4.3.7. Structure & Harness Structure and Harness is the final subsystem worksheet to be discussed as part of the Space Segment Model (the Propulsion subsystem is covered by the Space Transportation segment in IAAM). For bookkeeping purposes, the model restricts the scope of this subsystem to support structure only, since much of each satellite’s structural mass and cost are estimated elsewhere within the various subsystems. However, for the SSP concepts being considered in this study, a significant portion of the structural mass is tether-related, and this is a major component of the worksheet. One of the underlying assumptions that went into the development of both the SSP concepts and the model is the proposed use of a multi-stranded tether configuration called the Hoytether4. This braided space tether concept was developed by Dr. Robert P. Hoyt and Dr. Robert L. Forward of Tethers Unlimited, and is designed to prevent single point failures resulting from hits by micrometeoroids. It was also presumed that the Hoytether approach could be further enhanced through the use of HTSC materials in its construction, thus allowing the tether to be used as both a structural element and as a superconducting backbone for carrying power from the solar collectors to the power transmission subsystem Data characterizing such a system was supplied by Dr. James Powell5 as part of the Space Solar Power Technical Interchange Meeting that took place at NASA/LeRC in study Phase II. In operation, the model receives computed tether length information from the Solar Collection subsystem worksheet and computes tether mass in two ways - from the mass per unit length (kg/m) information obtained for the Hoyt tether and the specific mass value in kg/kW provided for the HTSC system It takes the larger of these two values as a conservative estimate, adds to it an additional percentage of the overall system mass to account for miscellaneous support structure, and computes a recurring cost for this subsystem using $ 100/kg as its basis. This mass and cost information for the Structure and Hamess subsystem is then combined with the corresponding estimates for the TCM/C&DH subsystem for inclusion in the Summary worksheet. 4.4. Estimating Manufacturing Requirements This section describes the methodology used to estimate the influence of manufacturing requirements on key SSP elements, issues and limitations, and an assessment of the results generated. The manufacturing requirements estimation methodology is represented on a Microsoft Excel worksheet that is attached to the SSP Space Segment Workbook and linked to any of the key SSP hardware elements. 4.4.1. Approach The selected approach for estimating manufacturing requirements is shown in the figure below. The starting point is the estimated cost of the 1st unit. This cost is then split into labor and materials using subsystem-specific total labor to total cost factors derived from the SAIC Life Cycle Cost Model 4 R. P. Hoyt and R. L. Forward, ‘Tailsafe Multistrand Tether SEDS Technology”, 4th International Conference on Tethers in Space, Washington, DC, April 1995. 5 J. R. Powell, “Application of High Temperature Superconductors to Solar Power Satellite Power Cabling - A Preliminary Evaluation”, Presented at Space Solar PowerTechnical Interchange Meeting, NASA/LeRC, Cleveland, OH, June 1996.
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