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Aviation History
1962
1962 - 1457.PDF
f LIGHT International, 16 August 1962 249 fact that 24 satellites would be required. With 50 per cent increase for possible failures, the satellites would take about three years to launch. With a five-year lifetime three or four additional satellites must be launched. This gives a total of 40 satellites, costing £60m to launch. The corresponding figures therefore are:— Polar orbital system Satellite development costs Development of satellite .. Nine development launchings Initial capital cost Three guidance stations 16 communication ground stations 40 satellites 40 satellite launchings Total £ millions 6.5 13.5 20.0 Total capital and development costs 4.5 24.0 8.0 60.0 Total 96.5 116.5 Within the accuracy of these estimates it could be assumed that these costs are roughly equivalent. A greater number of launchings would be required to keep the polar system in operation but this may be balanced by the overheads at Woomera being shared with other launchings. Similar costs would apply to a system with satellites in elliptical orbits at 63.4°. It is considered that changes in the various figures used in these estimates will not materially alter the rough equivalence of cost between these systems. Thus the major factor in deciding between them is the degree of confidence in their technical feasibility. There is no doubt that there is, at present, no confidence in the possibility of developing a reliable system other than with equatorial orbits. Future system developments The system proposed here can only be regarded as an interim system. It would provide several hundred circuits for telephony and a television channel. How ever, the complete requirement of 1,000 circuits for telephony and two television channels could probably be met with a payload of the order of 8001b. At the same time it might be advantageous to increase the height to 9,000 n.m. This agrees well with the pro posals for a liquid hydrogen-oxygen third stage for the ELDO launch vehicle, for which the estimated nominal payload for 9.000 n.m. is 1,2001b. It seems unlikely that payloads greater than about 8001b offer any advantage for communications satellites, at least for the orbital heights being considered here. Thus it would seem preferable to obtain increased communication capacity by the introduction of a second system rather than by the use of increased payloads. It is suggested, therefore, that a rough optimum exists for the satellite weight and that the ELDO launch vehicle with the new third stage will provide a suitable performance to put such a payload into a 4.000 n.m. orbit. At present it is difficult to envisage any major improvement to such a system apart from improvements in life. The latter may occur with the use of nuclear reactors as sources of power and new methods of propulsion for orbit adjustment over long periods of time. Both these would probably necessitate increased payloads. The limitation of height to 9,000 n.m. applies to telephony only (unless a means can be found of making the time delay acceptable) and not to television or data transmission. There would be certain advantages if it were possible for the first system to use the orbits proposed for the final system. As satellites of the first system failed, they could then be replaced by satellites of greater weight, gradually increasing the communications capability until the final system capability was achieved. With a first system using satellites in lower orbits this is not possible, since satellites in higher orbits cannot be synchronized with them. The objective of a common orbit for Phases 1 and 2 would require either a moderate improvement in the present ELDO launch vehicle or the acceptance of a further reduction in the communications capacity of the Phase I system. Conclusions Within the specified framework of launch vehicle and operational requirement it is believed that the choice of circular equatorial orbits is inevitable: it is considered that the relative technical simplicity of the whole system completely outweighs The GPO's Goonhiily Radio Station, Cornwall, already used successfully in the Telstar experiments, would form an important part of any future British Comsat system possible disadvantages. In fact, at the present time, station-keeping satellites in polar orbits or orbits inclined at large angles to the equatorial plane are not considered technically feasible for reliable performance. A system using random orbits would be more com plex, present severe operational difficulties and probably be more costly. The optimum height band for circular equatorial orbits appears to be about 6,000-9,000 n.m. for telephony; there may be advant ages in the 24hr orbit for telegraphy and television when this becomes technically feasible in terms of satellites controlled to be stationary with respect to the Earth. The present design of the ELDO launch vehicle precludes the use of 9,000 n.m. orbits, and for this reason a 6,000 n.m. orbit system is proposed, to be followed by a 9,000 n.m. orbit system when a new third stage (liquid hydrogen-oxygen) is available for the launch vehicle. If, however, the performance of the present launcher could be improved, or if a further reduction in the initial communications capacity could be tolerated, then it might be preferable to use some intermediate height between 6,000 and 9,000 n.m. from the beginning, followed by higher payloads at the same height when a new third stage (liquid hydrogen-oxygen) is available. The present nominal payload for 9,000 n.m. is 3501b but a safety margin of about 2001b is considered essential at present. The background concept of design is a minimum of electronics and mechanical moving parts for the satellite; hence the emphasis on passive and semi-passive methods of control and stabilization, although at present an active control system appears necessary to cope with initial conditions at injection and with orbit adjustment. However, from the engineering point of view, the satellite is simple in concept and construction—as it must be if a five-year life is to be achieved—and the same basic design can be used for the higher orbits. The doubtful component at present is the silicon solar cell, since it is susceptible to proton damage in the inner Van Allen belt. It is considered that the orbits proposed and the basic satellite design outlined are technically feasible; the proposed system is relatively simple and uses as far as possible inactive components for station-keeping and attitude stabilization. These factors should contribute significantly to long life and high reliability.
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