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{{Short description|British experimental V/STOL aircraft}}
{{Use dmy dates|date=June 2020}}
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{{Infobox aircraft
 |name= P.1127 / Kestrel
 |image= File:Hawker Siddeley P.1127 in flight at NASA Langley 1968 (cropped).jpeg
 |caption= Hawker XV-6A Kestrel in 1968
 |alt = Hawker P.1127 in 1968
 |type= Experimental [[V/STOL]] aircraft
 |national_origin= [[United Kingdom]]
 |manufacturer= [[Hawker Aviation]] <br/>[[Hawker Siddeley]]
 |designer= [[Sydney Camm]]<br/>[[Ralph Hooper]]
 |first_flight= 19 November 1960 (P.1127) <br/>7 March 1964 (Kestrel)
 |introduction = 
 |retired = 
 |status = 
 |primary_user= [[Royal Air Force]]
 |more_users= [[United States Department of Defense|US Department of Defense]]/[[NASA]] <br/>[[German Air Force]]
 |produced = 
 |number_built= 6 P.1127s <ref name="Production number">{{cite web |title=Hawker Siddeley - BAe Harrier |url=https://www.baesystems.com/en-uk/heritage/hawker-siddeley-harrier2 |website=BAE Systems |access-date=1 August 2023}}</ref> <br/>9 Kestrels <ref name="Production number" /> 
 |unit cost = 
 |developed_into= [[Hawker Siddeley Harrier]]
}}

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 [[Rolls-Royce Pegasus|Pegasus]] [[Thrust vectoring|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 {{HMS|Ark Royal|R09|6}} 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 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.<ref name = "mason 3">Mason 1967, p. 3.</ref> 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.<ref name = "mason 3"/>

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 [[Rolls-Royce Olympus|Olympus]] and [[Bristol Orpheus|Orpheus]] [[jet engine]]s to produce a [[Ducted fan|directable fan jet]].<ref name = "mason 3"/><ref>"VTOL Aircraft 1965". ''Flight'', 20 May 1965 p. 769.</ref> The projected fan jet harnessed rotatable ''cold'' jets which were positioned on either side of the [[Gas turbine engine compressors|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'',<ref group= "N">The name "[[Pegasus]]" for the engine was in keeping with Bristol's tradition of naming engines after figures from [[classical mythology]]</ref> was based came from [[Michel Wibault]], a French aviation consultant.<ref name = "mason 3"/><ref>Mason 1971, p. 369.</ref> Several adaptions and enhancements were made by Bristol to reduce size and weight over Wibault's original concept.<ref name= Jefford_21-22>Jefford 2006, pp. 21–22.</ref>

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 [[Hawker P.1121|''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.<ref name= Jefford_11>Jefford 2006, p. 11.</ref>{{refn|The development of a V/STOL jet was not Hawker's primary objective as it had put in a joint bid with [[Avro]] to meet the GOR.339 Requirement (which resulted in the [[BAC TSR-2]] development programme), but had been unsuccessful. The inability to obtain work on conventional aircraft in a hostile political climate was perhaps the greatest motivation for Hawker to proceed with the development of the Harrier.<ref name= Jefford_25>Jefford 2005, pp.&nbsp;11–12, 25.</ref>|group=N}} 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.<ref name = "mason 3"/><ref>[https://www.flightglobal.com/pdfarchive/view/1962/1962%20-%200216.html "Dr Hooker Speaks about the SST Olympus and VTOL Propulsion."] ''Flight International'', 8 February 1962. p. 218.</ref><ref name= "Mason370">Mason 1971, p. 370.</ref>

According to Air Chief Marshal [[Patrick Hine|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).<ref name= "Jefford_11–12">Jefford 2006, pp.&nbsp;11–12.</ref> 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.<ref name = "mason 3"/> 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 (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.<ref name= Jefford_23>Jefford 2006, p. 23.</ref>

===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.<ref name = "mason 3"/> This proposed aircraft soon received the internal designation ''P.1127''.<ref name = "mason 3"/> 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 [[landing gear|undercarriage]] could also be discarded in favour a conventional nose wheel-led undercarriage.<ref name = "mason 3"/> 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.<ref name = "mason 3 4">Mason 1967, pp. 3–4.</ref>

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.<ref name = "mason 4"/> 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 [[aileron]]s, tailplane, and [[rudder]] while the aircraft was held in a stationary hover, wingtip control jets were experimented with as an alternative reaction control approach.<ref name = "mason 4"/> This research included the development of an all-new control response simulator which linked a series of simple flying controls to a computer.<ref name = "mason 4"/> 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.<ref name= Jefford_28>Jefford 2006, p. 28.</ref>

