in the midst of World War II. They planned to use carbon-based RAM on the skin of their twin-engine, flying-wing Ho IX fighter/bomber to make it less susceptible to radar detection. The prototype first flew in 1944, but without the planned materials of the production model (Sweetman Lockheed Stealth 11). Perhaps fortunately for the Allies, the war ended before the Horten brothers were able to deliver any of the production models, known as the Gotha Go.229 (Richardson 59). Following the war, the United States and United Kingdom began to develop RAM. The UK focused primarily on using RAM in naval applications whereas the US, in the mid-1950s, quietly turned its attention toward using RAM on aircraft (Sweetman Lockheed Stealth 11).

In the mid-1950s, President Eisenhower was desperate for a glimpse behind the Iron Curtain to ascertain whether the Soviets were capable of launching a surprise nuclear attack, which the Joint Chiefs of Staff feared might be planned (Rich and Janos 8). This intelligence need led to the first significant attempt to reduce the radar return of a US airplane. This aircraft, the Lockheed U-2, was developed in complete secrecy by the Lockheed Advanced Development Projects division (a.k.a. “Skunk Works”) in California (Sweetman Lockheed Stealth 12). Its design mission was to obtain photos of Soviet territory as a spyplane for the Central Intelligence Agency, and it first overflew the Soviet Union on 4 July 1956. On that day, CIA pilot Harvey Stockman made a nine-hour flight from Germany, over Poland, into Soviet airspace, over Minsk and Leningrad, and then back to Germany (Rich and Janos 145).

Figure #1 A Lockheed U-2 spyplane soars above the clouds. Designed by Kelly Johnson and associates at the Skunk Works, the high-altitude reconnaissance aircraft became a workhorse for the U.S. Air Force in the 1950s and upgraded versions have been used ever since. The U-2 has the distinction of being the first airplane to make extensive use of radar-absorbing materials (RAM).

The original hope of Skunk Works chief engineer Kelly Johnson and CIA program head Richard Bissell was that the U-2, with its ceiling above 65,000 feet, would fly high enough to avoid detection by Soviet radar defenses for nearly two years (Sweetman “The Invisible Men” 11). These hopes however, were shattered as Stockman’s U-2 was tracked throughout its flight by Soviet radar and several interceptor aircraft had been scrambled in a vain effort to engage him. Johnson responded to the radar tracking of the first Soviet overflight mission with a briefing of his technical team. “‘Well, boys, Ike got his first picture postcard…but goddam it, we were spotted almost as soon as we took off. I think we’ve badly underestimated their radar capabilities…we always figured they wouldn’t even see us at sixty-five thousand feet. And you know why? Because we gave them lend-lease early-warning radar during World War II and presumed that, like us, they wouldn’t do anything to improve it. Obviously they have. I want you guys to brainstorm what we can do to make us less visible or help us go even higher’ (Rich and Janos 146).” With these words from Kelly Johnson, the struggle to reduce the radar signature of the U-2 began.

The U-2 flights continued and the Russians continued tracking them on radar and progressed in their attempts to intercept the spyplanes. U-2 pilot Mart Knutson recalls the Russian’s desperate struggle to down the high-flying U-2s. “‘They tried to stop us by trying to ram us with their fighters like a ballistic missile. They stripped down some of their MiG-21s and flew straight up at top speed, arcing up to sixty-eight thousand feet before flaming out and falling back to earth. Presumably they got a relight down around thirty-five thousand feet. I’m sure they lost some airplanes and pilots playing kamikaze missile. It was crazy, but it showed how angry and desperate they were becoming’ (Rich and Janos 151).” As the Soviets grew ever more agitated with the continued flights, and work progressed rapidly in their development of surface to air missiles (SAMs) capable of reaching the U-2’s altitude, Eisenhower and the CIA demanded the radar cross section (RCS) of the U-2 be lowered dramatically. The complexity of this monumental task is best illustrated by the relation between RCS and detection range. Radar cross section is calculated by first finding the size of a sphere that reflects the same amount of radar energy as the aircraft, and then the RCS is defined as the area of a circle having the same diameter as the aforementioned sphere (Richardson 27). Obtaining a meaningful reduction in RCS is inherently difficult as the detection range is a function of the fourth root of RCS. As a result, a fifty percent reduction in RCS would lead to only a sixteen percent decrease in detection range (Sweetman Lockheed Stealth 9).

Initiated near the end of 1956 and code-named Project Rainbow by the CIA, the efforts to cloak the U-2 from Soviet radar began. Two drastically different methods were proposed to accomplish this difficult task. Johnson summoned radar experts Dr. Frank Rogers and Ed Purcell to help, and they suggested stringing piano wire along the fuselage and tail to scatter radar energy in as many frequencies and directions as possible to minimize the useful return shown to the Soviets. This led to a much less aerodynamic aircraft and scraped some seven thousand feet off the original sixty-five thousand foot cruising altitude of the U-2 (Rich and Janos 152). The second approach was favored by Kelly Johnson and involved a special type of iron ferrite paint that would absorb rather than reflect radar waves. This signified the first extensive aerial use of radar-absorbent materials (Sweetman Lockheed Stealth 14).

Both of these concepts were implemented in modified U-2s, which came to be known as “dirty birds,” in the spring of 1957 (Sweetman Lockheed Stealth 14). This early RAM technology, which decreased the RCS tenfold, caused problems with heat dissipation through the airframe. On 2 April 1957, test pilot Robert Sieker was killed when the engine of his dirty bird overheated and failed at over seventy thousand feet. The cause of death: acute hypoxia and loss of consciousness after the faceplate on his pressure suit had blown off due to a faulty fifty cent clasp (Rich and Janos 153). Despite this tragedy, the RCS reduction effort and testing pressed on. Originally, the dirty birds had to fly over a test ground radar site to measure the RCS, but this process was soon abandoned for the less expensive and more efficient Skunk Works-developed static-pylon arrangement for measuring RCS on the ground from all possible angles.

Nine dirty bird missions overflew the Soviet Union, beginning in July 1957, resulting in vehement protests from Moscow, and the Soviet presentation to the US of detailed flight path descriptions (Sweetman “The Invisible Men” 15). By the end of 1959, Kelly Johnson knew that the U-2s days were numbered, as the Soviets would soon have the SAM technology to complement their advanced radar system and could then shoot down the spyplanes. The Soviets were rapidly developing their SA-2 installations around highly valued targets—precisely the targets of which Washington wanted U-2 photos. Morale of U-2 pilots stationed in Turkey and Pakistan plummeted as they saw through the sights of their craft the very weapons being readied to shoot them down. U-2 flights decreased in frequency in response to the growing Soviet missile capabilities, and the Skunk Works developed one of the first electronic counters to radar (Rich and Janos 157). This electronic counter-measure (ECM) system known as “Granger” was installed in the tail of the U-2 in hopes of preventing a radar-guided missile from obtaining a lock on the aircraft (Richardson 60).

The CIA pushed Eisenhower to authorize one final mission, and he reluctantly signed off on what was to become one of the most embarrassing moments in the history of US national defense. On that last mission, Francis Gary Powers was shot down on 1 May 1960 when one of fourteen SA-2 missiles launched in a barrage exploded close enough to the U-2 to knock off its tail. Powers bailed out (rather than ejecting) from his violently spinning craft, was captured, and imprisoned by the Soviets for two years before being swapped for a captured Russian spy. The incident effectively ruined the summit between Soviet leader Khrushchev and President Eisenhower scheduled for 14 May 1960 in Paris while simultaneously making Eisenhower look like a fool in the international spotlight (Rich and Janos 158-161). Johnson would later suspect that the radio emissions from the “Granger” electronic warfare system might actually have made it easier for the Soviet missiles to lock onto the U-2 (Richardson 60).

