become a controlling factor. Ref. E4 describes a structural design which has incorporated provisions for maintenance; Ref. E9 presents a detailed maintenance plan which even identified components down to the LRU (Lowest Replaceable Unit) level. Several documents include estimates for the number of vehicle flights that will be required for maintenance activities. n. Construction Approach The three overriding requirements for the construction in space of any SPS concept evident from all documents examined are: (1) Radiation protection in the Van Allen belts for radiation sensitive materials and for manned activities. (2) Development of construction/assembly approaches for large structures. (3) Development of automated, or robot/teleoperator construction mechanisms. With respect to the first, i.e., radiation in the Van Allen belts, Ref. A9 (a NASA briefing), states that construction would involve a high degree of remotely controlled activity from a protected environment, and that EVA would be restricted to special requirements. Several other documents discuss this, but are not so emphatic about restricted EVA. The development of construction/assembly approaches involves a number of factors. Ref. A9, for example, states there is less background in construction than in any other SPS area. Further efforts on design and technique options are recommended, as are evaluations to determine productivity rates in space. Ref. E5, the NASA forecast for 1980-2000, states that considerable effort will be required to learn how to assemble and control huge structures, and that this will undoubtedly require considerable advancement in our abilities to carry out human EVA's. This document further states that a program to meet these requirements can be laid out with confidence. The requirement for automated, or robot/teleoperator construction mechanisms was considered in Ref. A9. This document proposes a movable, orbital construction facility. Support equipment would include a traveling
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