The development of the Orpheus turbojet was undertaken by Bristol Siddeley in 1953 and one year later the first version (BOr.1) passed the 150-hour approval test with a thrust of 14.6 kN. In mid-1955 it was successfully installed in the Folland Gnat, a light fighter prototype. In 1956 some engines of the BOr.2 version were subjected to very demanding tests with ambient temperatures of +43 °C and 80% humidity and then again in an almost arctic climate with temperatures of -34 °C, demonstrating excellent reliability even in extreme operating conditions. Given the excellent results, in 1957 the engine was further developed in the BOr.3 version, which was chosen to equip the new FIAT G. 91 light fighter, used for 30 years in various NATO countries. In Italy it was used in various wings of the Air Force, including the Frecce Tricolori, the Italian national aerobatic team, up to 1982. The engine achieved considerable success and was also produced under license in India, France, Germany and Italy, where it was built by FIAT in the 803 version (BOr.3). Specifically designed for light fighter aircraft, the Orpheus features a high propulsive thrust-to-mass ratio, the best among engines in its category in 1957 (version 803/BOr.3). The front part is made of light alloy while the body of the compressor and all the accessories is in a single piece of magnesium and zirconium alloy. The engine shaft is supported by only two bearings located at the front and rear ends, i.e. respectively before the compressor wheels and downstream of the single-stage turbine. The combustion chamber is a cannular-type, i.e. composed of 7 direct-flow flame tubes.