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Aviation History
1964
1964 - 0397.PDF
248 FLIGHT International, 13 February 1964 EXPENSIVE NOISE —in Rockets and Turbojets: a Non-technical Discourse by Rolls-Royce's Chief Scientist S. L. Bragg, MA <C«nt»b), $ft (MIT), the author Fig 2 Blue Streak rocket powered by two Rolls-Royce RZ.2 liquid- oxygen/kerosine rocket engines, each giving 137,0001b thrust THIS is a paper of an unusual kind. Under the title "Noise and Oscillation in Jet Engines," it was given recently before the Royal Institution as a "discourse"—which may be defined as an informal lecture, on some aspect of science or engineering, designed to be readily understandable by those who may be regarded as laymen in the lecturer's particular field. Lucid in style, and avoiding all esoteric jargon, Mr Bragg's paper should afford non-technical readers of this journal a clear appreciation of phenomena of a sort that sometimes appears mystical and rather intimidating. It was presented in three sections; two, reprinted in the following pages with very slight abridgement, dealt with the causes of noise and oscillation in rocket engines and turbojets; the third—to be given in a subsequent issue—was concerned with the character and implications of aircraft noise. Fig I General arrangement of a typical rocket engine GEARING KEROSINE ! FROM MAIN TANK j L|QU|D OXYGEN (LOx) lMc"CI ' FROM MAIN TANK ' I!TURBINE TURBINE EXHAUST GAS GENERATOR. BLADE VALVE LOX REGULATORS START TANK SUPPLY MAIN FUEL VALVE ""-LOX PUMP START TANK SUPPLY PROPELLANT MAIN LINES NON-RETURN VALVE NON-RETURN VALVE MAIN LOX VALVE INJECTOR "-THRUST CHAMBER I 1 HP. LIQUID OXYGEN (LOX) HP KEROSINE | ANY of the advantages of jet engines—both gas turbine and rocket—stem from the fact that they are basically steady-flow machines. In the simplest type of turbojet, for example, such as Whittle's first unit, air is drawn steadily in by a fan or compressor: it then flows into a small furnace, or combustion chamber, where fuel is mixed with it and burns continuously: next, the hot gases expand steadily through a windmill, or turbine, that drives the fan: and finally they exhaust from a nozzle in a con- tinuous jet. The flow processes are basically similar in a rocket that uses liquid propellants, except that instead of a fan compressing air from the atmosphere we have a pump forcing a liquid oxidant from the vehicle tanks into the combustion chamber. Of course, the same processes of compression, combustion and expansion occui inside the conventional piston engine: but that is an unsteady- flow machine, because the actions occur in turn in the same part of the engine—the cylinder—instead of simultaneously, in different parts, as on the jets. Having emphasized that jet engines are basically steady-flow machines I am now going to contradict myself and say that the flow is never quite steady. There are obvious changes, of course, when the engine is started and stopped. But even when the unit is running at nominally constant speed all sorts of minor variations are super- imposed on the flow pattern. Some of these are regular oscillations driven by the combustion processes, and are known by des- criptive names such as "howling," "screaming" and "chugging." Some produce vibrations of critical parts of the engine, so that they may then fail by fatigue. And some are just random irregularities in the flow pattern, which produce a general and often unacceptable noise. My object is to take one or two examples of these unsteadi- nesses, whish may be loosely grouped as "oscillations and noise," and to describe their causes and cures in so far as these are known. ROCKET ENGINES Operation The rocket is probably the simplest form of jet engine, so let us start there. A rocket engine is nothing but a hollow cylinder filled with gas under pressure: the cylinder has a hole at one end, and the unbalanced pressure on the other end of the cylinder provides a propulsive force which thrusts the cylinder forward. The compressed gases tend to escape out of the hole at the
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