How Does Stealth Technology Actually Work?
At precisely 3 in the morning on January 17, 1991, a series of explosions rocked the Iraqi capital of Baghdad. The explosions were exact and devastating, destroying Iraqi military radars, command bunkers, and communications hubs with surgical precision. Immediately, Iraq’s aerial defence system sprang into action, its 3,000 anti-aircraft guns saturating the air with explosive shells. But the 60 surface-to-air missile batteries defending the country remained silent, their radars fruitlessly searching the skies for the unseen intruders. Little did the Iraqis know that they were up against an entirely new type of weapon: an aircraft designed to be almost invisible to radar, allowing it to penetrate deep into enemy territory and strike at critical targets with impunity. Known as the Lockheed F-117 Nighthawk, this revolutionary “stealth fighter” would go on to fly 1,250 sorties and drop more than 2,000 tons of bombs on Iraq without a single aircraft being shot down, helping bring Operation Desert Storm – the UN-backed expulsion of Iraqi forces from Kuwait – to a swift and decisive conclusion. Since then, the F-117’s revolutionary technology has proliferated throughout the world’s militaries, completely reshaping the face of modern warfare. But how does this technology work? How is it possible to render an entire modern jet aircraft nearly invisible? Well strap in as we dive into the fascinating history and technology of stealth.
In the early days of military aviation, little attention was paid to stealthiness. Anti-aircraft defences were primitive, and it was more important for aircraft to be easily recognized as friend or foe by ground forces and other aircraft. For this reason, during the First World War aircraft were prominently marked with national insignia like British and French tricolour roundels or the German Balkenkreuz. But as the War progressed and aviation technology improved, greater emphasis was placed on camouflage both on the ground and in the air. Most nations thus adopted various camouflage schemes, generally consisting of pale blue or tan lower surfaces to blend against the sky and green or brown upper surfaces to match the ground below. One unusually elaborate scheme was the Lozenge Camouflage used by the Luftsreitkräfte or Imperial German Air Force. This consisted of a pattern of four or five-sided polygons in a variety of colours including blue, grey, green, black, purple, and even pink, pre-printed on the fabric used to cover aircraft wings and fuselages. These disruptive patterns were intended to break up the aircraft’s outline against the ground and sky and were printed in a number of versions for use on upper and lower surfaces and for day and night operations. However, even late into the war many fighter pilots eschewed camouflage for garish colour schemes intended to aid unit cohesion, display national pride and individuality, and intimidate the enemy. For example, pilots of Jagdgeschwader I, the famous “Flying Circus” commanded by top German ace Manfred von Richthofen – AKA the “Red Baron” – painted their aircraft bright crimson, while those of Jasta 6 opted for zebra-like black and white stripes.
Despite this, the Germans did make the first attempt to build a truly stealthy aircraft. In 1916, they replaced the fabric covering of a Fokker E.III monoplane fighter with sheets of an early transparent plastic called Cellon, creating something that looked like the ancestor of Wonder Woman’s invisible jet. Unfortunately, testing revealed that while the aircraft was indeed difficult to spot against a clear sky, in cloudy conditions light reflecting off the shiny Cellon skin actually made the aircraft even more visible! Back to the drawing board. While the Germans chose not to pursue this approach any further, in 1935 the Soviet engineer Sergei Kozlov tried building his own transparent aircraft, a modified Yakovlev AIR-4 reconnaissance plane known as the Kozlov PS. However, the Soviets ran into the same problems as the Germans two decades before, and this project, too, was soon scrapped.