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".<ref name = "mason 4"/> In 1958, the design centred around a single Pegasus engine capable of generating {{convert|13,000|lbf|kN|abbr=on}} 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.<ref name = "mason 4">Mason 1967, p. 4.</ref> 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 [[List of Air Ministry specifications|Air Ministry specification]] being drafted around the project frequently echoed.<ref name = "mason 4"/>

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.<ref name= "Mason370"/><ref name= "Mason 91 p413">Mason 1991, p. 413.</ref> 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.<ref name= "Mason370"/><ref name = "mason 4"/><ref name= Jenkins_p13/> Hawker test pilot [[Hugh Merewether]] went to the U.S. at NASA's request to fly the [[Bell X-14]].<ref>Spick and Gunston 2000, pp. 359–360.</ref> In March 1959, the company's board of directors ([[Hawker Siddeley]] then) decided to privately fund two P.1127 prototypes.<ref>Spick and Gunston 2000, p. 358.</ref>

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 upon Thames|Kingston]], [[London]].<ref name = "mason 4"/> In April 1959, the Ministry of Supply formally issued a contract for the completion of a pair of P.1127 prototypes.<ref name= Jenkins_p13>Jenkins 1998, p. 13.</ref> However, there were critics amongst the Air Staff of the project, typically disliking the P.1127 for its [[Subsonic aircraft|subsonic]] speeds, favouring [[Supersonic speed|supersonic]]-capable aircraft instead; Mason attributes this as having caused considerable delay in the issuing of a contract to Hawker.<ref name = "mason 4 5">Mason 1967, pp. 4–5.</ref> On 23 July 1959, Hawker authorised the application of maximum effort to complete the development of the P.1127.<ref name = "mason 5">Mason 1967, p. 5.</ref>

===P.1127===
[[File:Hawker P.1127 62 (cropped).jpg|thumb|Third prototype at Farnborough 1962 with unswept trailing edges and rounded tips|alt=Prototype P.1127 with unswept trailing edges]]

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.<ref name = "mason 7"/> 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.<ref name = "mason 7"/> On 13 October 1960, the first Pegasus flight engine, capable of generating {{convert|11,300|lbf|kN|abbr=on}} of thrust, was delivered to Dunsfold.<ref name = "mason 7"/>

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.<ref name = "mason 7"/> 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 {{convert|70|kn|km/h}}.<ref name = "mason 7"/>

[[File:Hawker Siddeley P-1127, UK - Air Force AN2243306.jpg|thumb|left|P.1127 XP980 in use as a deck handling trainer in Royal Navy markings, 1989]]
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.<ref name = "mason 7"/> 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".<ref>[https://www.youtube.com/watch?v=by4lH2whhjk "Stanley Hooker: Legendary Rolls Royce & Bristol Engineer."] ''YouTube.com'', Retrieved 10 September 2016.</ref>

On 13 February 1961, XP831 performed its first conventional flight, flown by [[Bill Bedford]] and lasting for 22 minutes.<ref name = "mason 8">Mason 1967, p. 8.</ref> Soon after this, XP831 was refitted with a new model of the Pegasus engine, capable of generating {{convert|12,000|lbf|kN|abbr=on}} of thrust, prior to embarking on new hovering trials in May 1961. In June, XP831 attained another milestone in the program when it performed the first transition from vertical hover to horizontal flight, initially flying the length of Dunsfold's runway at a height of 50 meters.<ref name = "mason 8"/>

On 7 July 1961, the second prototype, ''XP836'', performed its first take off conventionally.<ref>[http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%200893.html "VTOL: A Flight International Survey."] ''Flight International'', 7 June 1962, p. 891.</ref> Continuing tests of the two prototypes proceeded to close the gap between vertical take off and flight, a feat which was achieved on 8 September 1961.<ref name= "Mason370"/><ref name = "mason 8"/> During September, the feat was repeated multiple times by both prototypes, transitioning from vertical to horizontal flight and vice versa, including instances in which the auto-stabiliser was intentionally disabled.<ref name = "mason 8"/>

During the flight test program, the issuing of [[NBMR-3|NATO Basic Military Requirement 3]] (NBMR-3) did not prove to be the opportunity as envisioned by Hawker, as NBMR-3 sought performance characteristics of which the P.1127 was not only unable to meet, but unlikely to be developed to meet in its current form either.<ref name = "mason 7"/> As such, in 1961, there was little military interest in the P.1127 program, although, in January 1961, Hawker was requested to provide a quote for the costs involved in a potential 100 production standard P.1127 aircraft.<ref name = "mason 7 8"/> Meanwhile, Hawker believed that the continuing development of the P.1127 would serve a successful demonstration, acting to dissuade potential customers from pursuing competing 'paper' VTOL aircraft projects.<ref name = "mason 8"/>

