Hawker Siddeley Kestrel (XV-6)

Following the end of World War Two, many strategists within the military services of the United States and its European allies realized that the large prepared runways required for jet aircraft were exceedingly vulnerable to nuclear or conventional attack. One surprise assault could substantially degrade the ability of the military to respond to a massive assault by waves of bombers or ground forces. The apparent solution was the development of fighter aircraft with Vertical and Short Takeoff and Landing (V/STOL) capability, which did not restrict them to vulnerable airfields. Throughout the 1950s, a number of defense contractors in the United States, Great Britain, and France experimented with a variety of V/STOL systems to determine the most practical option. The slower speeds of tilt-rotor and propeller driven systems eliminated them as possibilities. Jet-powered aircraft were clearly the answer, but a serious obstacle to their development seemed unsolvable. The thrust required for vertical takeoff and landing was far more than that required for cruise, which resulted in the airframe having to carry the considerable dead weight of either a much larger engine, or auxiliary lifting engines. The solution emerged out of Hawker-Siddeley Aviation in Great Britain with the development of the P.1127 and Kestrel designs, which evolved into an operational form as the versatile Harrier series of fighter and attack aircraft.

The first successful experiments with Vertical Takeoff in jets occurred in 1953 with the Rolls-Royce "Flying Bedstead." However, this design was not capable of transitioning effectively to forward flight. Frenchman Michel Wibault developed the breakthrough idea of using swiveling exhaust nozzles to vector the jet thrust, which eliminated the need for multiple engines. In 1956, Wibault's ideas found acceptance in the NATO Advisory Group for Aeronautical Research and Development chaired by noted engineer Theodore von Karman. By 1957, Hawker, desperate to find a new niche in the rapidly shrinking military aircraft market, launched a V/STOL fighter development program under the P.1127 designation. However, a number of hurdles remained. To develop a V/STOL aircraft requires that the designers of the aircraft and propulsion system solve five problems. These are:

1. The propulsion system must deliver thrust greater than the weight of the aircraft.

2. The propulsion system must have the capability of providing vertical thrust for hovering flight and horizontal thrust for cruise.

3. The propulsion system must negate the gyroscopic forces in the engine. Conventional aircraft alleviate this problem through aerodynamic forces, but other means are necessary to oppose it in V/STOL aircraft while hovering.

4. The engine must achieve fuel efficiency comparable to a conventional engine.

5. Flight controls must be capable of controlling aircraft attitude in high-speed, low-speed and hovering flight.

A breakthrough came with the development of the powerful Pegasus turbofan engine. Stanley Hooker and Gordon Lewis of Bristol Aero-Engines, Ltd. (later part of Rolls-Royce) conceived of a jet engine in which four swiveling nozzles on the side of the aircraft directed thrust from the engine exhaust and a bypass fan. The nozzles could rotate from a position slightly forward of the vertical to parallel with the horizontal axis of the fuselage. In so doing, they solved the problem of varying thrust for hovering, low-speed, and high-speed flight. The front pair of nozzles ducted "cold" bleed air from the initial low-pressure compressor stages, while the rear pair of nozzles ducted "hot" air directly from the turbine exhaust, though these provided a smaller share of the total thrust.

Under the able guidance of the Pegasus program manager, John Dale, the engine evolved with the level of performance required for the P.1127, and its successors - the Kestrel and Harrier. To solve the dual problems of achieving acceptable fuel efficiency and a high enough thrust to exceed the aircraft weight was a difficult task. To solve it, the designers settled on a water-injection system to generate short bursts of thrust beyond the normal rating of the engine. Thus, the Pegasus was an engine of normal thrust stressed to deliver the power needed for hovering flight for the very short period of time that a high performance tactical aircraft would need to land or take off vertically. Simultaneously, it retained the fuel efficiency required to achieve the range and payload for intended tactical missions. The high takeoff weights due to fuel and weapon stores of eliminated vertical takeoff, or hovering as a practical option for most battlefield scenarios, but even fully loaded, the aircraft could make a short-takeoff from a very small area.