In the interwar period, relatively few advancements were made in the field aircraft camouflage. Indeed, the growing doctrine of Air Power championed by theorists like Italian Army Major Giulio Douhet held that modern bomber aircraft flew too high and fast and were too heavily armed to be shot down, allowing them to attack their targets with impunity. This conceit, which British Prime Minister Stanley Baldwin described as “The bomber will always get through,” made camouflaging an aircraft’s approach redundant. However, several outside thinkers circles saw the advantages of aerial camouflage, and tried to sell the military on their theories. Among these was New Hampshire artist Abbott Handerson Thayer, who, after observing and painting countless animals in the wild, came up with a theory of camouflage known as counter-shading. According to Thayer, the dark upper and light lower colouration of most animals serves to camouflage them against their environment by countering the effects of light and shadow. Normally, direct sunlight makes an animal appear light above and dark below. Countershading, however, darkens the light areas and lightens the dark, reducing the animal’s contrast against its background and making it appear flat. Or, as Thayer himself described the effect in his 1909 book Concealing-Coloration in the Animal Kingdom:
Animals are painted by Nature darkest on those parts which tend to be most lighted by the sky’s light, and vice versa. … the fact that a vast majority of creatures of the whole animal kingdom wear this gradation, developed to an exquisitely minute degree, and are famous for being hard to see in their homes, speaks for itself.
Thayer’s conclusions are now disputed, with modern zoologists arguing that not all countershading in animals is related to camouflage and that certain colour schemes are instead intended for display, warning, or thermoregulation purposes. Nonetheless, based on his observations, in 1902 Thayer filed a patent on the use of counter-shading to camouflage ships at sea, and during the First World War tried to convince the U.S. Navy to adopt his methods. But the Navy was uninterested and ignored him. Shortly before the Second World War, zoologist Hugh Cott took up the cause and tried to sell the British armed forces on the concept of counter-shading. Unfortunately, Cott had the misfortune of being the protege of John Graham Kerr, a biologist who during the First World War had argued with artist Norman Wilkinson over the effectiveness of Dazzle Camouflage, a scheme to paint ships in bold black-and-white stripes to break up their outlines and protect them from submarines – and for more on this bizarre form of camouflage, please check out our previous video Why Were So Many British and American Warships in World War I Painted Like Zebras? In 1939, Norman Wilkinson was serving as the British Army’s Inspector of Camouflage, and thus held enormous influence over how military vehicles and equipment were painted. Despite this, Hugh Cott was invited to take part in an experiment in which two 12-inch railway guns were camouflaged – one using Cott’s counter-shading method, the other using conventional techniques. The guns were then photographed from the air from different angles. According to observer Peter Forbes, the contrast was striking:
“…[Cott’s gun was] invisible except to the most minute scrutiny by someone who knows exactly where to look and what to look for. The other gun is always highly visible.”
Though this should have vindicated Cott, the demonstration embarrassed the Army and they refused to adopt Cott’s ideas, arguing that his scheme was too complicated to apply properly. Then, in order to get the troublesome zoologist out of the way, the Army posted him to the Middle East. But while counter-shading was never formally adopted by any armed forces during the Second World War, Thayer and Cott’s ideas nonetheless influenced camouflage doctrine throughout the conflict. Most nations’ fighting aircraft were painted some combination of pale blue or grey below and darker colours matching the surrounding terrain above, roughly approximating a counter-shaded scheme. In addition, reconnaissance aircraft like versions of the British Supermarine Spitfire and American Lockheed P-38 Lightning were painted all over in a special dark azure colour known as “PRU Blue.” Developed by Australian inventor Sidney Cotton, the colour helped these unarmed aircraft blend against the dark sky at high altitudes.
The war also saw the development of an unusual camouflage system known as “Yehudi Lights.” From 1943 onwards, the introduction of long-range patrol aircraft allowed the Allies to turn the tide against German U-boats, which had been decimating merchant traffic in the North Atlantic since the very first day of the war. However, U-boat crews often spotted these slow-moving aircraft from several kilometres away, giving them enough time to crash dive and slip away. Some method was thus needed to conceal these aircraft’s approach, allowing them sneak up on unsuspecting U-boats. Developed by the U.S. Navy in 1943, Yehudi Lights consisted of a set of electric lights attached to the leading edges of an aircraft’s wings and powered by an onboard generator. When activated, these lights washed out and broke up the aircraft’s dark silhouette, making it harder to see against the bright sky. This is similar in principle to the technique of counter-illumination used by certain sea creatures like the mid-water squid. The underside of these squid are covered in light-producing organs called photophores, which when activated help lighten its and conceal its silhouette when seen from below. The system’s unusual name came from the late 1930s slang phrase “Who’s Yehudi?”, used to refer to a mysterious or absent person.
