Table V. Energy storage technologies and masses for 5 GW SPS. S-glass flywheel storage was preferred due to its low non-lunar mass. The energy density of the S-glass flywheel was estimated from the best performance to date of a steel flywheel, which was 3.125 W-hrs/kg [7]. The storage capacity of a flywheel is proportional to the working stress of the material divided by its density. Steel has a working stress of 124 MPa and a density of 7.8; S-glass has a working stress of 1379 MPa and a density of 2.4, so its capacity should be better by a factor of about 36. Not all the system's mass is in the flywheel, however, so improvement by a factor of 18 was assumed. All SPS studies to date have used a slipring-and-brush assembly as the electrical rotary joint. An aluminium slipring with a thin coin silver cladding, like that of the Rockwell design [7], was selected. This design minimizes non-lunar mass, yet provides low contact losses and low friction. Microwave Transmitter The microwave power transmission system (MPTS) converts electrical power to microwave radiation and transmits this radiation as a coherent beam which must be kept focused on a receiving antenna (rectenna) on Earth. Electrical power is converted to microwaves by RF amplifiers. Designs based on the klystron and magnetron amplifier concepts were evaluated. Solid-state amplifiers are unsuitable for a lunar SPS because their low efficiency would greatly increase the non-lunar coolant mass. Masses and projected efficiencies of the magnetron and klystron are listed in Table 6. The magnetron design uses more non-lunar material but is more efficient, reducing the size of the power conversion system. Thus the magnetron is preferred if the power conversion system has high non-lunar mass, and the klystron is preferred if the power conversion system contains little non-lunar mass. The klystron is preferred for both the silicon planar and the GaAs concentrator systems. Table VI. Comparison of klystron and magnetron.
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