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Aviation History
1956
1956 - 0531.PDF
FLIGHT, 4 May 1956 Power Supplies in Earth Satellites British Interplanetary Society Discuss Utilization of Solar Energy 531 SOME aspects of instrumentation suitable for use in satellitevehicles similar to some of those to be launched in the nextyear or so, and the means for providing power for their operation, were discussed in a lecture given by E. C. White, R. G.Wilkins, and J. Foley to the British Interplanetary Society at their April meeting at Caxton Hall, London. The authors had beenconsidering, independently of the official satellite programmes, the problems raised by the necessity to operate measuring and tele-metry equipment for much longer periods than have been called for in high-altitude research conducted with rockets. They con-cluded that where a satellite was to work for more than a few hours battery power would be quite impracticable, and that thebest solution was to use solar energy convened to electrical power by silicon photocell wafers.The first of the evening's papers considered the merits and disadvantages of different possible orbits for the satellite. Withsatellite heights of about 100 miles, atmospheric friction is suffi- ciently great to slow up and destroy the satellite in about an hour,or less than the time required for a complete circuit of the earth. As the height is increased the atmosphere becomes negligible ineffect and a satellite initially in orbit at a height of 200 miles has an estimated flight-time of 15 days. Where sunlight is needed asa source of power many orbits suffer the disadvantage of being partially shadowed by the earth, the worst case being that inwhich the satellite orbit is in the same plane as the earth's orbit round the sun. Interesting points about this case are that witha satellite height of less than 1,000 miles the satellite is in shadow for between one-third and a half of the time, and that the durationof shadow experienced by the satellite is a minimum at a height of about 800 miles. The shape of the earth's shadow, which alsoplays an important part, was shown to be effectively cylindrical, the penumbra having only a small effect. The purpose of the satellite was next considered, together withthe difficulties imposed by lengthy periods of operation. Even in the best circumstances, high-altitude rockets give only a fewminutes of flight when measurements of conditions outside the atmosphere can be made; but the satellite makes possible investi-gations into primary cosmic rays and observations of solar radia- tion and particle streams over periods of hours or days.Therefore equipment must be devised which will work, reliably and transmit accurate information over far longer periods thanhave been used in rocket flights so far. The need for conservative rating of components and great economy in power consumptionled to the proposal that a receiver and data-storage circuits should be included in the satellite and that information should be trans-mitted only when the ground receiving-station was ready for it, and had transmitted the appropriate signal to the satellite. Certainof the more highly rated components, in particular the trans- mitter, would have to be duplicated in order that failure of onecomponent should not render the great expense of launching the satellite a complete waste. A selection of instruments and equipment which would form a likely load for a satellite was taken and their weights and powerrequirements estimated. On account of their small dimensions and their reliability transistors were suggested as very suitablecircuit elements for the data-storage equipment (a pulse counter), for the modulator for the transmitter, and for power supplyconverters. The table below shows the weight and power required for two satellites performing similar duties. The first is intendedto transmit continuously and the second to transmit only when it receives a "request for information" in the form of a radio signalfrom the earth. The final paper was concerned with the methods of convertingand storing the sun's energy. After pointing out that it was much more important to get a large output from a small weight, ratherthan have a unit which was highly efficient as a power converter, the speaker went on to eliminate heat engines using solar boilersas being far too complex for satellites of the small weight and size being considered. The other alternatives were thermopiles and photocell con-verters. The former could be made to give 6 Watts output per pound weight, and might have a conversion efficiency of 7 percent, but it was necessary to direct one set of intermetallic junc- tions towards the sun, and keep the other set in shadow. Themechanical arrangements to ensure this were not likely to be worth while. Photocell converters could give an output of 17Watts per pound weight and convert solar energy with an efficiency of 12 per cent. These cells were made from silicon containinga carefully introduced trace element in its surface layer; a voltage was developed across the interface between this layer and the puresilicon when solar radiation fell on the cell. The whole surface of the sphere could be covered with a mosaic of cells, which wouldalso need a transparent protective shield to prevent damage by micrometeorites. The need for measurements to be made, and possibly for infor-mation to be transmitted, while the satellite is in the earth's shadow made it necessary to store some energy in the satellite.It was found that the weight of the necessary cells reduced the Watts per pound figure from 17 to 14. Component Two Geiger counter tubesResistance thermometer Transmitter, duplicatedSwitch motor Datastorage Power converters Totals ContinuousTransmission Meanpower (Watts) 2.75 1.53 0.82 8 6 1 3 23.1 Weight (Ib) 0.8 0.25 0.35 1.5 1 1 2 6.9 PeriodicTransmission Meanpower (Watts) 2.75 1.53 0.82 0.48 0.36 0.06 0.31 2.85 1.6510.8 Weight (Ib) 0.8 0.25 0.35 1.5 1 1 1.6 0.35 2.5 9.4 MINUS FOUR TURBO JETS, PLUS TWO TURBOPROPS: AN UNUSUAL B-47 This XB-47D is one of two modified—by Boeing for the U.S.A.F.—to serve as test beds for the new Curtiss- Wright T49 turboprop, two of which replace the usual inboard turbojets in their double pods. The outboard engines are the normal General Electric J47s. Four- blade Curtiss turbo-electric airscrews, each 15ft in dia- meter, with paddle-type blades Uin wide and ducted spinners, are used in con- junction with the T49s.
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