substituting three solid elements, arbitrarily adding supports every 12 inches to balance 33 lb each, the weight is Fig. 3.4-4. Assuming the 12 in. supports add an additional 50% weight penalty, the total weight would be 0. 0643 Ib/ft. Figure 3.4-5 shows the limitations of both structural elements with respect to the Shuttle cargo bay, and it is clear that further optimization can result in less weight penalty while yielding an efficient packaging density. This assembly concept would require the largest work force in orbit of any of the options considered. When the cost of the required Space Station, crew rotation and materials and life support logistics is factored into the equation, this approach does not appear desirable. Figure 3.4-6 summarizes an estimate of the rate of assembly of a typical structural tri-beam in which the caps and intercostal members have been shipped in an efficient Shuttle packaging arrangement to a Space Station. Skylab 3 data on the rate of assembly of the twin pole sunshade was used to establish the degree of human skills in space environment. In the Skylab 3 mission, a single man assembled two 55 ft poles in 5 ft sections in 137 minutes. This represents an assembly rate of 6. 2 min/operation. A typical MPTS 18 meter structural member would require 78 operations. Assuming a 90% learning curve improvement in skills relative to Skylab performance, a 2. 5 min/operation could be considered plausible. At this rate 5.7 Ib/m-hr rate of assembly could be achieved. Twenty-four 12-man Space Stations would be required to support the assembly crew at 5.7 Ib/n-hr. A total of 470 klb of MPTS antenna structure could be fabricated using a crew of 275 in the allotted 2-month period. This high manpower requirement with associated Space Station support equipment tends to eliminate this approach as a viable detailed assembly approach (Fig. 3.4-7). Complete fabrication and assembly in orbit can achieve 100% Shuttle load factor by transportation of raw materials to the fabrication/assembly site. This concept requires a free flying "factory". It is not unreasonable to assume that one could be designed and built with little technical risk. Figure 3.4-8 shows a concept for in-orbit fabrication and assembly of a typical girder. Considering the factors involved, that is, volume limitations of the Shuttle and the desire to minimize on-orbit personnel, this approach appears to be the most promising. An operations analysis of this process has tentatively established a rate of assembly of 420 Ib/hr for the MPTS structural elements. At this rate, eight manufacturing modules would be required to meet assembly time tables.
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