The negation of the gyroscopic forces was an engineering marvel. While in a hover or low-speed flight the mass of the spinning engine components, combined with the massive airflow passing through the engine would normally have been so great that the rolling tendency would have been impossible to counter. To solve this problem, the two sections of the engine, the bypass fan stage and the high-pressure compressor, along with their associated turbine stages, rotated in opposite directions, thereby zeroing out the gyroscopic forces.

The solution to aircraft control in low-speed and hovering flight required an exceptionally high level of cooperation between the engine and airframe designers. Ultimately, a system of reaction controls emerged, which bled air off the engine compressor and ducted it to the wing tips, nose and tail. Then, reaction control valves, known as "puffer jets" vented bleed air to cause a reaction that rotated the aircraft in the desired direction. A normal stick and rudder operated the conventional aerodynamic controls in cruise. However, when the pilot moved the nozzle level on the throttle towards the Short and Vertical Takeoff settings, the hydraulic system of the aircraft proportionally phased in the reaction controls. This arrangement resulted in a flight control system in which the transition from normal forward flight, using aerodynamic control surfaces to low speed and hovering flight, using reaction controls, was seamless. From the pilot's standpoint, he flew the aircraft in a conventional manner all the way from high-speed flight to a hover with no change in technique, other than having to position the nozzle lever.

Hawker-Siddeley designed the P.1127 around the revolutionary Pegasus engine, once Dale and his team had finalized its design. The design team took great care to minimize the weight of the aircraft with the incorporation of innovative features such as a tandem landing gear system that included retractable outriggers.

By October 21, 1960, the first P.1127 had begun tethered hover testing and on November 19, 1960, it made its first untethered flight. The first successful transition from cruise to vertical mode took place on September 12, 1961. The P.1127 also demonstrated supersonic dives in 1961, but these tests ended after one of the forward fiberglass nozzles failed because of the aerodynamic forces, resulting in the loss of the aircraft.

After construction of the first two P.1127 demonstrators, four more "development" models continued the test program. Support for the Hawker-Siddeley program within the British military vacillated between enthusiasm and complete disinterest. The Royal Air Force endorsed, then cancelled a supersonic variant, the P.1154. However, by 1962, the "Tripartite" team of Germany, Great Britain, and the United States agreed to fund construction of nine P.1127s, optimized for evaluation of their military potential, under the Kestrel designation. The sixth P.1127 acted as a prototype for the improvements, which included a new wing form and the Pegasus 5 engine with an additional 4,500 pounds of thrust. The Kestrels began flight tests in 1964. The United States, which had funded a wide range of V/TOL programs, was eager to evaluate its investment, and the U.S. Army, Navy, and Air Force evaluated six Kestrels under the XV-6 designation. The Royal Air Force, Royal Navy and the U.S. Marine Corps (USMC) found that the Kestrel met their needs for a fighter and attack aircraft capable of operating away from prepared airfields or on surface vessels other than aircraft carriers. By 1969,Great Britain had formed its first squadron based on the production version of the Kestrel - known as the Harrier. In 1971, the USMC followed suit when it adopted the Harrier under the AV-8A designation.

One of the characteristics of the vectored thrust design was the unintended consequence of "Vectored Maneuvering". During testing of the Kestrel pilots discovered that they could Vector In Forward Flight (VIFF) to radically increase the maneuverability of the aircraft. By rotating the nozzles downward in normal flight, the pilot could execute turning maneuvers that are impossible for an aircraft that is limited to conventional aerodynamic controls. Kestrel XS689/64/NASA 521, after participating in the Tri-Service program of the U.S. armed forces, operated at NASA Langley for VIFF evaluation, where the US. Marine Corps learned much of the technical knowledge needed to apply VIFF as an effective combat tactic. On June 20, 1974, at the conclusion of the VIFF evaluation at Langley, NASA transferred this Kestrel to the National Air and Space Museum.

