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Aviation History
1957
1957 - 1753.PDF
FLIGHT, 29 November 1957 843 MISSILES AND AIRCRAFT Inaugural Lecture of the R.Ae.S. Guided Flight Section SIMILARITIES and differences in the respective developmentof guided weapons and manned aircraft were discussed byMr. J. E. Serby, C.B.E., B.A., F.R.Ae.S., in the first lecture to be held by the newly formed Guided Flight Section of the RoyalAeronautical Society in London on November 21. The title of the lecture was Guided Weapons and Aircraft—Some Differencesin Design and Development. The defensive guided weapon had throughout its evolutionarylife been dependent completely on a radar environment, the lec- turer pointed out. In the interception phase, however, the mannedfighter had closely approached g.w. philosophy, and the need for a human pilot had rested more and more on the need to deal withtake-off and landing. In the field of aerodynamics, the most striking difference wasthat the g.w. opted out of all the difficulties of take-off and landing. It was not concerned with landing, and as regards take-off it couldbe accelerated very rapidly by a launching boost which would take it up to a speed at least one-half of its desired cruising speed. Theg.w. was fortunate in another major respect, in that it avoided almost all transonic problems by virtue of its high acceleration. In the disposition, shape and size of wings and tail, the g.w.was at first sight markedly different, but much of this apparent difference arose from the fact that the missile could operate at amuch higher wing-loading, so that the relative wing area was much smaller. A number of missiles incorporated moving wings. Theinertia effects were here much lower, and so lift and hence manoeuvre acceleration could be achieved more quickly; in addi-tion the body incidence remained low, which in turn gave a number of advantages. These were offset by the increased wingarea needed for die same lift because of the lift interruption at the centre section; and an added weight penalty arising from theheavier wing-actuating mechanism. As usual, the designer's choice was a compromise. The importance of minimizing the drag of a missile dependedon the type of weapon. At one extreme there was the ballistic missile where, on average, there was a considerable excess ofthrust over drag and the important factor was a high acceleration before the thrust was cut off. Only while the missile was climbingthrough the atmosphere did the drag have significance, and die result was that a ten per cent increase in drag would reducethe range by about 0.5 per cent only. At the other extreme was the longish-range surface-to-air g.w., where a considerable part of theflight was likely to be at cruising speed and in the atmosphere. In these conditions aerodynamic efficiency was demanded and,since the percentage decrease in range approximated to the per- centage increase in drag, a reasonable lift/drag ratio must beachieved. As far as weight was concerned, a study of growth factors couldgive much information. The growth factor was the rate of increase of total missile weight per unit increase in payload weight (war-head, fuse and guidance), assuming the restoration of the original performance. For air-to-air and short-range surface-to-air missiles,this growth factor F, or dW/dP, was between 2.5 and 3 (see Fig. 1), compared with values of 10-15 in modern fighter andbomber aircraft. In the case of the ballistic missile, savings in the fig. J (left). Weight growth factor for 12 types of missile. Growth factor F plotted against total-weight/payload ratio. Mean line gives F=0.8 W/P; limit lines (dotted) give F=0.9W/P and 0.7W/P. fig. 2. Variation of all-burnt velocity V and range with Wp/W, where ^v=weight of usable propellent on any stage, and W = total weight of that stage. Payload assumed to be 1/25 of the initial weight. 24.000 22,000 2a ooo 18.000 Bl«.000 14.000 12,000 tO, 000 8.000 weight of structure, powerplant and tankage were even more impor-tant, since the objective was to give the missile the maximum kinetic energy and hence the longest ballistic range, and these weightsdetermined directly the final acceleration imparted to the end stage. The tremendous premium attaching to reduction of these weightswas shown in the diagram (Fig. 2). If Wo was the all-up weight at the start of the flight and Wi the residual weight when all fuel isconsumed, ballistic missiles could expect to achieve a Wo/Wi ratio of 20 or so, while the long-range aircraft was doing well to achievea value of 2. In the missile case the range was proportional to the square of the logarithmic weight function, whereas in the case ofthe aeroplane it was directly proportional to it. The development cycle of missiles and of aircraft (see outlinebelow) was the next topic discussed by Mr. Serby. A prototype aircraft needed to be complete as a flying machine before its firstflight, whereas widi missiles each individual element (such as propulsion, drag, control, guidance) could be tested separately oneby one, and only when each was satisfactory did the whole system need to be "buttoned together." Including test vehicles for indi-vidual tests, not necessarily of die same form as the final prototypes, approximately 200 missiles had to be flown in die course of thedevelopment of one type; for aircraft development between two and six prototypes would serve. This followed from the fact thatmissiles normally made one flight only. DEVELOPMENT CYCLE AircraftChoice of layout, W/S, power- plant, etc.; type of controls,flaps, etc. Wind tunnel model work. Scaled flying models.Flying test-beds to prove power- plants, etc Prototypes 2-6.A. and A.E.E. testing. Production. Missile Choice of guidance system in rela-tion to warhead and fuse; choice of layout, powerplant, etc., in light ofabove. Wind tunnel model work togetherwith ground launched models (rocket-propelled). Propulsion rounds provided simplyto develop boosts and sustainer. Test vehicles to develop power-plants, explore aerodynamics, boost separation, etc.R. and D. rounds 100-200. Acceptance rounds 50-200. 'JProduction. -40 6.000 Missile development simply could not proceed without tele-metry. The total flight-time available for the collection of data might be only ten seconds, during which time information aboutthe propulsion, aerodynamics, and guidance and control responses to a variety of naturally evolved or commanded manoeuvres mightall have to be signalled back to the ground. Parameters to be examined varied from transient phenomena such as wing flutter,which required continuous channel recording, through combus- tion chamber pressures requiring less frequent sampling, to lateraland longitudinal g measurements, angular yaw, pitch or roll and voltage readings, which required only infrequent sampling. Reliability was coming to be one of the most debatable charac-teristics of military equipment in general and of guided weapons in particular. It should be remembered that the overall reliabilityof a complex equipment was not the average, but was the product, of die individual reliabilities of its components. If, for example,an equipment had 100 components, each with a reliability of 99 per cent, the overall reliability would be about 35 per cent. To achieve80 per cent overall reliability in a 400-component equipment, each item should have a failure rate of only one in 1,200—a figure thatwas high but surely attainable. Although the aeroplane was highly complex, it could survive thefailure of quite a few of its complex components—^due partly to alternative or standby equipments and a human pilot to operatethem. The picture changed as automatic devices took over more and more duties, however, particularly in the case of military air-craft, where target recognition, target seeking, bomb-aiming and even stability were relying more and more on electronics. In otherwords the military aircraft was coming more in line with the g.w. where, in general, failure of one component would prevent asuccessful mission. Dealing with the subject of the techniques of missile recovery,the lecturer disclosed that the Weapons Research Establishment at Woomera, South Australia, was working on a system designedto recover missiles in flight. If this were successful, and it had reached diis stage for small vehicles, there were wide possibilitiesfor re-use. In conclusion, Mr. Serby said that g.w. design had much tolearn from aeroplane design and vice versa. The time was ripe for a greater sharing of experience. 0 75 OB 0-85 <W
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