f. Power Output Levels Bounds can be placed on the range of potential SSPS power levels as shown in Figure 6. The SSPS design can be adjusted to provide from 4 to 40 mW/cm2 of rectified power at the receiving antenna. This range of power level postulates a receiving antenna diameter of 10 to 20 km and a transmitting antenna diameter of 1 to 2 km. An idealized Gaussian distribution was chosen to establish the transmitting and receiving antenna diameters. There is an additional cutoff established by the inability of the transmitting antenna's passive thermal control system to reject the waste heat of the microwave generator when the microwave power density of the transmitting antenna rises above 4.13 W/cm2. Thus, in principle, an SSPS could be designed to generate electrical power on Earth at power outputs ranging from about 2,000 to 20,000 MW. It is likely that a narrower range of power output, ranging from 3,000 to 15,000 MW will be more effective. A nominal power output level of 5000 MW at the receiving antenna falls about in the middle of the range of interest represented by the shadowed area in Figure 6, and therefore it was chosen as representative for the SSPS baseline design. The overall capacity of the future transmission grid system will place an upper boundry on the SSPS power output to allow for the possibility that one SSPS has to be taken out of service. SSPS Flight Control Although the SSPS is orders-of-magnitude larger than any spacecraft yet designed, its overall design is based on present principles of technology. Thus, its construction and the attainment of a 30-year operating life require not new technology, but substantial advances in the state of the art. The SSPS structure is composed of high-current-carrying structural elements whose electromagnetic interactions will induce loads or forces into the structure. Current stabilization and control techniques are capable of meeting the requirements of spacecraft now under development. Most of these spacecraft have comparatively rigid structures and the amenable to control as a single entity by reaction jets or momentum storage devices. But the large size of an SSPS suggests that new structural and control system design approaches may be needed to satisfy orientation requirements. This study, however, indicates that present analytical techniques/tools are adequate and that an SSPS can be controlled to better than ± 1 degree. Low-thrust, ion propulsion systems appear promising for SSPS control, because their performance characteristics are compatible with the potential lifetime required of the SSPS. Earth-to-Orbit Transportation A high-volume, two-stage transportation system will be required for an SSPS: (1) a low-cost stage capable of carrying high-volume pay loads to low-Earth orbit (LEO); and (2) a high-performance stage capable of delivering partially assembled elements to synchronous or some intermediate orbit altitude for final assembly and deployment. The factors affecting flight mode selection include payload element size, payload assembly techniques, desirable orbit locations for assembly, time constraints, and requirements for man's participation in the assembly. The choice of transportation
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