The Harrier VTOL aircraft had its origin back in the 1950s. Hawker Aircraft Ltd of Kingston-upon-Thames, Surrey, UK was approached by Dr. S. G. Hooker of the Bristol Engine Company with an approach for a radical new type of jet engine, one which would allow for vertical takeoff and landing flight. This engine was based on a Bristol B.E.25 Orion turboprop core that drove four centrifugal compressors via a series of shafts and gearboxes. The compressors were enclosed in a rotatable exhaust that could produce either forward or vertical thrust. It seems that the idea was originally that of a French engineer named Michel Wilbaut.
The Bristol design staff soon disposed of the gearboxes and blowers and replaced them with a front fan from an Olympus turbojet to which were attached two rotating ducts that could direct the cold air either horizontally or vertically. With this change, the engine was designated B.E.48. An Orpheus turbojet was later substituted for the Orion component. The new engine then became known as the B.E.53. The hot exhaust from the Orpheus was still directed horizontally aft in the conventional manner.
Hawkers assigned a young project engineer, Ralph Hooper, to the task in mid-1957. The team worked on the idea for a year, and came up with four fundamental changes to Hooker's design. First, a common intake was used for both the compressor and the gas generator. Second, a bifurcated tailpipe (similar to that used on the Hawker Sea Hawk) was substituted for the horizontal single exhaust, which permitted the hot exhaust to be directed vertically as well as the cold exhaust from the compressor. This had the advantage of allowing the aircraft to rest horizontally while on the ground. Third, aerodynamic cascasdes were used in the rotatable nozzles, permitting much smaller and neater nozzles to be used. Finally, the two spools of the engine were arranged to rotate in opposite directions, cancelling out gyroscopic effects. The new engine became known as the Pegasus. Hawkers thought enough of the idea that in March of 1958 they decided to launch at its own expense the construction of two prototypes under the designation P.1127.
The Ministry of Defence White Paper of April 1957 had announced that missiles rather than manned aircraft would fulfill the needs of the Royal Air Force in the future and that the UK would not need any more fighters. In addition, the British government was in a fit of extreme austerity at the time, and Hawkers was unable initially to attract any official British funding for the P.1127. However, considerable interest was attracted from the United States, particular from NASA. NASA, in fact, provided a considerable amount of funding to Hawkers and did a lot of of the model testing. Hawkers decided in March of 1958 to build at its own expense two flight-test articles and one static test article, while they continued to work with Bristol on the B.E.53 engine.
The first B.E.53 engine was bench-tested at Patchway, Bristol on September 1, 1959. In June of 1960, the Ministry of Supply finally relented and provided some financial support. Specification ER.203 was retrospectively drawn up, designating the P.1127 as an experimental aircraft powered by a 11,000 lb.s.t Pegasus 2 vectored-thrust turbofan. Metal was cut on the first of three prototypes.
The aircraft was to some extent based on a perceived NATO requirement for a replacement for the Fiat G.91 light tactical close-support aircraft. It was a single-seat aircraft with a high-mounted wing with a swept leading edge and a straight trailing edge. A swept vertical fin was fitted, with a horizontal tailplane with no dihedral. The test aircraft a fuselage with the B.E.53 engine mounted amidships, with the engine being fed by large intakes on each side of the fuselage. The cold-air rotateable exhausts were immediately aft of the air intakes, adjacent to the forward edge of the high-mounted wing. The hot-air rotateable exhausts were on the sides of the rear fuselage, underneath the wing center section. The single seat cockpit was located well forward on the fuselage, giving the pilot a good view during vertical landings. There were a series of reaction control nozzles situated in the nose, at the wingtips, and in an extension protruding from the tail that were fed by hot air bled from the engine. These were used to steer the aircraft during vertical landings and takeoffs. A bicycle retractable landing gear was provided, with a two-wheeled main member located underneath the fuselage aft of the hot-air exhaust and a nose member underneath the fuselage adjacent to the air intakes. Small retractable outrigger wheels were attached to the wingtips.
By this time the B.E.53 engine had been given the name Pegasus, and was offering a thrust of about 10,400 pounds. The first flight prototype, XP831 was ready in September of 1960. On October 21, it was lifted off the ground for a few inches while attached to a tether. With test pilot Bill Bedford at the controls, the aircraft achieved free hovering flight for the first time on November 19.
The second aircraft, XP836 was ready in the winter of 1960-61. It flew for the first time on July 7, 1961. On September 8, 1961, the first successful transition from vertical takeoff to conventional horizontal flight was made. Four days later, both prototypes had made complete accelerating and decelerating transitions. The pilots were Bill Bedford and Hugh Merewether.
XP836 was lost at Yeovilton on December 14, 1961 when a cold nozzle failed and detached in flight, and pilot Bill Bedford was forced to eject.
Four more test aircraft were ordered in 1961 (XP972, XP976, XP980, and XP984). They were powered by the 13,500 lb.s.t Pegasus 3, which offered 1500 lb more thrust than the Pegasus 2 of the first two prototypes. XP972 first flew on April 5, 1962. It was heavily damaged on October 30, 1962 in an inflight fire caused by the failure of an engine main bearing. The plane had to be landed at an unused airfield at Tangmere. Test pilot Hugh Merewether escaped uninjured. XP976 flew for the first time on July 12, 1962, XP976 flew on July 12, 1962 and incorporated streamwise wingtips and introduced inflatable air intake lips for increased airflow at low forward speeds and a fin-mounted pitit head. It was scrapped at RAE in 1970. XP980 flew for the first time in May of 1963 and introduced a taller fin and an anhedral tailplane. XP984 flew for the first time in October of 1963 and featured a wing with increased sweep and a sweep on the trailing edge. Wing leading edge extensions were fitted and cold steel nozzles were tested. The aircraft force landed on Thorney Island on March 19, 1965 but was repaired. It crashed and was destroyed in a landing at RAE Bedford on October 31, 1975.
Early in the test program, a pair of ventral strakes was added to increase pressure beneath the fuselage when low-hovering.
XP831 had been flown to the Paris Air Show in 1963 to show off its capabilities. On June 16, 1963, Bill Bedford was making a turn in front of the crowd after trying to transition from hover into conventional forward flight, when the plane rapidly lost height and crashed. Bedford walked away from the crash unhurt. An investigation later showed that a particle of dirt had penetrated the air motors that rotated the nozzles, so that without adequate forward speed and wing lift the aircraft simply fell out of the sky out of control. XP831 was later repaired and is now at the RAF Museum, Hendon.
The VZ-12 designation was allocated to two P.1127 Kestrels, which were given the USAF serials 62-4507 and 62-4508 but never delivered
One Bristol Pegasus vectored-thrust turbofan, 13,500 lb.s.t. Performance: Maximum speed 720 mph at sea level. Initial climb rate 20,000 feet per minute. Service ceiling 50,000 feet. Dimensions: Wingspan 24 feet 4 inches, length 49 feet 0 inches, height 10 feet 3 inches. Weights: 8900 pounds empty, Loaded 11,000 pounds. Later aircraft operated at loaded weights of up to 14,500 pounds. Armament: none carried.