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Aviation History
1956
1956 - 0566.PDF
-m 566 THE SUPERSONIC TURBOJET . . . driving ball-bearing screw jacks. By this means the actuatingmechanism could be made irreversible and quite light, and the probable slow movement of the moving parts would be no greatdisadvantage. In the hypothetical VG engine the nozzle throat area is varied by injecting compressed air upstream from anannular slot at the throat. Even discounting the possible requirements of variable profile,perforated or slotted walls, thrust-reversal and noise-suppression, it is not going to be easy to make efficient con-di nozzles ofreasonable weight. The fact that the whole assembly will operate at almost red heat is another large straw on the designer's back;and it may even be found desirable to cool the nozzle by a flow of fuel, as is common practice with rocket motors. The AmericanBell company have, as noted on page 588, found a way of using light alloy (with excellent thermal conductivity) for rocket-motorchambers, and efficient cooling might allow such construction in supersonic turbojets. In many supersonic aircraft it may be found that it would payto drive a high proportion of th^e accessory systems from an auxiliary power unit remote from the main powerplant. Never-theless, it is clear that a considerable volume probably will be available between the tip of the intake bullet and the front bear-ing of the engine. In order to recover the maximum pressure up- stream of the compressor, the diffuser length may have to be asmuch as 12ft, on a large engine, and for the whole of this distance the central tube behind the intake centre-body will have sufficientdiameter for all kinds of pumps, valves and electric equipment. Mechanical drives could readily be obtained from the front ofthe engine, probably through a gearbox. One important factor is that the interior of the bullet and rear-wards extension should be as cool as any other part of the aircraft— although, for certain critical items such as the majority of theelectronics, some form of artificial cooling will be necessary. Even the fuel itself may need to be cooled, although how this couldbe done is by no means apparent since it is generally regarded as being the ultimate heat sink of the aircraft. The point must befaced, however, that the mean temperature-rise across the intake and compressor may be as much as 500 deg C and if the fuel werealready warm from skin-friction heat transmitted through the airframe, excessive vapour pressures would result. This in turncould cause trouble in the seals, pumps and control system, par- ticularly at high altitudes when the ambient pressure is very lowand the fuel flow is much reduced. In this respect, therefore, the problem resolves itself into oppos-ing factors: the need to employ a fuel which readily vaporizes Two recent, and highly successful, super- sonic aircraft are the Fairey FJ).2 (right), holder of the World Speed Record at 1,132 m.p.h., and the Chance Vought XF8U Crusader (below), also a 1,000 m.p.h.-plus aircraft, designed as a fighter for the U.S. Navy. The former has sharp-edged wing- root intakes; the latter has a double-shock chin-intake. Both are powered by a single, "partly variable" afterburning turbojet. FLIGHT, 11 and burns in a high-velocity airflow conflicts with the require-ment for a fuel which can be stored in a hot tank and pumped through hot pipes without aeration or vapourization. In the rela-tively distant future (say beyond 1965) these problems of hydro- carbon liquid fuels may be only of historical interest since atten-tion is being increasingly devoted to entirely new types of fuel. Most of the lighter metals have at one time or another beeninvestigated to determine their suitability for employment as a fuel for heat engines. In particular, lithium and aluminium areat present the centre of a great deal of activity, and an article on the use of aluminium as an aircraft fuel was published by us onNovember 21st, 1952. In the U.S.A., and probably in Great Britain, extensive research programmes are afoot aimed at theultimate employment of such metals, either in a finely divided form as a powder or as a "slurry" mixed with a liquid fuel, or asstrip or wire fed from a coil. Other work is going ahead with com- pounds of boron, of which boron hydride is outstandingly attrac-tive. The Boeing Airplane Company envisage the use of this fuel, or ethyl borane, in their Model 110 supersonic bomber project. At present, at least, it looks doubtful whether the gas turbinein squadron service will ever renounce the liquid hydrocarbon fuels on which it has grown up. A conclusion could be thatsophisticated fuels will be confined to ramjets and similar con- trivances in which there are possibilities of greater specific heatrelease. The ramjet is a strong competitor to the turbojet in several fields and has a definite edge in short-life applications, in-cluding guided missiles, and for all applications in which per- formance exceeding Mach 3 is required to be matched with morethan a very limited flight endurance. As with the turbojet, de- signers are raising their sights with the ramjet, and Mach 4is now considered well within its capabilities. Nevertheless in the Mach range up to about 2.5 to 2.7—which is the spectrumfor which the hypothetical turbojets of this article are visualized— the ramjet would be much fatter than the turbojet and of at least comparable weight, besides suffering from the drawback of beingalmost valueless at less than sonic speed. The other major aircraft prime mover, the rocket, is indispens-able to most missile programmes and has clear applications in manned aircraft where range at maximum thrust is of minorimportance. In interceptors, rocket propulsion can make a truly enormous difference to climb, manoeuvrability, acceleration and, inparticular, performance at altitude. If properly used, a rocket can actually reduce the quantity of fuel burned during a sortie. Yet in spite of the manifold prospects now unfolding for theapplication of new forms of power, the author would be quite happy to herald the familiar turbojet as the dominant form ofpower for manned aircraft for all flight speeds up to about three times that of sound. As he has attempted to show, the super-sonic turbojet will be quite a different animal from anything we have today and in most respects it will be a more intransigent one.Even basic bench-testing and flight-development of such power- plants must cause many a furrowed brow, and almost the onlyfact that can be predicted with certainty is that their development will cost enormous sums of money. To all tax-payers this will bea familiar state of affairs. W.T.G.
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