Johnson had already begun to envision a supersonic successor to the U-2 long before it was rendered obsolete by the advancements in Soviet missiles. In 1959, the CIA solicited proposals from Lockheed and Convair for a low RCS, high altitude, supersonic replacement for the U-2. The basis for these requirements was the “blip-scan” theory—stating that a small radar reflection, which changed position dramatically between scans, would be viewed as noise by Soviet radar operators (Sweetman Lockheed Stealth 16). Johnson was a step ahead of the game at this point, having formed a small team of incredibly talented engineers in April 1958 to begin work on the U-2’s successor. Johnson told his engineers, “‘We’ll fly at ninety thousand feet, and jack up the speed to Mach 3. It will have a range of four thousand miles. The higher and faster we fly the harder it will be to spot us, much less stop us’ (Rich and Janos193).” Ben Rich, Johnson’s head propulsion engineer for the program remarked, “Had I really thought about it, in complexity the U-2 was to the Blackbird [the resulting Mach 3 spyplane] as a covered wagon was to an Indy 500 race car.” As the design of his paradigm shattering supersonic spyplane progressed, Johnson lobbied the CIA to fund his new program (Rich and Janos 192).

Johnson and his Skunk Works team developed a series of designs, with ever improving radar signatures, and ultimately produced the A-12, which was selected over the Convair Kingfish for the CIA supersonic spyplane program (Sweetman Lockheed Stealth 16). Approval to construct the first A-12 prototype came in August 1959 and as the prototype was developed and tested, constant alterations were made to the design in order to improve its low RCS characteristics. Key features included a blended wing/body, RAM integrated into the wing leading and trailing edges, and a coating of improved ferrite radar-absorbent paint. The engines would have cone-shaped inlets that compressed the thin air for combustion at extremely high altitudes while providing the additional benefit of masking the highly reflective engine from radar (Richardson 61). The Skunk Works also strived to keep the aircraft’s twin tails, necessarily large to provide yaw control at Mach 3 speeds, as small as possible and to make them entirely of composite, radar-absorbing materials (Rich and Janos 197). Lockheed developed its own radar-absorbent composite honeycomb materials for use on the elevons, chines, and twenty percent of the wing planform area. As the Skunk Works designs progressed from A-1 through A-11 and the final A-12, these key features contributed heavily to a ninety percent reduction in RCS (Richardson 61).

Figure #2 The pioneering J-58 engines developed for the A-12 and SR-71 required an unorthodox method for starting. As Keith Beswick recalls in Ben Rich’s book, “We rigged up two big 425-cubic-inch Buick Wildcat race car engines, and estimated 500 horsepower each, to turn the massive starter shafts and those suckers did the trick. The hangar sounded like the damned stock car races, but starting those huge engines was tough. (218)”

In a daring move, Johnson and his head structures engineer, Henry Combs, decided to build the first titanium airplane in history. With the exception of stainless steel weighing twice as much, this exotic alloy was the only material with enough strength and heat resistance to be used at Mach 3 flight velocities. Typically, aluminum (which loses structural integrity at three hundred degrees Fahrenheit) was used in airframes, but this aircraft would routinely experience temperatures exceeding eight hundred degrees Fahrenheit at the nose and twelve hundred degrees at the engine cowlings. To complicate the situation further, only a single, small US firm produced the metal and very little knowledge existed on methods to extrude, weld, rivet, or drill titanium (Rich and Janos 202).

Johnson’s protégé, Ben Rich described the problem of limited titanium supplies, “the CIA conducted a worldwide search, and using third parties and dummy companies, managed to unobtrusively purchase the base metal from one of the world’s leading exporters—the Soviet Union. The Russians never had an inkling of how they were actually contributing to the creation of the airplane being rushed into construction to spy on their homeland (203-204).” Rich later proposed using black paint on the plane’s skin to radiate the heat generated by air friction and allow use of a softer, more easily workable, and less heat resistant titanium alloy (Rich and Janos 202-203). The A-12’s dark paint scheme led to its acquisition of the unofficial moniker “Blackbird” (Richardson 61).

The radar signature reduction measures taken in designing the Blackbirds worked relatively well, however Johnson’s initial assessment that the Soviets would have the radar technology to detect any US airplane by the early 1960s proved correct (Sweetman “The Invisible Men” 17). A hard pill to swallow, Rich recounts Johnson’s concession to the CIA that, “I’m convinced that current improvements in Russian radar will allow them to detect any airplane built in the next three to five years. Radar technology is far ahead of antiradar technology, and we’re just going to have to live with that fact. We’ll never achieve the zero degree of visibility the president [Eisenhower] seems so stuck on (Rich and Janos 198).” The Soviet Union had just completed a new, incredibly powerful P-14 “Tall King” early warning radar, which was computer controlled and capable of detecting even the RAM cloaked, high-flying A-12 (Sweetman Lockheed Stealth 17).

Soon after Kennedy won the 1960 election, Johnson grew anxious about how the young incoming President would react to the Blackbird program, especially with its increasing costs and the Russian development of the Tall King radar, which by all indications could detect a target with an RCS one-third that of the Blackbird (Rich and Janos 215). Kennedy, his Air Force chief of staff, General Curtis LeMay, and Secretary of Defense Robert McNamara were all impressed by the CIA A-12’s capabilities. Kennedy left the June 1961 Vienna summit with Khrushchev feeling as though the US and USSR were on the brink of a massive war and was eager to permit the Air Force to order two-seat updated versions of the A-12, which would become the venerable SR-71 spyplane.

Figure #3 The SR-71 Blackbird in flight. An extremely successful spyplane, the SR-71 set numerous speed and altitude records, made a tremendous impact on U.S. national security, and its performances either in speed or spying will not soon be eclipsed.

This version of the Blackbird would have a second crewmember, the Reconnaissance Systems Officer, to manage the electronic eavesdropping equipment and non-automatic cameras (Rich and Janos 227-233). Washington also approved a study of a Mach 3 interceptor, the YF-12A, based on the Blackbird design; however, after flight-testing in the mid-1960s it never entered active service (Richardson 61). President Johnson transferred the responsibility of aerial reconnaissance entirely to the Air Force in 1968 to reduce the cost associated with having the CIA and Air Force both operating fleets of the high-performance Blackbird aircraft (Rich and Janos 239). The A-12 quickly faded out of the spotlight; the SR-71 and its two-man Air Force crews then carried the Mach 3 spyplane torch for the next twenty-five years.

Figure #4 The two seat SR-71 Mach 3 spyplane which served US intelligence needs in the Air Force, setting countless world speed and altitude records and earning Kelly Johnson his second Collier trophy, aviation’s most prestigious award.

Figure #5 Both the SR-71A (left) and the YF-12A interceptor prototype shared the Blackbird’s characteristic double delta-wing, with a blended wing/body intended to reduce RCS, and twin inward canted tails. These sleek, titanium alloy structure airplanes made innovative use of RAM on the leading edges of the wings and chines and were powered by massive ramjet engines with the trademark conic inlets visible here. Their black paint was designed to help radiate the massive amounts of heat generated by atmospheric friction and produced the nickname “Blackbird.”

The Blackbird family of aircraft, though designed to have the lowest RCS that was technically feasible in such a high-altitude, high-speed airplane, ultimately relied on its height and speed to survive (Sweetman “The Invisible Men” 17). Enemy radar could track the Blackbird, but SAMs would have to lead the plane by some 30 miles to have a chance of hitting it. Typically, by the time enemy defense radar located the Blackbird and fired a SAM at it, the Blackbird would easily outrun the missile with its phenomenal cruise speed over 2,000 miles per hour and cruise altitude in excess of 80,000 feet. Intelligence gathering was this plane’s destiny—it could survey 100,000 square miles of territory per hour (Lockheed SR-71A). Blackbird RSO Captain Norbert Budzinske recalls the imaging capabilities of the aircraft in Rich’s Skunk Works: “Sometimes after a mission, I’d get a look at our photo take to see how well we did. It was unbelievable! You could actually see down the open hatches of a freighter unloading in Haiphong’s harbor. In fact, the photo interpreters claimed that they could tell what was down in those hatches, it was that sharp and clear from 85,000 feet. They’d blow up our photos to the size of a table. (247)”

Figure #6 Mine eyes have seen the glory! The camera porthole on the underside of the SR-71A is the window through which the US intelligence community gathered a wealth of information about numerous adversaries. This equipment was capable of recording in minute detail the activities of hostile forces from 85,000 feet altitude while the airplane thundered through the sky at a blistering 3,000 feet per second.

