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Aviation History
1955
1955 - 0197.PDF
FLIGHT, 18 February 1955 197 ROCKET PROPULSION for AIRCRAFT Professor Baxter Discusses Prospects and Problems in R.Ae.S. Lecture THE Saunders-Roe Club House at Cowes was the venuefor a Royal Aeronautical Society "main" lecture givenbefore the Isle of Wight Branch of the Society on the evening of February 10th. The lecturer was Professor A DBaxter, M.Eng., F.R.Ae.S., M.I.Mech.E., F.Inst.Pet., Pro- fessor of Aircraft Propulsion, College of Aeronautics, and hissubject was that signified by the title above. The successful development of the gas turbine for aircraft pro-pulsion, said Professor Baxter, had stimulated much thought, some serious and a great deal speculative, on the possibilities of evenmore advanced forms of propulsion. The ramjet and the rocket had immediately sprung to mind. The former was a naturaldevelopment of the turbojet; it could extend the useful field of the turbojet, but the bounds of that extension could be clearlyseen. On the other hand, the rocket was not restricted by the same fundamental limitations, and its eventual, although stillsomewhat remote, exploitation in wider spheres could be prophesied with some confidence.Before die 1939-45 war, the stage of development of the rocket motor was comparable with that of the internal combustion enginea century ago. Pioneers like Tsiolkovski in Russia, Esnault- Pelterie in France, Oberth in Austria and Goddard in Americahad laid down the basic theory and had made some small-scale experiments in support, but the first major practical steps hadcome from the Germans during the war. They had demonstrated the potentialities of rocket propulsion very effectively and hadpointed the way to future developments. As a result, post-war progress had been gready accelerated, but unfortunately muchof it had been shrouded in secrecy. Nevertheless, it was probably fair to liken the present status of the rocket motor to that of thepetrol engine in the early days of powered flight. The latter, only forty years ago, had been recognized as a practical, but com-paratively inefficient, aircraft powerplant; its usefulness had been limited and its future possibilities, except as a weapon of war, hadbeen no more than visions, rejected by die great majority. It was not surprising, therefore, to find that only two mainlectures of the Royal Aeronautical Society had been devoted to the subject of rockets. The first, by Perring, had reviewed thewar-time German work, and the second, by Cleaver, had revealed some lines of development with particular reference to assistedtake-off. The present paper endeavoured to follow up those earlier ones by considering the possible roles of the rocket motorin the near future and by putting forward some of the problems which had to be solved before the more ambitious proposals couldbe achieved. The discussion would be confined to earth-bound piloted craft, although there was much in common both withguided-weapon propulsion and the requirements of interplanetary flight.To determine the best uses of the rocket motor, said the lec- turer, it was first necessary to consider its special features, par-ticularly in relation to the corresponding characteristics of other propulsion units. They could be enumerated as (i) thrust prac-tically independent of altitude and forward speed; (ii) thrust- weight and thrust/frontal area ratios very high; (iii) specific con-sumption very high. Prof. Baxter illustrated the first and second features by figures SO 16 O 8 0 ROCKET -RAMJET- Fig. 1. Specific fuel con- sumption of high-speed powerplants at Mach 2.0. REHEAT TURBOJET Fig. 2. Break-even times at which weights of rocket and propellant equal those of fully reheated turbojet (at Mach 2.0). 2QOOO 4QOOO ALTITUDE (ft) 6O.OOO 20,000 40,000 6QOOO ALTITUDE (ft) which compared the thrust per square foot of frontal area at varyingforward speed and a constant altitude of 40,000ft for, respectively, the rocket and the air-swallowing engine. It was seen that inthose circumstances the rocket had almost twice the thrust of the ramjet even at its peak performance, and approaching four timesthe best the turbojet could give with reheat to the maximum tem- perature. Another figure gave the same comparison for varyingaltitude and a constant Mach number of 2.0. The decreasing thrust of the air-swallowing engine became serious above 60,000ft, andvery little net dirust was produced; in contrast, the rocket increased its thrust by 15 per cent or more. Near ground level the differ-ence was not so marked, and a well-designed ramjet could give more thrust than the rocket. Generally, however, high supersonicflight speeds near the ground were unlikely, because of aircraft stressing considerations. It should also be borne in mind, continued Prof. Baxter, thatto obtain the optimum performance over a range of flight speeds called for variable intake-geometry and exhaust nozzle shape forboth ramjet and turbojet. To do this might involve considerable extra weight and mechanical complication, and generally it wouldbe preferable to design for a specified flight speed and accept a reduction in performance at other conditions. The rocket didnot suffer intake problems at all, nor were exhaust expansion ratios affected by forward speed. Present-day turbojets had weight/dirust ratios of about 0.3at static sea level conditions, rising to 0.75 at 30,000ft and 3.0 at 60,000ft. Rocket motors, on the other hand, should be not morethan 0.06 at all altitudes: that is, up to fifty times lighter than the turbojet. With reheat, the turbojet specific weight would bebetween 50 per cent and 66 per cent less. Litde information on the weight of ramjets was available, and they were difficult tocompare because of the large variation in thrust with forward speed. At sea level, M = 2, they would probably be lighter thanrockets, but at 60,000ft they would be eight to ten times heavier. Showing (Fig. 1) variation of specific thrust wim altitude forthe various engines, the lecturer said it was at once apparent that this was the rocket's one serious drawback, making its use as aconventional powerplant almost impossible. This large propel- lant-weight penalty was, however, more than compensated bythe low motor-weight for short times of operation, and Fig. 2 showed the running times and altitudes for which it had an advan-tage over the best that a reheated turbojet would give. Without reheat the "break-even" times would be approximately doubleuiose shown, and in bom cases the time was increasing rapidly with height. Some virtue might be claimed, too, for the rocketbecause the final weight of an aircraft of the same initial weight would be much less than for die turbojet version. In view of the limited time of operation for which the rocketgave a lower total weight, it might be asked why it should be considered at all. Unless it had some potential advantages, therewas no case for it and, considered merely as one propulsion unit replacing another for straightforward flight, it obviously hada very limited claim. This direct substitution was a trap that, with other forms of engine, had caught designers more than oncein the past; it was essential to compare units when each was installed in the most appropriate airframe and was using itsoptimum flight plan. For the rocket, this would demand new and unormodox operating techniques. The rocket's main advantages were that it could develop larger
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