The jet age
Beginning in the 1920s, steady advances in aircraft performance had been produced by improved structures and drag-reduction technologies and by more powerful, supercharged engines, but by the early 1930s it had become apparent to a handful of farsighted engineers that speeds would soon be possible that would exceed the capabilities of reciprocating engines and propellers. The reasons for this were not at first widely appreciated. At velocities approaching Mach 1, or the speed of sound (about 1,190 km [745 miles] per hour at sea level and about 1,055 km [660 miles] per hour at 11,000 metres [36,000 feet]), aerodynamic drag increases sharply. Moreover, in the transonic range (between about Mach 0.8 and Mach 1.2), air flowing over aerodynamic surfaces stops behaving like an incompressible fluid and forms shock waves. These in turn create sharp local discontinuities in airflow and pressure, creating problems not only of drag but of control as well. Because propeller blades, describing a spiraling path, move through the air at higher local velocities than the rest of the aircraft, they enter this turbulent transonic regime first. For this reason, there is an inflexible upper limit on the speeds that can be attained by propeller-driven aircraft. Such complex interactions in the transonic regime—and not the predictable shock-wave effects of supersonic flight, which ballisticians had understood since the late 19th century—presented special problems that were not solved until the 1950s. In the meantime, a few pioneers attacked the problem directly by conceiving a novel power plant, the jet engine.
While still a cadet at the Royal Air Force College, Cranwell, in 1928, Frank Whittle advanced the idea of replacing the piston engine and propeller with a gas turbine, and in the following year he conceived the turbojet, which linked a compressor, a combustion chamber, and a turbine in the same duct. In ignorance of Whittle’s work, three German engineers independently arrived at the same concept: Hans von Ohain in 1933; Herbert Wagner, chief structural engineer for Junkers, in 1934; and government aerodynamicist Helmut Schelp in 1937. Whittle had a running bench model by the spring of 1937, but backing from industrialist Ernst Heinkel gave von Ohain the lead. The He 178, the first jet-powered aircraft, flew on Aug. 27, 1939, nearly two years before its British equivalent, the Gloster E.28/39, on May 15, 1941. Through an involved chain of events in which Schelp’s intervention was pivotal, Wagner’s efforts led to the Junkers Jumo 004 engine. This became the most widely produced jet engine of World War II and the first operational axial-flow turbojet, one in which the air flows straight through the engine. By contrast, the Whittle and Heinkel jets used centrifugal flow, in which the air is thrown radially outward during compression. Centrifugal flow offers advantages of lightness, compactness, and efficiency—but at the cost of greater frontal area, which increases drag, and lower compression ratios, which limit maximum power. Many early jet fighters were powered by centrifugal-flow turbojets, but, as speeds increased, axial flow became dominant.
Early jet fighters
Subsonic flight
Though Whittle was first off the mark, the Germans advanced their programs with persistence and ingenuity. The Messerschmitt Me 262, powered by two Jumo engines and with wings swept back 18.5°, was capable of 845 km (525 miles) per hour. Armed with four 30-mm cannon and unguided rockets, it was an effective bomber destroyer, but it entered service too late to have a major effect on the war. The Gloster Meteor entered service on July 27, 1944, about two months before the Me 262; though it was less capable than the German fighter, it was effective in intercepting V-1 “buzz bombs.” Desperate to combat Allied bombers, the Germans also turned to rocket propulsion, fielding the tailless Me 163 Komet in the final months of the war. Powered by a hydrogen peroxide rocket designed by Hellmuth Walter, the Komet had spectacular performance, but its short range and ineffective cannon armament made it an operational failure. In addition, the propellants were unstable and often exploded on landing.
Meanwhile, the U.S. aviation industry entered the jet race with the receipt by General Electric of a Whittle engine in 1941. The first U.S. jet, the Bell P-59A Airacomet, made its first flight the following year. It was slower than contemporary piston-engined fighters, but in 1943–44 a small team under Lockheed designer Clarence (“Kelly”) Johnson developed the P-80 Shooting Star. The P-80 and its British contemporary, the de Havilland Vampire, were the first successful fighters powered by a single turbojet.
