adaptation, and performance achievement. In addition, it presents the possibility of utilizing the highest jet velocities (specific impulses) of any system and may provide a much longer operating life. Therefore, it is suggested that additional study is needed before the ion/argon device is discarded as a dependent SPS propulsion system. Specific issues recommended for consideration are given in the next section, Technology Issues. 1.4.4 TECHNOLOGY ISSUES The following issues must be resolved before a high performance electric propulsion system concept can be selected and operational hardware development initiated. 1.4.4.1 MPD THRUSTER BASIC FEASIBILITY Resolution of the basic feasibility and gross performance of the MPD thruster requires laboratory research including development, test, and operation of a variety of laboratory thruster hardware configurations. Detailed definition and establishment of a suitable research program is indicated as a next step if the MPD device is to receive additional consideration for use in the SPS orbital transfer operation. 1.4.4.2 ION PROPULSION SYSTEM OPTIMIZATION As discussed in section 1.4.3.3, selection of ion thrusters for the SPS orbit transfer function is dependent not so much upon basic thruster feasibility as upon thruster system performance and cost optimization and upon SPS design in enhancement of the thruster system. The following considerations are particularly significant: o Ion thruster and SPS power source design to minimize power conditioning requirements and hardware complexity, weight, and cost. o Applicability and achievement of very high jet velocities. o Increased engine thrust level and thrust to weight ratio. o Validation of expected efficiencies, shown as up to 85% for the 100 cm size thrusters in Table VI-D-1-7. o Define expected engine life and refurbishment requirements. This effort is expected to be a study by knowledgeable personnel and supported by a limited amount of laboratory testing. 1.4.4.3 MPD PROPULSION SYSTEM OPTIMIZATION After demonstration of MPD thruster basic feasibility as described in section 1.4.4.1, a system optimization can be accomplished. This will be based upon the findings of the basic research laboratory testing.
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