In 1945, Yehudi Lights were tested aboard a Grumman TBF Avenger torpedo bomber. Promisingly, while a regular aircraft was detected at a range of 19 kilometres, the Yehudi-equipped Avenger got within 3 kilometres (around 30 seconds’ flying time) of the test ship before being spotted – too close for a U-boat to crash-dive. A similar system called Diffused Lighting Camouflage was also tested by the Royal Canadian Navy, and consisted of a series of spotlights aimed at the sides of a ship to counter-illuminate and conceal its silhouette against the sky. Between 1941 and 1943, the system was installed experimentally on a number of RCN vessels including the Flower-class Corvette HMCS Rimouski, which in September 1943 spotted and intercepted the German submarine U-536 in the Baie des Chaleurs, Quebec. The U-boat appeared not to see Rimouski approaching, and the ship moved in for the kill. Unfortunately, at the last moment a stray radio communication from the shore alerted U-536’s commander, Kapitänleutnant Schauenburg, and the submarine succeeded in crash diving and escaping. Despite these promising results, neither Yehudi Lights nor Diffused Lighting Camouflage were used operationally during the war, though the idea may be making a comeback. In 2008, researchers from the University of Kansas covered a small drone in electroluminescent panels whose colour could be matched to the sky. Flying at 300 metres, the drone became almost invisible to the naked eye.
By the time such visual camouflage systems came of age, however, they had been rendered all but redundant by one of WWII’s game-changing technological innovations: Radar. Developed independently by the Germans and the British in the late 1930s, Radar – short for RADio Detection And Ranging – works by emitting a beam of microwaves, which bounce off the target and are detected by the radar receiver, allowing the target’s position and speed to be measured. For the first time, Radar allowed incoming aircraft to be detected far beyond visual and auditory range, giving ample warning of an attack and sufficient time for antiaircraft defences to be deployed. Indeed, the British Chain Home radar system and its associated ground-controlled interception network, the Dowding System – was a pivotal factor in allowing the Royal Air Force to defeat the German Luftwaffe during the 1940 Battle of Britain. Meanwhile, the German Kammhuber Line, a network of Freya and Wurzburg radars, radar-controlled searchlights and antiaircraft guns, and night fighters, made the skies over Germany deadly for RAF bombers attempting to pound Nazi industry into submission. Early attempts at radar countermeasures, however, focused not on making aircraft less detectable but rather on jamming and confusing the radar itself. The simplest of these countermeasures was code-named Window, which consisted of thin strips of aluminium foil dropped in bundles out of aircraft. These clouds of aluminium returned massive radio echoes, overwhelming and blinding the German radar operators. Window turned out to be one of the Allies’ most effective secret weapons, being used to devastating effect during the firebombing of Hamburg in July 1943 and the massive deception campaign in the leadup to the D-Day landings in June 1944 – and for more on this, please check out our previous video Operation Bodyguard: the Grand Deception That Made D-Day Possible. Modern versions of Window, known as “chaff,” are still carried by military aircraft today, and along with high-temperature infrared flares, are launched in order to defeat radar and infrared-guided antiaircraft missiles. The Allies also developed a number of active electronic jamming systems including Moonshine, Cigar, Grocer, and Jostle, which broadcast powerful radio systems intended to overwhelm German radar and night-fighter radio systems.
While no WWII aircraft were deliberately designed to be invisible to radar, a few managed to achieve this ability by accident – including the British de Havilland Mosquito light bomber. Due to wartime metal shortages, the Mosquito was built almost entirely of laminated plywood, which happened to be transparent to German radar. While the Mosquito’s large metal engines and propellers still returned radar echoes, the wooden airframe gave the aircraft the radar cross-section of a much smaller aircraft, making it much harder to detect and track. A fundamental term when discussing stealth technology, radar cross-section or RCS refers to the size of a spherical metal reflector that would return the equivalent radar echo of a particular vehicle. While it is impossible to make an aircraft completely invisible to radar, modern stealth techniques can reduce the radar cross-section of an aircraft the size of the Northrop-Grumman B-2 Spirit – a bomber with a 52 metre wingspan – down to that of a tennis ball.
