To date the thermal model is not as highly developed as the photovoltaic model but has been used for preliminary analysis in determining the optimum number of concentrator modules. The development of these models began in-house in April 1976 and modifications and improvements have continued since. As promising ideas appear they are incorporated into the models so that their overall impact on the SPS can be evaluated. Current plans are to continue in this activity so that better SPS designs can evolve. 6.2.2 TRANSPORTATION MODEL The transportation model computes flight rates, inventories, and engine and vehicle buy dates per year for the support vehicles of a given SPS with a designated assembly location. If the SPS is assembled in low Earth orbit (LEO), the COTV only transfers the logistics (attitude control propellant and spare parts) needed to resupply the SPS's that are on-line. If the SPS is assembled in geosynchronous Earth orbit (GEO), both logistics and construction materials must be transferred. The model computes the COTV construction and logistics flights per year based on the quantity of material to be transferred and the payload of the vehicle. This flight rate combined with the propellant per flight, engine life, time to change engine, checkout time, and trip time yields the propellant, engine buys, and inventory required per year to support the transfer of cargo from LEO to GEO. The POTV flights per year, engine life, and propellant per flight are inputs to the model and yield POTV engine buys and propellant required per year to support the transfer of personnel from LEO to GEO. The HLLV flights can be divided into five categories, i. e., construction, logistics, orbital transfer vehicle (OTV) engine, OTV propellant, and OTV delivery flights. Construction and logistics flights per year are based on the HLLV payload and the construction or logistics material required. The OTV engine flights per year are based on the COTV and POTV engine buys for that year along with the mass of each and the HLLV payload. The OTV propellant flights per year are based on the number of COTV and POTV flights for that year and the propellant required for each along with the HLLV payload. The OTV delivery flights per year are based on the required COTV and POTV inventories for that year along with the engine and stage weights for each and the HLLV payload. The HLLV inventory is based on the total flights per year and the average turnaround time between flights. Vehicles expire after 10 years or 500 flights, whichever comes first, and buy dates are computed on this basis.
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