been made is that the entire structure exists at the beginning of the orbit decay analysis time frame. An orbit-keeping module has been sized to maintain the SSPS at 190 n mi altitude. It represents a small version of the ion propulsion system proposed to transport the SSPS to geosynchronous orbit, and as such, has been sized using the procedure discussed in Subsection 3.1.4. Power from the stationkeeping module ion engine is derived from solar photons impinging on solar cells. As the SSPS circles the earth in the 190 n mi assembly orbit, it will be in the earth's shadow approximately 40% of the time. Since the power source for the stationkeeping module will be inoperative during the shadow traverse, the force from the engine will drop off and the SSPS orbit will decay slightly. To compensate for this effect, the thrust required from the stationkeeping module has been increased from 11 to 16 lb. Characteristics of the stationkeeping module, which was sized to keep the SSPS at 190 n mi altitude under nominal air drag conditions, are as follows: 3.1.4 SEPS (Ion Engine) Sizing 3.1.4.1 Sizing Procedure The factors affecting ion system size and a sizing procedure flow logic are depicted in Fig. 3.1.13. Maximizing payload ratio ( ) is the fundamental goal in sizing the ion propulsion. Unlike chemical propulsion, this is not achieved with maximum specific impulse ( ). The reduced propellant weight requirement with associated high I must be traded against the increase in weight of the power supply required to achieve it. The factors affecting that trade are:
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