The SR-71 first flew on 22 December 1964, entered service in January 1966, and was retired in 1990 (Lockheed SR-71A) by [then] Defense Secretary Dick Cheney due to defense budget cuts and its high operational cost (Rich and Janos 259). The SR-71 saw a brief resurrection as it was recalled from retirement in 1995 and flew operational missions again in the late 1990s before being retired permanently (Lockheed SR-71A). In retrospect, the Blackbird’s operational career gleams with success. It flew some 3,500 surveillance sorties over hostile territory in North Vietnam, North Korea, Libya, Cuba, the Soviet border, and the Persian Gulf. Drawing hundreds of SAM missile firings, it made history as the first military plane never to be downed or lose a flight crewmember to enemy fire. During its career, the Blackbird flew some 65 million miles; most of them ticked off at Mach 3 and over fifteen miles high (Rich and Janos 241).

The originally ultra-secret Blackbird program gave birth to another more highly classified, yet less famous Skunk Works project. Lockheed produced one of the first operational unmanned aerial vehicles (UAVs) in the form of its Mach 3 D-21 drone. The relatively small project, code-named Tagboard, was according to Ben Rich, the “most sensitive project during [his] years at the Skunk Works (262).” Powered by a single ramjet engine similar to the SR-71 and A-12 aircraft, it was originally intended to be launched from a piggyback position on a modified SR-71 airplane. After a mid-air collision on 30 July 1966 of the carrier SR-71 aircraft and the D-21A, which resulted in the drowning death of the launch officer, the supersonic launch concept was abandoned (Rich and Janos 266-267).

The D-21B was then developed to be launched from an under-wing pylon of a B-52H. This design required a solid rocket to speed the drone up to supersonic velocities where the ramjet could operate. Several operational D-21B launches occurred from 1969 to 1971 when the program was cancelled. Due to the high level of secrecy involved with the D-21, few details of these operational missions have been released (Lockheed D-21B). According to Ben Rich, four D-21Bs flew surveillance missions over China’s nuclear test facilities, undetected by Chinese radar, but suffering from several technical and logistics failures resulting in the loss of all photo intelligence gathered on the flights (267-270).

Figure #7 The D-21B drone shown in three different views above was the modified version of the D-21A originally launched from the back of a modified A-12 Blackbird. These Mach 3 reconnaissance drone flew several operational missions launched from B-52s and boosted to speed by a solid rocket motor. In many ways, it is the forerunner of modern unmanned aerial vehicles and stealth aircraft.

The often-overlooked D-21 drones foreshadowed two significant future military aviation trends. First, with its small size and use of RAM, the D-21 had a RCS considerably smaller than the Blackbird. Rich recalls, “The drone had the lowest radar cross section of anything we had ever developed (264).” This concept would contribute to Lockheed’s later assuming the industry lead in developing airplanes designed completely around stealth technologies. Also, this ushered in a new era in the realm of unmanned aerial vehicles. Development of remotely piloted and robotic vehicles for surveillance has continued in an effort to minimize the risks to American service men and women.

In 1974, the Defense Advanced Research Projects Agency (DARPA) invited Northrop, McDonnell Douglas, General Dynamics, Fairchild, and Grumman to answer two questions. First, what would it take to make an aircraft undetectable to radar during operation? Second, is your company able to accomplish this? DARPA received answers from McDonnell Douglas and Northrop; each company was subsequently awarded a government contract worth approximately 100,000 dollars to pursue the goal of developing the first true stealth aircraft. Ben Rich, Lockheed’s new president of Skunk Works, was informed of the study by Ed Martin, Lockheed’s California Companies director for science and engineering, who was networking with contacts at the Pentagon and Wright-Patterson Air Force Base. Lockheed wasn’t invited with the original companies purely on the fact they hadn’t produced a fighter in nearly a decade, but Rich, especially with all the research which had gone into the U-2, A-12, and SR-71, wasn’t going to let this opportunity slip away. Rich, with Kelly Johnson and Ed Martin obtained a letter from the CIA granting permission for Skunk Works to discuss results from previous research on low-observation characteristics with DARPA. Rich then persuaded DARPA to let Lockheed into the competition—with the stipulation that they would receive no government contract. The Skunk Works simply had to secure the capital necessary to proceed with design development, which Larry Kitchen, Lockheed’s President, promised would not be a problem (Crickmore and Crickmore 9-10).

Denys Overholster, a Skunk Works mathematician and radar specialist, presented the major breakthrough for stealth technology to Ben Rich. Overholster, offered a method of making an aircraft undetectable to radar from the most heavily guarded targets in the world. Ironically, the research Overholster used to form his ideas came from Moscow! The source was an unclassified technical publication, Method of Edge Waves in the Physical Theory of Diffraction, written by Pyotr Ufimtsev, a Russian expert in theoretical optics (Sweetman “The Invisible Men” 20). Ufimtsev had used formulas derived by Scottish physicist James Clerk Maxwell, which were subsequently refined by German electromagnetics expert Arnold Johannes Sommerfeld. Taking these formulas a step further, Ufimtsev could calculate RCS across the surface of a wing and at the edge of the wing and then sum the calculations for an accurate total. At the time, the notion of accurately calculating a RCS was compared to “medieval alchemy.” The solution was to make an aircraft out of flat panels angled so none would be perpendicular to radar, a process also known as faceting (Rich and Janos 19-20).

With theory in hand, the work commenced to produce a computer program that would be able predict the RCS of various designs. Overholster teamed with two other engineers and worked around the clock for 5 weeks to produce the RCS prediction program known as “Echo 1.” With Echo 1, Skunk Works was able to evaluate over 20 different aircraft designs, choosing the best purely from the RCS perspective (Crickmore and Crickmore 11-12).

The faceted delta-wing design produced the lowest RCS and began its course through history. Lockheed and Northrop were challenged to produce full-scale wooden models of their designs for a “pole off,” or RCS testing duel. The model having the lowest RCS would win—Lockheed prevailed and continued development of their stealth design. The program code named Have Blue officially started on 26 April 1976. Due to Lockheed’s enormous success in reducing RCS, it was upgraded to top secret, making it a completely unacknowledged program. Lockheed was to build two flight-worthy prototypes to demonstrate their technology and prove the RCS values shown on the models could be reproduced on functioning aircraft (Sweetman Lockheed Stealth 28-30). Never had such an aesthetically unpleasing design been proposed, and it was labeled the “Hopeless Diamond” as many experienced aerodynamicists thought it would never fly. After seeing the sketch of the design, an irritated Kelly Johnson barged into Ben Rich’s office saying, “‘Ben Rich, you dumb shit,’ he stormed, ‘have you lost your goddam mind? This crap will never get off the ground’ (Rich and Janos 28).”

For the aerodynamicist Have Blue was a nightmare, but the electrical engineers would save the day by hiding the plane from radar and then developing the computer control systems necessary to fly it. The airplane would have multiple computers which could perform thousands of electro-hydraulic adjustments per second, giving it the stability needed to sustain controlled flight. This system is known as “fly by wire” since electric sensors, wires, and actuators replaced traditional mechanical control systems consisting of cables, shafts, and gears (Rich and Janos 32). The two Have Blue prototypes had different functions. The first prototype’s 1001 primary purpose was to demonstrate the airplane could fly safely, while the second Have Blue 1002 would include a more complete compliment of stealth features including RAM and nontraditional nose probes. The Have Blue 1001 was painted in a desert camouflage scheme in order to keep prying eyes (i.e. Soviet spy satellites) from discovering the new faceting technique and Have Blue aircraft 1002 was painted a light gray color (Sweetman Lockheed Stealth 30).