On 2 November 1960, the Ministry of Supply issued a contract for a further four prototypes to be produced, which were intended to develop the aircraft further towards being a realistic combat design, such as the refinement of the wing, engine improvements, and of accompanying operational equipment.<ref name = "mason 7 8">Mason 1967, pp. 7–8.</ref> Throughout this period, improved models of the Pegasus engine were rapidly developed, such as the Pegasus 3 being capable of {{convert|15,000|lbf|kN|abbr=on}} of thrust. Apart from the improved powerplants, the first four P.1127 prototypes were quite similar; the fifth prototype, ''XP980'', introduced the taller fin and tailplane anhedral which were later used on the production Harrier.<ref name= "Mason371">Mason 1971, p. 371.</ref> The fourth machine was partially used to provide Hawker production test pilots with type familiarisation.<ref name= "Mason371"/> The first carrier vertical landing was performed by the first prototype on {{HMS|Ark Royal|R09|6}} in 1963.<ref>Mason 1971, p. 372.</ref><ref name = "mason 10">Mason 1967, p. 10.</ref> The last P.1127, ''XP984'', introduced the swept wing.<ref name= "Mason371"/> It was eventually fitted with the {{convert|15,000|lbf|kN|abbr=on}} Pegasus 5 and functioned as the prototype Kestrel.<ref>Mason 1971, p. 373.</ref>

The first three P.1127s crashed, the second and third occurring during development. In 1963, the first prototype, ''XP831'', publicly crashed at the [[Paris Air Show]]; the accident had been caused by a speck of dirt in the air feed lines of the nozzle control motor, which had caused the engine nozzles to stick. ''XP831'' was later fully repaired and resumed development flying. All the pilots involved survived.<ref>Mason 1971, pp. 371–2.</ref><ref name = "mason 9 10">Mason 1967, pp. 9–10.</ref>

===Kestrel FGA.1===
[[File:Hawker Siddeley XV-6A Kestrel USAF.jpg|thumb|Hawker Siddeley XV-6A Kestrel in USAF livery|alt=An aircraft landed on a runway]]

By late 1961, Hawker had been able to demonstrate the validity of its design concept, despite a lack of support from the RAF and little from the [[Civil Service (United Kingdom)|Civil Service]].<ref name = "mason 8 9">Mason 1967, pp. 8–9.</ref> In early 1962, official support emerged in the form of the Operational Requirements branch of the [[Ministry of Aviation]], which approached the Treasury seeking its sanction of a batch of nine production-standard aircraft to be operated by an evaluation unit to be administered by the [[Central Fighter Establishment]] at [[RAF West Raynham]]. In light of open interest expressed by figures within the U.S. and [[Germany|West Germany]], the British government approached these nations with an offer to collaborate on the project and to seek contributions towards the cost involved.<ref name = "mason 9"/> Following the acceptance of all three nations, on 22 May 1962, Hawker received a formal Instruction to Proceed with the procurement of materials for the construction of the nine aircraft.<ref name = "mason 9">Mason 1967, p. 9.</ref>

The nine aircraft were ordered as the ''Kestrel FGA.1'', which was essentially an improved version of the P.1127. On 7 March 1964, ''XS688'' became the first Kestrel to conduct its [[maiden flight]], flown by Bill Bedford.<ref name = "mason 10"/> The Kestrel had fully swept wings and a larger tail than the early P.1127s,<ref name= "Mason375">Mason 1971, p. 375.</ref> and the fuselage was modified to take the larger {{convert|15,000|lbf|kN|abbr=on}} Pegasus 5 engine as in the P.1127/Kestrel prototype ''XP984'', in addition to some other changes, such as addition of [[ultra high frequency]] (UHF) radio and assorted operational equipment in a bay house within the rear fuselage.<ref name = "mason 10"/> Prior to the availability of the Kestrel for testing, a pair of NASA pilots visited Dunsfold to perform a complete set of handling trials using the earlier P.1127 in its place.<ref name = "mason 9"/>

On 15 October 1964, the Tri-partite Evaluation Squadron (TES) was formed at RAF West Raynham, staffed by a diverse mix of military test pilots from Britain, the United States and West Germany.<ref name= "Mason375"/><ref name= Jefford_41>Jefford 2006, p. 41.</ref> The personnel comprising the squadron were highly experienced pilots; prior to flying the Kestrel, each received a week's ground training at Bristol's in-house facility and a week's ground instruction at Dunsfold prior to a three-hour flight conversion instructed by Bill Bedford.<ref name = "mason 10"/> The purpose of the squadron was to evaluate the suitability of V/STOL aircraft for field operations, compare competing styles and methods of taking off/landing, develop normal flight operating procedures, perform instrument flight assessments, conduct night flight operations, and explore jet-borne maneuvering throughout the Kestrel's flight envelope.<ref name = "mason 10"/>

