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Aviation History
1960
1960 - 0634.PDF
634 FLIGHT, 6 May •'60 Power Supplies for Small Missiles . . . In a total-loss system it is obviously uneconomical in weight topressurize the hydraulic fluid by compressed gas stored in the missile; but, for short durations where the initial weight of fluidis small, it can be pressurized by a slow-burning cordite charge, or preferably by a bleed from the rocket motor, with reasonableweight economy. A circulating system may have a particular advantage, if the pump is electrically driven, in that all the missileauxiliary power can be derived from a compact electric battery. However, the system must be assessed in relation to equivalentpneumatic systems to determine which best suits the particular application. In both pneumatic and hydraulic application adequate filtrationis essential to avoid clogging the system, and very fine filters are standard parts of the various components. Also, filtration duringcharging has to be carefully observed. A major factor affecting the design of the component parts isthe environment that the missile has to experience. All parts of a pneumatic or hydraulic system can be designed without excep-tional difficulty to meet with high reliability flight and transport vibration specifications and the temperature environment. Special cases arise depending upon the role of the missile andthe type of system used. For instance, an air-fired missile can usually be kept warm while the aircraft is flying, whereas aground-fired missile may have to operate in extreme arctic and tropical conditions, when heating or cooling of the requiredmagnitude is not easily achieved. Hydraulic systems must there- fore be designed to allow for fluid expansion and contraction andfor adequate actuator response over the relevant viscosity range. In addition, heating caused by the work done on the fluid, or bythe use of high-temperature gas, has to be allowed for. The environmental temperature range has little effect on the operationof the components in a pneumatic system, but the pressure vessel must be designed to provide adequate flight endurance at thelowest temperature and to have adequate strength for the much higher pressures that develop at the higher temperatures. It is obviously undesirable for hydraulic or pneumatic systemsto leak. Loss of working fluid means topping-up, which is operationally undesirable—particularly if the missile has toremain in a ready-to-use condition for a protracted period. Electric power. For small-missile applications electric power isderived either from batteries, from a turbo-alternator driven from the missile pneumatic system or from a slow-burning corditecharge. The choice between these supplies depends mainly on the type of components used in the missile. Miniature secondary cells have been developed specially formissile application, and can be obtained in standard low-voltage battery packs giving high, medium or low current outputs depend-ing on size. A typical medium-size silver/zinc alkaline battery can provide 60A at 12V or 120A at 6V for three minutes,the volume being less than 25 cu in. A battery of this sort can adequately provide both 1-t. and h-t. supplies for a large numberof thermionic valves, current for the valve heaters being obtained directly from the battery and the h-t. and bias supplies via tran-sistor oscillators and associated rectifiers and voltage stabilizers. Frequently, transistors can advantageously be used in place ofvalves. Transistor circuits normally consume about one-twentieth of the power required for equivalent valve circuits; furthermore,low-voltage supplies only are required. Dry Leclanche primary cells built into compact h-t. batteries canbe used for thermionic-valve circuits when the lead is small, but h-t. will still have to be provided by a secondary-cell battery. Un-doubtedly batteries provide the most suitable power supply for transistor circuits. Secondary or primary cells can be used, butthe secondary-cell battery will in most cases be lighter in weight and will certainly provide a better-regulated supply. Shelf-life of the batteries considered so far is inferior to that ofthe other components in the missile. Dry batteries have a shelf- life of several months in missile applications. Secondary cellshave good shelf-life when stored without electrolyte, but when filled require periodic recharging and replacing. A further diffi-culty experienced with these batteries is their poor performance at low temperatures. This is not necessarily a great disadvantage in air-fired missiles which can draw hot air from the air aftengines. Ground-fired missiles may require a small amour: ofpower from the launcher to heat the battery. This is undcii". ,i ebecause ground-fired missiles are expected to remain ready for instant use for lengthy periods, and ideally should require pcveronly a second or two prior to launch. Batteries capable of met np the full environmental specification for guided missiles, as we J