The Hawker P.1127 and the Hawker Siddeley Kestrel FGA.1 are the British experimental and development aircraft that led to the Hawker Siddeley Harrier, the first vertical and/or short take-off and landing (V/STOL) jet fighter-bomber.
Development began in 1957, taking advantage of the Bristol Engine Company's choice to invest in the creation of the Pegasus vectored-thrust engine. Testing began in July 1960 and by the end of the year the aircraft had achieved both vertical take-off and horizontal flight. The test program also explored the possibility of use upon aircraft carriers, landing on in 1963. The first three aircraft crashed during testing, one at the 1963 Paris Air Show.
Improvements to future development aircraft, such as swept wings and more powerful Pegasus engines, led to the development of the Kestrel. The Kestrel was evaluated by the Tri-partite Evaluation Squadron, made up of military pilots from the United Kingdom, the United States, and West Germany. Later flights were conducted by the U.S. military and NASA.
Related work on a supersonic aircraft, the Hawker Siddeley P.1154, was cancelled in 1965. As a result, the P.1127 (RAF), a variant more closely based on the Kestrel, was ordered into production that year, and named Harrier – the name originally intended for the P.1154 – in 1967. The Harrier served with the UK and several other nations, often as a carrier-based aircraft.
Development
Background Following the end of the Korean War, a number of aircraft companies in both Europe and America separately decided to investigate the prospective of vertical take-off and landing (VTOL) aircraft, which would eliminate the requirement for vulnerable runways by taking off and landing vertically as opposed to the conventional horizontal approach.Mason 1967, p. 3. In addition to military applications, the prospect of applying such technology to commercial airliners was also viewed with considerable interest by the mid-1950s, thus the value of developing viable vertical take-off systems was judged to be substantial. However, even during this era, few companies had envisioned that a VTOL aircraft would also be realistically compatible with the characteristics of high performance military aircraft.
In 1957, jet engine engineer Stanley Hooker of the Bristol Engine Company informed aeronautics engineer Sydney Camm of Hawker Aircraft that Bristol had been working a project that combined major elements of their Olympus and Orpheus jet engines to produce a directable fan jet."VTOL Aircraft 1965". Flight, 20 May 1965 p. 769. The projected fan jet harnessed rotatable cold jets which were positioned on either side of the compressor along with a 'hot' jet which was directed via a conventional central tailpipe. The original concept upon which the engine, which had been named Pegasus,The name "Pegasus" for the engine was in keeping with Bristol's tradition of naming engines after figures from classical mythology was based came from Michel Wibault, a French aviation consultant.Mason 1971, p. 369. Several adaptions and enhancements were made by Bristol to reduce size and weight over Wibault's original concept.Jefford 2006, pp. 21–22.
Around the same point as Hooker's approach, Hawker had been working upon the development of a replacement fighter aircraft for the Hawker Hunter, designated as the P.1121. However, the P.1121 was cancelled shortly after the publishing of the 1957 Defence White Paper, which had advocated a policy shift away from crewed aircraft and towards missiles.Jefford 2006, p. 11. In light of this cancellation, Hawker found itself with the available resources to commit to a new project, and thus decided to study the use of the projected Pegasus engine as a basis for a new military aeroplane that would be able to conform with an active NATO specification that sought a new Light Tactical Support Fighter to replace the Fiat G.91. Particular attention was paid to meeting the specification's performance and load requirements."Dr Hooker Speaks about the SST Olympus and VTOL Propulsion." Flight International, 8 February 1962. p. 218.Mason 1971, p. 370.
According to Air Chief Marshal Sir Patrick Hine, Hawker's interest may have also been stimulated by the presence of Air Staff Requirement 345, which sought a V/STOL ground attack fighter for the Royal Air Force (RAF).Jefford 2006, pp. 11–12. Aviation author Francis K. Mason expressed a contrary view, stating that Hawker's decision to proceed was independent of British government initiatives, and that the P.1127 project was primarily based upon the NATO requirement instead. Hawker had a keen ally in its development in the form of Bristol, but by that point the latter was experiencing financial difficulties, and the lack of foreseeable commercial applications for the Pegasus engine in particular, coupled with refusals from HM Treasury, meant that development would have to be financed by NATO institutions instead. The close cooperation between Hawker and Bristol was viewed by project engineer Gordon Lewis as a key factor which had enabled the P.1127's development to proceed in spite of technical obstacles and political setbacks.Jefford 2006, p. 23.
Origins Senior project engineer Ralph Hooper at Hawker promptly set about establishing an initial layout for a theoretical aircraft to take advantage of the Pegasus engine, using data provided by Bristol. This proposed aircraft soon received the internal designation P.1127. In July 1957, a modification made to the design was the incorporation of a bifurcated tailpipe, similar to the Hawker Sea Hawk, which was equipped with rotatable nozzles for the hot exhaust, similar those already used for the cold exhaust. The switch from a single tailpipe meant that the initial tailwheel undercarriage could also be discarded in favour a conventional nose wheel-led undercarriage. The design process extended throughout 1958, being financed entirely by Hawker, while approaches were made to NATO headquarters (Belgium) to better establish the tactical requirements sought, particularly between the conflicting demands for a lightly armed supersonic fighter and a simpler multipurpose subsonic one.Mason 1967, pp. 3–4.
