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Aviation History
1960
1960 - 0589.PDF
FLIGHT, 29 April 1960 589 Power Supplies for Small Missiles PART 1: AN ALTERNATOR ENGINEER'S VIEW By R. MANSEY THIS is the first of two articles in which—at "Flight's" invitation—views on a little-aired topic are expressed by senior missile engineers. The author of this instalment, Roy Mansey, is alternator projectengineer, de Havilland Propellers. This company is actively engaged on Firestreak guided weapons and has been responsible for the develop-ment of Blue Streak. In addition, the firm's Alternator Division is now producing more than 80 per cent of the electrical power units fittedto guided weapons in this country. Its activities embrace the use of hydraulic fluids, high-test peroxide, iso-propyl-nitrate, cordite andhigh-pressure bottled gases. VARIOUS types of guided missiles have been publicized somuch that they are now common knowledge, from thesimple wire-controlled artillery weapon to the multi-stage space vehicle. Each species of missile has its own electrical powerdemand, not necessarily related to its physical size, but rather to the performance specification it is required to fulfil. Ballisticmissiles fired from a stable platform may often have a relatively low electrical demand, compared with a smaller missile having aradar-controlled launcher and a complicated manoeuvre pattern such as is required for anti-aircraft purposes. At this stage, missile designers have usually experienced oneor more forms of supplying electrical power and are, therefore, able to assess new requirements in the light of this experience.Thus a system having once proved reliable will be used again, although modified to bring in later techniques or to cover specialfunctions. In this way, the reliability factor is increased, but new methods are always carefully assessed and adapted in the constantsearch for weight reduction. The missile electrical power unit is generally an integratedconception, containing its own source of energy and method of control. It must normally be self-starting and usually have acomparatively short life, but there are considerable exceptions in both cases. Sources of Power. There are, briefly, four types of prime energysource available; batteries, bottled gases, monofuels and hydraulic fluids. A further alternative is the use of an existing speed-controlled drive. Batteries. Battery techniques have developed rapidly in recentyears, and various types of secondary cell are now available. Nickel/cadmium and silver/zinc batteries have a remarkablepower/weight ratio, silver/zinc having the greater capacity per unit volume and nickel/cadmium the higher reliability factor.The operating temperature is very restricted, however, with a range of only —10 to +45°C for most practical purposes; andthis severely limits their application, owing to the complication necessary to afford adequate temperature protection. Storage andcharging also need careful provisioning. For one shot devices operating for a very short duration, a battery with a high tempera-ture chemical action could be a practical possibility. Battery developments comprise too vast a subject for more than a cursorymention here, but there is no lack of information available for the designer. Regeneration of d.c. supplies to closely controlled a.c, eitherwholly or partly, is carried out by transistor inverters. Their rapid development has resulted in an increased demand for batteryinstallations for many types of missiles, but a major problem is their temperature limitation. The latest silicon transistors operate satisfactorily with junctiontemperatures of 150°C. Where more severe conditions are met, some control is possible by careful design of a heat sink.This can take many forms, from mass metal to evaporation tech- niques, or coolant circulation through hollow mountings. Forlong durations, it is usually necessary to contain the whole inverter in a regulated temperature. This can either be done locally byfeeding a temperature-regulated air supply into the inverter box, or by mounting the unit in a temperature-regulated bay, togetherwith all other components having similar limitations. From the electrical aspect a further problem is the use ofpower-factored loads. The resulting inefficiency can be catered for by increasing the size of the inverter, but the heat generation canthen become an acute embarrassment. It is not advisable to use inverters in this manner except under very restrictedcircumstances. Bottled Gas. A simple method of providing rotary power isthe use of a high-pressure air bottle feeding a single-stage turbine drive. Single-impulse turbine efficiency is not high—probablyless than 30 per cent—but the simplicity of operation compensates lor this. Overall considerations would not normally warrant morecomplication, although a re-entry system can be introduced fairly Sl*nply. A relatively low speed and direct drive gives an admir- able power unit which can be controlled by a load feed-back to the throttle. Various examples of this method have been pub-lished, the most notable being that of the de Havilland Propellers' Firestreak, where high-pressure air is used to feed actuators inaddition to the power unit. The clean gas supply removes the need for expensive filters or complicated servo systems and allowssimple rotary or sliding throttles to be used. One secondary feature of this system is that the two-stage pressure reduction (i.e.,reducing valve and turbine) provides cool air from the turbine exhaust which can easily be directed to solve a cooling problem. Monofuels. The next stage in the power unit is the use ofmore exotic fuels to feed the turbine. A large number of mono- propellants have now been developed throughout the world, allwith their own characteristics and problems. For a British missile designer, however, two main fuels are readily available—iso-propyl-nitrate (IPN) and hydrogen peroxide (HTP)—although alternatives will shortly be in production. Cordite is also avail-able, but the problems associated with its use for controlled power have rendered it unsuitable compared with other fuels. Eachmonofuel contains approximately the same energy per unit weight, and poses many problems in converting energy to electrical power. IPN requires ignition and burns at a temperature in excessof 1,000° C. This means that a coolant must be introduced, probably water/methanol, and careful control is necessary in main-taining the correct ratio. Small quantities of potassium cyanide are left in the exhaust—which, if allowed to collect, could bedangerous—and a gum sediment in the tyrbine system necessi- tates cleaning after very few hot runs. The fluid is safe to handleunder all normal conditions, and a number of successful systems operate by it. It is interesting to note that the A.W.A. Seaslugwas converted from a partially successful cordite system to highly successful IPN operation.HTP, on the other hand (Fig 1), is slightly corrosive to handle, but presents few problems if copious supplies of waterare available. It requires no ignition, being converted to super- HTP SPEED 5K3NAL Fig 1. A controlled HTP drive. A Barskc impeller is used to obtain feed pressure and speed signal heated steam and oxygen by the action of a catalyst, the steamtemperature being approximately 600 °C. Continuous running is limited only by the life of the catalyst pack, which will slowly"poison"; and catalyst design is, therefore, most important. Some self-decomposition occurs, requiring vented storage tanks andfrequent tests to prove correct fluid strength. Where weight is of prime importance, this would mean that the fuel containercould only be installed just prior to firing, or the seif-decomposi- tion would result in reduced energy and increase in fuel content.Hydraulic Fluid. In cases where a pump drive is readily available, a constant-speed drive- can be obtained by the use ofa controlled hydraulic motor. A simple example is that of a pump driven by a ram-air turbine, which is limited only by the need forsufficient forward airspeed and thus may be well suited to missiles carried by a parent vehicle. However, it is possible to provideaccumulators to start the system if high accelerations can guaran- tee sufficient airspeed in a short period from take-off. The Bristol/Ferranti Bloodhound typifies the use of hydraulics in this manner. The complications of a hydraulic system normally render ituneconomic for electric-power generation only, and it is usual to include as many functions as possible, e.g., actuators for con-
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