The successful Harrier and Harrier II (designated AV-8B in U.S. service) embodied most features of the Kestrel and were similar in external appearance despite the fact that Hawker-Siddeley redesigned 95 percent of the components. The additions of radar, modern avionics, and precision guided munitions have allowed the descendents of the Kestrel to remain an integral part of front-line NATO units. Spain and Italy, India, and even the Royal Thai Navy have joined the RN, RAF, and USMC in operating these remarkable aircraft. These aircraft made important contributions to the Falklands campaign, where they formed the backbone of British airpower, the 1991 Gulf War with the USMC, NATO operations in Bosnia-Herzegovina and Kosovo, and the campaign against the Taliban and Al Qaeda in Afghanistan. The Harrier's ability to operate without the need for massive aircraft carriers and prepared runways has allowed it to remain in service for over thirty years in spite of its limitations in payload, speed, and weapon systems relative to other modern military aircraft. However, like the Kestrel before it, the Harrier requires skills beyond those of conventional fast-jet pilots and only intensive and rigorous training programs have kept accident rates within acceptable levels.

With the end of the Cold War and the reshaping of regional alliances, the United States and its allies can no longer rely on the availability of prepared runways necessary for fielding tactical aircraft. Thus, V/STOL capabilities have become an essential element of the Joint Strike Fighter developed by Lockheed for the military services of the United States and Great Britain. New advances in lift-fan technology have helped to reduce to the penalties for the inclusion of V/STOL capabilities. Although the technology has evolved, the missions pioneered with the Kestrel remain an integral component of the concepts for the next generation of tactical aircraft. That these aircraft are one of the few types of foreign aircraft adopted by the U.S. military for widespread use and production illustrates the revolutionary nature of the Kestrel and Harrier program.

Wingspan: 7.0 m (22 ft 10 in)

Length: 12.8 m (42 ft)

Height: 3.3 m (10 ft 9 in)

Weights: Gross, Vertical Takeoff, 5,897 kg (13,000 lb)

Gross, Short Takeoff, 7,711 kg (17,000 lb)

Gross, Normal Takeoff, 8,618 (19,000 lb)

Empty, 4,790 kg (10,560 lb)

Engine: Rolls Royce Pegasus 5 (15,200 lb thrust)

References and Further Reading:

Davies, Peter E. and Anthony M. Thornborough. The Harrier Story. Annapolis, MD: Naval Institute Press, 1996.

Jenkins, Dennis R. Boeing/Bae Harrier. North Branch, MN: Specialty Press Publishers, 1998.

XV-6 curatorial file, Aeronautics Division, National Air and Space Museum

Roger Connor, John Braddon

Following the end of World War Two, many strategists within the military services of the United States and its European allies realized that the large prepared runways required for jet aircraft were exceedingly vulnerable to nuclear or conventional attack. One surprise assault could substantially degrade the ability of the military to respond to a massive assault by waves of bombers or ground forces. The apparent solution was the development of fighter aircraft with Vertical and Short Takeoff and Landing (V/STOL) capability, which did not restrict them to vulnerable airfields. Throughout the 1950s, a number of defense contractors in the United States, Great Britain, and France experimented with a variety of V/STOL systems to determine the most practical option. The slower speeds of tilt-rotor and propeller driven systems eliminated them as possibilities. Jet-powered aircraft were clearly the answer, but a serious obstacle to their development seemed unsolvable. The thrust required for vertical takeoff and landing was far more than that required for cruise, which resulted in the airframe having to carry the considerable dead weight of either a much larger engine, or auxiliary lifting engines. The solution emerged out of Hawker-Siddeley Aviation in Great Britain with the development of the P.1127 and Kestrel designs, which evolved into an operational form as the versatile Harrier series of fighter and attack aircraft.

The first successful experiments with vertical takeoff jet thrust occurred in 1953 with the Rolls-Royce "Flying Bedstead." However, this design was not capable of transitioning effectively to forward flight. Frenchman Michel Wibault developed the breakthrough idea of using swiveling exhaust nozzles to vector the jet thrust, which eliminated the need for multiple engines. In 1956, Wibault's ideas found acceptance in the NATO Advisory Group for Aeronautical Research and Development chaired by noted engineer Theodore von Karman. By 1957, Hawker, desperate to find a new niche in the rapidly shrinking military aircraft market, launched a V/STOL fighter development program under the P.1127 designation. However, a number of hurdles remained. To develop a V/STOL aircraft requires that the designers of the aircraft and propulsion system solve five problems. These are:

1.The propulsion system must deliver thrust greater than the weight of the aircraft.

