Fig. 3. Thermal insulation for SC cable. The space environment is advantageous for thermal insulation, because no vacuum vessels are necessary for multilayer insulation. Moreover, thermal shield of cryogenic temperature is available with a proper radiator system. Consequently, the structure of thermal insulation can be simple. In this case, the shroud temperature is supposed to be 77 K (-321° F) for the estimation of the heat-in-leak. Figure 3 shows the conceptional view of the thermal insulation. The cable itself does not have thermal insulation layers and it will be laid surrounded by blankets of multilayer insulation installed on the shroud. Many merits can be counted in this structure, for example, smaller diameter of the cable, easier connecting and laying. The level of the orbit for the construction is not yet decided, but it can easily be carried out by robots because of the simplified coaxial structure of the cable. The connected cables can be laid without attention to the electromagnetic force and the environment, such as plasma, electric charge and so forth. REFRIGERATORS According to investigations performed so far, refrigerators seem inevitably to be vibrational heavy machines which need periodic inspection. And considering that large ones are absolutely but exclusively necessary for initial cooling down, it is beneficial to make a refrigerator compact combining its components, i.e., a compressor, heat exchangers, expanders, etc., into a unit for easy changeout. Large ones will be transferred to another SPS newly built on GEO to avoid unnecessary redundancy, and ordinary units which need maintenance will be brought down to LEO where human labor is available if necessary, and replaced with new ones. The power for refrigerators is estimated as up to 19 MW. Its main components, the load by the heat-in-leak at the cables and the one by the heat through the power leads, are almost comparable. The weight of this set of refrigerators of 19 MW is extrapolated to 7x 105 kg (1.5x 106 lbs) from ordinary ones used on the ground. It is so heavy for the use on a SPS that lighter ones must be designed specifically for the space use, but this should not be very difficult. Moreover, if low temperature compressors are available, not only the weight but also the power consumption will be improved greatly.
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