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Aviation History
1958
1958 - 0432.PDF
448 FLIGHT, 4 April 1958 Guided-weapon Development Problems A Lecture by Vickers-Armstrongs G.W. Chief Engineer FOR his lecture to the Guided Flight Section of the RoyalAeronautical Society, on March 20, Brigadier J. Clemow,M.A., chief engineer (guided weapons) of Vickers-Armstrongs (Aircraft), Ltd., chose as his subject the title above.In his opening remarks the lecturer stressed that the fundamental objective of such development was the production of efficientweapons of war, able either to produce destruction sufficient to neutralize an enemy threat, or to destroy the enemy should hedare to attack. Thus the development should be rapid and effi- cient; the product must be reliable and not over-complicated; mustbe able to stand up to long storage, rough handling and severe environmental conditions; and must be ready for action after littlewarning. Brig. Clemow said that the missile was only one component of the overall weapon system, but in view of the limitedtime at his disposal he would confine his remarks to the missile only. Problems associated with the development of the missile tendedto fall into two broad categories, technical and organizational. A complex missile design embodied many closely interrelated, butwidely different, engineering techniques, and a fundamental prob- lem was the successful integration of these technologies. It wasall too easy to fall into the habit of thinking that the field in which one was particularly expert was the most important one in theproject. "Not only," he said, "is it important to get away from this erroneous attitude; it is important to go further and takepositive steps, certainly if you occupy any senior position in the guided weapon team, to understand the problems and method-thinking of the other fields of science and technology which are concerned in the weapon system." What was required was a newrace of engineers, each individual being perhaps a specialist in one subject but having a broad understanding of the whole field. The principle of organization favoured was one in which therewas progressive sub-division from the chief engineer downwards into branch, group and section, so that, at the various levels, seniorpersonnel would have four, or at most six, subordinates directly responsible to them. Guided-weapon development fell naturallyinto four main divisions: design, experimental manufacture, test and trials and administration. These, in turn, could convenientlybe broken down into four or five branches of activity, and then sub-divided again if necessary. For example, design could bebroken down into aerodynamics, structure and propulsion, guid- ance and control; and each could then be sub-divided. A guided-weapon project naturally progressed through thephases of assessment, design study, detailed design and develop- ment, manufacture, and testing and field trials. For optimumloading of the technical resources of the department it therefore followed that a guided-weapon organization should be handlingmore than one project at spaced stages of development. The question then arose as to whether the organization should be split"project wise" or by functions. Brig. Clemow favoured the latter system, a project engineer, with no interest other than one project,being appointed to co-ordinate the activities of the specialist groups. A system whereby there were, say, two electronics depart-ments, one working on Project A and one on Project B, was wasteful of technical effort, and prevented experience gained onone project being applied to the other. A separate assessment group was also considered undesirable,for the members of such a group rapidly got out of touch with realities and a design group would usually have considerablereservations about blindly developing to the requirements of an independent group. For similar reasons a separate research groupwas not favoured. It was preferable to form the assessment team ad hoc by choosing the project engineer and selecting other mem-bers of the team from the various specialist groups. After the assessment phase the specialists would revert to the various groupsand continue straight on into the detailed design and development. From then onwards the project engineer would be responsible forspecifying what was to be done, while the specialist-group leaders would be responsible for seeing that the various technical taskswere carried out to the requirements of the project engineer. Real development planning was essential, and a developmentprogramme—not merely in phases, but broken down into details of component design and development—was one of the firstrequirements. The lecturer firmly believed in the principle of extensive ground and environmental testing, allied with simulatorstudies for the bulk of the development, with resort to flight trials as the ultimate proof of success. From the simulator study,the designer should know what to expect from the trial. Reliability was very important, but it was "tending to becomea fetish in some quarters." Brig. Clemow did not favour the idea of a large, independent organization monitoring reliability andexercising mandatory powers. Turning to the technical problems of missile development, the lecturer stated that the primary objective was kill-probability,which was a function of miss-distance and warhead