[[File:Harrier P1127 NAS Pax NAN9-66.jpg|thumb|upright=1.4|A pair of Kestrels at the [[Naval Air Station Patuxent River]], [[Maryland]], US, 1966]]
During the course of the evaluation, the Tri-partite pilots developed a typical sortie routine for the Kestrel of conducting short take-offs (STO) and returning to base on vertical landings.<ref name= Jefford_43>Jefford 2006, p. 43.</ref> This manner of operation ([[STOVL]]) was judged to be the optimal practice.<ref name= Jefford_47>Jefford 2006, p. 47.</ref> Operating from rough airstrips was also trialled at nearby [[RAF Bircham Newton]], where the aircraft proved adept at traversing boggy ground and taking off from a variety of temporary ground coverings.<ref name= Jefford_44-45>Jefford 2006, pp. 44–45.</ref> During testing one aircraft was lost when a pilot tried to do a rolling take-off with the parking brake on;<ref name= "Mason375"/> the evaluations were finalised in November 1965.<ref>Spick and Gunston 2000, p. 362.</ref>

Six of the eight surviving evaluation aircraft (the three allocated to the U.S. plus those allocated to Germany) were transferred to the U.S.<ref name= "Mason375"/> for evaluation by the Army, Air Force, and Navy as the ''XV-6A Kestrel''. After Tri-Service evaluation they were passed to the USAF for further evaluation at [[Edwards Air Force Base]], except for two that were assigned to NASA.<ref>Evans, A: "American Harrier – Part One", ''Model Aircraft Monthly'' Vol.8, Iss.4, pp. 36–39.</ref> One of the two remaining British-based Kestrels was attached to the ''Blind Landing Experimental Unit'' (BLEU) at [[RAE Bedford]] and the other, ''XS693'', went to [[Blackburn]] for modification to take the uprated Pegasus 6 engine.<ref>Mason 1971, pp. 375–6.</ref>

In addition to some strengthening, there were alterations to the air intake, which had throughout the P.1127 and Kestrel series featured an inflatable lip to smooth the intake airflow when the aircraft was almost stationary. There were concerns about the service life of these devices, so they were replaced with conventional suction relief doors.<ref>Mason 1971, p. 376.</ref> Experience gained during naval testing on board the [[commando carrier]] [[HMS Bulwark (R08)|HMS ''Bulwark'']] in 1966 convinced project officers that less reactive materials would be substituted for all uses of [[magnesium]] in the Kestrel's airframe, in any further prototypes and production aircraft.<ref>Jefford 2006, pp. 65–66.</ref> The Kestrel became the prototype for pre-production Harriers.<ref name= "Mason377">Mason 1971, p. 377.</ref>

===P.1127 (RAF)===
{{main|Hawker Siddeley Harrier}}
In 1961, [[NATO]] issued [[NBMR-3|NATO Basic Military Requirement 3]] (NBMR-3) which specified a VTOL aircraft, albeit one with the supersonic performance of an aircraft such as the [[McDonnell Douglas F-4 Phantom II|F-4 Phantom II]], along with the VTOL capability.<ref name = "mason 7"/> Recognising that the P.1127 would not satisfy this, Hawker commenced work upon a new project, designated ''P.1150'', which was effectively a supersonic derivative of the P.1127, as well as another version, designated [[Hawker Siddeley P.1154|''P.1154'']], which would meet the specified requirements of NBMR-3. The P.1154 was declared the winner of the NATO competition; prototype construction was already underway at the point of cancellation in 1965. The French government had withdrawn shortly after the P.1154's selection over the [[Dassault Mirage IIIV]].<ref name= Jefford_12-13>Jefford 2006, pp. 12–13.</ref>

Following the termination of the P.1154, the RAF then began studying a straightforward upgrade of the Kestrel as a production aircraft; accordingly, it issued Requirement ASR 384, which sought a V/STOL ground attack jet,<ref name= Jefford_13>Jefford 2006, p. 13.</ref><ref name= "Mason378">Mason 1971, p. 378.</ref> known as the ''P.1127 (RAF)''.<ref>Spick and Gunston 2000, pp. 362–363.</ref> In late 1965, the RAF placed an order for six pre-production P.1127 (RAF) aircraft.<ref name= Jenkins_p21>Jenkins 1998, p. 21.</ref> On 31 August 1966, the first of these P.1127 (RAF) aircraft made its [[maiden flight]].<ref name= "Mason378"/><ref name = "mason 10 12">Mason 1967, pp. 10, 12.</ref> In early 1967, an order for 60 production aircraft was formally received by Hawker Aviation; at this time the aircraft received the ''[[Hawker Siddeley Harrier|Harrier GR.1]]'' designation.<ref name= Jenkins_p21/><ref name= "Mason harrier p81">Mason 1986, p. 81.</ref><ref>[http://www.flightglobal.com/pdfarchive/view/1966/1966%20-%201559.html "VTOL Aircraft 1966".] ''Flight International'', 26 May 1966. p. 884.</ref> The Harrier went on to become a successful aircraft in British service, and was exported to several nations, often seeing usage as a carrier-based aircraft.{{citation needed|date=November 2021}}