The prototypes were flight tested at “Area 51,” a top-secret flight-test facility near Groom Lake Nevada, to ensure the project stayed secret. Have Blue 1001’s first flight was on 1 December 1977. HB1001 made 36 total flights, 24 by Lockheed test pilot Bill Park and 12 by USAF pilot Norman “Ken” Dyson. Dyson was selected as a test pilot upon recommendation from his wing and deputy wing commanders. After being ordered to General Tom Stafford’s office, he was told to shut the door and asked if he would volunteer for a highly classified flight test job. In Major Dyson’s own words, “‘I thought about it for not very long and I said, “Well, I’d like to do that General,” and he put me to work.’ (Crickmore and Crickmore 16).” On the 37^th flight of HB1001 4 May 1978, after conducting fly-bys to determine the visual signature of the aircraft, Park’s approach exceeded the maximum 17-degree glide slope. A few feet from the ground the emergency nose-down flap activated. Ironically, this safety feature caused the HB 1001 to slam into the runway. Park increased the thrust and climbed away from the runway, retracting the landing gear. The impact had damaged the starboard landing gear, and it jammed in a half extended position. Park tried in vain to get the landing gear extended, but fuel was running out, and he was ordered to eject. After he climbed to a safe altitude, he ejected from the damaged prototype. He struck his head during the ejection, and was unconscious when he landed—his parachute then dragged him across the desert floor. It was determined that he suffered a concussion, and his test pilot career ended due to the injuries he sustained (Sweetman Lockheed Stealth 31-32).

Figure #8 Have Blue prototypes 1002 (top) and 1001 prior to the flight testing incidents which destroyed each aircraft. The Have Blue aircraft proved that a faceted, stealthy design could in fact be implemented in a functioning, controllable airplane.

Subsequently, Have Blue 1002 needed to be rendered flight worthy. By 20 July 1978, HB 1002 was ready to enter flight tests. This time, Lockheed would be able to take RCS measurements and finally put its stealth secrets to the test. Overall, HB 1002 made 52 flights, all piloted by Ken Dyson. On 11 July 1979, while Dyson was flying against an F-15’s radar, the pressure in a hydraulic system began to oscillate and then a fire warning light illuminated (Crickmore and Crickmore 23). He shut down the engine and pointed his plane towards home when he lost all pressure in the hydraulic system (Sweetman Lockheed Stealth 33). Major Dyson describes the event in his own words, “The plane pitched violently down, something like 7 negative gs. It then pitched up; the pitch rates were just eye watering, something only an unstable machine could do. I was somewhere around 225 knots and above 20,000 feet, and the airplane was tossing me up and down and actually got near vertical nose down and near vertical nose up. I began to try and reach for the ejection seat ring that was between my legs. I got my hand on it and pulled. The canopy blew off, the seat went out, and I found myself floating under a chute at about 20,000 feet (Crickmore and Crickmore 23).”

Have Blue 1002 crashed near the Tonopah Test Range (TTR) and some TTR workers rushed into vehicles and started toward the crash site. These curious witnesses were deterred from stumbling onto one of the US Department of Defense’s most precious secrets however, when “the F-15 buzzed them at zero feet (Sweetman Lockheed Stealth 33).” Fortunately, the Have Blue program had only one scheduled test flight remaining and had achieved all of its test objectives; as a result, Have Blue is considered an enormous success despite the loss of both prototypes (Crickmore and Crickmore 22-23). Most importantly, the Have Blue flight tests proved that a stealth aircraft, with its faceted design ignoring traditional aerodynamics, could in fact be engineered to fly in a controlled manner (Sweetman Lockheed Stealth 33).

On 1 November 1978, Ben Rich, director of Lockheed’s Skunk Works, signed a contract for five full-scale F-117s, the first being ready in July 1980. They had only 22 months to get the first aircraft built and flying; the far less complicated Have Blue had taken 18 months to build. Alan Brown, F-117 program manager, commented on the tight deadline, “Ben said ‘Okay.’ The rest of us said, ‘Oh, shit’ (Rich and Janos 71).” Production of the F-117 ran into significant problems. The Skunk Works was engineering an airplane which in order to be stealthy had to veer away from practical methods of engineering. Some of the huge obstacles included, but were not limited to, the intake and exhaust system, air data measurement, and the canopy glass (Crickmore and Crickmore 27).

Engine air intake proved difficult for multiple reasons. First, an engine often produces a large RCS, with electromagnetic waves bouncing off fan blades easily. To combat this, Lockheed covered the air inlets with grills made of RAM. The grills absorbed some radar energy and reflected the rest in directions least likely to be observed by enemy radar installations. The grills caused a reduction in air pressure resulting in diminished engine performance. Some of this disadvantage is countered by the flow straightening qualities of the grill which ensure clean, laminar air flow to the engine (Dornheim). While the grills worked sufficiently well for Have Blue, in real world scenarios, they presented the problem of ice build up. “The inlets, as Alan Brown put it later, ‘not only looked like ice-cube trays but acted that way’ (Sweetman Lockheed Stealth 54).” After experimenting with different methods to prevent ice build up, the Skunk Works settled on the not particularly innovative idea of adding a simple wiper blade along with a de-icing jet to de-ice the grilles (Crickmore and Crickmore 27).

Figure #9 The grill-covered engine inlet on the port side of the F-117A. The grills effectively managed radar reflections, but decreased the pressure entering the engine and caused icing problems. The Skunk Works engineers solved the icing problem and decided to live with the drop in pressure as the grills ensured laminar airflow at angles of attack up to 20 degrees.

Figure #10 (previous page) The exhaust system of the F-117 is comprised of two nozzles, one on either side of the V-tail, extending several feet wide on the top of the plane near the trailing edge. This “platypus tail” cloaks the hot exhaust gases from ground-launched IR targeting weapons, and decreases the temperature of the exhaust by mixing it with the airflow around the plane as it passes by the vortices generated by the V-tail (top right of photo).

The air data measurement system was exceptionally tricky. Since the probes protrude from the nose of the plane and provide vital data during flight, they not only had to be stealthy but also functional. The system required four separate channels for use in measuring airspeed, angle of attack, and sideslip (Sweetman Lockheed Stealth 55). If this array of pitot probes quit receiving data, the fly-by-wire system would not be able to make the microsecond flight adjustments, rendering the plane hopelessly out of control within seconds. The solution to this problem came in the form of four probes, each with a pyramid-shape tip to give the lowest possible radar return while still collecting the critical flight data (Sweetman Lockheed Stealth 55).

Figure #11 The design of this stealthy air data measurement system was one of the largest challenges faced in the development of the F-117.

Additionally, with regard to the canopy, radar waves bouncing around in the cockpit were extremely undesirable. A material that would allow the pilot to see out, but prevent the electromagnetic radar waves from coming in, had to be developed (Rich and Janos 81-82). The windows of the canopy were coated with a film of indium-tin oxide that reflects radar signals away from the cockpit interior where a large return would otherwise be produced (Sweetman Lockheed Stealth 54-55). Other considerations included minimizing the F-117s own electronic emissions. An aircraft’s electronic emissions (i.e. active radar) signature can inform hostile forces of its location, speed, and direction of motion. Therefore, the most clear-cut answer was to omit radar from the F-117. The first military aircraft since the 1950s not to have onboard radar, the aircraft would instead use laser targeting, which would not reveal the airplane’s location, in order to deliver its payload of 2,000-pound laser-guided bombs (Sweetman Lockheed Stealth 41-42).

Figure #12 A 2,000-pound laser guided bomb extends from the weapons bay of the

F-117A. Note the serrated edges on the weapons bay door—specifically designed to minimize the forward reflection of radar. The F-117, having no targeting radar, relies on laser-guided precision munitions.

The first F-117 “Nighthawk” was flight tested in May 1981. It had one major problem, which confirmed one of Ben Rich’s doubts. The all-moving V-shaped tails were too small, and halfway through the flight test, one of the tails broke lose and free fell to the desert floor. Skunk works engineers redesigned the tail for subsequent production models—the original tail was 15 percent too small and exceedingly flexible. Overall, the plane handled well and within 5 years of the initial contract, a stealth squadron of 18 units and a few spares existed, with only one major accident (Rich and Janos 89). On 20 April 1982, Bob Ridenauer, a Lockheed test pilot, barely climbed off the ground before his aircraft flipped over landing upside down (Sweetman Lockheed Stealth 55). Technicians later discovered, when the aircraft was rewired, the pitch and yaw controls had been reversed. Ridenauer survived but spent seven months in the hospital—due to a mistake, which should have been caught in the inspection process. Ben Rich was extremely upset, believing a high price is too often paid when too many inexperienced workers do such vital work on an airplane (Rich and Janos 89-91).

Figure #13 The Lockheed F-117A Nighthawk was the world’s first operational stealth aircraft. Capable of carrying two 2,000-pound laser-guided bombs, it can execute precision strikes on high-value, heavily defended targets. With no radar or defensive countermeasures, the F-117 relies on its stealth to avoid hostile fire.

