The ion thruster is a demonstrated technology for low power systems (kilowatts), and extrapolations can easily be made from this base (30 cm thruster) to a more appropriate ion thruster (100 cm) for SPS cargo orbital transfer. The ion system, in addition to being state-of-art, has the advantages of high efficiency, long life, and wide operating range. Power processing for the ion thruster, however, makes this system expensive and heavy per kilowatt input to the thruster. Future analyses can profitably be applied to methods of returning the ion thruster assemblies back to LEO to lower operating costs through reuse of these long life and relatively expensive components. Argon is used as the propellant for this study. Magnetoplasmadynamic thruster concepts are relatively speculative. A significant amount of additional technology work is needed to specify configurations, performance characteristics, and costs. Conceptually, however, the MPD is a relatively simple device with low power processing requirements and, consequently, low hardware costs. An additional advantage of the MPD thruster is that the thrust density (power density) is significantly greater than that of the ion engine; therefore, fewer thrusters are required to perform the SPS orbital transfer to GEO. Argon is the propellant assumed here. Resistojets are basically the electrical resistance parallel of the nuclear (solid core) engine, NERVA. The performance characteristics of such an engine are similar, i. e., specific impulse of approximately 800 s. Resistojets are essentially state-of-art, but are at significantly lower power levels than will be required for the SPS application. In addition to the relatively low specific impulse (in comparison the ion and MPD, specific impulse is in the 2 000 to 20 000 s range), the propellant is liquid hydrogen and long term (several months) storage of LH2 is substantially more difficult than storing argon. From the candidate electric thruster discussed and a relative transportation performance/cost comparison shown in Figure 12-13, the MPD thruster warrants increased technology attention. The relative performance/ cost comparisons are based upon total costs from Earth surface to GEO. The results of the HLLV study indicate $20 per pound is easily achieved with the fully reusable system; therefore, this is used for the Earth launch cost. The performance of the ion system is indicated by the launch cost contribution, and the ion system cost (expendable) pushes it to the highest value of the three options evaluated. The MPD and resistojet have relatively low cost hardware and, even though the launch costs predominate (especially for the resistojet), the totals are less.
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