d. Transportat ion A ballistic single stage to low earth orbit with a payload of 500,000 Ibm (225,000 kg) is assumed for this stage of the transportation cycle. Orbit transfer is to be self-powered because the satellite modules themselves are the most efficient available mode for moving large masses to geosynchronous orbit. In view of the relatively long trip times (1-3 months), a chemical orbit transfer vehicle employing LO^/LH^ propellants and advanced space engines is needed for personnel and priority cargo. e. Assembly Low orbit assembly is seen to provide simpler logistics for material and personnel and a relatively low drag penalty. It imposes no penalty on the design of any of the concepts considered and together with self-powered transfer results in minimum mass transport to low orbit (i.e., minimum orbit burden factor). f. Maintenance As with the solar photovoltaic system, the design criterial, choices of materials, component life, and expected operating conditions will determine reliability and hence the needed maintenance. It need only be added here that the Brayton cycle turbogenerators involve moving parts and hence present a higher risk than the more passive systems based on thermionic diodes or solar cells. There is also some concern that the mirror surfaces will degrade due to radiation although tests to date have been inconclusive. g. Cost The four alternative systems costs are compared in Exhibit 11. Total program costs range from 1.8 to 3.5 trillion dollars for 62 satellites placed on orbit by the year 2011. The first operational unit would be placed on orbit in 1991. The first unit costs of the satellites range from 22 to 53 billion dollars.
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