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Aviation History
1960
1960 - 0280.PDF
280 FLIGHT, 26 February FIRESTREAK apparent sight-line movement in the plane of the body oscillation.Accordingly, the strictest attention has been given to IR dome aberration, bearing friction levels and balancing. The forecasts ofhigh-altitude instability arising from aberration or viscous friction have not proved true, partly because the frequency response ofthe control system changes with altitude. t The precession torques which must be applied to the head gyroare proportional to the space rates of spin of the sight-line. The precession currents in the torque motors therefore give straight-forward measurements of the spin rate, resolved into the plane of the torque motors, and this can be used for proportional naviga-tion. All that remains is for the missile to be made to turn, in each of the two wing planes, at rates proportional to theseprecession currents. Firestreak is launched in a pursuit course, i.e. straight at thetarget. Dispersions and errors are then removed by a navigational system which varies only slightly from the classic proportionaltype. In order to achieve this, the precession torques applied to the homing-head gyro must be proportional to the rate of spinof the missile sight-line, resolved into the wing planes. The precession currents are therefore direct measurements; but inpractice it is more satisfactory to sense rate of turn by accelero- meters placed normal to the body axis. A rate gyro would senserate of turn of body axis, which differs from that of the flight-path by the rate of change of angle of attack. To some extent thiscould be overcome by suitable filtering, but the wide range of firing altitudes and speeds renders simple filtering inapplicable.Accordingly, a high-gain accelerometer feedback is stabilized by a rate gyro in pitch and yaw, while filtering decouples the controlsystem from the body-vibration mode. Since the control system produces acceleration in response tosight-line spin, the ratio of flight-path turn rate to sight-line spin (the k-factor) increases as the weapon slows down. Although noeffort has been made to counteract this, the navigation system works well. One point, however, needs particular attention. Asangle of attack builds up after launching, the rocket thrust com- ponent normal to the flight-path helps the weapon to turn in thedesired direction, but this is not sensed by the lateral accelero- meters. At low altitudes this effect is negligible, but at high altitudethe angle of attack is much greater and could result in the missile oversteering violently during the boost phase. The missile leaves the fighter with fins fixed centrally, and ifthe demands from the guidance system are increased steadily from zero to the full k-factor value over the boost phase, themissile navigates smoothly. If the navigation system were switched on suddenly this would result in a violent acceleration oscillationpersisting throughout flight (the average acceleration achieved being relatively small). The k-factor is therefore designed toincrease gradually and thus provide a smooth build-up of accelera- At the upper end of the speed range body lift accounts for a high pro- portion of the total (arbitrary vertical scale); this point is relevant to discussion of guidance and control on this page tion. Roll-control requirements are fairly simple, it beingnecessary only to keep the steady roll-rate sufficiently low for the output acceleration to take place in substantially the same planeas the sight-line spin. Firestreak has two sets of planes of symmetry, the wing planesand the planes at 45 ° to them; and in die latter lateral stability is considerably less than in the wing plane. Thus the control systemmust produce adequate performance over a wider range of static margin than would ideally have been the case. In particular, themissile must be safe with fixed fins at low fighter speeds (when the missile stability is poor), yet must have adequate trim powerat high Mach numbers. It is fortunate that a rolling torque develops which prevents the missile from remaining stable at highangles of attack other than in the 45° plane. At low angles of attack Firestreak can be trimmed without rolling torque in any plane. During high-altitude firing, the weapon rolls until theplane of the demand is contained in the 45 ° plane; all that the roll control system then needs to do is to damp this motion so thatthe roll rates are acceptable. Signals from the guidance loop demand accelerations whichmust be made very rapidly and be very close to those demanded In the lateral system the most powerful disturbing factor ischange of atmospheric pressure with altitude, and so fin angles for the same demand signals are arranged to increase with altitude.The effect on wing performance is unaltered and, despite the presence of the high accelerometer loop gain, the missile tends tobe more sluggish at altitude than at sea level. Provision is there- fore made for altitude compensation. Although the missile is capable of pulling very high g at lowaltitudes, the acceleration is prevented from exceeding a specific level by limiting the guidance demand signal. The latter is Typifying the prime contractor's "pot- ting" techniques for electronic compon- ents is the guidance electronics as- sembly, located just aft of the forward IR fuzing bay amplified by the preamplifier, the output from which is againamplified by a driver magnetic amplifier. The latter is used to drive a Laws relay or transducer, which governs a pneumaticvalve controlling a fin actuator. In each lateral plane are three feedback loops, comprising an accelerometer stabilized by a rategyro and actuator-piston position. Roll control is relatively simple, roll-rate being measured by a gyro, the amplified signalsfrom which are fed to the driver amplifiers for each fin. This represents a zero-demand servo loop, with disturbances fed backas corrections to the fins as ailerons. Development of the pneumatic valve took some time. The verysmall leakage rate could be obtained only by small clearances. These varied with extremes of temperature, and in the coldcondition the driving current for unit movement increased. This was rectified by introducing dither, but it was not easy to find thematerials which would stand up to the environment and still not pick up under the high-frequency unlubricated dither imposed. To improve resistance to vibration and climate, electronic com-ponents are anchored between printed circuits and sealed into a can. Terminations generally come to the surface through glassinsulators, and the whole is potted in a light foamed resin. While this is undoubtedly reliable, it hampers the changing of faultycomponents. When several sub-units are mounted on one chassis, they are loomed together and dipped in a varnish which binds allloose cables to the surface, protecting them against vibration and moisture (photograph above). There are two versions of the fire-control to suit aircraft armedwith two or four rounds. To cut down sudden asymmetric drag during an attack, provision is made for launching missiles insequence from alternate wings, particularly when two pairs are selected. Should a missile hang up, or misfire, its presence is takeninto account during subsequent firings. The equipment passes cold sir to the missiles when armingtakes place and indicates ?ny failure of the aircraft power suoolies which would make for an abortive launching. It locks the missilehead to the AI radar, provides a scan to increase the beatn angle to improve aiming tolerances, samples the IR signals to pick outthe true target from noise, and provides pulses which launch the missile on the pressing of the firing button. The fire-control alsotells the pilot when the missile is seeing the target with a signal ot sufficient strength to ensure that it will lock-on when the firmsbutton is pressed. Pressing the button changes over from aircraft to missile electric supplies, and ignites the boost motor. Fighter-borne fire-control is packaged in four boxes (three 101a two-missile installation, as on the Lightning F.l). Each box has two walls, between which cold air is passed, and the electronicsgive up their heat to the inner skin. When supersonic flight duration is brief, the cooling air may be discarded.
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