The Bristol Siddeley Orpheus is a single-spool turbojet developed by Bristol Siddeley for various light fighter/trainer applications such as the Folland Gnat and the Fiat G.91. Later, the Orpheus formed the core of the first Bristol Pegasus vectored thrust turbofan used in the Harrier family. Design and development The engine had its genesis in a 1952 request by Teddy Petter of 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. The Orpheus incorporated the novel feature of a large-diameter shaft for its single spool which then needed only two bearings. The weight savings from deleting a bearing and associated parts listed below gave an engine with a thrust of 5,000 lbf but weighing only 800 lb. Eliminating the usual centre support bearing for the shaft joining the compressor and turbine meant the shaft would whirl, assuming a bowed shape, and damage the engine. Whirling was prevented by using a large diameter tube in place of the usual small diameter shaft. The large-diameter thin-walled tube, more than 8 inches in diameter was stiff enough to raise the whirling speed beyond the engine running range using only two bearings instead of the usual three. Two bearings gave a further advantage: previous engines had also needed a coupling in the shaft to allow for any misalignment between the static parts of the three bearings. With two bearings, the shaft simply followed the straight line between them. So the introduction of the large diameter tube allowed the removal of a bearing, a coupling, the engine's support structure for that bearing together with its lubrication system and cooling air supply. A cannular combustor was used with seven flame tubes. This was a recent development in jet engines and the Orpheus also included the innovation of incorporating the turbine entry duct and its stator vanes into the flame tube outlet, each flame tube providing one seventh of the overall duct. This had two advantages, it simplified the manufacture of a complicated and unreliable component, also the segmented design allowed easier allowance for thermal expansion. Developing a Sea Level Static thrust of ,