and mass characteristics. The data of Table VI-D-1-9, based on the assumptions of Table VI-D-1-IQ are therefore considered as representative as is available at this time. The mass breakdown is as follows: Total vehicle and payload mass characteristics as a function of switchover altitude are shown in Figure VI-D-1-17. OPERATIONS The COTV is not reused after SPS orbital transfer. Consequently, COTV operations are limited to standard launch to LEO, installation of COTV modules on the SPS structure, and the LEO to GEO transfer flight. During the transfer mission, the COTV operates integrally with the SPS and depends primarily upon SPS ground support capacities and facilities, supplemented as required for specialized COTV functions. COSTS Recurring cost estimates are shown in Table VI-D-1-11 and summarized on Figure VI-D-1 -16. R&D costs to achieve a satisfactory and dependable thruster are unknown but are expected to be relatively high. 1.4.3.3 ION DEPENDENT COTV Assuming the more optimistic MPD thruster performance predictions of Table VI-D-1-7, ion/argon propulsion systems appear to be less desirable than MPD/argon systems for the SPS dependent orbit transfer operation. This may be seen by comparing Boeing's ion/argon system of Table VI-D-1-11 and Figure VI-D-1-18 with their MPD systems. Note, however, that Boeing assumes a relatively pessimistic ion system with a specific impulse of only 5,100 seconds, an efficiency of only 57%, and power pro- cessiong hardware five times heavier than for the MPD system. Relatively small shifts in either ion or MPD assumptions can have a significant impact on the relative merits of these systems. The ion system, based on extrapolation of actual hardware experience and a much better theoretical understanding, generates a considerably higher confidence level in ultimate feasibility, design
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