For a given value of sizing parameter and mission there exists an optimum I which maximizes the non-propulsive payload ratio. Figures 3.1-14 and 3.1-15 show, respectively, the optimum specific impulse and corresponding maximized payload fraction as a function of. Once the optimum I and maximized payload fraction are estab- lished, many of the important propulsion system characteristics can be determined. The propulsion system weights, total thrust requirement, and total source power requirement can be calculated, as shown below, on the basis of a given non-propulsive payload weight ( ): 3.1.4.2 SEPS Sized For Geosynchronous Delivery From 190 N Mi Following the sizing procedures outlined in Subsection 3.1.4.1, the characteristics of a representative ion propulsion system for the SSPS delivery mission can be determined. The mission parameters assumed are: The assumed mission delta-V corresponds to an ascension to geosynchronous orbit by continuous thrusting from an initial orbit altitude of approximately 190 n mi. A representative trip time of one year was selected to improve the sizing parameter while keeping within the bounds of thruster system capability for continuous propulsion. Durations of approximately 8000 and 35 00 hr have been demonstrated in ground and space tests, respectively. A three-year continuous-propulsion capability can readily be projected, at this time, for the SSPS time frame.
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