==Design==
[[File:Hawker P.1127 ‘XP831’ (19253036156).jpg|thumb|Port lower fuselage of the first prototype P.1127, showing the rotating jet nozzles]]

The P.1127/Kestrel was an experimental V/STOL aircraft, which served as the forerunner for the production of the Hawker Siddeley Harrier and the wider [[Harrier jump jet|Harrier]] family. It served to demonstrate an entirely original technique of flight, as well as to trial a brand new type of engine in the form of the Pegasus [[turbofan]] engine.<ref name = "mason 7">Mason 1967, p. 7.</ref> Despite this, the aircraft employed a principally conventional structure which, according to Mason, lent itself favourably to performing the intended ground attack operations envisioned for production aircraft.<ref name = "mason 5"/>

The location of the large Pegasus engine, which was housed within the relatively small fuselage, meant that the cockpit was positioned directly forward of it, in between the later "elephant-ear" air intakes to the engine itself.<ref name = "mason 5"/> The 'cold' jet exhaust was directed from the large fan forward of the engine compressor to rotatable [[fiberglass]] nozzles located on the sides of the main fuselage aft of the air intakes; the 'hot' jet exhaust from the rear of the engine was channeled via a bifurcated exhaust channel to two steel rotatable nozzles located somewhat aft of those used for the 'cold' jets. Additionally, [[bleed air]] was also extracted from the compressor and sent via ducts within the fuselage and wings to control nozzles, known as "puffers", located upon the aircraft's nose, tail, and wing tips, which were used for stabilisation during mid-air hovers.<ref name = "mason 5 6">Mason 1967, pp. 5–6.</ref>

The aircraft was furnished with a single-piece shoulder-mounted wing mounted across the top of the fuselage; the shape of the wing progressively alters from a clipped [[delta wing|delta]] to a fully [[swept wing|swept]] configuration, complete with extended [[leading edge]]s and spaced out [[Leading-edge extension#Dogtooth extension|sawtooth]] extensions.<ref name = "mason 6">Mason 1967, p. 6.</ref> A fully movable single-piece tailplane was also adopted, which was hinged upon the rear fuselage. The P.1127 had an atypical undercarriage arrangement, known as a "zero-track" [[Tricycle landing gear|tricycle undercarriage]], which supported the majority of the aircraft's weight upon a pair of centrally mounted main wheels; steering was performed via a conventional nose wheel while balance was provided by a pair of wing tip-mounted [[Undercarriage arrangements|outriggers]].<ref name = "mason 6"/>

[[File:Hawker Siddeley FGA.1 Kestrel (27873975992).jpg|thumb|left|Air intake of a P.1127]]
The P.1127 was equipped with entirely conventional flying controls when operated within the normal flying regime; to avoid aerodynamic [[Stall (fluid mechanics)#Fixed-wing aircraft|stall]] conditions when decelerating to slow speeds, control would be gradually and automatically transferred to the reaction jets, or "puffers", by bleeding more air from the compressor as the engine nozzles were moved towards the downwards position.<ref name = "mason 6"/> An auto-stabilisation system was installed upon all aircraft; the necessity of such a feature is arguable, as countless transitions between vertical and horizontal flight were performed with the auto-stabiliser equipment being entirely disabled.<ref name = "mason 6"/>

The P.1127 lacked any in-built armaments, the ethos of tactical flexibility meant a reliance upon underwing [[hard point]]-mounted munitions and equipment, which included multiple [[2-inch RP|2-inch (51 mm) rocket]] batteries, 30-mm [[ADEN cannon]] [[gun pod]]s, and {{convert|1,000|lb|kg|abbr=on}} [[bomb]]s, [[napalm]], and range-extending [[drop tank]]s.<ref name = "mason 6"/> The prototypes were furnished with long nose-mounted instrumentation booms, which were replaced on the Kestrel with a much smaller counterpart on the [[fin (aeronautics)|fin]] to enable the nose to accommodate a tactical [[Aerial photography|camera]] instead. An unusual feature of the P.1127 was the installation of a [[ram air turbine]] upon the rear fuselage just forward of the fin to provide power for ancillary services in the event of engine failure.<ref name = "mason 6"/>

==Variants==
[[File:Hawker P.1127 prototype Brooklands.jpg|thumb|The last of the six P.1127 prototypes (XP984) and the only one retrofitted with a swept wing (seen here with an earlier straight wing<ref name="Hieminga">{{Citation |last=Hieminga |first=Jelle |title=P1127_wingchange |date=2016-10-23 |url=https://www.flickr.com/photos/102686263@N02/32606744511/ |access-date=2022-08-26}}</ref>). Later converted to the first Kestrel prototype with Pegasus 5 engine.|alt=Prototype on museum display]]