The jets of World War II inaugurated the first generation of jet fighters, in which turbojet propulsion was applied to existing airframe technology and aerodynamics. (Indeed, some early postwar jets—notably, the Soviets’ Yakovlev Yak-15 and Yak-23 and the Swedish Saab 21R—were simply reengined propeller-driven fighters.) These aircraft generally outperformed their piston-engined contemporaries by virtue of the greater thrust that their jets provided at high speeds, but they suffered from serious deficiencies in range and handling characteristics owing to the high fuel consumption and slow acceleration of early turbojets. More fundamentally, they were limited to subsonic speeds because the relatively thick airfoils of the day were prone to the compressibility problems of transonic flight—especially at high altitudes, where the higher speeds required to produce lift in thin atmosphere brought aircraft more quickly to transonic speed. For this reason, first-generation jets performed best at low altitudes.
Other first-generation fighters included the U.S. McDonnell FH Phantom and the British Hawker Sea Hawk (the first jet carrier fighters), the McDonnell F2H Banshee, and the French Dassault Ouragan. These single-seat day fighters were in service by 1950, while first-generation all-weather fighters, burdened with radar and a second crew member, entered service through the late 1950s.
Transonic flight
As the first generation of jet fighters entered service, many aerodynamicists and engineers believed supersonic flight a practical impossibility, owing to transonic drag rise or compressibility, which threatened to tear an aircraft apart. Nevertheless, on Oct. 14, 1947, U.S. Air Force Capt. Charles Yeager, flying a rocket-powered Bell X-1 launched from the bomb bay of a B-29 Superfortress bomber, became the first human to exceed the speed of sound. Designed exclusively for research, the X-1 had thin, unswept wings and a fuselage modeled after a .50-inch bullet. Yeager’s flight marked the dawn of the supersonic era, but it was only part of a broad wave of testing and experimentation that had begun during World War II. Germany had experimented then with swept-back and delta-shaped wings, which delayed transonic drag rise, and after extensive testing these configurations were widely adopted in the postwar years. At the same time, the development of slats, slotted flaps, and other sophisticated high-lift devices for landing and takeoff enabled designers to use smaller wings, which in turn allowed them to achieve higher speeds. Turbojets became more powerful, and in the late 1950s afterburning, or reheat, was introduced. This permitted large temporary thrust increases by the spraying of fuel into hot exhaust gases in the tailpipe—in effect turning the turbojet into a ramjet.
As these developments took hold, a second generation of fighters appeared that were capable of operating in the transonic regime. These aircraft had thinner lifting and control surfaces than first-generation jets, and most had swept-back wings. Aerodynamic refinements and more powerful, quicker-accelerating engines gave them better flight characteristics, particularly at high altitudes, and some could exceed the Mach in a shallow dive. In addition, airborne radars became more compact and reliable, and radar-ranging gunsights began to replace the optically ranging sights used in World War II. Air-to-air missiles, using radar guidance and infrared homing, became smaller and more capable (see rocket and missile system: Tactical guided missiles). Outstanding fighters of this generation were the U.S. North American F-86 Sabre and its opponent in the Korean War (1950–53), the Soviet MiG-15. The F-86 introduced the all-flying tail (later a standard feature on high-performance jets), in which the entire horizontal stabilizer deflects as a unit to control pitch, yielding greater control and avoiding the compressibility problems associated with hinged surfaces. This and a radar-ranging gunsight helped the F-86 achieve a favourable kill ratio over the MiG-15, despite the Soviet fighter’s greater speed, higher service ceiling, and heavier armament. Other jets of this generation were Britain’s superlative Hawker Hunter, the MiG-17, and the diminutive British-designed Folland Gnat. The latter two, introduced in the mid-1950s, later became successful low-altitude dogfighters—the Gnat against Pakistani F-86s in the Indo-Pakistani conflict of 1965 and the MiG-17 against U.S. aircraft in the Vietnam War (1965–73).
Modern jet fighters
Supersonic flight
A third generation of fighters, designed around more powerful, afterburning engines and capable of level supersonic fight, began to enter service in the mid-1950s. This generation included the first fighters intended from the outset to carry guided air-to-air missiles and the first supersonic all-weather fighters. Some were only marginally supersonic, notably the U.S. Convair F-102 Delta Dagger, an all-weather interceptor that was the first operational “pure” delta fighter without a separate horizontal stabilizer. Other aircraft included the Grumman F11F Tigercat, the first supersonic carrier-based fighter; the North American F-100 Super Sabre; the Dassault Mystère B-2; the Saab 35, with a unique double-delta configuration; and the MiG-19.
