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Bristol Siddeley Orpheus
Fiat 4023
Fiat 4024




The engine had its genesis in a 1952 request by Folland for an engine in the 5,000 pounds (22 kN) class to power a new trainer and lightweight fighter-bomber they were developing. Stanley Hooker, relatively new to the company after an earlier career at Rolls-Royce, took the project under his wing. He delivered a relatively simple and easy to maintain engine, which was put into use in the Folland Gnat, flying in 1955. Developing a Sea Level Static thrust of 4,520 lbf (20.1 kN), the Orpheus 701 had a 7 stage axial compressor driven by a single stage turbine.

Other users, mostly trainers, soon followed, including the Fuji T-1, Hindustan Marut, HA-300,and the experimental Hunting H.126 and Short SB5. In 1957 NATO ran a competition for a light fighter design, asking for entries in both engine and airframe categories. The Orpheus was the unanimous winner of the engine contest, and was thus selected to power the Fiat G.91R and G.91T using Fiat-built versions of the engine.

Many companies in the 1950s were looking at ways of producing a vertical take off and landing aircraft. Michel Wibault had the idea of using a turboshaft engine to drive four large centrifugal blowers which could be swivelled to vector the thrust. Hooker's engineers decided on using the Orpheus to drive a single large fan that would supply air to a pair of rotating nozzles, while the exhaust flow from the Orpheus was split into two and would supply another pair of nozzles at the rear of the engine. This experimental system developed into the Pegasus.


Fiat G.91, Folland Gnat, Canadair Sabre – all Bristol Siddeley Orpheus powered



First run on 17 December 1954, rated at 3,285 lbf (14.61 kN) by Spring 1955, powered the prototype Folland Gnat.

(Mk.701 / Mk.703)

(Mk.801 / 803 / 805)


With a simplified reheat system the BOr.12 was rated at 68,100 lbf (302.92 kN) dry and 8,170 lbf (36.34 kN) with afterburning.

De-rated to improve reliability and fuel consumption and increased engine life, rated at 4,230 lbf (18.82 kN) for the Fiat G.91T and Hawker Siddeley Gnat T Mk.1.

Rated at 4,520 lbf (20.11 kN), the Mk.701 was used in the production Folland Gnat F Mk.1 for Finland and India.

The Mk.703 rated at 4,850 lbf (21.57 kN) powered the Hindustan HF-24 Marut Mk.1.

The Mk.801, rated at 4,520 lbf (20.11 kN), powering G.91s. The Mk.801 was identical to the BOr.2 engine except for accessories.

The Mk.803, with improvements to the compressor, rated at 5,000 lbf (22.24 kN), replaced earlier marks used in G.91s.

The Mk.805, de-rated to 4,000 lbf (17.79 kN), powered the Fuji T-1 trainers of the JASDF and the Hunting H.126 jet-flap research aircraft.

FIAT 4023
Mk.803 engines Licence built by FIAT.

FIAT 4023
Mk.803 engines Licence built by FIAT with added fire detection system.


Breguet Taon
Fiat G.91
Folland Gnat
Fuji T-1
Hindustan Marut
Hunting H.126
Short SB5



Orpheus BOr.3 / Mk.803
Type: Turbojet
Length: 75.45 in (1,916 mm)
Diameter: 32.4 in (823 mm)
Dry weight: 835 lb (379 kg)
Compressor: 7 stage axial compressor
Combustors: Can-annular with 7 flame tubes
Turbine: Single stage turbine
Fuel type: Aviation kerosene
Oil system: Pressure spray and metered feed to gearbox and rear bearing. Oil from the rear bearing is lost overboard to the jet efflux.
Maximum thrust: 5,000 lb (2 kN)
Turbine inlet temperature: 1,184 °F (640 °C) maximum continuous
Specific fuel consumption: 1.08 lb/lb/hr(110.1 kg/kN/hr)
Thrust-to-weight ratio: 5.988 lbf/lb (0.0587 kN/kg)



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