;P.1127
:Experimental V/STOL fighter, two prototypes and four development aircraft.<ref name= Jenkins_p13/>
;Kestrel FGA.1
:Aircraft for the tripartite evaluation squadron, nine built, six later transferred to the United States where they were designated XV-6A.<ref name= 'evans'>Evans, A. "American Harrier – Part One." ''Model Aircraft Monthly'', Vol. 8, Issue 4, pp. 36–39.</ref>
;P.1127 (RAF)
:Development V/STOL ground attack and reconnaissance fighter, six built as pre-production evaluation aircraft before the type was ordered into production as the [[Hawker Siddeley Harrier|Harrier GR.1]]. First aircraft flew from Dunsfold on 31 August 1966.<ref name= "mason harrier p78">Mason 1986, p. 78.</ref>
;XV-6A
:United States military designation for the six Kestrel FGA.1 transferred to the U.S.<ref name= 'evans'/>
;VZ-12
:U.S. Army designation for two P.1127 development aircraft, not delivered.<ref>Swanborough and Bowers 1990, p. 338.</ref>

==Operators==
;{{UK}}
* [[Aeroplane and Armament Experimental Establishment]] P.1127 and Kestrel trials and evaluation
* [[Royal Air Force]]
** Central Fighter Establishment – Kestrel (P.1127) Evaluation Squadron (also known as the Tripartite Evaluation Squadron), nine Kestrel aircraft allocated for evaluation in 1965. One aircraft was lost in an accident, six aircraft later passed to the United States Army, one to the [[Royal Aircraft Establishment]] and one to Hawker Siddeley for further trials.<ref name= "Mason 91 p419-0">Mason 1991, pp. 419–420.</ref>
* [[Royal Aircraft Establishment]] P.1127 and Kestrel trials and evaluation.<ref name= "Mason 91 p419-0"/>

;{{USA}}
* [[United States Army]] (aircrew participated in both the Tri-partite Evaluation Squadron and as part of the American XV-6A Tri-service evaluation team. Had three Kestrel aircraft allocated after the evaluation at West Raynham and also acquired the three allocated to Germany. Four later transferred to the United States Air Force and two to the National Aeronautics and Space Administration).<ref name= 'evans'/>
* [[United States Air Force]] (aircrew participated in both the Tri-partite Evaluation Squadron and as part of the American XV-6A Tri-service evaluation team but had no aircraft allocated after the evaluation at West Raynham, four former United States Army aircraft operated for trials).<ref name= 'evans'/>
* [[United States Navy]] (aircrew participated in both the Tri-partite Evaluation Squadron and as part of the American XV-6A Tri-service evaluation team but had no aircraft allocated after the evaluation).<ref name= 'evans'/>
* [[NASA]] operated two former United States Army Kestrels.<ref name= 'evans'/>

;{{FRG}}
* [[German Air Force]] (''Luftwaffe'') (aircrew participated in the Tri-partite Evaluation Squadron, three allocated Kestrel aircraft not delivered and passed to United States Army)<ref name= 'evans'/>

==Aircraft on display==
[[File:XV-6A Kestrel 64-18266 NASA 521 VASM.jpg|thumb|XV-6A Kestrel on display at the [[Virginia Air and Space Center]]|alt=Kestrel on museum display]]

