which showed that, at peak SPS production rate (seven/yr, scenario B) on the 9 order of 12 x 10 metric ton-km/yr of transport would be required. This 12 compares with a total U.S. transportation projection of 5 x 10 metric ton-km/yr in 2000. Therefore, the maximum SPS impact would be less than 1 percent of the year 2000 total requirement. Warehousing and handling facilities at intermediate and end points should be considered in future studies. E. Energy Payback A preliminary study was conducted to assess the energy required for the implementation of an SPS and the time required for the SPS to generate an equivalent amount of energy. Table IX-2 is a list of the energy- intensive materials required for the implementation of an SPS of "nominal" weight and the energy involved in its production. To process the material for an SPS, 6.7 x 10^ kWh are required. Approximately 80 percent of the total is involved in the production of the necessary propellants and aluminum for the rectenna. Previous comments regarding the use of aluminum for the support structure of the rectenna should be noted. The SPS produces 8.1 x 10^ kWh/yr; consequently, the 6.7 x 10^ kWh used in implementation of the SPS would be generated or "paid back" in less than 10 months. This figure compares favorably with estimates for conventional ground systems, which range from 0.2 to 1.0 year for payback.
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