On 10 November 1989, the F-117 Nighthawk went public after over a decade of being top secret. As US action against Iraq in Operation Desert Storm loomed, the Nighthawk would soon be put to the test. A squadron of F-117s flew to King Khaid Air base in southern Saudi Arabia (the location specifically chosen outside the range of Saddam’s Scud missiles), which was built specifically for the airplane. The Air Base featured an air purification system, eliminating threats from nuclear, biological, and chemical weapons. In addition, fortified hangars with huge blast doors designed to survive direct hits were built—all in order to keep the Nighthawks sheltered from attack. On the first night of bombing Iraqi targets in Kuwait and Iraq, more than 1000 sorties were flown and it marked the first time a stealth aircraft was used as a frontline weapon. The Nighthawks delivered laser guided bombs, even flying over Baghdad (reportedly the most heavily defended airspace in the entire world), to destroy key targets. The F-117s flew over 1000 sorties and took out over sixty percent of Iraq’s strategic targets (Goodall 47 51-55).

Figure #14 The F-117A Nighthawk in flight. Clearly visible: its faceted stealth geometry, grill-covered engine inlets, stealthy four-probe air data measurement system, and indium-tin oxide coated cockpit windows which prevent radar waves from entering the cockpit.

Since the activation of the Nighthawk, it has undergone upgrades to keep it competitive. First, The Weapon System Computational Subsystem program was replaced. Next, the Offensive Capability Improvement Program added an improved flight management system, a digital moving map, and a digital situation display. Finally, new cockpit instrumentation with Honeywell color multi-function displays, a digital auto throttle, and a pilot-activated recovery system were added (Richardson 117). In order to protect the advantage and effectiveness of the Nighthawk for decades to come, it is necessary to withhold information from the public, including missions and technical data. It will likely be years, if ever, before the US public will truly be able to appreciate the F-117 Nighthawk.

During the 1970s, Lockheed was not the only company working on stealth technology; Northrop (later Northrop Grumman after a merger) was also working steadily on stealth. Northrop’s goals were even loftier than Lockheed’s; they were challenged with producing an “all aspect stealth.” All aspect stealth meant they were including airborne radar in their aircraft. Typically, radar is about as easy hide in battle as a searchlight is in complete darkness. The aircraft was known as the Battlefield Surveillance Aircraft – Experimental (BSAX) and would need to loiter over the battlefield for extended durations, therefore exposing it to radar from many different directions. After initial RCS testing of the BSAX produced horrendous results, Fred Oshira, a Northrop electromagneticist, worked diligently to solve the radar problem. He began carrying a piece of model clay with him, molding new shapes and analyzing them wherever he was. Oshira, while watching his children on the teacup ride during a family vacation at Disneyland, molded the clay into a new shape. It had a rounded top with flat sloped sides that flared down and outward into a knife-edge. The new design not only worked, but also provided a breakthrough in stealth design. Instead of scattering the electromagnetic waves like a mirror, it caused the waves to flow around the curved body of the aircraft. The result was a design that remained stealthy from nearly every viewing direction and defeated a wider range of radar frequencies (Sweetman “The Invisible Men” 25-26).

The new design of the BSAX easily passed RCS requirements and Northrop was given the go-ahead to build two examples—one would be used for flight testing and the other as a spare—the prototypes were code-named Tacit Blue. Tacit Blue had a bluff nose, a bulky body to accommodate onboard radar, walled sides sloping downward to knifelike edges, and V-tails with curved tips to prevent radar from picking them up. If enemies viewed the aircraft from below, they would see no sign of its two engines, as they were buried towards the rear of the fuselage behind a flush inlet on the topside. Tacit Blue’s first flight was in February 1982. It would fly 134 more times over a 3-year period and remained highly classified for over a decade. Ultimately, the lessons learned from Tacit Blue would help Northrop emerge victorious over Lockheed for the largest defense contract of the 20^th Century—the B-2 Advanced Technology Bomber (ATB) (Pace 17-18 and Sweetman “The Invisible Men” 27).

Figure #15 The Tacit Blue prototype was the first airplane to use rounded surfaces to direct radar energy in predictable directions to minimize the probability of detection by enemy radar.

Figure #16 The dorsal exhaust nozzle (left) on Tacit Blue shielded the hot exhaust gases from detection by hostile ground forces. The exhaust was mixed with air flowing through the tail to rapidly reduce its temperature and minimize the plane’s IR signature. Tacit Blue’s curved features are evident in the photo of the cockpit (right). The knowledge Northrop gained in using curved surfaces to create a stealthy design paid huge dividends in the design of the Advanced Tactical Bomber, Northrop Grumman’s B-2.

The official acknowledgment and unveiling of Northrop Grumman’s B-2A “Spirit” did not come until 22 November 1988, but the roots of the B-2 can be traced back to the late 1970s when President Jimmy Carter canceled production of the proposed 240 B-1As. The Air Force wanted the supersonic, swing-wing bomber to complement its B-52 heavy bomber in projecting US global nuclear bombing capabilities to potential adversaries. At the time, President Carter was highly criticized, but only by people who had no knowledge of the United States’ top-secret stealth technology under development (Pace 15).

Less than a week into Northrop’s design studies they decided on a flying wing, a design they had not seriously considered for over 35 years. Company founder Jack Northrop’s flying wings of the 1940s proved uncontrollable—it was decades before computerized fly-by-wire systems were available to make flight control adjustments thousands of times faster than humans. The B-2A would use a quadruple-redundant fly-by-wire flight control systems to monitor the aircraft in real time. The B-2A had ambitious design specifications including long range, high payload, and low observability. Midway through Northrop’s design process the USAF added operating at a low altitude, terrain following mode. This set the first flight back by roughly two years and added in excess of one billion dollars to the cost of the program (Goodall 69 75).

The B-2A, revealed to the world only twelve days after the F-117A unveiling (10 November 1988), differed dramatically in the shape used to achieve its remarkably low radar cross signature. Whereas the F-117A made extensive use of faceting, the larger B-2A relies primarily on carefully sculpted curves and rounded surfaces (Pace 27). Both aircraft have sharp leading edges, use RAM heavily, and have serrated edges on their landing gear and weapons bay doors. The B-2A’s engine inlets have serrated edges where they protrude above the wing and direct the intake air through an S-shaped duct to the engines. These serpentine inlets allow the highly reflective engines to be buried about one engine diameter below the intake opening—out of direct line of sight of enemy radars (Goodall 84-85). The exhaust nozzles are located on top of the wing, several feet forward of the trailing edge. This causes hot exhaust gases to mix with the boundary layer air flowing around the airframe; resulting in a temperature reduction, which lowers the infrared signature of the B-2A and limits the effectiveness of IR seeking missiles.

The effort that went into building such an advanced bomber was enormous; the B-2 industrial team developed almost 900 new materials and processes. These included, high speed milling machines that reduced processing time and increased quality, a computer-controlled drill currently marketed by Cooper Industries that improved quality and decreased chance of rework, and machines developed to cut composite material three times faster and with much greater precision. As a result, many of these developments have been implemented in other sectors of United States industry (Pace 45-46).

The B-2A was first flown on 17 July 1989 and six aircraft were produced for development testing. In his January 1992 State of the Union address, President George H. W. Bush announced the reduction from the original order of 133 B-2As to 21 aircraft (Goodall 89). With costs much higher than originally anticipated and the end of the Cold War, it has become increasingly difficult to justify 2.14 billion dollar unit cost of the B-2 (Pace 41). The B-2 is by far the world’s most expensive airplane—costing nearly half as much as a Nimitz class nuclear-powered aircraft carrier (Goodall 89). Currently, 21 B-2As are operational with the first operating unit, the 393^rd Bomb Squadron, formed on 27 August 1993. The B-2A was built to have a service life of 40,000 hours, 10,000 hours longer than the B-52 “Stratofortress” which has flown since the mid 1950s and is predicted to be retired around 2050 (Richardson 118-119).

Figure #17 The curvaceous Northrop Grumman B-2 Spirit stealth bomber makes use of RAM in addition to its carefully planned geometry to reduce its RCS to 0.1 m².