* P.1127 [[United Kingdom military aircraft serial numbers|''XP831'']] on display at [[Science Museum (London)|The Science Museum]], London, England.<ref>{{cite web |url= http://www.sciencemuseum.org.uk/objects/aeronautics/1992-796.aspx |title= Hawker P 1127 VSTOL Experimental Aircraft |publisher= Science Museum |year= 1960 |url-status= dead |archive-url= https://web.archive.org/web/20080423225828/http://www.sciencemuseum.org.uk/objects/aeronautics/1992-796.aspx |archive-date= 23 April 2008}}</ref>
* P.1127 ''XP980'' (fitted with a Harrier GR.1 wing) is on display at the [[Fleet Air Arm Museum]], Yeovilton, England.<ref>{{cite web |url= http://www.airmuseumsuk.org/museum/Yeovil1/800/pages/331%20Hawker%20P.1127%20XP980.htm |title= Hawker P.1127 |publisher= airmuseumsuk.org |access-date= 8 October 2010}}</ref>
* P.1127 ''XP984'' (restored with a Harrier GR.1 wing in 2016<ref name="Hieminga"/>) is on display at the [[Brooklands Museum]], Surrey, England.<ref>{{cite web |url= https://www.flickr.com/photos/imagesofaviation/2907299714/ |title= Hawker-Siddeley P1127 Kestrel (XP984) |publisher= Flickr |access-date= 8 October 2010|date= 22 August 2008 }}</ref>
* Kestrel FGA.1 ''XS695'' on display at the [[Royal Air Force Museum Cosford]] England.<ref>{{cite web |url= http://www.aeroflight.co.uk/museums/raf-museum-cosford.htm |title= The Royal Air Force Museum, Cosford |publisher= aeroflight.co.uk |access-date= 8 October 2010 |archive-date= 15 March 2013 |archive-url= https://web.archive.org/web/20130315094403/http://www.aeroflight.co.uk/museums/raf-museum-cosford.htm |url-status= dead }}</ref>
*Kestrel FGA.1 XS694 in storage and under restoration at [[Wings Museum]] England, former NASA 520 Aircraft
* XV-6A Kestrel [[United States military aircraft serial numbers|''64-18262'']] on display at the [[National Museum of the United States Air Force]] at Wright-Patterson AFB, Ohio, United States.<ref>{{cite web |url= https://www.flickr.com/photos/afmuseum/4151051672/in/photostream/ |title= Hawker Siddeley XV-6A Kestrel |publisher= Flickr |access-date= 9 March 2011}}</ref>
* XV-6A Kestrel ''64-18263'' with NASA [[livery]] on display at the [[Virginia Air and Space Center]], Hampton, Virginia, United States
* XV-6A Kestrel ''64-18264'' formerly held in storage by the [[United States Army Aviation Museum]], Alabama, United States;<ref>{{cite web |url= http://www.airport-data.com/aircraft/photo/136188.html |title= 64-18264, 1964 Hawker Siddeley XV-6A Kestrel |publisher= airport-data.com |access-date= 9 March 2011}}</ref> now on display at the [[Pima Air & Space Museum]] in [[Tucson]], [[Arizona]].<ref>{{Cite web | url=http://www.pimaair.org/aircraft-by-name/item/kestrel | title=Pima Air & Space Museum – Tucson, Arizona – Pima Air and Space Museum – Tucson, Arizona | access-date=31 October 2016 | archive-date=1 November 2016 | archive-url=https://web.archive.org/web/20161101164444/http://www.pimaair.org/aircraft-by-name/item/kestrel | url-status=dead }}</ref>
* XV-6A Kestrel ''64-18266'' with NASA livery on display at [[Air Power Park]], Hampton, Virginia, United States.<ref>{{cite web |url= https://www.flickr.com/photos/mikelynaugh/4279705907/ |title= Hawker Siddeley XV-6A Kestrel – Air Power Park – Hampton, VA |publisher= Flickr |access-date= 9 March 2011|date= 16 January 2010 }}</ref>
{{clear}}

==Specifications (Kestrel FGA.1)==

{{Aircraft specs
|ref=Hawker Aircraft since 1920<ref name= "Mason377"/> ''The Hawker P.1127 and Kestrel''<ref name = "mason 12">Mason 1967, p. 12.</ref>
|prime units?=kts
<!--   General characteristics
-->
|crew=1
|length ft=42
|length in=6
|length note=
|span ft=22
|span in=11
|span note=
|height ft=10
|height in=9
|height note=
|wing area sqft=
|wing area note=
|aspect ratio=<!-- sailplanes -->
|airfoil=
|empty weight lb=9800
|empty weight note=
|gross weight lb=14000
|gross weight note=VTO
|max takeoff weight lb=17000
|max takeoff weight note=STO
|fuel capacity=
|more general=
<!--    Powerplant
-->
|eng1 number=1
|eng1 name=[[Bristol Siddeley Pegasus 5]]
|eng1 type=vectored-thrust [[turbofan]] engine
|eng1 lbf=15000
|eng1 note=
<!--    Performance
-->
|max speed kts=617
|max speed note=
|max speed mach=0.92
|cruise speed kts=
|cruise speed note=
|stall speed kts=
|stall speed note=
|never exceed speed kts=
|never exceed speed note=
|range nmi=
|range note=
|combat range nmi=
|combat range note=
|ferry range nmi=
|ferry range note=
|endurance=<!-- if range unknown -->
|ceiling ft=55000
|ceiling note=
|g limits=<!-- aerobatic -->
|roll rate=<!-- aerobatic -->
|climb rate ftmin=30000
|climb rate note=
|time to altitude=
|lift to drag=
|wing loading lb/sqft=
|wing loading note=
|fuel consumption lb/mi=
|power/mass=1.04
|more performance=
<!--    Armament
-->
|guns= 
|bombs= 
|rockets= 
|hardpoints=2 wing pylons
|hardpoint capacity=
|hardpoint rockets=2x 19 round [[SNEB|2.7" (68 mm) SNEB]] launchers / 2x 32-round [[2-inch RP|2" (51 mm) Microcell]] launchers

|avionics=
}}

==See also==
{{Portal|United Kingdom|Aviation}}
{{aircontent
|related=
* [[Hawker Siddeley P.1154]]
* [[Hawker Siddeley Harrier]]
* [[Short SC.1]]
|similar aircraft=
* [[Bell X-14]]
* [[Hunting H.126]]
* [[Ryan XV-5 Vertifan]]
* [[Yakovlev Yak-36]]
|lists=
* [[List of VTOL aircraft]]
|see also=
* [[Harrier jump jet]], an overview of the Harrier family
* [[Ralph Hooper]]
}}

