At the time that the MoA ordered the development batch pf P.1127s the RAF still had GOR345 outstanding for a V/STOL tactical strike aircraft. Throughout 1960 and into 1961 the Operational Requirements Branch continued to study how best to meet this requirement. To all intents and purposes this meant how best to develop the P.1127, as this was the only possible contender to replace the Hunter in the timeframe envisaged - service entry was planned for 1964-65. It was clear that a version of the P.1127 equipped with a Pegasus rated at 18,000 lb. could meet GOR345. Such an engine was in prospect, Bristol-Siddeley proposing their Pegasus 5 with a new fan, annular combustion chamber and other improvements. However, almost as soon as Hawker had submitted proposals for such an aircraft to the MoA the RAF upgraded the performance they required, with the level speed demanded going from Mach 0.95 to Mach 1.2. This was beyond the ability of the P.1127, even with the more powerful engine, and by spring 1961 the RAF was rapidly losing interest in the P.1127 as its Hunter replacement.

Nevertheless, the MoA was keen to develop the aircraft as they hoped it would fulfil not only RAF needs but also the emerging requirements of many NATO air forces. At SHAPE in Paris GOR-2 was still under consideration, although the equipment and mission radius required was already beyond even the P.1127 variant offered to fulfil GOR345. At the same time, several countries were drafting requirements for V/STOL combat aircraft. One of the most interested was West Germany, which hoped to replace the Fiat G.91 and Lockheed F-104G with advanced subsonic and supersonic strike fighters. The former requirement was given the designation VAK 191. With the decline in RAF support for the P.1127, the MoA turned its attention to the VAK 191 requirement. Early in 1961 the Minister of Aviation, Peter Thorneycroft, negotiated an Anglo-German agreement to study the development of V/STOL fighters. This was aimed by the British at leading to German adoption of the P.1127, although the Germans saw the P.1127 as only one of several candidates - they designated it VAK191A along with the Focke-Wulf 1262 (VAK191B), EWR 340 (VAK191C) and Fiat G.95/4 (VAK191D). Hawker also designed several 'clean sheet of paper' aircraft, the P.1159, P.1162 and P.1163, to meet the German requirement. In order to give the agreement some teeth, the MoA exerted pressure on the RAF to accept 30 P.1127s, which the Service grudgingly agreed to do if the Germans bought the aircraft.

While these political machinations were taking place in Europe, interest in the P.1127 was being stirred in the United States. There the US Army was trying to get control of its own tactical air support and saw the P.1127 as the ideal aircraft to achieve this. Throughout 1961 and 1962 they studied the implications of using the P1127, with a senior Army official, Dr Larsen, visiting Kingston in May 1962 to discuss the proposal. Part of the US Army requirement was that the P.1127, if selected against the Lockheed XV-4A and Ryan XV-5A, should be built in America. To this end, an agreement was announced in January 1963 that Hawker and Northrop were to co-operate in the development of the aircraft. A team from Northrop arrived at Kingston in July 1963 to gain first-hand knowledge of the P.1127, also touring Germany to see US Army operations in the field.

General arrangement of the Kestrel FGA.1.	Detail of the throttle and nozzle angle controls.

By late 1961 the MoA in Britain had got wind of US Army interest in the P.1127 and was stirred into action by the 'astonishing rumour' that the Americans might order 1000 aircraft. In light of the German requirement it was proposed in early 1962 that the three countries should get together to procure a developed version of the P.1127 for service evaluation purposes, agreement to this being reached in May. For the MoA this was a way of firming up foreign interest in the aircraft and committing them to its development. However, the Germans saw it solely as a technical evaluation, with their interest already firmly fixed on the Fw 1262 as the aircraft most likely to meet VAK191, the P.1127 being considered incapable of meeting the critical sortie even if fitted with a proposed 21,000 lb. Pegasus 5-6A. For the RAF the possibility of developing a Tripartite version of the P.1127 was an opportunity to get themselves off the hook for their commitment to buy 30 aircraft as part of the Anglo-German agreement. To add further to the confusion, the US Air Force was starting to oppose US Army plans to acquire fixed wing air power. With so many conflicting points of view, it is unsurprising that negotiations took the whole of 1962 to be finalised. After initial proposals for between four and 18 aircraft had been considered, the final agreement signed in Paris on 16 January 1963 was for nine developed P.1127s, costs being divided equally between Britain, the Federal German Republic and the United States of America.

At Kingston it was decided that in order to incorporate the latest standard of modifications in the trials aircraft their construction should be delayed as late as possible. Assembly was begun during 1963, with these aircraft incorporating all the modifications of the last development batch machine, including the new wing, Pegasus 5 and fuselage stretch. In addition, all nine aircraft were equipped with inboard pylons for fuel tanks and possible weapons, a nose mounted recce camera and a simple weapons sight for target tracking, all intended to facilitate operational trials. The first of the Tripartite evaluation aircraft flew in March 1964, the others following along at regular intervals until the last flew in March 1965. Initially these aircraft were known as P.1127s but in November 1964 they were given the name Hawker Siddeley Kestrel FGA Mk.1. Not only did the Kestrel have many of the limitations of the P.1127 removed, it was also, like the earlier aircraft, fully stressed for combat operations, with provision for extra pylons and equipment, such as a Doppler navigator and auto-stabiliser - in fact, it was essentially the aircraft the RAF would have received had they pursued GOR345. However, as money was limited for the trials, this equipment was never fitted.

The Tripartite programme involved pilots from the RAF, Luftwaffe, and three US services - Army, Navy and Air Force. Training on the Kestrel began at Dunsfold in the latter part of 1964, with the Tripartite squadron officially forming at RAF West Raynham on 1 April 1965. The objective of these trials was to establish the ground-rules for the operation of jet V/STOL fighters. This meant that emphasis was placed on operating techniques such as short take-off, dispersed site operations and concealment, rather than on flying attack missions. Although such sorties were simulated, no weapons were ever carried by Kestrels. In total 1,367 sorties were flown during the trials, which lasted until 30 November 1965. A huge amount was learnt about operations in the field, including the effects of hot gas re-ingestion, ground erosion and ways to alleviate both effects. At the end of the trials six Kestrels went to the United States, the Luftwaffe having no further use for their three allotted aircraft. In the USA the aircraft were designated XV-6A and were involved in many years of trials with the Services and at NASA. In Britain the Kestrel had initially been seen as the precursor of the V/STOL strike fighter the RAF had come to adopt after the withdrawal of GOR345 - the supersonic P.1154. It was only with the cancellation of this aircraft in February 1965 that the RAF was forced to adopt a developed Kestrel for front line service, ultimately named Harrier, the name originally selected for the ill-fated P.1154.