Alternatives to Solar Arrays To reduce the ballistic coefficient, other more compact power generation methods can be considered. Potentially, solar dynamic generator (SDG) systems, involving Brayton. Stirling or Rankine cycles, could be suitable candidates. [10] The first advantage of SDG systems is that their higher efficiency would reduce their exposed area compared with a solar array, thus reducing the propellant load. According to some estimates [11] an SDG would enable about a one-third reduction in aerodynamic drag compared with a solar array. If correct, this smaller exposed area would effectively counteract the losses of beamed power. Another potential advantage not possible with solar arrays is the SDG’s ability to continuously supply power during eclipse periods due to the thermal and mechanical inertia built-up by the generator when in sunlight. Thus, SDGs could take the place of Freedom's batteries from a functional standpoint. Freedom would still, of course, require batteries in the event of a Powersat outage. However, the Powersat offers the potential to reduce the batteries’ cycling rates and. as a result, the rate at which they will have to be replaced. This demonstrates the advantages to the Powersat developers of finding a market niche that produces revenues as a result of annual recurring cost savings, as opposed to initial investment costs, as in the case of comsats. The major disadvantages of SDG systems are their higher development costs, more complex packag ing for launch requirements, and the fact that none have yet flown in space. The mechanical systems used for power generation may cause maintainability concerns. However, the potential of SDGs is clear for future space applications that require increasing power demands. The Japanese NASDA has proposed flying an experimental SDG system on the ETS bus and launching it on an H-2 booster (See Figure 4.2-5 and the Appendix). A
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