In March 1999, less than two years after it had reached Initial Operational Capability (IOC) the B-2A made its combat debut in Kosovo. Although the missions, some lasting up to 44 hours, were highly successful and allowed the ground troops to move in, the lines still remain drawn on whether to produce more B-2s. Howard P. “Buck” McKeon, R-California and House Armed Service Committee member is quoted from December 2001 as saying, “’To me, it’s a no brainer…If we go to places like Iraq, we’re going to need those planes’(McCutcheon).” Whether or not more B-2s will be purchased remains to be seen, but without a doubt—when that much money is involved—the B-2 will continue to be an extremely controversial weapons system.

Even within the short operational life span of the B-2A, many improvements have been made. The original configuration of the B-2A had two bays immediately outboard of the main wheel wells built to carry highly corrosive chemicals that were to be sprayed in the exhaust to eliminate contrails. Northrop Grumman could not get the system to work in testing and opted instead for a rearward pointed scanning device, which notifies the pilot when the airplane produces contrails. This allows the aircraft to be moved to an altitude where contrail production will cease. Instead of merely leaving the two bays unused, the idea of carrying decoy missiles, miniature missiles, or even jamming missiles that could render enemy radar useless is being explored (Fulghum and Wall 51).

Figure #18 A B-2 Spirit in flight—clearly showing its intakes with serrated protruding edges and the exhaust nozzles on top of the flying wing design.

Figure #19 The “Spirit of Indiana” is dedicated at Grissom AFB on 22 May 1999.

The U.S. Air force is testing a new magnetic radar-absorbing material (MagRAM), which has the potential of dramatically reducing downtime due to maintenance. The new Alternate High-Frequency Material (AHFM) is a permanent coating, which reduced the gap-size enough to avoid making the RCS larger. Dramatic time saving comes from the permanent spray on material compared to the current tedious method of caulking and taping joints and fasteners. “For example, today it takes about 36 hr. to remove old tape and coatings, open a skin panel, service a system, close the panel, tape and coat joints and fasteners, then cure the low-observable materials properly. In contrast, AHFM cuts the same task to an hour or less (Scott and Wall 117).” Using the AHFM would make accessing the interior of the bomber more closely resemble a traditional aircraft. Some other known improvements being tested include, improved satellite communications gear and new software which will increase the radar and navigation systems’ capabilities, reduce false-alarms, and cut the crew’s workload (Scott and Wall 117).

Following the successes of the first two operational stealth aircraft in the world, the F-117A light tactical bomber and the B-2A heavy bomber, the US Air Force focused its energy on a new stealthy fighter, dubbed the Advanced Tactical Fighter (ATF), to replace the aging fleet of F-15s as America’s air superiority fighter. According to James Goodall, “A 1981 Northrop paper defined a stealth fighter as one that could detect its opponent first and fire first, even if the hostile aircraft had a more powerful radar and a longer range missile (91).” The ATF program, which was conceived as early as 1976 but reached its final organizational form in 1986, called for two competing prototypes to be produced and demonstrated in flight tests in 1990. The two designs chosen to square off against each other were Lockheed’s YF-22 and Northrop’s YF-23 (Richardson 78-80).

The ATF program called on the competing manufacturers to combine stealth technologies, advanced avionics and weapons systems for extreme maneuverability and beyond-visual-range (BVR) engagement of adversaries, and supercruise capability [flying for extended periods at supersonic speeds without using fuel-guzzling afterburners] (Richardson 79). A request for proposal (RFP) was sent to General Electric and Pratt & Whitney soliciting designs for a Joint Advanced Fighter Engine to power the ATF (Williams 5). The Air Force selected Lockheed and Northrop from a half-dozen manufacturers submitting concepts to each produce two concept demonstration prototypes while General Electric and Pratt & Whitney produced competing engines (the YF120 and YF119 respectively) for the ATF. One prototype from each airframe manufacturer was fitted with P&W YF119 engines and the other with GE YF120 engines and flight testing of the four different combinations commenced in 1990 (Lockheed … YF-22 “Raptor”).

General Electric produced a potentially revolutionary engine with a variable bypass ratio and, at least in the case of the YF-22; GE’s YF120 engines seemed to have a slight edge on their P&W counterparts in the area of supercruise performance. Lockheed’s YF-22 made use of thrust-vectoring nozzles which improved maneuverability by deflecting as much as twenty degrees up or down whereas the Northrop YF-23 focused on a lower infrared signature from the rear of the plane and instead used only aerodynamic control surfaces (Richardson 82).

Visually, the competing models were strikingly different. The YF-22 bears a slight resemblance to the F-15 it is slated to replace. Inlets feeding air to the two buried engines are located on either side of the fuselage. It has three missile bays for internal storage of less-than-stealthy munitions, twin outward slanted tails, and a pair of all-moving stabilators (surfaces that serve as both horizontal stabilizers and elevators) at the rear. Its weapons bay and landing gear doors have serrated edges to reduce their radar signature in both the closed and open positions.

Figure #20 The Lockheed YF-22 “Raptor” prototype. The winner of the Advanced Tactical Fighter competition, the production version of the F-22 Raptor will reign the skies of the 21^st Century as America’s premier air-dominance fighter.

Figure #21 The weapons bay and landing gear doors of the YF-22 clearly illustrate its stealthy design intent. The serrated edges are specifically designed to reflect radar away from enemy positions.

The YF-23 is about seven feet longer than the YF-22 and features a distinctive diamond-shaped wing planform. Its rudder and elevators are combined into an all-moving V-tail configuration, with the tails slanted outward at fifty degrees from vertical (Richardson 82). The YF-23 fuselage was considerably more slender than the YF-22, with inlets gently protruding from the nearly flat underside. The engines were housed in two separate nacelles blended with the upper surface of the wing and the non-vectoring nozzles were designed to mask hot exhaust gasses from infrared detection underneath and behind the aircraft.

Figure #22 Both Northrop Grumman YF-23 prototypes in flight—the unsuccessful entrant in the ATF competition.

The competing designs can be summarized as: “Lockheed built an agile dogfighter’s fighter that was stealthy. Northrop built and designed a stealth aircraft that flew like a fighter. (Goodall 110)” Final proposals from the competing teams were submitted on 2 January 1991, and on 23 April 1991, the US Air Force announced the selection of Lockheed’s F-22 powered by Pratt & Whitney F119 engines for development into the production ATF (Richardson 83). Doug Richardson speculates that although the reasoning for the decision remains classified, the Air Force preferred the maneuverability of the thrust-vectoring YF-22 to the stealthier YF-23. Also, he comments that having experienced difficulty with radically new engine designs in the F-15 and F-16 programs, the Air Force opted toward the more conservative design in hopes of encountering fewer difficulties (83).

Figure #23 The YF-22 featured exhaust nozzles (one is shown here) that are capable of vectoring the YF 119 engine’s thrust 20 degrees up or down for enhance maneuverability. The powerful YF 119 engine allows for non-afterburning supercruise at speed of Mach 1.5.

The ATF program then entered the engineering and manufacturing development stage, where the final design airframe, avionics, and various other flight hardware are being flight tested and refined before low-rate production of service-ready aircraft begins (Richardson 83-84). The development of such an advanced combat aircraft has been troubled by testing delays, technical problems (mostly involving the software integration of all the aircraft’s advanced war-fighting electronic sensors and targeting systems), and government budget and procurement reductions. One substantial issue is that only a single F-22 test aircraft currently is configured for the critical role of expanding the plane’s flight envelope, thus delaying performance results needed to secure Pentagon and congressional support. Engineers are working steadily on resolving the many technical hurdles still plaguing the F-22 while the Pentagon has recently considered cutting the planned final production to 180 aircraft from the original number of 762. This number represents less than half the 380 airplanes that the U.S. Air Force deems absolutely necessary to meet defense requirements (Wall 48-49).