Another WWII aircraft commonly believed to have stealth capabilities was the experimental German Horten Ho-229 flying wing jet fighter. A futuristic design, the Horten lacked a fuselage or vertical tail surfaces, parts of regular aircraft that are excellent radar reflectors. Like the Mosquito, it was also built largely of wood, and was even coated in paint containing carbon particles intended to absorb radar waves. Theoretically, all this should have made the Horten the world’s first true stealth aircraft, but the war ended before it could see service. In 2008, however, the Northrop-Grumman Corporation and the National Geographic Society teamed up to evaluate the advanced German jet’s anti-radar capabilities. The team constructed a full-sized non-flying mockup, took it to Northrop-Grumman’s radar test range at Tejon, California, and exposed it to radar waves at various frequencies. These tests revealed that the model had a radar cross-section around 40% that of the standard German WWII fighter, the Messerschmitt Bf-109. However, this experiment failed to account for two important factors: first, the real aircraft was not built entirely of wood and featured a large internal steel structure; and second, its twin jet engines – especially their spinning compressor blades – would have been strong radar reflectors. These two factors combined would have made the Horten’s stealth capabilities marginal at best. Even the innovative carbon-laced paint was found to be largely ineffective against WWII-era radars.
Indeed, the first deliberate application of radar stealth technology took place not in the air, but at sea. The development of centimetric airborne radar gave the Allies a huge advantage in the Battle of the Atlantic, allowing patrol aircraft to detect objects as small as a submarine periscope on the ocean surface from several kilometres away. In response, the Germans developed innovative radar-absorbing materials codenamed Schornsteinfeger and Sumpf, which was applied to U-boat periscopes and the snorkels they used to run their diesel engines while submerged. These materials were effectively versions of the Salisbury Screen developed by American engineer Winfield Salisbury in the early 1940s. This consisted of a sheet of insulating material of a particular thickness sandwiched between a metal screen and a solid metal sheet. When a radar wave hit the screen, it split into two components: one half of the wave reflected off the screen, while the second traveled through it and bounced off the solid metal layer. The two waves then cancelled each other out through destructive interference, causing the object to which they were applied to return a much weaker radar signature. The Germans also pioneered a form of acoustic stealth technology to protect U-boats against ASDIC, the Allied codename for active sonar. Codenamed Alberich after the invisible dwarf from Germanic mythology, this took the form of special rubber tiles applied to the hull of the U-boat. The tiles were covered in small, specially-shaped holes which would absorb any ASDIC sound waves that struck them. Alberich was applied experimentally to a handful of U-boats including the U-480. The coating proved highly effective, with U-480’s commander, Oberleutnant Hans-Joachim Förster, managing to sink four ships in five days in August 1944 without ever being detected by the Allies. Unfortunately, the War ended before the technology could be implemented on a wider scale. Today, however, anechoic tiles like Alberich are a common feature on many military submarines.
The development of true stealth technology would have to wait until the post-war period – but only after some hard lessons were learned in combat. In the early years of the Cold War, the main Western strategy for defeating Soviet air defences was simply to fly too high and fast to be intercepted. In 1956, Lockheed Aircraft introduced the U-2, a spy plane designed to penetrate deep into Soviet airspace and photograph strategic military installations. Essentially a jet-powered glider with enormous 31-metre wings, the U-2 was hardly stealthy, but it could cruise at an altitude of 24,000 metres – far higher than any Soviet fighter or missile could reach. So while Soviet radar easily tracked the U-2 overflights, for four years the aircraft enjoyed relative immunity from attack. Then, on May 1, 1960, U-2 pilot Francis Gary Powers was shot down by a Soviet S-75 Dvina surface-to-air missile while on a mission over Sverdlovsk. In addition to sparking a major diplomatic incident, the shootdown revealed that Soviet missile technology had caught up with the U-2, and that some method was now needed to conceal future overflights. Thankfully, Lockheed had already initiated a project codenamed RAINBOW with the goal of making the U-2 less visible to radar. Ultimately three solutions were developed, codenamed Trapeze, Wires, and Wallpaper. Trapeze and Wires consisted of fine steel wires mounted on short fibreglass pylons strung along the leading and trailing edges of the wings and along the sides of the fuselage, which broke up and scattered radar waves; while Wallpaper was a Salisbury Screen-based radar absorbing material applied to the aircraft’s fuselage.