To this point, jet fighters had been designed primarily for air-to-air combat, while older aircraft and designs falling short of expectations were adapted to ground attack and reconnaissance. Since land-based surface attack was to be carried out by bombers, the first operational jets of fighter size and weight designed to attack surface targets were based on aircraft carriers. These paralleled the third generation of fighters, but they were not supersonic. One example was the British Blackburn Buccaneer, capable of exceptional range at low altitudes and high subsonic speeds. The Douglas A-4 Skyhawk, entering service in 1956, sacrificed speed for ordnance-delivery capability. One of the most structurally efficient aircraft ever built, it carried the burden of U.S. Navy attacks on ground targets in North Vietnam and was often used by Israeli pilots in the Middle Eastern conflicts. The A-4 Skyhawk was still in use with the Kuwaiti air force during the Persian Gulf War (1990–91), an astonishingly long service life. The Grumman A-6 Intruder, which entered service in the 1960s, was another subsonic carrier-based aircraft. The first genuine night/all-weather low-altitude attack aircraft, it was highly successful over North Vietnam and continued to be in service until the late 1990s. The electronic warfare version, the EA-6B Prowler, was projected to remain in service well into the 21st century.
Mach 2
A fourth generation of fighters began to appear in the 1960s, capable of maximum speeds ranging from about Mach 1.5 to 2.3. Top speeds varied with the intended mission, and increasing engine power, aerodynamic sophistication, and more compact and capable radars and avionics began to blur the differences between two-seat all-weather fighters and single-seat air-superiority fighters and interceptors. By this time military designers had become convinced that air-to-air missiles had made dogfighting obsolete, so many interceptors were built without guns. This generation included the first land-based jet fighters designed with surface attack as a secondary or primary mission—a development driven by the appearance of surface-to-air missiles such as the Soviet SA-2, which denied bombers medium- and high-altitude penetration. Precursor to this generation was the Lockheed F-104 Starfighter, designed by a team under Kelly Johnson and first flown in 1954. Capable of speeds well above Mach 2, this interceptor was built with short and extremely thin wings to reduce the generation of shock waves. However, light armament, limited avionics, and poor maneuverability made it an ineffective air-to-air fighter, and only with the installation of up-to-date bombing and navigation systems in the 1960s did it become a useful low-level attacker.
The truly outstanding fighters were the U.S. McDonnell F-4 Phantom II and the MiG-21. A large twin-engined two-seater, the F-4 was originally a carrier-based interceptor armed only with air-to-air missiles, but it was so successful that the U.S. Air Force adopted it as its primary fighter. When combat in Vietnam showed that gun armament was still valuable for close-range dogfighting, later versions of the F-4 were fitted with an internally mounted 20-mm rotary cannon. The MiG-21 was a small delta-wing, single-seat aircraft designed as a specialized daylight interceptor, but it soon proved amenable to modification for a broad range of missions and became the most widely produced jet fighter ever. It was a formidable threat to U.S. airmen over North Vietnam and to Israeli pilots over the Sinai Peninsula and Golan Heights in 1973.
Also outstanding was the Republic F-105 Thunderchief, one of the largest single-engined fighters ever built. Designed to carry a nuclear bomb internally as a low-altitude penetrator and therefore exceptionally fast at low altitudes, the F-105, with heavy loads of conventional bombs under the wings, carried out the brunt of U.S. Air Force attacks against North Vietnam. Also noteworthy in this generation were the British Electric Lightning, one of the first Mach-2 interceptors to enter service and one of the fastest at high altitudes; the Soviets’ twin-engined all-weather Yak-28 Firebar; the Convair F-106 Delta Dart, a single-seat air-defense interceptor with superior speed and maneuverability; the Dassault Mirage III, the first successful pure delta in the air-to-air role and an enormous export success; the Soviet Sukhoi Su-21 Flagon, a tailed-delta single-seat all-weather interceptor; and the Vought F-8 Crusader, an outstanding carrier-based dogfighter over Vietnam.
