difficult to manufacture in space. The total mass of this component is so low that no attempt was made to substitute lunar materials. WBS 1.1.2.6 MPTS Maintenance The reference MPTS maintenance system is heavily oriented to manned operation. Technology being developed for the manned space station suggests that many of these operations will be automated by the time an SPS could be functional. The impact of this change has not been estimated for the maintenance system, but it seems likely that the reduced need for life support systems and structures would result in a simpler and less massive system. Because of the uncertainties, the estimated mass of this subsystem was left unchanged. WBS 1.1.2.6.1 Gantries This component is mostly aluminum structure. It also includes motors and related control systems, which are assumed to be non-lunar. WBS 1.1.2.6.7 Tracks These aluminum tracks support the gantry. WBS 1.1.2.7 Antenna Pointing The most massive item in this system is the Control Moment Gyros (CMG), which are essentially spinning blocks of aluminum or iron. It is assumed that at least 90% of the CMG mass can be easily replaced with lunar material. The star scanner has low mass and high complexity, so no attempt was made to use lunar material in it. WBS 1.1.3 Info Management The major mass component is cabling. As described in the introduction to this section, cables are assumed to be 100% lunar. The reference design includes five tons of computers, but this estimate probably includes much more than computers. WBS 1.1.4.2 Electric Propulsion The most massive component of the electric propulsion system is power processing and associated thermal control. These components are assumed to be similar to those on the MPTS. The rest of the electric propulsion system includes electric thrusters and fuel systems which are assumed to be made on Earth. The reference design uses argon propellant in ion thrusters. The resupply of 50 tons of argon annually would more than triple the non-lunar requirements of the SPS over a 30-year lifetime. It is assumed here that some lunar material, perhaps aluminum, calcium(3), or oxygen, will be used as propellant. The effect of this propellant change on the rest of, the system is beyond the scope of this study. The propulsion system and fuel requirements used here are for the silicon planar configuration, which flies perpendicular to the orbital plane. All other power conversion systems must fly perpendicular to the ecliptic plane, which would significantly increase the fuel requirements. WBS 1.1.4.3 Chemical Propulsion The chemical propulsion system includes 20 tons of maneuvering reserve fuel
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