Hawker Siddeley P . 1127 The Hawker P . 1127 and the Hawker Siddeley Kestrel FGA . 1 were the 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 . Kestrel 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 Britain , 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 " in 1967 . The Harrier would go on to serve with the UK and several nations , often as a carrier - based aircraft . Design and development . Background . In 1957 , the Bristol Engine Company informed Sydney Camm of Hawker that they had a project to combine their Olympus and Orpheus jet engines to produce a directable fan jet . The original idea on which the engine , named " Pegasus " , was based came from Michel Wibault , a French aviation consultant . Several adaptions and enhancements were made by Bristol to reduce size and weight over Wibault ' s original concept . Hawker took the planned Pegasus engine as a basis for a plane that could meet the current NATO specification for a Light Tactical Support Fighter . Prior to the P . 1127 project , Hawker Aviation had been working on a replacement for the Hawker Hunter — the Hawker P . 1121 . However , the P . 1121 was cancelled shortly after the 1957 Defence White Paper , which advocated a policy shift away from manned aircraft and towards missiles . 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 , the reaction control system - this was resolved by April 1959 . As the P . 1127 had been developed at a time of deep UK defense cuts ; Hawker had to seek commercial funding , significant engine development funding came from the U . S . Wind tunnel tests conducted by NASA Langley Research Center using a sub - scale model showed acceptable flight characteristics . Hawker test pilot Hugh Merewether went to the U . S . at NASA ' s request to fly the Bell X - 14 . In March 1959 , the company ' s board of directors ( Hawker Siddeley then ) decided to privately fund two P . 1127 prototypes . In late 1959 the British Ministry of Supply contracted for two P . 1127 prototypes . P . 1127 . The first prototype P . 1127 , serial " XP831 " , was delivered in July 1960 for static engine testing , and in October the Pegasus flight engine was made available . The first tethered flight took place at Dunsfold Aerodrome on the 21 October and free flight hover was achieved on 19 November , after which the first publicity photos were released . The second prototype , " XP836 " , made its first take off conventionally on 7 July 1961 . The two aircraft proceeded to close the gap between vertical take off and flight , which was achieved by 8 September 1961 . Four more prototypes were ordered . Throughout this period improved Pegasus engines were being developed , with the Pegasus 3 being capable of 15 , 000 lbf ( 67 kN ) of thrust . Apart from this , the first four aircraft were quite similar , but the fifth , " XP980 " , introduced the taller fin and tailplane anhedral later seen on the Harrier . The fourth machine was used , in part to give the Hawker production test pilots P . 1127 familiarisation . The first carrier vertical landing was performed by the first prototype on in 1963 . The last P . 1127 , " XP984 " , introduced the swept wing . It was eventually fitted with the 15 , 000 lbf ( 66 . 7 kN ) Pegasus 5 and functioned as the prototype Kestrel . The first three P . 1127s crashed , the second and third during development . The first prototype ( XP831 ) crashed at the Paris Air Show in 1963 but was fully repaired and resumed development flying . All the pilots involved survived . Kestrel FGA . 1 . Nine evaluation aircraft were ordered as the " Kestrel FGA . 1 " , an improved version of the P . 1127 , the first flying on 7 March 1964 . The Kestrel had fully swept wings and a larger tail than the early P . 1127s , and the fuselage was modified to take the larger 15 , 000 lbf ( 85 kN ) Pegasus 5 engine as in the P . 1127 / Kestrel prototype " XP984 " . Due to interest from the U . S . and Germany , the Tri - partite Evaluation Squadron ( TES ) was formed on 15 October 1964 at RAF West Raynham , staffed by military test pilots from Britain , the United States and West Germany . The Kestrel ' s first flight was on 7 March 1964 . These equipped the Tripartite Evaluation Squadron formed at RAF West Raynham in Norfolk in March 1965 . The Evaluation pilots developed a typical sortie routine for the Kestrel of conducting short take - offs ( STO ) and returning to base on vertical landings . This manner of operation ( STOVL ) was judged to be the optimal practice . 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 . During testing one aircraft was lost ; the evaluations were finalised in November 1965 . 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 . 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 . 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 . 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 . Experience gained during naval testing on board the commando carrier 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 . The Kestrel became the prototype for pre - production Harriers . P . 1127 ( RAF ) . NATO requirement NBMR - 3 specified a VTOL aircraft , but one with the performance of an aircraft such as the F - 4 Phantom II along with VTOL capability . Hawker drafted the P . 1150 , a supersonic P . 1127 , and the P . 1154 , which would meet NBMR - 3 . The P . 1154 was a winner of the NATO competition and prototype construction was already underway at the point of cancellation in 1965 . The French government withdrew shortly after its selection over the Dassault Mirage IIIV . The RAF then began looking at a simple upgrade of the Kestrel , and issued Requirement ASR 384 for a V / STOL ground attack jet , known as the P . 1127 ( RAF ) . In late 1965 , the RAF ordered six pre - production P . 1127 ( RAF ) aircraft . The first P . 1127 ( RAF ) flew on 31 August 1966 . An order for 60 production aircraft was formally received by Hawker Aviation in early 1967 ; at this time the aircraft received the " Harrier GR . 1 " designation . The Harrier would go on to be a successful aircraft in British service , and was exported to several nations , often seeing usage as a carrier - based aircraft .