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Aviation History
1955
1955 - 1746.PDF
9 December 1955 » 869 The Bristol Olympus 1/2B, first run in December of 19S1. This engine was rated at 9J50 Ib thrust and weighed 3J20 Ib. BRISTOL OLYMPUS The Western World's Most Powerful Production Turbojet IN any branch of engineering, the design and development ofa completely new project which markedly pushes out theboundaries of existing knowledge is particularly difficult and hazardous. In the case of the Bristol Olympus turbojet, boththe thrust and the specific fuel consumption were required to be considerably better than anything previously achieved; more-over, the design was formulated at a time when gas turbine tech- nology was hardly more advanced dian was the science of flightin 1910. In fact, drawing the parallel more directly, the projection ofthe Olympus in 1946 can be likened to the position which would have existed had the Bristol Box-kite been followed directly bythe Bulldog. Had such a thing been attempted, the history of fixed-wing aircraft would probably have advanced more rapidly,if less surely; yet quite comparable advances have been attempted —successfully—in the aircraft gas turbine industry, and thisindustry has consequently gone ahead with the strides of a giant. This unprecedented rate of progress is due in no small measureto the efforts of such firms as the Engine Division of the Bristol Aeroplane Company. This Engine Division started investigation into the design ofa high-thrust turbojet in 1946. In order to set the scene, the company at that time were in the process of running down thehuge wartime output of Hercules sleeve-valve piston engines and initiating production of the larger Centaurus. At the sametime, under the general direction of Mr. F. M. Owner, C.B.E., M.Sc, the first, exploratory steps were being taken into the newworld of gas turbines with the Theseus turboprop and the Phoebus pure jet, the latter paving the way for the first family of Proteusturboprops. The design study for the new, and powerful, turbojet wasinitiated at approximately the same time as the formulation ol design parameters for a projected Bristol long-range bomber, todie same general M.o.S. specification as that to which the Vulcan and Victor were first designed. For approximately eight months, a number of alternative lay-outs for both the engine and the aircraft were examined in some detail. The most obvious form of power seemed to be four largeturbojets buried within the wing, and preliminary "optimization" calculations for pressure ratio and turbine-entry temperatureindicated that the former should be set at roughly 9:1, a very high pressure ratio at mat time. Information from the AircraftDivision showed that the corresponding thrust required would be approximately 9,000 lb, assuming four engines per aircraft.Other alternatives were the use of a greater number of smaller engines (for example, one study involved die use of 10 turbojetseach of about 3,500 lb thrust) and die possibility of employing a turbojet with a lower pressure ratio was also investigated in somedetail. It was calculated that reduction of pressure ratio would have significantly increased the engine diameter; and not onlywould this engine have required a greater fuel load to be carried, but the increased diameter, when translated into wing thicknessand consequent aircraft drag, was a significant disadvantage. During this early period of calculation, the Ministry of Supplyissued one of their rare engine specifications. This was for a turbojet of generally similar performance to that in which Bristol were already interested. The requirements were severe, the specificconsumption, in particular, being considerably lower than any- thing previously achieved. The Bristol designers considered thattheir work on die large turbojet could be combined widi the requirements of this specification. Their studies indicated mat four would be the optimum numberof powerplants, and accordingly set the thrust requirement at 9,000 lb per engine. At the same time, the pressure ratio wasfixed at what was then an exceptionally high value, the resultant weight penalty being, on paper, more than outweighed byimproved efficiency and consequent better range performance. Parenthetically, it may be remarked diat the work was dien beingcarried out by an incredibly small number of engineers; and it is still debatable whether it is better to put a large number ofmen on to a programme in its early stages, or whether this can best be handled by four or five designers working exceptionallylong hours, but capable of viewing the project as a whole with- out being constrained by the work done by those responsible foranother part of the project. As far as can be ascertained, no gas turbine in 1946 had runat a pressure ratio exceeding 5.5:1. The precedents for high- pressure were the abortive Northrop-Hendy Turbodyne and thevery early and remarkable three-shaft turbine suggested by Haync Constant. Work was also well advanced on the Rolls-Royce Clydetwo-shaft turboprop, but data on this engine were not available to the Bristol designers. Bristol themselves had achieved a pressureratio of approximately 5 :1 in the Theseus by employing an axial compressor mounted on the same shaft as a final centrifugalstage. This engine was also designed to incorporate a heat ex- changer to improve die overall efficiency, but a heat exchangercould scarcely be employed on a turbojet (in saying this we are referring to 1946, and not to present developments with nuclearpower). In the Clyde, however, Rolls-Royce had employed low- pressure and high-pressure compressors in series, each drivenby its own turbine through a mechanically independent shaft. It seemed to Bristol that, since the required pressure ratio couldnot then be obtained from a single axial compressor without making unacceptable sacrifices in flexibility of operation, thetwo-shaft design was the best alternative, and such a layout was, in fact, envisaged from the outset. An oudine of the advantages to be gained from dividing up thework of compression between two independent compressors is given later in this account, but the fundamental advantages werethe prospect of reasonable surge-free operation over an extreme range of r.p.m. and altitude. The Bristol design engineers werenaturally anxious to make the engine capable of efficient operation over a wide range of——, and the self-matching characteristics ofv'T such a compressor seemed to provide an excellent answer. Inaddition, a cogent argument for the choice of a high pressure ratio was provided by the fact diat, at that time, virtually every gasturbine was in severe combustion trouble at altitudes which would now be considered below operational requirements for a bomber.In 1946, the greatest pressure ratio then achieved by a single axial assembly in regular running was 5.5:1; this unit required14 stages of blading, and was (and is) employed in the Armstrong
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