Unfortunately, testing revealed that these modifications were only marginally effective against Soviet Radar. They also produced significant drag and seriously impacted performance, costing the U-2 1,500 metres in altitude and 2,200 kilometres in range. Even worse, the Wallpaper material acted as a thermal insulator and caused the engine to overheat – with fatal consequences. During an April 2, 1957 test flight, heat buildup caused the aircraft’s engine to fail. This in turn caused the cabin to depressurize and test pilot Robert Sieker’s pressure suit helmet to pop open. Sieker immediately lost consciousness and the aircraft entered a flat spin and spiralled towards the ground. Though Sieker eventually regained consciousness and bailed out, he was too close to the ground and was killed. Nonetheless, operational flights of the modified U-2s, codenamed Dirty Birds or Covered Wagons, began in July 1957. A total of nine missions were flown before the system was deemed ineffective and the project was abandoned.
With the U-2 quickly becoming obsolete, Lockheed decided to take a different approach to defeating Soviet air defences. Rather than out-climbing Soviet fighter and missile, they would instead out-run them. The result was the A-12 OXCART, a futuristic titanium spy plane capable of flying at more than three times the speed of sound. While the A-12 and the later SR-71 Blackbird’s main defence was their sheer speed, great effort was also taken to minimize their radar cross-section – the first aircraft to be so-designed from the ground up. Stealth features included angled vertical tail surfaces and a sharp “chine” around the fuselage to deflect radar waves away from the aircraft, the use of radar-absorbing composite materials in many components, and fuel additives to make the jet exhaust less detectable to infrared sensors. The result was an aircraft with the radar cross-section of a Piper Cub – a civilian recreational aircraft half its size.
However, the first stealth aircraft as we understand the term today would not appear until the 1980s, with the technology that made it possible coming – ironically – from the Soviet Union. In 1962, Soviet physicist Pyotr Ufimtsev published a groundbreaking paper titled Method of Edge Waves in the Physical Theory of Diffraction, in which he described how to accurately calculate the radar cross-section of any shape and how to design a shape that would reflect the majority of radar waves away from its source. Though it attracted little interest in Russia, the paper was secretly translated into English, and in 1975 came to the attention of Lockheed radar specialist Denys Overholser. Overholser realized that Ufimtsev’s theories could be used to render an aircraft nearly invisible to radar. Based on his recommendations, Lockheed and the US government approved a development program under the codename Have Blue. The design of the specially-shaped stealth airframe was undertaken by engineer Bill Schroeder. As 1970s computers were not powerful enough to analyze complex curved shapes, Schroeder designed the aircraft entirely out of flat angled planes to make the analysis easier – a technique now known as faceting. This, however, resulted in a shape that was completely unstable in flight. Thankfully, recent developments in computerized stability augmentation systems – also known as fly-by-wire – allowed the radical new aircraft to be made flyable.
