requirements for a lightweight interceptor to counter the threat of escorted Russian Tu-4 bombers (AST OR/303). To address this, Petter developed a number of concepts. His early designs used jettisonable engines but by late 1951 he had focussed on more conventional designs; the Fo 139 and Fo 140 (which would eventually be developed into the Midge and Gnat respectively). With concept design work on the light fighter ramping up, Petter decided not to proceed with the Red Dean and the MoS cancelled the contract in November 1951. Petter sent a brochure describing the Fo.140 to the Air Staff in January 1952, but beyond this, further development of the light fighters was hindered by the lack of engines with a high thrust-to-weight ratio. The Bristol Saturn was cancelled, the Armstrong Siddeley Viper had insufficient thrust and Rolls-Royce had neither a suitable engine nor the inclination to develop one. To address this problem Petter approached Stanley Hooker at Bristol in late 1952 to discuss the design of a suitable engine. This meeting would lead to the development of the Orpheus. By 1952 Government interest in a light fighter had waned. Despite this Petter began the design and construction of the prototype as a private venture. The Fo 139 design was revised, adopting a shoulder-mounted wing and a low-set tailplane. In common with his previous designs, Petter made use of magnesium alloys in the structure, using material surplus from the RTV2 Missile programme in the construction of the Midge prototype. In June 1953 Petter presented his manifesto on light fighter design at the AFITA congress at the Paris Air show. In this paper, titled "Design for Production", Petter compared a 2500 kg light fighter with a 7500 kg standard fighter and concluded that although the weight ratio was 3:1, four times as many light fighters could be manufactured for the same cost. To achieve this he explained how the fuselage, wing structure, engine and services could all be simplified. Colonel Johnnie Driscoll, head of the NATO Mutual Weapons Development Program noted Petter's light fighter concept. NATO was interested in ground-attack aircraft that could be operated from improvised airfields and could be manufactured by postwar European industry. Driscoll formalised a NATO requirement in August 1954 that would become the NATO light fighter competition. It was based on Petter's concept but included the requirement to use low-pressure tyres. However, the Gnat was designed to operate