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Aviation History
1958
1958 - 0684.PDF
700 FLIGHT, 23 May 1958 Combustion and Propulsion POINTS FROM PAPERS AT AGARD MEETING Part 1 THE Combustion and Propulsion Panel of the NATOAdvisory Group for Aeronautical Research and Develop-ment (AGARD) held its third "colloquium" in Palermo, Sicily, recently. Unlike the two earlier meetings of the combus-tion panel (Cambridge 1953 and Liege 1955) the papers presented were concerned with general propulsion problems as an indica-tion of the extended scope of the panel under its new title. The moving spirit of the meeting, and leader of the panel's activities,is the indefatigable Dr. Theodore von Karman. The eighteen lengthy papers, and the accompanying discussions, are to be pub-lished in full in the autumn, under the title Selected Combustion and Propulsion Problems 1958, by AGARD, from the Palais deChaillot, Paris XVIe. The following selective summaries of certain of the papers are an indication of the contents of this volume. The papers were presented in five sessions entitled: I, power-plant requirements related to aircraft mission; II, interaction of combustion systems with other engine components; III, noise;IV, combustion; V, aerophysical chemistry. Most of the speakers came from the U.S.A., but there were some redoubtable contribu-tions from the U.K. and France and one from Germany. In Propulsion by Air Breathing Engines, Mr. A. A. Lombard,chief engineer of Rolls-Royce, Ltd., surveyed the present scene in similar terms to his recent R.Ae.S. lecture (Flight, January 17),but introduced some new technical information, particularly with respect to high-energy fuels and the ejector nozzle. There maywell be no money today for a new British military turbojet, which is a pity, since the fully developed reheated turbojet reaches itspeak at M = 2.5-3.0. Following upon the thrust increases obtainable by adoptingdouble or multi-shock intakes, constant compressor-blade Mach number (N\/T control), r.p.m. increase to reduce spillage and aconvergent/divergent nozzle, there comes the ejector nozzle. This is little less efficient than a fully-variable con-di mechanical nozzle(see curve) and it is very much lighter, although there may be some loss in gain due to the absence of pressure from the expandinghigh-pressure efflux on the basal area of the afterbody. While at high supersonic speeds L/D might, with close attentionto interference effects, be kept as high as 50 per cent of subsonic values, the engine efficiency at M = 2.5 will be twice that at M = 1.0,so that ranges akin to those achieved today should be attainable. It is unfortunate that a wing suitable for range flying at M = 2.5 wouldnot permit take-off in a reasonable distance, unless the aircraft were supported by direct jet-lift until its forward speed exceededthe high stalling speed. "Advances made in the construction of lightweight jet engines make it quite feasible to provide the neces-sary thrust for VTO without uneconomic weight penalty." Discussing the parameters of air-intake conditions, pressure ratioand engine weight, Mr. Lombard reiterated his suggestion of the turbo-rocket engine as one of the most promising hybrids to meetthe conflicting requirements. Turning from improvement in thermodynamic efficiency bydesign refinement to the use of "exotic" fuels, Mr. Lombard said that there are three special fuels which may offer advantages overkerosine: fuels of higher calorific value on either a weight or density basis, e.g. other hydrocarbons; fuels of high calorific valueon both weight and density, e.g. boron compounds; fuels enabling higher temperatures to be reached, e.g. light metals. A plot of the calorific values for hydrocarbon fuels againstdensity reveals a virtually unique curve—whether volume or weight is considered—all the way from pure liquid hydrogen tosolid carbon because the heat of formation of the compounds is low. Where the heat is large, as in the triple-bond acetylene, thefuel is significantly above average—but is hazardous for that very reason. Boron, with the same density as carbon, has a much highercalorific value and forms compounds akin to the hydrocarbon series—hydrogen-diborane (B2H6), pentaborane (B5H9), decabor-ane (B10H14), etc. Unlike CO2, the boron oxide product (B2O3) does not vaporize at normal operating temperatures; it may thereforecause deposition and erosion problems, and when it boils it absorbs much latent heat. Burning boron powder at the high intensityneeded in a jet engine would be difficult, since the oxide might condense on unburnt particles and inhibit combustion. Diboraneand pentaborane are toxic gases under normal atmospheric condi- tions. Hydrocarbons with a few carbon atoms replaced by boronatoms could conceivably combine the satisfactory physical proper- ties of kerosine with some of the higher calorific value and goodburning properties of boron. Boron: 13,800 C.H.U./lb, if oxide is solid (below 800 deg K); 13,500 C.H.U./lb, if oxide is liquid;10,300 CH.U./lb, if oxide is gaseous (above 1,800 deg K); carbon: 7,840 C.H.U./lb. Maximum temperature possible with kerosine is 2,300 deg K,and it is the heat-release per unit mass of total products which sets the limit of gas velocity and thrust. Magnesium and aluminiumhave very high heat values on this basis, plus high density; it has been suggested that a paint-like slurry of metal powder in kerosinemight be stabilized and pumped—though erosion by metallic oxides might prove troublesome. It is necessary to investigate the ease of combustion of the varioushigh-energy fuels so as to take full advantage of high combustion intensities and so reduce combustion chamber and afterburner sizesby making full use of higher reaction rates and flame speeds. Fuels of higher stability range could be burnt at turbine-inlettemperature conditions so that the dilution zone could be eliminated. Liquid hydrogen has a calorific value about 2.7 times, and adensity only 0.087, that of kerosine. It must be evaporated before entering the combustion chamber and this process could be usedfor structural cooling—but the aircraft would have to be specially designed to provide the large volume and minimise the surfaceinsulation. The following table is indicative of the effect of two high-energy fuels upon aircraft characteristics (kerosine = unity). Liquid Hydrogen-carbon-hydrogen boron (A) Same take-off weight „. ,and fuel load:— : Range 2.80 1.41Fuel volume 11.70 0.96 (B) Same fuel volume:—Take-off weight 0.63 1.02 Range 0.30 1.45(C) Same payload/range:— (1) Development:Take-off weight 0.72 0.86 Fuel load 0.30 0.65Fuel volume 3.50 0.63 (2) Re-design:Take-off weight 0.46 0.76 Fuel load 0.19 0.51Fuel volume 2.22 0.49 Combustion problems resolve themselves into current andfuture, i.e., M = 2.5 + . In the present, high-altitude relight methods are still haphazard, but "the problems of controlling BY-PASS AIR FUSELAGE SHROUD, 30000 JETPIPE- -- CREHEAT OFF)' FINAL NOZZLE (REHEAT ON) Semi-cross-section through the ejector nozzle discussed by Mr. Lombard. (Centre) Increase in performance of a typical supersonic turbojet due to A, mechanically variable nozzle, and B, ejector nozzle. (Right) Performance of some hydrocarbon fuels, plotted against weight and volume. 4O 5 3Ou S20| < o -1O IDEAL _v>- // / j> O=?B» 26,000 1 21000 18X100 I 4,000 10.000 LIQUID HYDROGEN 1200,000 O5 10 AIRCRAFT 15 2O MACH 2-5 05 10 15 20SPECIFIC GRAVITY -1,000/000 600^100 600000 £ 400,000 200,000
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