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Aviation History
1955
1955 - 1335.PDF
FLIGHT, 9 September 1955 449 The Gyron in display form: 46%in diameter and Win long to the turbine shroud or 154in with a standard jet-pipe. TOMORROW'S ENGINES —at the Farnborough Show ALTHOUGH it would be neither fair nor accurate todescribe this year's S.B.A.C. Exhibition as an "enginek Farnborough," the fact remains that it is the occasion for the first public showing of British powerplants of the ut- most significance and consequence. As always, these engines cannot be fully described at their first showing; details of their mechanical design are allowed to be discussed only where they are externally evident, and practically no per- formance figures may yet be published. Nevertheless, we have prepared the following discussion in order that something of the background of these most important units may be placed in true perspective. On a number of counts the de Havilland Gyron deserves tohead the list. As is already known, this engine is a large turbojet designed particularly for supersonic flight. As we mentioned last week, it is now eight years since the deHavilland Engine Co. were able to display a new gas turbine. During most of this time the company's engine designers, underthe direction of the late Major Frank Halford, were looking sufficiently far ahead to enable them to by-pass the entire stageof development represented by present axial engines and concen- trate upon something that would mark as great an advance as didthe introduction of the turbojet itself. It was, in fact, five years ago that the company's thoughts crystallized into the design ofa turbojet suitable for supersonic applications; and by "super- sonic" de Havillands intended to go the whole hog and reach areally worthwhile Mach number and not something of the order of 1.2 or 1.3. It was obvious from the outset that a great deal of thrust wasgoing to be needed. It was also obvious that the whole pro- gramme was going to break a lot of completely fresh ground.Accordingly, in the basic formulation of design parameters for the new turbojet—which, incidentally, was originally designatedHalford H.4—the de Havilland Aircraft and Engine companies joined forces. The Aircraft company conducted research intovarious "paper" fighter designs, and kept the Engine company informed of probable thrust requirements. During 1950. there-fore, the H.4 design became progressively more clearly defined, until eventually it was possible to issue drawings for the manu-facture of prototype parts. It must be remembered that in 1950-1951 supersonic flying wascompletely unknown in this country, and the de Havilland Engine Company had to base their original design of engine on ideaswhich were sometimes conflicting and frequently ill-determined. From the outset, however, it was evident that the Gyron wouldhave to be an entirely new sort of engine and not just a bigger axial of the Avon-Olympus-Sapphire type. At high supersonicMach numbers it is possible to utilize the kinetic head of the intake air to do a greater proportion of the compression; for ex-ample, at 1,500 m.p.h. the ram pressure ratio (isentropic) can be as much as 7.5 :1 at sea level and 12:1 at the tropopause. As a result, it is possible to relieve the engine compressor ofa proportional amount of work, thus making it possible to dis- pense with a number of compressor stages and making the wholeengine significantly shorter, more compact, rrore rigid and much lighter. It was soon proved to de Havilland's satisfaction that a BY THE TECHNICAL EDITOR msdium-co/npression engine of this nature would have an exceed-ingly competitive specific weight coupled with a low specific fuel consumption at high supersonic speeds and altitudes. Between 1950 and 1952, the company conducted a considerableamount of research, much of the actual testing being on a scale hitherto unknown in this country. Various applications wereconsidered for various types of Gyron, with and without an after- burner and with and without a supplementary rocket motor. Asa result of this work, the de Havilland Enterprise now seem to have supersonic formula: completely "buttoned up." Neverthe-less, they wished to prove the Tightness of their thinking before offering the Gyron for sale, and the engine was accordingly onlyput forward to the Ministry of Supply after prototypes had been made and successfully run. The engine then received Govern-ment backing and it has since been developed under M.o.S. con- tract. It can now be stated that the Gyron has been fully type-testedat a rating of 15,000 1b thrust; and it is patently obvious that this is a conservative figure even for the engine as it exists at present.The establishment of such a rating at this stage is a certain indi- cation that the Gyron will be capable of type-approval at 20,000 1bthrust without exceeding present established limits of turbine inlet temperature, and it is likely that the de Havilland Engine Com-pany can see considerably higher values than this on the horizon. It is a fundamental precept that one of the principal factorsgoverning the power that can be obtained from a given size of engine is the turbine-inlet temperature. This temperature, whichtoday is normally less than 1,250 deg K, is obtained partly by compressing the air and partly by the burning of fuel in thecombustion chamber. In the past, the need for good specific fuel consumption has led to the employment of compressors of high-pressure ratio (up to 13:1) which raise the temperature of the air by more than 250 deg C. At Mach numbers now envisagedthe ram compression alone can raise the intake air temperature from 0 to 200 deg C, so that the delivery at the downstream endof the compressor may have a temperature as high as 500 deg C, leaving under 500 deg C available for the combustion of fuel.Speaking generally, thrust is proportional to the rate at which fuel is burned and it is therefore highly desirable to increase themaximum rate at which fuel can be injected without overheating the turbine. In the Gyron, the reduced pressure ratio must leaveat least another 100 deg C available for the combustion of fuel so that the engine can produce proportionately higher thrust atsupersonic speed. In such engines, therefore, high pressure ratio is no longer an advantage, and the new philosophy is to make ramcompression do the work. Subsonically, of course, the Gyron cannot equal the s.f.c. ofhigh-pressure engines like the Olympus and J57. On the other hand, its efficiency is considerably better than that of centrifugalturbojets and must correspond roughly to that established by the early axials—which, incidentally, required a larger number of com-pressor stages to achieve the same result. No weight has been revealed for the Gyron, but it has beenstated that it is an exceptionally light engine for its thrust. This is a remarkable fact when it is borne in mind that almost theentire structure must be of steel (or, perhaps, in later versions of the engine, alloys of titanium) in order to obtain the required
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