lethality. For a given kill-probability, an increase of miss-distance meant a con-siderable increase in warhead weight, which in turn produced a threefold increase in all-up weight. Thus the miss-distance wasa major fundamental. Miss-distance arose from a number of causes and the optimumsolution was often a compromise of conflicting demands. A com- mon contribution to miss-distance was inability to enforce therequired trajectory, owing to the limits of manoeuvre of the missile. Sometimes, as in die case of beam-riding, there was a bias miss-distance inherent in the system. For example, the arrangement might be that the missile developed an acceleration towards thebeam axis proportional to the distance of the missile from it. If the beam was moving with an acceleration in order to track thetarget, then the missile would lag behind the beam. The major cause of random miss-distance was "noise." This could be gener-ated within the missile equipment by vibration or other causes, or it could be external—as in the case of radar noise, or "glint." Inthe latter case, if a filter was used to reduce the noise the resultant lag produced a miss-distance. On the other hand, without a filterthe system limits cut off the extremes of the fluctuating demands caused by the excessive noise, so that a compromise was necessary.A similar case where compromise was necessary arose in the case of a missile radome. A pointed radome produced a destabilizingeffect due to aberration. If a round nose was used, the aberration was reduced but at the expense of increased drag. Three requirements existed for the control system: fast response,stable response, and insensitivity to external conditions. The last requirement was also an illustration of the need for compromise,since on the one hand a large gain was required to minimize variations of aerodynamics, but on the other hand too large a valueof gain gave rise to instability. THE DISCUSSION MR. CUSS (Gloster Aircraft) remarked that the problems mentionedhad been mainly in the fields of guidance and control. With the aid of slides he described a structural problem he had encountered and thesolution that had been evolved. The particular requirement was for a cylindrical body with a high natural frequency. Cutting away thestructure to provide a 160 deg access panel reduced the bending stiffness to one-seventh of the original, thus presenting a formidable technicalproblem if a prohibitive increase of weight was to be avoided. In reply, Brig. Clemow stated that he had experienced structural problems butthat these had usually been of the type affecting the control system. Troubles often arose from local resonances, but a solution could usuallybe found by placing accelerometers at nodes and also by spacing of frequencies. MR. HARRISON (Armstrong-Whitworth) referred to the philosophy ofanticipating the results of field trials by extensive simulator exercises, and asked what determined the stage at which the missile should befired. Brig. Clemow replied that he favoured extensive simulator work and ground testing with relatively few firings, rather than a large numberof firings. At the peak of development trials about 1J trials per fortnight might be achieved. He believed in conducting a phase of field trials withidentical pairs of rounds. One pair with average values of system parameters, and two pairs with each of the extreme values, fo bracketthe range of system performance, often proved to be adequate. MR. DUNNING (R.P.D.) noted that propulsion had only been brieflymentioned in the lecture. Did this mean that no problems had been experienced in this field? Brig. Clemow stated that, in his experience,propulsion had caused few problems. This was true of solid p:opcllants, although liquid propellants were another story. Big problems had beenexperienced with boosts, but these were problems of separation and not of the propellant as such. He had never experienced flame-attenuationproblems. MR. TARRANT (M.O.S.) thought Brig. Clemow had "debunked" reli-ability too much. Reliability was most important, and the requiremeni was not merely to produce a system which would meet the performancespecification, but that this performance could be guaranteed and obtained consistently. In reply, Brig. Clemow said that he was not againstreliability, but merely the means—notably large independent organiza- tions—which were sometimes used to achieve it. Reliability should resultfrom good design and engineering practice, and quality-control during manufacture. He firmly believed in a system of defect reporting andanalysis, with a strong feedback to the design groups. MR. CRONIN (Elliot Brothers) asked what were the relative merits offixed and moving wings. Brig. Clemow said that moving wings were alleged to produce lift more quickly, but were heavier and needed greateractuating powers. No body lift was available and there was little to choose on aerodynamic grounds, but radome aberration was less. Withramjets, moving wings allowed body-mounted engines to obtain the fuli ram. He personally generally preferred fixed wings, as they were lighterand simpler and easier to attach. In conclusion, MR. PRITCHARD (Elliot Bros.) who, at very short nonce,had acted as chairman, made a plea that the many visitors in the audienc should seriously consider becoming members of the Society.
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