The development process had involved extensive use of physical models; for one series of blowing trials, mixtures of focused hot and cold air were directed onto ground platforms to simulate the ground effect upon take-off. This work was considered to be critical to the project as there was very little knowledge of the adverse effects which could influence the aircraft during the vertical takeoff process; as there was no airflow over the ailerons, tailplane, and rudder while the aircraft was held in a stationary hover, wingtip control jets were experimented with as an alternative reaction control approach. This research included the development of an all-new control response simulator which linked a series of simple flying controls to a computer. By the end of 1958, barely eighteen months after the start of the project, all the main features of the P.1127 were developed with one exception, that being the reaction control system, the development of which was completed by April 1959.Jefford 2006, p. 28.
Throughout the development, Camm heavily emphasised the importance of the design's simplicity, observing that "Sophistication means complication, then in turn escalation, cancellation, and finally ruination". In 1958, the design centred around a single Pegasus engine capable of generating of thrust; when fully equipped, the aircraft was to weigh slightly less than the maximum thrust, thereby allowing vertical take-offs to be performed under all nominal conditions.Mason 1967, p. 4. During late 1958, the rapid progress of the P.1127 project had been noticed by technical advisors at NATO, who began promoting the acceleration of the aircraft's development and that member nations should skip over the next generation of support fighters in favour of the emergent P.1127 instead. In Britain, support for the program was also growing within the British Air Staff, from January 1959 onwards, rumours of a pair of P.1127 prototypes being ordered by the Ministry of Supply alongside those of an Air Ministry specification being drafted around the project frequently echoed.
As the P.1127 had been developed at a time of deep UK defense cuts, Hawker had to seek commercial funding, and significant engine development funding came from the U.S.Mason 1991, p. 413. Research assistance was also provided by the U.S., including a series of wind tunnel tests conducted by NASA's Langley Research Center using sub-scale models, which demonstrated acceptable flight characteristics. Hawker test pilot Hugh Merewether went to the U.S. at NASA's request to fly the Bell X-14.Spick and Gunston 2000, pp. 359–360. In March 1959, the company's board of directors (Hawker Siddeley then) decided to privately fund two P.1127 prototypes.Spick and Gunston 2000, p. 358.
In February 1959, Hawker had completed practically all of the design work and thus passed the entirety of its manufacturing design work to the company's Experimental Design Office at Kingston, London. In April 1959, the Ministry of Supply formally issued a contract for the completion of a pair of P.1127 prototypes.Jenkins 1998, p. 13. However, there were critics amongst the Air Staff of the project, typically disliking the P.1127 for its subsonic speeds, favouring supersonic-capable aircraft instead; Mason attributes this as having caused considerable delay in the issuing of a contract to Hawker.Mason 1967, pp. 4–5. On 23 July 1959, Hawker authorised the application of maximum effort to complete the development of the P.1127.Mason 1967, p. 5.
P.1127
On 15 July 1960, the first "P.1127 Prototype V/STOL Strike Aircraft", serial XP831, was delivered to Dunsfold Aerodrome, Surrey, to commence static engine testing. On 31 August 1960, the Pegasus engine was run for the first time while inside the airframe. Some of the tests were performed from a purpose-built platform at the aerodrome which functioned to deflect the hot exhaust gases away from the aircraft during early hovering trials while more powerful versions of the engine were developed. On 13 October 1960, the first Pegasus flight engine, capable of generating of thrust, was delivered to Dunsfold.
On 21 October 1960, the initial tethered flight, performed by XP831, was conducted at Dunsfold; at this stage of development, this feat had required the airframe to have been stripped of all extraneous weight and restrictions on the engine meant it could not be run at full power for more than 2.5 minutes at a time. Several tethered flights took place, partially so that the test pilots could familiarise themselves with the hovering controls; on 4 November, the first tethered flight without use of the auto-stabiliser system was accomplished. In mid-November, conventional taxying trials were performed at speeds of up to .
On 19 November 1960, the first un-tethered free-flight hover of XP831 was achieved; a week later, the first publicity photos of the P.1127 were released. Prior to the first flight, Hooker is claimed to have asked of Camm "I suppose you are going to do some conventional flying first Sydney?" and Camm replied "What for?" Hooker said "Well you know, just to make sure the aeroplane is a nice aeroplane, and everything under control." Camm replied, "Oh, Hawker aeroplanes are always beautiful, nothing wrong with a Hawker aeroplane, not going to bother with that. Vertical first time"."Stanley Hooker: Legendary Rolls Royce & Bristol Engineer." YouTube.com, Retrieved 10 September 2016.