[19] The design used a single Bristol Siddeley BS100 engine with four swivelling nozzles, in a fashion similar to the P.1127, and required the use of plenum chamber burning (PCB) to achieve supersonic speeds.

This ambition was complicated by the conflicting requirements between the two services—while the RAF wanted a low-level supersonic strike aircraft, the Navy sought a twin-engine air defence fighter.

[23] Following the election of the Labour Government of 1964 the P.1154 was cancelled, as the Royal Navy had already begun procurement of the McDonnell Douglas Phantom II and the RAF placed a greater importance on the BAC TSR-2's ongoing development.

[23] Work continued on elements of the project, such as a supersonic PCB-equipped Pegasus engine, with the intention of developing a future Harrier variant for the decades following cancellation.

[24][N 3] Following the collapse of the P.1154's development the RAF began considering a simple upgrade of the existing subsonic Kestrel and issued Requirement ASR 384 for a V/STOL ground attack jet.

[N 6] The capability to scatter Harrier squadrons to dozens of small "alert pads" on the front lines was highly prized by military strategists and the USMC procured the aircraft because of this ability.

[56] The engine is equipped for water injection to increase thrust and takeoff performance in hot and high altitude conditions; in normal V/STOL operations the system would be used in landing vertically with a heavy weapons load.

[57] The water injection function had originally been added following the input of US Air Force Colonel Bill Chapman, who worked for the Mutual Weapons Development Team.

It is viewed by senior RAF officers as a significant design success, that to enable and control the aircraft's vertical flight required only a single lever added in the cockpit.

[69] Changes included the removal of all magnesium components, which corroded quickly at sea, and the integration of American radios and Identification Friend or Foe (IFF) systems; furthermore the outer pylons, unlike the RAF aircraft, were designed from delivery to be equipped with self-defence AIM-9 Sidewinder heat-seeking air-to-air missiles.

[72][73] All RAF GR.1s and the initial AV-8As were fitted with the Ferranti FE541 inertial navigation/attack suite, but these were replaced in the USMC Harriers by a simpler Interface/Weapon Aiming Computer to aid quick turnaround between missions.

[35][36] The USMC upgraded their AV-8As to the AV-8C configuration; this programme involved the installation of ECM equipment and adding a new inertial navigation system to the aircraft's avionics.

Substantial changes were the Lift Improvement Devices, to increase VTOL performance; at the same time several airframe components were restored or replaced to extend the life of the aircraft.

[N 9] The Sea Harrier was also fitted with navigational aids for carrier landings, modifications to reduce corrosion by seawater and a raised bubble-canopy covered cockpit for better visibility.

[23][82] An early demonstration of the Harrier's capabilities was the participation of two aircraft in the Daily Mail Transatlantic Air Race in May 1969, flying between St Pancras railway station, London and central Manhattan with the use of aerial refuelling.

In 1977, these three squadrons were moved forward to the air base at Gütersloh, closer to the prospective front line in the event of an outbreak of a European war.

[93] The Harrier GR.3s focused on providing close air support to the ground forces on the Falklands and attacking Argentine positions; suppressing enemy artillery was often a high priority.

[91] From 10 to 24 May 1982, prior to British forces landing in the Falklands, a detachment of three GR.3s provided air defence for Ascension Island until three F-4 Phantom IIs arrived to take on this responsibility.

The first generation Hawker Siddeley airframes were replaced by the improved Harrier II, which had been developed jointly between McDonnell Douglas and British Aerospace.

[It] had limited capability, but that's how the first-generation automobile, boat, or other major systems evolved... it brought us into the world of flexible basing and the Marine Corps into the concept of vertical development" The United States Marine Corps began showing a significant interest in the aircraft around the time the first RAF Harrier squadron was established in 1969, and this motivated Hawker Siddeley to further develop the aircraft to encourage a purchase.

[107] Although there were concerns in Congress about multiple coinciding projects in the close air support role,[N 11] the Marine Corps were enthusiastic about the Harrier and managed to overcome efforts to obstruct its procurement.

[114][115][N 12] Other exercises were performed to demonstrate the AV-8A's suitability for operating from various amphibious assault ships and aircraft carriers, including a deployment of 14 Harriers aboard USS Franklin D. Roosevelt for six months in 1976.

[117][N 13] When the reactivation of the Iowa-class battleships was under consideration, a radical design for a battleship-carrier hybrid emerged that would have replaced the ship's rear turret with a flight deck, complete with a hangar and two ski jumps, for operating several Harriers.

However, the USMC considered the need for naval gunfire support to be a greater priority than additional platforms for carrier operations, while the cost and delay associated with such elaborate conversions was significant, and the concept was dropped.

[126] Due to the Harrier's unique characteristics it attracted a large amount of interest from other nations, often as attempts to make their own V/STOL jets were unsuccessful, such as in the cases of the American XV-4 Hummingbird and the German VFW VAK 191B.

[131] During tests in November 1972, the British pilot John Farley showed that the wooden deck of their aircraft carrier Dédalo was able to withstand the temperature of the gases generated by the Harrier.

[133][134][N 18] The Royal Thai Navy's AV-8S Matadors were delivered as part of the air wing deployed on the new light aircraft carrier HTMS Chakri Naruebet.

After considering American, Italian, and Spanish designs, the Australian government accepted a British offer to sell HMS Invincible, which would be operated with Harriers and helicopters.

[143][144][145] However, the Royal Navy withdrew the offer after the Falklands War, and the 1983 election of the Australian Labor Party led to the cancellation of plans to replace Melbourne.

In 1977 Li Chiang, the Chinese Minister of Foreign Trade, visited the UK and British Aerospace organised a Harrier flying demonstration.