2.The propulsion system must have the capability of providing vertical thrust for hovering flight and horizontal thrust for cruise.

3.The propulsion system must negate the gyroscopic forces in the engine. Conventional aircraft alleviate this problem through aerodynamic forces, but other means are necessary to oppose it in V/STOL aircraft while hovering.

4.The engine must achieve fuel efficiency comparable to a conventional engine.

5.Flight controls must be capable of controlling aircraft attitude in high-speed, low-speed and hovering flight.

A breakthrough came with the development of the powerful Pegasus turbofan engine. Stanley Hooker and Gordon Lewis of Bristol Aero-Engines, Ltd. (later part of Rolls-Royce) conceived of a jet engine in which four swiveling nozzles on the side of the aircraft directed thrust from the engine exhaust and a bypass fan. The nozzles could rotate from a position slightly forward of the vertical to parallel with the horizontal axis of the fuselage. In so doing, they solved the problem of varying thrust for hovering, low-speed, and high-speed flight. The front pair of nozzles ducted "cold" bleed air from the initial low-pressure compressor stages, while the rear pair of nozzles ducted "hot" air directly from the turbine exhaust, though these provided a smaller share of the total thrust.

Under the able guidance of the Pegasus program manager, John Dale, the engine evolved with the level of performance required for the P.1127, and its successors - the Kestrel and Harrier. To solve the dual problems of achieving acceptable fuel efficiency and a high enough thrust to exceed the aircraft weight was a difficult task. To solve it, the designers settled on a water-injection system to generate short bursts of thrust beyond the normal rating of the engine. Thus, the Pegasus was an engine of normal thrust stressed to deliver the additional power needed for hovering flight during the very short period of time that a high performance tactical aircraft would need to land or take off vertically. Simultaneously, it retained the fuel efficiency required to achieve the range and payload for intended tactical missions. The high takeoff weights due to fuel and weapon stores of eliminated vertical takeoff, or hovering as a practical option for most battlefield scenarios, but even fully loaded, the aircraft could make a short-takeoff with only dozens of meters available.

The negation of the gyroscopic forces was an engineering marvel. While in a hover or low-speed flight the mass of the spinning engine components, combined with the enormous airflow passing through the engine would normally have been so great that the rolling tendency would have been impossible to counter. To solve this problem, the two sections of the engine, the bypass fan stage and the high-pressure compressor, along with their associated turbine stages, rotated in opposite directions, thereby zeroing out the gyroscopic forces.

The solution to aircraft control in low-speed and hovering flight required an exceptionally high level of cooperation between the engine and airframe designers. Ultimately, a system of reaction controls emerged, which bled air off the engine compressor and ducted it to the wing tips, nose and tail. Then, reaction control valves, known as "puffer jets" vented bleed air to cause a reaction that rotated the aircraft in the desired direction. A normal stick and rudder operated the conventional aerodynamic controls in cruise. However, when the pilot moved the nozzle level on the throttle towards the Short and Vertical Takeoff settings, the hydraulic system of the aircraft proportionally phased in the reaction controls. This flight control system resulted in a seamless transition from normal forward flight, using aerodynamic control surfaces for low-speed flight and reaction controls for hovering flight. From the pilot's perspective, he flew the aircraft in a conventional manner all the way from high-speed flight to a hover with no change in technique, other than having to position the nozzle lever.

Hawker-Siddeley designed the P.1127 airframe around the revolutionary Pegasus engine, once Dale and his team had finalized its design. The design team took great care to minimize the weight of the aircraft with the incorporation of innovative features such as a tandem landing gear system that included retractable outriggers.