ashaving long shelf life, are, however, in an advanced stag of development. Turbo-alternator power packs can satisfy the missile envi•-in-mental requirements, including operation at high and ow temperatures, and they have excellent shelf life. They are con-siderably heavier than equivalent battery supplies, but for outputs in excess of about 200W a cordite-driven turbo-alternator ma'v beattractive, particularly when shelf life and low-temperature opera- tion are of especial importance. A turbo-alternator driven from the pneumatic system is fre-quently used in small missiles requiring a substantial amount of a.c. power regulated in frequency to about one per cent accuracyThis is achieved by means of an electropneumatic valve in the input line to the turbine controlled by the alternator output viafrequency-sensitive networks. Voltage control to the same order of accuracy can also be achieved, and a d.c. supply is then obtainedvia transformers and rectifiers. Both types of turbo-alternator can deliver correctly controlled power approximately one secondafter initiation. Gyroscope Power Requirements. The missile gyroscopes needto be run up to speed prior to launch. Air-fired gyros can be powered from the pneumatic or electric services of the parentaircraft. Pneumatic operation is obtained by blowing the air over small buckets cut into the periphery of the gyro wheel, and electricoperation is usually obtained by running the wheel as part of a synchronous motor. The gyroscope may therefore be runningfrom aircraft take-off until the missile is fired. On the other hand, gyroscopes in ground-fired missiles requirequick run-up devices to operate just prior to launch. This can be achieved by means of a high-pressure air blast from a bottle onthe launcher, or the required energy may be stored in the missile. In the latter case the gyro can be quick-started by the missilepneumatic system, by a small inbuilt gas cylinder or cordite charge or by a wound-up spring. A quick-start device will gener-ally run the wheel up to speed in less than 0.1 sec. Owing to the short flight times of small missiles the gyro wheels usually coastduring flight, but if r.p.m have to be maintained this can readily be done pneumatically or electrically. Conclusions. For small missiles, pneumatic control-surfaceactuation is generally preferred at the present time and the com- ponent parts are in a highly developed state. Future trends mayfavour either hot-gas actuation using the rocket motor or electro- mechanical actuation. Both are attractive as they do not requirepneumatic storage, but considerable research and development must be carried out before their advantages can be properlyassessed. Electric power supplies are also well advanced in development,and battery supplies are usually more attractive than turbo- alternators. However, where batteries are used, the ready-to-useshelf life of the missile would at present appear to be governed primarily by these components. With the introduction of silicon transistors a major break-through towards the ideal small missile has been achieved. Such a missile may be broadly defined as having a shelf life measuredin years without the need for periodic component changing or topping-up at specified intervals. It should be able to operatewithout heating or cooling in extreme arctic and tropical con- ditions, the latter with full solar radiation. It should not requireany power supplies until a second or two prior to launch, and the total activation time should be no more than two seconds underall conditions. It is obvious that a battery to fit this specification is now the keyto the ideal small missile—and, as already stated, is in an advanced stage of development. In all other respects the specifi-cation can certainly be met in the foreseeable future using existing techniques. EQUIPMENT SALES ABROAD CJINCE 1946 some £24m worth of aircraft equipment has been^ exported to over a hundred countries by three companies, Ferranti Ltd, Rotax Ltd and the Dunlop Rubber Co Ltd. Rotaxhave exported more than £6m worth of actuators, ignition, starting and generating equipment, switch gear and other items to morethan 50 countries in Australasia, Africa, the Far East, North, Central and South America and the Middle East and Europe.Rotax equipment is fitted to 16 different types of overseas aircraft and seven types of British aircraft manufactured by companiesabroad under licence agreements. Ferranti artificial horizons, airborne fire control and airbornesecondary radar equipment worth nearly £6m have been sold to 38 foreign countries and fitted to 11 different types of aircraftbuilt overseas. , The same export pattern is followed by Dunlop tyres, wheels,brakes and operating equipment as well as windscreen wipers ana de-icing equipment. Since the war, these products have earned£12m and their annual sales (£lm in 1954, £1.8m in 1959) are still growing. Dunlop products have been fitted to 24 types oiaircraft in a dozen countries. '. " *
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