The greatest concerns are with the operational problems associated with two spacecraft that must orbit within 5-20 km of each other over many years. This will almost certainly require the use of an ion propulsion system to overcome the high system drag. Chemical propulsion systems for any Powersat are mandatory to ensure simultaneous reboosting when necessary. A combination of both SDGs and ion propulsion systems would seem the best method for reducing the station-keeping propellant needs even more and. potentially, the rate of battery replacement. Although neither of these technologies are currently spaceflight proven, much ground development work is being conducted. 4.2.2 Option 2: The High Orbit Laser Solution Overall Configuration The success of Powersats in space station operations depends on the ability to increase the station’s power supply while simultaneously and significantly reducing the annual propellant and battery upload to the station. As an alternative to the co-orbiting Microwave Solution, another possibility involves placing three Powersats in a much higher Earth orbit, around 7,500 km above the Earth, (Figure 4.2-6) allowing the Powersat to effectively replace the function of the Sun when Freedom is in eclipse. Thus, rather than half of Freedom’s solar arrays being used to continuously charge the batteries, all of this power could be dedicated to use. When in eclipse, Freedom would be powered entirely by the Powersat fleet The 7,500 km orbit is the minimum altitude which can be used to ensure nearly total coverage during Freedom’s eclipse period. It is also synchronised so that one orbit of Freedom is equivalent to one-third of an orbit of a Powersat. Thus, as soon as Freedom crosses the terminator into shadow
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