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Aviation History
1961
1961 - 1017.PDF
FLIGHT, 27 July 1961 117 LEFT E RIGHT DOWN RANGE STICK POSITION MISSILE COURSE rig 3. Idealized lateral-acceleration system ACROSS RANGE LEFT RIGHT DOWN RANGE STICK POSITION MISSILE COURSE Fig 4. Idealized lateral-velocity system ACROSS RANGE WIRE DISPENSER IGNITER PROTECTION CAP BLAST TUBE PROTECTION CAP FINS PLUG N«.3 IGNITER PRIMER IGNITER & FLARE ASSEMBLY CONNECTIONS TO MISSILE PRINTED CIRCUIT BOARD "COVER AND R.F SCREEN The autopilot also arranges that the missile automaticallylevels out at a convenient height after launch, thus considerably easing the operator's task. It is also so designed that in the case of awire breakage the missile automatically turns on to the original launch heading towards the enemy lines, thus minimizing risks tothe troops using it. In the case of an acceleration-type missile (broken line in Fig 5)attempting a similar task, the curved-path acquisition on to the sight-line is a longer and more difficult task, with a much greaterlikelihood of overshoot. It will be seen that the minimum possible target range is appreciably greater. The operator also has difficultyin holding the missile on to the sight-line. There typically results a weaving motion for a period of 3-4sec. The amplitude of themotion determines the error remaining in the system. This effect is due to the time-lag between the operator's applying an accel-eration command and observing the effects in terms of displace- ment and velocity—the two parameters which he can most easilyappreciate and interpret. In the case of velocity control, the operator commands velocityand observes the effects much more rapidly. In other words, the time-lag of the system is much shorter, and the weave about thesight-line is negligible. The importance of a small time-lag cannot be over-emphasized,since it is one of the most important factors in ease of control. Also, the residual displacement error varies as the square of the lag.The Vigilant lag is less than lsec as compared with about 2sec for a typical acceleration system. This means that, allowing for the lagin perception and response of the operator, the inherent accuracy of Vigilant is greater by a factor of about four—enabling not just thetank, but the most vulnerable portion of the tank to be attacked. The Vigilant closed-loop velocity control system has beendesigned to produce an easy-to-learn system, accurate over a wide range and angular coverage. Thus the first big advantage is that anysoldier can be trained, and special selection is unnecessary. Accuracy, and ease of training, also have a most importantbearing on operational costs. The unil cost of the more sophis- ticated Vigilant must necessarily be higher than that of its acceler-eration-control counterparts. Analysis shows, however, that the overall costs of training in peace time and cost per kill in war-timeshow an advantage to Vigilant by a factor of about two. This is because of the much smaller number of training missiles required,and the superior single-shot kill probability allied with greater coverage. (b) The missile must be forced to turn on to its new courseas sharply as possible. This is one of the reasons why Vigilant has been designed to have a high manoeuvrecapability—that is, a small turning circle. The response must also be as fast as possible, and phase advance isused to reduce the missile time-lag. A plan view of a typical Vigilant flight-path is shown in Fig 5.It will be seen that it departs from the ideal in that, instead of showing an instantaneous change to the new straight-line heading,ihe course is rounded at the "corners." If the radius of curvature or time lag were allowed to increase, the system would revert moreand more to the acceleration type. The missile can be pulled rapidly round on to the sight-linewith negligible overshoot, thus enabling short-range targets to be engaged even when there is appreciable operator separation andthe target is off the line of fire. When the sight-line has been acquired the missile can be flown accurately and smoothly along astraight path. If there should be a disturbance—e.g., caused by a gust or grazing an obstruction such as a bush—a change of attitudewill result; but it will be immediately detected by the gyro. The missile is then automatically restored to its original heading,having been subjected to a small lateral displacement which the operator immediately observes and corrects. Fig 5. Velocity and acceleration control systems compared GATHERING PHASE TERMINAL GUIDANCE PHASE _ INHERENT SYSTEM ACCURACY ACCELERATION.--! SYSTEM VELOCITY SYSTEM CVIGILANT) ACCELERATION SYSTEM VELOCITY SYSTEM (VIGILANT)
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