The result of Have Blue was a pair of sub-scale technology demonstrator aircraft that looked unlike anything the world had ever seen. Wedge-shaped and covered in sharp angled facets, the aircraft looked so un-aerodynamic that its builders jokingly nicknamed it “the hopeless diamond.” In addition to its trademark faceting, Have Blue integrated a number of other RCS-reducing features, including sharply-angled vertical tail surfaces like the A-12 and SR-71. On regular aircraft, the most radar-reflective features tend to be cavities like the cockpit, engine intakes, and wheel wells as well as moving parts like spinning jet engine compressor blades. Thus, on Have Blue, great care was taken to hide or fair over these features. The steeply-angled canopy was coated in a thin layer of gold, which reflected radar waves away from the cockpit while not affecting the pilot’s vision. The engines were buried deep in the fuselage, with protective screens covering the intakes, while heat-absorbing tiles lining the exhaust ducts reduced the exhaust temperature and the aircraft’s infrared signature. Finally, the whole aircraft was covered in special paint impregnated with tiny iron spheres, which would absorb radar waves and dissipate their energy in the form of heat.
Have Blue made its maiden flight on December 1, 1977 above Groom Lake, Nevada – better known as Area 51. The test program revealed that the aircraft worked exactly as intended; the stability augmentation system allowed for smooth, controllable flight, while the stealth features reduced the aircraft’s RCS to that of a marble. While both prototypes were destroyed by mechanical problems unrelated to their stealth features, the program was considered successful enough for Lockheed to proceed to the next phase, codenamed Senior Trend. The result was the F-117 Nighthawk, which made its maiden flight on June 18, 1981. Though popularly known as the “Stealth Fighter”, the F-117 was designed from the outset as a bomber capable of penetrating deep into enemy territory and carrying out precision strikes using laser-guided munitions. In addition to the stealth features developed by Have Blue, the F-117 lacked radar or any other transmitters that could betray its presence and carried its weapons in an internal bomb bay. 64 F-117s were produced, the type first seeing combat during the US invasion of Panama in 1989. The Nighthawk remained in service for 25 years, finally being retired in 2008. In that time, only one Nighthawk was ever lost in combat: an aircraft flown by Lieutenant Colonel Darrell Zelko, shot down by a missile over Yugoslavia on March 27, 1999.
The success of the F-117 paved the way for further advances in stealth technology. In 1989, the same year the F-117 was first deployed, the Northrop Corporation – now Northrop Grumman – introduced the B-2 Spirit Stealth Bomber. Echoing the design of the wartime German Horten 229, the B-2 is a pure flying wing, with no vertical tail surfaces and a crew compartment, engines, and internal bomb bay smoothly blended into its chevron-shaped wing. As previously mentioned, these features, along with its buried engine intakes and radar-absorbing paint, help give the B-2 an RCS equivalent to a tennis ball. Though originally designed to penetrate Soviet airspace and destroy strategic targets using nuclear weapons, the end of the Cold War relegated the B-2 to the conventional bombing role, the type being used for air support in Kosovo, Afghanistan, Iraq, and Libya. However, the aircraft’s unique capabilities come at a tremendous cost, with the 21 strong B-2 fleet being valued at nearly $2 billion apiece.
In recent years, stealth technology has been incorporated into an increasing number of military vehicles. Aircraft like the American Lockheed F-22 Raptor and F-35 Lightning II, the Russian Sukhoi Su-57, and Chinese Chengdu J-20 incorporate many of the stealth features pioneered by the F-117 including faceting, internal weapons bays, and metal-coated canopies, though modern computer software has allowed engineers to design more curved, aerodynamic airframe shapes that do not require as much computer correction to render stable. Many of these aircraft also incorporate what are known as radar-absorbing structures, with special cavities that absorb and dissipate incoming radar waves. The effect is what one engineer likens to an “electromagnetic roach motel” – radar waves check in, but they never check out. Many modern naval vessels also feature a stealthy, faceted design, including the French Lafayette Class, Dutch De Zeven Provinciën Class, and the Swedish Visby Class.
But while stealth has been a game-changer in the fields of aerial and naval warfare, if history has taught us anything, it is that on the technological battlefield no tactical or strategic advantage is maintained for very long. New weapons lead to new countermeasures and so on, and it is only a matter of time before new technologies render stealth as obsolete as the introduction of radar made visual camouflage during the Second World War. Indeed, developments in multi-beam and low-frequency radar and enhanced infrared imaging are already beginning to chip away at modern stealth’s effectiveness. Who knows what the future will hold?
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