By October 21, 1960, the first P.1127 had begun tethered hover testing and on November 19, 1960, it made its first untethered flight. The first successful transition from cruise to vertical mode took place on September 12, 1961. The P.1127 also demonstrated supersonic dives in 1961, but these tests ended after one of the forward fiberglass nozzles failed because of the aerodynamic forces, resulting in the loss of the aircraft.

After construction of the first two P.1127 demonstrators, four more "development" models continued the test program. Support for the Hawker-Siddeley program within the British military vacillated between enthusiasm and complete disinterest. The Royal Air Force endorsed, then cancelled a supersonic variant, the P.1154. However, by 1962, the "Tripartite" team of Germany, Great Britain, and the United States agreed to fund construction of nine P.1127s, optimized for evaluation of their military potential, under the Kestrel designation. The sixth P.1127 acted as a prototype for the improvements, which included a new wing form and the Pegasus 5 engine with an additional 4,500 pounds of thrust. The Kestrels began flight tests in 1964. The United States, which had funded a wide range of V/TOL programs, was eager to evaluate its investment, and the U.S. Army, Navy, and Air Force evaluated six Kestrels under the XV-6 designation. The Royal Air Force (RAF), Royal Navy (RN) and the U.S. Marine Corps (USMC) found that the Kestrel met their needs for a fighter and attack aircraft capable of operating away from prepared airfields or on surface vessels other than aircraft carriers. By 1969,Great Britain had formed its first squadron based on the production version of the Kestrel - known as the Harrier. In 1971, the USMC followed suit when it adopted the Harrier under the AV-8A designation.

One of the characteristics of the vectored thrust design was the unintended consequence of "Vectored Maneuvering". During testing of the Kestrel pilots discovered that they could Vector In Forward Flight (VIFF) to radically increase the maneuverability of the aircraft. By rotating the nozzles downward in normal flight, the pilot could execute turning maneuvers that are impossible for an aircraft that is limited to conventional aerodynamic controls. Kestrel XS689/64/NASA 521, after participating in the Tri-Service program of the U.S. armed forces, operated at NASA Langley for VIFF evaluation, where the US. Marine Corps learned much of the technical knowledge needed to apply VIFF as an effective combat tactic. On June 20, 1974, at the conclusion of the VIFF evaluation at Langley, NASA transferred this Kestrel to the National Air and Space Museum.

The successful Harrier and Harrier II (designated AV-8B in U.S. service) embodied most features of the Kestrel and were similar in external appearance despite the fact that Hawker-Siddeley redesigned 95 percent of the components. The additions of radar, modern avionics, and precision guided munitions have allowed the descendents of the Kestrel to remain an integral part of front-line NATO units. Spain and Italy, India, and even the Royal Thai Navy have joined the RN, RAF, and USMC in operating these remarkable aircraft. These aircraft made important contributions to the 1982 Falklands campaign, where they formed the backbone of British airpower, the 1991 Gulf War with the USMC, NATO operations in Bosnia-Herzegovina and Kosovo, and the campaign against the Taliban and Al Qaeda in Afghanistan. The Harrier's ability to operate without the need for massive aircraft carriers and prepared runways has allowed it to remain in service for over thirty years in spite of its limitations in payload, speed, and weapon systems relative to other modern military aircraft. However, like the Kestrel before it, the Harrier requires skills beyond those of conventional fast-jet pilots and only intensive and rigorous training programs have kept accident rates within acceptable levels.

With the end of the Cold War and the reshaping of regional alliances, the United States and its allies can no longer rely on the availability of prepared runways necessary for fielding tactical aircraft. Thus, V/STOL capabilities have become an essential element of the Joint Strike Fighter developed by Lockheed for the military services of the United States and Great Britain. New advances in lift-fan technology have helped to reduce to the penalties for the inclusion of V/STOL capabilities. Although the technology has evolved, the missions pioneered with the Kestrel remain an integral component of the concepts for the next generation of tactical aircraft. That these aircraft are one of the few types of foreign aircraft adopted by the U.S. military for widespread use and production illustrates the revolutionary nature of the Kestrel and Harrier program.