per MWe of thruster input power for a generally similar thermionic converter vehicle with the following characteristics: o Payload: 500,000 lb. (226,757 KG) o Tug lifetime: 97.2 months o LEO to GEO transfer time with payload: 123 days minimum o Specific impulse: 2040 sec. o MPD thruster efficiency = 50% Extrapolating from this data, the tug of this section is estimated to have a recurring, hardware cost of $400M and a cost per flight (excluding refurbishment and assuming 20 months of useful, Earth orbit transfer lifetime) of $400M/5 trips or $80M/flight. 1.4.3.2 MPD-ARCJET DEPENDENT COTV Recent studies within industry and NASA suggest that the MPD-arcjet design becomes a leading contender for the low thrust COTV application if the upper range of MPD thruster efficiency, jet velocity, and operational life can be achieved. Boeing FSTSA studies have defined the system presented in this section. This system, considered broadly representative of the more optimistic MPD designs, derives its basic electrical power from an external source, and is based on assumptions shown in Table VI-D-1 -10. Important factors leading to this design include the following: EFFICIENCY Predictions of attainable MPD-arcjet thruster efficiencies vary from 10% in the earlier literature to current estimates up to 80%. The resulting MPD system assessment for the COTV function thus ranges from worthless to possibly the best of all choices. JPL assumes a constant efficiency of between 40 to 50%, whereas Boeing considers the efficiency variable with jet velocity, from about 35% to 70%. The MPD-arcjet COTV of this section incorporates the latter assumption and thus may achieve an improved thruster compared to the JPL concept and a reduction in required SPS power at any particular specific impulse design point. SPECIFIC IMPULSE Early estimates of the optimum specific impulse (or jet velocity) for MPD-arcjet thruster operation in the dependent COTV mode were in the 2000 to 2500 second range. Based on the assumptions of Table VI-D-1-10 and the factors of this section, however, an optimum specific impulse of between 6100 and 8200 seconds (60 and 80 KM/sec) is indicated. This is presented in Table VI-D-1 -11 and Figure VI-D-1-16. Note that for the constant efficiency thruster assumption, a much lower specific impulse and jet velocity of about 4100 seconds and 40 KM/second, respectively, is indicated.
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