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Aviation History
1951
1951 - 0879.PDF
55<> THE SECRET YEARS An Account of Jet Developments prior to the Official Disclosure in January, 1944 ¥>UGHT TT7E have it on the authority of Sir Frank Whittle that Vy he first began to consider the possibilities of gas * " turbines and jet propulsion for aircraft in 1928, when a flight cadet at the R.A.F. College, Cranwell. Neither the power unit nor the method of propulsion were novel ideas; indeed a number of attempts had already been made to produce the one and to achieve the other, but without success. The early gas turbines were operated at ground level and suffered from low compressor and turbine effi- ciencies and from materials which enforced a relatively low maximum gas-temperature. Jet-propulsion projects—a number appeared about the end of the first World War— mostly employed a high-velocity jet of gas to induce through a venturi-shaped nozzle a flow of air which enveloped the jet and "augmented" the thrust. In 1929, when at the Central Flying School, Wittering, Whittle conceived the idea of using the gas turbine in combination with jet propulsion. Hitherto they had been unrelated, but he realized that a number of favourable factors would accrue to such a system. At the low air temperatures obtaining at high altitudes the improved ratio of positive work (expansion) to negative work (compression) for a given maximum gas temperature would yield a substantially improved overall efficiency. Provided operation was affected at high aircraft speeds, some part of the work of compression could be accomplished by ramming the air in a divergent intake duct: by that means the efficiency of the whole compression process would be raised. Finally, not the entire expansion process but only that part necessary to drive the compressor would be subjected to turbine losses. A patent application was lodged on January 16th, 1930, and it is of interest to note that in addition to the turbojet a claim was made for a ramjet. This, however, was disallowed by the Examiner on the grounds of anticipation by other inventors and was, per- force, deleted. The specification was brief and, in the customary legalistic phraseology, the declared object was : "obtaining pro- pulsive force in the one sense of direction by the reaction caused by expelling fluid in the opposite sense of direction." It expressed the conviction that "an embodiment of this invention will provide a large thrust in proportion to its weight, that it will perform at greater altitudes than are at present obtainable, that it makes possible higher speeds than have up to the present been obtained, that it will operate with any fuel now in use, and that it will have N The first potent, A—Shaft. B — Com p ressor Whittle 1930. stator blades. F—Centrifugal im- peller. G—Diffuser vanes. H—Air collector. J — Combustion chambers. K—Fuel sprayer. L—Gas collector. M—Turbine stator blade*. N—Turbine rotor blades. P —Jet nozzles. Assembly of the first experimental unit, wip) a single combustion chamber. a reasonably low fuel consumption. Further, that simplicity and convenient external form is achieved." Quoting again, it was stated "it can be demonstrated that the efficiency of this device conceived as a propulsive engine will not be reduced by reduction of the density of the atmosphere, and owing to the low temperature of the upper atmosphere may actually be enhanced." The first approach was the obvious one—it was to the Air Ministry, and there began the series of rebuffs and discourage- ments. In the same year, 1930, a number of attempts were made to enlist the support of engineering firms. The verdict was always the same— the practical difficulties were too great, the gas turbine was unproved, there were prospects of substantial ad- vances in the conventional piston engine. Regrettable as it was, this attitude is understandable in view of the prevailing conditions. Known turbines and rotary compressors were of low efficiency, technicians were pessimistic regarding improvements, and industry was undergoing a period of depression. Hope of ever reaching the stage of practical development receded, but Whittle retained his faith and never ceased to study and design on paper. A minor tragedy of this discouraging period was the lapsing of the original patent. As a result of failing to arouse any interest in the idea the necessary renewal fees were not paid. Financial Support Whittle specialized as an engineer officer and, while taking a course at Cambridge in 1935, was contacted by two ex-R.A.F. officers, R. D. Williams and J. C. B. Tinling, who considered that another attempt should be made to launch the project. An agreement was concluded and immediate efforts were made to obtain financial backing. After initial lack of success they secured the interest of an independent consulting engineer, M. L. Bramson, on whose technical opinion—differing from current orthodox disbelief—the investment-banking firm of O. T. Falk and Part- ners agreed to lend their support. A sum of £2,000 was subscribed for working expenses and forthwith an order was placed with B.T.H. for engineering and design work to Whittle's require- ments for a first experimental unit. Negotiations proceeded for the formation of a company, Power Jets, Ltd., and as Whittle was a serving officer of the R.A.F. the project came under the examination of the Air Ministry. Even at that date the Ministry deemed it unlikely that the suggested power unit would be of military value and consequently decided there was no need to impose secrecv. However, in March, I93^> an agreement between Whittle, Williams and Tinling, the Presi- dent of the Air Council, and O. T. Falk and Partners established the new company. A contract was placed with B.T.H. to build the compressor and turbine and another with Laidlaw, Drew and Co. of Leith, for the combustion equipment. Here must be emphasised one of the most difficult hurdles that Whittle had to surmount. His designs and his estimates of per- formance were based solely on aerodynamic theory, yet at that time techniaues for rotary compressors and turbines, although well established, were based on empirical rules and experimenral precedents. He was breaking new ground and had to refute long- cherished traditions; to win first the confidence and then the support of the acknowledged experts. Up to that time the highest efficiency attained by a centrifugal compressor was about 68 per cent with a pressure ratio of about 2.5 : r. As a minimum, Whitt-c expected 70 per cent efficiency but with a pressure ratio of 4 • This, it was felt, and stated, was asking too much. How right ne was became evident when eventually the Whittle compressor
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