continuously operating propulsion system for station-keeping with respect to Freedom. • Controlling the positioning of the Powersat with respect to Freedom will be a complex operational problem. Understanding the Drag Problem If the Powersat uses solar arrays to generate power, then these arrays will need to be significantly larger than those that would be required to augment Freedom directly due to efficiency losses of the microwave generator, and the transmission and reception process. As a result, the ballistic coefficient (proportional to the exposed Area/Mass) of the Powersat will be much larger than the increase in drag coefficient of Freedom with the 75 kW solar array capability. Therefore, the effects of atmospheric drag on the Powersat would be much more pronounced, requiring practically continuous station-keeping manoeuvres to keep up with Freedom and stay within power-beaming range. How can there be a net savings in the total amount of propellant mass used by Freedom and the Powersat, especially if the size of the Powersat’s solar arrays are three times the size of those otherwise needed to augment Freedom? Adding another pair of solar arrays to Freedom would approximately double its ballistic coefficient, leading to a doubling of the drag and, very approximately, a doubling in the propellant requirements. Critically, however, the Powersat w ould be as much 15-20 times less massive than Freedom, i.e. 15-20 tonnes for the Powersat versus 300 tonnes for Freedom. As a result, the Powersat’s propellant needs would be somew'hat less than the increase Freedom would otherwise need, even though its drag is far higher. Although the Powersat’s solar array size would be three times that for an equivalent Freedom capability, the specific amount of propellant mass needed for station-keeping would be less because the Powersat is so much lighter. How much the propellant could be reduced would need to be the subject of detailed analysis. (Figure 4.2-4)
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