Hawker P.1121 Staff having been unable to understand the concept of the multi-purpose functionality of the aircraft, which would become commonplace on later aircraft. While seeking a new project to take the place of the P.1121, Camm soon began work upon another new and radical aircraft design, designated as the Hawker Siddeley P.1127. The P.1127 would become the Hawker Siddeley Harrier, the first effective vertical take-off and landing (VTOL) combat aircraft. The Hawker P.1121 was envisioned as being one of the first British-built supersonic fighter aircraft, being projected to be capable of speeds of up to Mach 2.5. It was to have
Hawker Siddeley Harrier function had originally been added following the input of US Air Force Colonel Bill Chapman, who worked for the Mutual Weapons Development Team. Water injection was necessary in order to generate maximum thrust, if only for a limited time, and was typically used during landing, especially in high ambient temperatures. The aircraft was initially powered by the Pegasus 6 engine which was replaced by the more powerful Pegasus 11 during the Harrier GR.1 to GR.3 upgrade process. The primary focus throughout the engine's development was on achieving high performance with as little weight as possible, tempered by the amount of