The P.1127, as proposed, was sufficiently promising to initiate a self-funded collaborative effort between Hawker Aircraft at Kingston and Bristol Engine Company at Patchway, Bristol, in the autumn of 1957, although at the time both companies had other more important projects on hand.
US Mutual Weapons Development Program representatives were shown a brochure at Farnborough in September 1957, and commented that twice as much flying radius would be required if the P.1127 was to have a useful military potential. A considerable increase in thrust was clearly necessary to achieve such an improvement in range. The Government had selected the Short SC.1 powered by Rolls-Royce RB.108 lift engines to provide V/STOL know-how – and said that no alternatives would be supported. If British Government policy had been imposed on Bristol and Hawker, there would have been no Pegasus engine and no Harrier.
By March 1958, the design of the Hawker P.1127 was broadly similar to the form of the prototype that was to commence hovering trials about thirty months later. The centre fuselage was filled by the relatively large diameter of the engine, which was to receive its air through two intakes that were extremely short (roughly equal in length to one engine diameter) and of semicircular cross-section.
The pilot was accommodated in a rather cramped cockpit, which was not raised to give a rear view. Early attempts to have two main undercarriage units mounted in the fuselage sides just ahead of the rear nozzles were abandoned because of possible blast damage to the tyres. They were replaced by a single main unit mounted just behind the engine, resulting in what was termed for marketing purposes a ‘zero-track tricycle’ configuration. The P.1127 under-carriage load distribution was somewhat between that of a ‘bicycle’ and a ‘tricycle’ arrangement: the nose unit of the Harrier GR1 typically supported 38 per cent of the aircraft weight. There were ‘outrigger’ wheels at the wingtips to provide lateral balance on the ground.
The angular position of the four engine nozzles was to be selected by a single lever, mounted just inboard of the quadrant on the left-hand console. The lever controlled the supply of high-pressure air to two motors, mounted under the engine. These would turn the nozzles by a system of rotating shafts, bevel gears, chains and sprockets.
In September 1959, the Pegasus 1, rated at 9,000lb thrust, first ran on the bench. It should be appreciated that many V/STOL concepts had by that stage proved unflyable and that a massive question mark hung over the viability of the P.1127. Fortunately for Britain, there had been a series of American V/STOL failures, as a result of which both NASA and the USAF became seriously interested in foreign projects.
On 15 July 1960, the first P.1127 prototype (XP831) was transported by road from Kingston to Dunsfold airfield, where the final items of equipment were installed and various systems tested. The first tethered hover took place on 21 October 1960. Untethered hovering followed on 19 November and the first conventional take-off and landing were recorded on 13 March 1961. The second prototype (XP836) went straight into conventional flying trials, making its maiden flight on 7 July 1961. The initial flight trials with the prototypes showed promise, although they were short of thrust, directional stability and reaction control power in V/STOL.
A variety of modifications were applied to the six P.1127s built. The final development aircraft (XP984) had a 15,500lb static thrust (st) Pegasus 5 and this aircraft served as a trials installation for some features of the nine Kestrel evaluation aircraft (XS688–696) that followed. The Kestrel had an uprated new swept wing, a drooped tailplane and a 9in extension to the rear fuselage. Although it had no armament provisions, it did have two hard points under the wings.
The first Kestrel had its maiden flight on 7 March 1964, and the last took to the air on 5 March 1965. A tripartite squadron, funded by Britain, West Germany and the USA, undertook evaluation of the Kestrels at RAF West Raynham, from 1 May to 30 November 1965. After the trials had been successfully completed, Britain retained two of the Kestrels (XS693 and 695), while Germany sold its three examples to America, where six were used by NASA at Edwards AFB under the designation XV-6A for a variety of V/STOL research projects.