Lockheed’s F-22 design, dubbed the “Raptor”, underwent several significant alterations in moving from the YF-22 prototype to the production F-22. Lockheed learned from the 1981 mishap in which severe control problems resulted from the undersized F-117A V-tails—one of which parted company with the plane as test pilot Hal Farley flew it the first time. Largely influenced by that mistake, Lockheed made the YF-22 tails seventy percent larger than the F-15 (Goodall 108). Following flight testing, it was determined that the vertical tails for the production F-22 could be decreased in size by about twenty-five percent. The Raptor’s wing sweep has been reduced by about six percent at the leading edge and low-observable considerations led to a redesign of the stabilators and trailing edge of the wing. The cockpit has been moved forward slightly and the engine inlets moved aft to improve pilot visibility to the front and sides of the Raptor (Williams 6).

Figure #24 One of two vertical tails on the YF-22 Raptor prototype. The tail size has been reduced by an estimated 25% for the production models of the Raptor.

The production model of the Raptor will be designated the F-22A. An industry team consisting of Lockheed Martin, the Boeing Company, and General Dynamics is responsible for manufacturing the Raptor. Primary contractor Lockheed Martin touts the F-22A as the only truly stealthy air defense fighter in the world. Unlike the F-117A Nighthawk and B-2 Spirit, the F-22A, will use its low observable characteristics in a different fashion. The preceding stealth airplanes combined their miniscule radar cross sections with careful mission planning to slip through gaps in enemy air defenses. The airplanes’ low RCS causes these gaps in the radar net by drastically reducing the distance at which a radar installation can detect them (Williams 19).

The F-22A pilot will not have the luxury of detailed mission planning to ensure her plane remains invisible to enemy radar. The role of the Raptor will be varied and include such tasks as escorting other attack aircraft to their targets, hunting enemy fighters, and destroying air defenses such as SAM sites (Williams 19). As a result, the Raptor will use its marble-sized RCS (Fulghum, “USAF Plans…” 24) in conjunction with its advanced radar targeting system to ensure that it can detect, and kill an enemy before it even sees the Raptor on its own radar (Fulghum, “New F-22 Radar…” 50).

Figure #25 The F-22 in flight. This aircraft is an engineering and manufacturing development prototype, used for flight testing before low-rate production of service-ready aircraft begins.

The most expensive and most capable fighter in the world, the F-22 will soon bring a new level of aerial dominance capabilities to the US Air Force inventory. Fulghum reports, “The breakthrough difference [rather than simply speed or stealth]… is its expanded range of lethality provided by the stealth fighter’s hard-to-detect and highly integrated radar and sensor suite (“New F-22 Radar…” 50).” With its classified [BAE Systems produced] ALR-94 passive receiver system, the F-22 will be able to passively detect an enemy (assuming the enemy has its radar on) at a range of 250 nautical miles (Williams 9). The Raptor can use its own, nearly undetectable, radar to spot an enemy aircraft at 120 nautical miles and then (as soon as the new AIM-120 air-to-air missiles enter service) fire and eliminate the threat from 100 nautical miles—well before the enemy detects the F-22 (Fulghum, “New F-22 Radar…” 50). The F-22 holds its largest advantage in this type of BVR confrontation—as specified by the original ATF requirements; it was designed to destroy enemies before they can even see it.

The ability to transmit radar signals and remain undetected is a giant improvement upon previous generations of stealth aircraft (recall that the F-117A has no radar). The major breakthrough, still highly classified, lies in the Northrop Grumman/Raytheon APG-77 radar. This active, electronically scanned array (AESA) consists of an interlinked set of small transmitters and receivers and is believed to be the most advanced of its kind in existence. Offering an inherently smaller RCS than traditional radar and cleverly shrouded by RAM, the AESA is well suited to the Raptor’s low-observable nature. The AESA is extremely efficient, sending narrow beams of energy in repeated bursts to identify and track targets rather than scanning the entire sky (Williams 8). Information the AESA collects is stored and compiled for use in targeting enemy aircraft. The F-22’s radar activity is hidden by meticulous use of short pulses of energy directed in narrow beams that are “randomly” distributed over the frequency spectrum (Fulghum, “New F-22 Radar…” 50). The AESA confuses an enemy’s radar interception system as it varies its beam and pulse width, scan rate, and pulse-repetition frequencies. Such inconsistent and varied radar energy does not allow an adversary enough information to recognize the Raptor as a threat (Williams 9). Gen. Richard Hawley remarked that the APG-77 will be able to “detect other airplanes and smaller targets at significantly greater ranges than today’s radar…Perhaps more importantly, it will be integrated into an avionics architecture that will give the pilot a better and more synthesized view of the battlefield (Rhea).”

In another effort to improve battlefield awareness, the Raptor will also make use of a secure intra-flight datalink (IFDL) to communicate with other F-22s and further increase the lethality of a group of Raptors. The sensors of several aircraft can be linked together with the IFDL, allowing some clever pack-hunting tactics to be used. For instance, a single F-22A that is outside of missile range tracks an enemy fighter with its AESA radar—possibly even disclosing his location in doing so. This Raptor sends the necessary targeting information via the secure datalink to another Raptor closer to the enemy but not using its own AESA radar, and the undetected Raptor fires a missile to eliminate the enemy aircraft without ever being located (Williams 9).

The US Air Force envisions using the F-22A alongside the B-2 as pivotal members of a “kick-down-the-door” force consisting of America’s most potent stealth aircraft. This special unit would likely include twelve B-2s and forty-eight F-22s and would provide a massive strike task force capable of destroying 270 to 426 targets anywhere on the globe in the first day of a conflict. The F-22s would rid the skies of enemy aircraft, paving the way for the B-2s to drop their precision munitions. After aerial dominance is obtained, attention would focus on destroying SAM missiles and other air defenses, which would wreak havoc on conventional strike aircraft (Fulghum, “USAF Plans…” 24). Fulghum reports that, “Because of its small radar return, …high operating altitude (40,000 ft.), and fast cruising speed (Mach 1.5), the F-22 has about ‘12 times more airspace’ than conventional aircraft in which to operate safely. In that space, antiaircraft radar cannot detect it with enough precision to shoot it down, said retired USAF Gen. Richard Hawley (“USAF Plans…” 24).”

Whereas the B-2 program was the most expensive military aircraft program of the 20^th Century, the Joint Strike Fighter program could potentially be the largest of the 21^st Century. The Joint Strike Fighter (JSF) program grew out of several previous US Department of Defense aircraft development programs dating as far back as the 1980s. In 1987, design began on an advanced short-takeoff, vertical landing (STOVL) airplane as part of the ASTOVL program that would result in a replacement for the US Marine Corps’ AV-8B Harriers (subsonic STOVL attack fighters) and F/A-18s (Williams 27). By 1990, this technically troubled program came under the direction of DARPA, which outlined new requirements for the project—the main two being a 24,000-pound maximum empty weight and use of the Joint Advanced Fighter Engines then under development for the YF-22 and YF-23. As the Air Force became interested in a modified version of the ASTOVL plane which would have an extended range by using conventional takeoff and landing techniques, the program became known as the Common Affordable Lightweight Fighter (CALF) (Sweetman Lockheed Stealth 119-122).

The Air Force and Navy were in the midst of jointly developing a heavy attack aircraft known as the A/F-X when the Clinton administration cancelled the program in 1993. During the same year, the Pentagon formed the Joint Advanced Strike Technology (JAST) program to investigate all aspects of a next-generation strike airplane. Under congressional urging, the CALF program was placed under the direction of JAST (Williams 26-30). After the setback of having the A/F-X program cancelled, the Air Force and Navy were keen on gaining involvement in JAST in order to acquire a new attack aircraft to replace the aging fleets of F-16s and F-14s. The JAST program took shape under the direction of Maj. Gen. George Muellner who had a goal of creating a single fighter plane capable of meeting the requirements of the Air Force, Navy, and Marines in slightly different variants while maintaining a high degree of commonality among them in order to cut costs (Sweetman Lockheed Stealth 122). In 1996, prototype proposals were solicited for the JAST fighter, and the program name was soon changed to Joint Strike Fighter (Williams 31).

