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Aviation History
1957
1957 - 0917.PDF
FLIGHT, 5 July 1957 21 Orpheus BOr.3 for Fiat G91. PROGRESS WITH THE ORPHEUS BRISTOL'S LIGHTWEIGHT TURBOJET ADVANCES TO HIGHER POWERS THE completion last month by the Orpheus BOr.3 of itsOfficial Type Test at the design rating of 4,850 1b thrustrepresents a remarkable achievement. The design of the engine was started in December 1953 and the first Orpheus ran on December 17, 1954. The fact that only two and a half years elapsed between the first run and the completion of the Official Type Test at the design thrust emphasizes not only the soundness of die development programme, but also the dependable nature of the original design. In fact, not since the Derwent and Nene— whose development was in the hands of the present chief engi- neer of Bristol Aero-Engines, Dr. S. G. Hooker—has an engine been designed and developed so rapidly. Incidentally, it is per- tinent to note that the Orpheus is in a sense a replacement for these centrifugal engines, being in the same thrust class though of much lighter weight. The thrust/weight ratio of the Orpheus BOr.3 is 5.99 lb thrust per lb-weight, undoubtedly the highest figure recorded for any type-tested turbojet engine in the world. As is often the case with successful engines, the demand for increased operational performance has caused the aircraft designer to ask for still more thrust. The development of the Orpheus is therefore already proceeding beyond the original design figure. The Orpheus BOr.3 was developed under Ministry of Supply contract for the joint Mutual Weapons Development Programme, as a result of agreements between the United States and the United Kingdom. The Mutual Weapons Development Pro- gramme is the agency charged with the administration of United States funds for the purpose of assisting the development and production of defence equipment for the NATO countries. Besides the 5,760 lb Orpheus BOr.ll announced in March, Bristol Aero-Engines are also developing, under the auspices of the MWDP, an advanced version of the engine designated BOr.12, for which the design thrust is 6,810 1b dry, a figure which can be boosted to nearly 8,000 lb by the use of reheat. This engine is primarily intended for lightweight strike aircraft to meet the requirements of SHAPE. Since the basic design of the BOr.12 is the same as that of earlier Orpheus engines, it will be appreciated that the increase of weight over the figure of 810 lb applicable to the BOr.3 will be relatively small. The resulting thrust/weight ratio will therefore be altogether exceptional. The use of the Orpheus engine in European light fighters has resulted in considerable interest from Continental engine manu- facturers, and it was strongly rumoured during the Pans Show that a licence agreement had actually been signed between flie French national aero-engine concern, SNECMA, and Bristol for the manufacture of the engine in France. No confirmation of this suggestion has been issued; but, on the other hand, it has not been denied. Moreover, in Italy, Fiat of Turin have an agree- ment for technical collaboration with Bristol, which includes an option to manufacture the Orpheus in that country. As is well known, the Orpheus is also the powerplant of the Folland Gnat, which is to be built at the parent factory at Hamble, as well as in India, and Finland, as a result of the adoption of the Gnat for the air forces of those countries. In India the Gnat will be built by Hindustan Aircraft, Ltd., at Bangalore, and the Orpheus will also be built under licence at Bangalore in accordance with an agreement signed between the Indian Govern- ment and Bristol Aero-Engines in September last. In a brochure issued at the Paris Show by North American Aviation, the Orpheus is specified for the NAA-249 jet trainer as an alternative to the Westinghouse J34. This aircraft is envisaged for training pilots through various stages, from primary to advanced, and may well be widely adopted where economical training is required. A two-seater, it has a design gross weight of 9,500 lb and a maximum speed of 495 m.p.h. with the 3,400 lb- thrust J34. A substantial number of these aircraft have already been ordered by the U.S. Navy as the T2J-1, and although these aircraft may be powered with the J34 further orders are expected to follow. Proposals are to be made by N.A.A. to the Royal Canadian Air Force concerning the Orpheus-powered version, and the company has also approached the U.S. Air Force. The latter has inspected the mock-up of the aircraft at North Ameri- can's Columbus, Ohio, Division. It has also been reported from foreign sources that the Lock- heed Aircraft Corporation are interested in a version of the Orpheus to power a prototype twin-engine jet utility transport (the GL-135, by the California Division); and, indeed, the Orpheus is likely to have an important future in this field. A special derated version, known as BOr.4, has been developed from the BOr.3 especially for trainers, transports and executive air- craft. This version will have a low specific fuel consumption and an extended life between overhauls, as a result of the reduction of both the turbine operating temperature and the compressor r.p.m. The BOr.4 is rated at 4,230 lb. One class of trainer for which this engine is likely to be widely adopted is the two-seater developed from the corresponding fighter or strike aircraft, and an example of this is the transonic Fiat G91-T, a tandem two- seater developed from the G91 lightweight strike machine. The G91-T can be used for advanced training, and it can also take off from short, semi-prepared strips or grass fields, and would therefore make an ideal trainer for pilots scheduled for posting to lightweight strike squadrons. The G91-T has evidently been designed with this requirement very much in mind. The Orpheus was originally planned, as a private venture, for lightweight fighters and similar aircraft. Detail design, which began in December 1953, was given a flying start by the numerous calculations and design studies which had already been made on this class of engine. The first aircraft for which the Orpheus was chosen was the Folland Gnat; and, as the very similar Midge was already flying with a different powerplant, there was a natural desire to bring the Orpheus to the flight stage as soon as possible. All the detail drawings were issued within six months, and the manufacturing simplicity built into the design was such that an engine was assembled and run by December 17, 1954. Com- pressor rig tests were carried out before and during the running
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