The energy density of the S-glass flywheel was estimated from the current best performance of a steel flywheel, which is 3.125 Whrs/kg.(5) 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 of the system’s mass is in the flywheel, however, so improvement by a factor of 18 was assumed. The estimated non-lunar composition in the flywheel is probably quite conservative. 4.5 ELECTRICAL ROTARY JOINT All SPS studies to date have used a slipring-and-brush assembly as the electrical rotary joint. In the reference design (2, vol 2, p. 76), a slipring carrying half the load of a 10 GW SPS consisted of 11,810 kg. of coin silver.(90% silver, 10% copper) The brush assemblies (aluminum, spring steel, and brushes) total 1970 kg.; the brushes are 85% silver, 12% molybdenum disulfide, and 3% graphite. The slipring’s cross-section is a hollow hexagon; each of the two contact surfaces is a 1.0 cm x 50 cm bar of coin silver.(2, vol. 3, Fig. 4.6-9) There are 832 brushes and three concentric sliprings in the reference design. The current density in this design is about 10 A/cm*2. The power loss for a 5 GW SPS with 40 kV power distribution is 40 kW. (6, p. 104) The use of silver molybdenum sulfide brushes on silver was chosen by Boeing to minimize abrasion, drag, and electrical losses. In that study, copper and aluminum were considered as slipring materials - both were less massive and less expensive - but were rejected due to limited space experience with them. Selection of new slipring/brush material combinations is not within the scope of this study, so the same materials were used at all sliding contacts. New materials, such as copper or aluminum sliprings, should be reconsidered when more space experience with them has accumulated. Three options were considered to decrease the use of Earth material in the electrical slipring. The first was to use a thin plating of coin silver on an alnminum slipring, as in the Rockwell design. (3, vol. 7, Table 3.1-13) The second was to eliminate the sliding contact entirely by using conductive vapor in a sealed enclosure. This option was rejected because of insufficient data on rotary seals in a high-voltage space environment. This Gytion should be reviewed when more space experience with rotary seals has accumulated. The third option was to replace the sliding contact with beams of electromagnetic radiation, such as microwaves or light. This option was rejected because its low efficiency (< 90%) would significantly increase the mass of the power collection and conversion systems, and because it would require considerable non-lunar mass to build and cool the electronics. The option of using a thin coin silver cladding on aluminum was selected. The design uses a 1 mm layer of coin silver on a 1.6 cm thick bar of
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