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Aviation History
1960
1960 - 0590.PDF
590 FLIGHT, 29 April I960 Power Supplies for Small Missiles . . . trols and radar scanners. Accurate frequency control is obtainedby the use of input pressure signals in conjunction with amplified error signals. The accepted hydraulic fluid in this country is still the mineral-based DTD 585, although many alternatives are available. Although notorious for its low flash point of 93 °C, this figureis now regarded as unrealistic under normal conditions; DTD 585 has been used at up to 200 °C without its lubricating quaiitiesbeing greatly impaired. Bearing-lubrication and cooling ceases to be a problem if thedrive fluid is fed through the generator on its return to the reservoir. Silicone-rubber seals would be essential for such asystem, but all the techniques necessary for satisfactory operation are available. It is often preferred to have a local hydraulic circuitwithin the missile by driving the pump from a missile power unit using other methods of drive—a reversal of the systemdiscussed above. Direct Drive. The last major classification is that of a directdrive from some other equipment. In small missiles of short dura- tion, this is seldom practical, but offers a satisfactory method wherelarge pumping capacity or other rotary drives are necessary. There is no problem, providing that the speed of rotation can be readilycontrolled. Frequency Control. Having covered the various drive possibili-ties, methods of frequency control can be considered. In the case of transistor/inverters, provided that battery volts are main-tained at a satisfactory level, a suitable oscillator poses no difficult problem. A vast amount of literature is now available and, withinthe limitations previously mentioned, this type of power supply will become increasingly popular. Frequency control of bottled-gas units has already been brieflymentioned. With a controlled-pressure input, such as would be given by a reducing valve, it is only necessary to compensatefor load changes by feeding an amplified error signal to a throttle actuator. For monofuels, the control problem is more complex, owingto the temperature range and fuel characteristic. Careful filtration is essential, especially at critical points in the system where close-tolerance servos are used. A reasonably constant input pressure and temperature allows the use of a compensatory load or avariable brake instead of a throttle. Various methods are in opera- tion, the most notable being the use of a slave alternator whichsupplies power to a dummy load varying inversely with change of main alternator load. Another method is to control a turbinewheel by an eddy-current brake mounted integrally with the wheel (Fig 2). Such systems, however, are limited by power output:the higher the power the more uneconomical the excess weight becomes. Above 1.5kW, therefore, some form of throttle isnecessary. An in-line throttle is generally unsatisfactory for very close tolerances, and a more practical solution is to use a by-pass Fig 2 (left). Monofuels may be controlled with a turbine wheel and eddy current brake Fig 3 (right). Typical electrical filter with resonating capacitor, parallel inductance and parallel filter tuned to fundamental frequency system bleeding excess energy through a controlling valve intothe exhaust. With hydraulic fluids, a throttle is obviously ideal. It can eitherbe electrically controlled when exceptionally close frequency tolerances are necessary, or hydraulically by obtaining a signalacross a calibrated orifice. The latter system is only possible with fixed-capacity motors. In some cases it is desirable to use avariable-capacity motor, especially when heat generation into the oil is a problem. In this case, an electrically operated throttlewould provide close control and a pressure-drop signal across the throttle could be used for varying the capacity. Thus, the heatgenerated in the motor would be constant under all conditions and would allow accurate design of the oil cooling system. Missile Arming. In the various drive methods reviewed, a further consideration is that of missile arming prior to launch, with the inevitable changeover problem. Some missile designs may require to have the missile power unit running some time before actuallaunch, especially where a stand-by condition is a feature. Hydraulically driven power units present no problem, providedthere is a ground supply point with self-sealing couplings which separate on launch. With monofuels and turbine-driven powerunits it is possible to use piped cold gas, such as compressed air or nitrogen, for prolonged ground running before changing overto monofuel on launch. Some slight delay would normally bt allowed in order to ensure satisfactory operation of the monofuelprior to launch. On the other hand, a simple rectified mains supply or batteriesmay be the only available ground power and the firing procedure must have an allowance for the power unit to run up and stabilize.It is, therefore, essential for a very rapid run-up condition to be specified in the design, and it is interesting to note that somerotary machines will run up from rest and stabilize at 24,000 r.p.m. in 300 milliseconds (0.3sec). The Generator. The generator part of the power unit can bebasically classified in four ways: salient-pole or inductor type, each either regulated or unregulated. At speeds up to approxim-ately 8,000 r.p.m. a regulated salient-pole machine is practical if the rotor diameter is under 2in. Slip-rings and coils must becarried on the rotor for regulation, and both pose balancing problems. These factors normally rule out the machine for mostmissile applications. Regulated machines are normally of the inductor type, in which the voltage control is obtained by vary-ing the field current in coils wound round the stator, the rotor being a simple laminated structure. Where reasonably constantload is applied, permanent-magnet fields are possible; but it should be appreciated that voltage will vary considerably withchanges in load. One popular missile system is to use a controlled inductor-type single-phase high-frequency generator for suchitems as guidance units and amplifiers, and a small three-phase salient-pole machine in tandem, having a permanent-magnet rotor.The latter machine is thus uncontrolled but would be used for supplying gyro motors and similar units. Another combinationis to have two separate output windings in the controlled machine, each giving a separate voltage. Thus, a low-voltage high-frequencysupply can be locally rectified for d.c, no transforming being necessary. Electrical Filters. With inductor-type alternators, the voltagewaveform is far from sinusoidal and the machine is essentially very inductive. Consequently, a great deal of thought is necessary inorder to ensure that loads requiring harmonic currents do not distort the voltage excessively. For this reason, it is essentialthat fully representative missile loads be supplied to the power- unit designer at a very early stage in order that the necessary filterscan be checked. Filters (Fig 3) can vary from single resonating condensers tuned to the fundamental frequency, to advanced con-trol loops covering generator, filter, regulator and load. The subject of filters is complex and lengthy in the extreme. In oneinstance, it is necessary to have excessive distortion into a resistive load in order that the combination of filter and missile load is heldto acceptable limits. Power Unit Comparison. As a final indication of the varioussystems available, Fig 4 shows a comparison based on a kW-min basis. These curves represent a vast over-simplification of thewhole problem and can only be used as a rough guide. They must inevitably be open to criticism because of the multiplicity offactors that can influence the designer. Considerable improvement can be effected by using the prime energy source for more thanone function, or by combining different systems to cover excessive environmental conditions. The curves shown, however, are basedon a fully regulated and controlled system used solely for electric- power generation. The hydraulic system is omitted in this instancebecause of the large variations necessitated by special conditions,
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