From Maj. Gen. Muellner’s vision emerged the concept of a multi-service attack fighter that would provide the flexibility and war-fighting ability to serve three branches of the US military as well as British and other allied nations well into the 21^st Century. Three different variants of the Joint Strike Fighter would be designed. The first, a conventional takeoff and landing aircraft for the US Air Force, will replace the F-16 as the primary light fighter and attack aircraft. Although its performance is only slightly better than the F-16 it will replace and significantly slower and less maneuverable than the F-22, the Air Force is anxious to reap the rewards of the JSF's low-observable design. The Navy, which would rather jump off a carrier without a life jacket than buy a single-engine fighter, had little choice but to accept the single-engine JSF (Sweetman Lockheed Stealth 123). The Navy variant has larger wings and a reinforced structure to withstand the stresses of carrier catapult launches and arrested landings. The third and most technologically challenging version is the Marine Corp STOVL variant. This aircraft is designed to take off and land from the decks of the Marines’ small carriers and from forward operating locations where full-length runways are not available. Making this engineering problem even more difficult, the Marines required that the STOVL variant (unlike the Harrier) be supersonic, stealthy, and capable of landing with a 5,000-pound payload (Hadingham 71).

In November 1996, Lockheed and Boeing were selected to construct two concept demonstration aircraft to undergo flight-testing beginning in 2000. Boeing’s design, designated the X-32, featured a high, delta-wing arrangement with a V-tail, and single inlet under the nose of the aircraft. Boeing’s Dr. John McMasters commented on the appearance of the X-32 in a visit to Prof. Crossley’s A&AE 251 class on 8 November 2002, saying, “We nicknamed it the Angry Frog.” Dr. McMasters also expressed his personal opinion that the X-32 was “the world’s second ugliest airplane.” Boeing chose to use a relatively low-risk direct lift approach similar to the Harrier for its STOVL variant. “A direct lift system redirects the thrust from the engine through a series of downward-pointing nozzles on takeoff and landing (Hadingham 71).” This decision caused Boeing to require incredible amounts of thrust from the engine while at the same time having to focus their design efforts on minimizing the weight of the aircraft (Hadingham 71).

Figure #26 The “Angry Frog,” Boeing’s entry into the JSF competition, officially designated the X-32.

The Lockheed Martin design, designated the X-35, is reminiscent of the F-22 Raptor with its mid-wing, twin tails, dual stabilators, and an inlet on each side of the fuselage (Williams 34). Lockheed’s STOVL design relied on an innovative lift fan system designed by Purdue University Aeronautics & Astronautics alumni Paul Bevilaqua. Bevilaqua, working for Lockheed’s Skunk Works, patented the lift fan, which was powered by an astonishing 28,000 shaft horsepower from the aircraft’s jet engine. An extremely durable clutch and gearbox had to be developed in order to make the 90-degree turn from the horizontal jet engine to power the vertically mounted lift fan, located just behind the cockpit (Hadingham 71-72). The lift fan utilizes two counter-rotating fans to produce 18,000 pounds of thrust—nearly half the thrust needed for the X-35 to hover (Sweetman “Lockheed Stealth” 131). The largest challenge Lockheed faced was proving the lift fan system to be reliable enough for use in a front-line jet fighter (Hadingham 72).

Figure #27 Lockheed Martin’s X-35 lands after completion of a flight testing mission.

After the completion of a rigorous flight test involving all three variants of both the Boeing X-32 and Lockheed X-35, on 26 October 2001, the Pentagon announced the Lockheed Martin X-35 as the winner of the Joint Strike Fighter competition (Hadingham 76). Following the decision, many in the aerospace industry expressed the sentiment that Lockheed’s innovative STOVL design, built upon Bevilaqua’s patented lift fan, was the key factor in the decision. Both companies’ Air Force and Naval variants performed similarly, but the X-35 outclassed the X-32 in STOVL tests. Lockheed’s lift fan design produced more lifting power than Boeing’s direct lift system in a hover in and expelled exhaust about 1,000 degrees (Fahrenheit) cooler—reducing risk to ground crewmembers and damage to landing surfaces. These two factors gave Lockheed’s design a huge edge in power and safety, sealing the contract victory (Hadingham 76).

Figure #28 The Pratt & Whitney F 119 engine installed in Lockheed Martin’s X-35 is tested, resulting in a dramatic nighttime afterburner display.

Lockheed has been working diligently with Pratt & Whitney to reduce the infrared, acoustic, and radar signatures of the JSF engine. In addition, knowledge acquired in the F-22 program undoubtedly will be integrated into the JSF. Specifically, experts speculate that the F-22 engine exhaust radar blocker will be applied to the JSF as well. Engineers have toiled to develop a device to block the radar return of the exhaust cavity and simultaneously survive the extreme temperatures (Fulghum “Stealth is Still…” 46). Researchers from Lockheed claim to have developed a method of blocking the radar return of the F-22’s nozzles without compromising the fighter’s IR signature—a technique almost certainly to be borrowed for the JSF program (Fulghum “Stealth Engine Advances…” 90). A great deal of attention has been focused on designing stealthy inlets for the JSF engine. The rotating blades of a fighter engine and the inlet that feeds air to it are among the largest contributors to an aircraft’s RCS. Michael Dornheim quotes an anonymous stealth expert as saying, “The front-aspect radar cross section of an F-15 [the current USAF air superiority fighter] is about 10 sq. meters, and of that about 10 sq. meters comes from the inlets (92).” As a result, the JSF design uses serpentine inlets coated with RAM and using special inlet vanes designed to absorb radar energy in a similar fashion as the F-22 (Dornheim 99).

The JSF will use an active electronically scanned array (AESA) radar system similar to but smaller than the F-22. The smaller AESA will have a range of about 90 miles and be optimized for targeting enemy ground installations, but the JSF will also be capable of fending off aerial threats if necessary. The AESA’s range is dictated by the number of transmitter/receiver modules it has—the JSF has fewer than the F-22 to reduce size and cost, hence the shorter effective range (Fulghum “F-22, JSF Designed…” 53).

The Joint Strike Fighter is planned to compliment the F-22 Raptor as a lower flying, primarily ground attack aircraft. Major General John L. Hudson described the different roles of the JSF and F-22 on 18 November 2002 in a speech at Purdue University. Maj. Gen. Hudson described the F-22 as a key part of the “kick-down-the-door force,” that would obtain air superiority while B-2 bombers eliminated key strategic ground targets in the first few days of a conflict. Then, a “persistence force” consisting largely of Joint Strike Fighters would continually seek and destroy mobile ground targets including mobile missile launchers, tanks, and armored troop carriers. Having been designed for this ground attack role, Hudson explained, the JSF combines survivability and lethality with its blend of stealth, speed, defensive countermeasures, and weapons systems capabilities.

The F-117, B-2, F-22, and JSF will all serve as strong deterrents to potential adversaries of the United States. The F-117 and B-2 have proven the effectiveness of stealth weapons delivery systems in combat. In addition to the F-22 and JSF (both due to enter service within the next decade), these aircraft will provide the U.S. with the ability to inflict decisive, unilateral blows in any near future conflict. Modern radar defense systems have been effectively countered by the United States’ new breed of warplanes featuring RAM, stealthy geometry, and electronic jamming. Undoubtedly, these new aircraft, which trace their ancestry back to the U-2 and SR-71 spyplanes, will be challenged in the coming decades by more advanced defense systems. As a result, research into stealth technology will not cease in the foreseeable future but instead will continue in an effort to remain ahead of the detection technologies of potential adversaries. The traditional course of military advancement indicates that stealth airplanes will be challenged by a new generation of defensive aircraft detection systems. However, the United States will, in all likelihood, maintain current research in the field of low-observable technology in order to retain the current tactical advantage offered by stealth aircraft.

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Photo Credits

Pollock, George and Bart Hott at the USAF Museum Dayton, OH: taken on

26 October 2002 Figures: 2, 4, 5, 6, 7, 9, 10, 11, 12, 13, 15, 16, 20, 21, 23, 24

Ultimate Military Collection obtained from <http://www.military.cz/usa/air/in_service/aircraft/f117/pics/pics_en.htm>. 27 October 2002 Figure: 9

United States Air Force Photos <http://www.af.mil/photos/>. 27 October 2002

Figures: Cover, 1, 3, 14, 17, 18, 19, 22, 25, 26, 27, 28

Credits

Followed the rules and convention for citation instituted by the Modern Language Association (MLA). Instruction viewed on 2 November 2002 at the following:

<http://campusgw.library.cornell.edu/newhelp/res_strategy/citing/mla.html#mla>.

<http://www.bedfordstmartins.com/online/cite5.html>.