Figure 5. Thruster Support for Bicycle Wheel SPS. 9. Comparison of Old and New Reference Designs For the bicycle wheel SPS, the total of the rim plus fiber mass for the most massive case is about 28 kg. Since the hub weighs 600 kg and the struts that support the thrusters weigh 1000 kg, the rim plus fiber mass is negligible. Total mass is therefore approximately equal to the array mass plus 1.6 metric tonnes for a terrestrial or lunar bicycle wheel whose diameter is approximately 2 km. For an inflatable sphere, the mass of the inflation gas (even enough for 30 years) is negligible compared to the array. The 10 GHz designs from Table 2 can be viewed as new reference designs and compared to the NASA/US DOE reference designs1. This comparison is presented in Table 4. Note that the figure of merit shown here is specific mass (kg/kW), rather than specific power (kW/kg), since the former more easily lends itself toward the discussion of global energy requirements in Section 10. By examining the specific masses in Table 4, it is seen that thin-film SPS’s allow for a substantial improvement over conventional designs. The best case scenario is the terrestrial bicycle wheel (without mirror), which is better than the NASA/US DOE Type 1 by a factor of more than 20 (i.e., a 95% savings in mass per unit of power delivered). For the terrestrial bicycle wheel with mirror, this factor is only 15, but, as stated earlier, this may be an acceptable price to pay for a more steady supply of power. The inflatable sphere
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