==References==

===Notes===
{{reflist |group= "N"}}

===Citations===
{{Reflist|30em}}

===Bibliography===
*Buttler, Tony and Jean-Louis Delezenne. ''X-Planes of Europe: Secret Research Aircraft from the Golden Age 1946-1974''. Manchester, UK: Hikoki Publications, 2012. {{ISBN|978-1-902-10921-3}}
* Cowan, Charles W. (ed.) ''Flypast 2''. Windsor, Berkshire, UK: Profile Publications Ltd., 1972. {{ISBN|0-85383-191-2}}.
* Swanborough, Gordon and Peter M. Bowers. ''United States Navy Aircraft since 1911''. Putnam Aeronautical, 1990. {{ISBN|0-87021-792-5}}.
* Hannah, Donald. ''Hawker FlyPast Reference Library''. Stamford, Lincolnshire, UK: Key Publishing Ltd., 1982. {{ISBN|0-946219-01-X}}.
* James, Derek N. ''Hawker, an Aircraft Album No. 5''. New York: Arco Publishing Company, 1973. {{ISBN|0-668-02699-5}}. (First published in the UK by Ian Allan in 1972)
* Jefford, C.G., ed. [https://web.archive.org/web/20110105084744/http://www.rafmuseum.org.uk/research/documents/Journal%2035A%20-%20Seminar%20-%20the%20RAF%20Harrier%20Story.pdf ''The RAF Harrier Story.''] London: Royal Air Force Historical Society, 2006. {{ISBN|0-9530345-2-6}}.
* Jenkins, Dennis R. ''Boeing / BAe Harrier''. North Branch, Minnesota: Specialty Press, 1998. {{ISBN|1-58007-014-0}}.
* Mason, Francis K. ''Hawker Aircraft since 1920.'' London: Putnam, 1991. {{ISBN|0-85177-839-9}}.
* Mason, Francis K. ''Harrier''. Wellingborough, UK: Patrick Stephens Limited, Third edition, 1986. {{ISBN|0-85059-801-X}}.
* Mason, Francis K. ''Hawker Aircraft since 1920''. London: Putnam Publishing, 1971. {{ISBN|0-370-00066-8}}.
* Mason, Francis K. ''The Hawker P.1127 and Kestrel (Aircraft in Profile 93)''. Leatherhead, Surrey, UK: Profile Publications Ltd., 1967.
* Spick, Mike and Bill Gunston. ''The Great Book of Modern Warplanes''. Osceola, WI: MBI Publishing, 2000. {{ISBN|0-7603-0893-4}}.

==Further reading==
*{{cite journal |last1=Hayward |first1=Keith |title=The One that Got Away: How the Hawker P.1127 Escaped Healey's Axe |journal=The Aviation Historian |date=2019 |issue=29 |pages=48–56 |issn=2051-1930}}

==External links==
{{Commons category}}
* [http://www.harrier.org.uk/history/history1_1.htm History – The Origins of the P.1127, at Harrier.org.uk] {{Webarchive|url=https://web.archive.org/web/20110928082832/http://www.harrier.org.uk/history/history1_1.htm |date=28 September 2011 }}
* [http://www.flightglobal.com/pdfarchive/view/1960/1960%20-%202814.html "P.1127 – Hawker's V/STOL Strike Aircraft"] a 1960 ''Flight'' article
* [http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%202326.html "The P.1127 Analysed"] – a 1962 ''[[Flight International]]'' article on the Hawker P.1127
* [http://www.flightglobal.com/pdfarchive/view/1963/1963%20-%200628.html "P.1127 Development"] – a 1963 ''Flight International'' article on the development of the Hawker P.1127

{{Hawker Aircraft aircraft}}
{{Harrier variants}}
{{US Army VTOL}}
{{US STOL and VTOL aircraft}}

{{good article}}

[[Category:Hawker aircraft|P1127]]
[[Category:V/STOL aircraft by thrust vectoring]]
[[Category:1960s British experimental aircraft]]
[[Category:Single-engined jet aircraft]]
[[Category:High-wing aircraft]]
[[Category:Harrier Jump Jet]]
[[Category:Aircraft first flown in 1960]]
[[Category:Aircraft with retractable bicycle landing gear]]