7.1. 5. 3 ORBITAL TRANSFER PROPULSION Assuming the SPS is assembled in low Earth orbit (435 kip) with subsequent transfer to its geosynchronous orbit, an integrated SPS propulsion system will be required to perform the transfer as well as maintain attitude control at all times. The SPS propulsion system will also have to provide orbital drag makeup during assembly in the low orbit as well as correct for orbit distortion because of solar pressure. Geosynchronous station keeping propulsion requirements include counteracting the effects of solar pressure, Sun and Moon induced inclination changes, Earth ellipticity induced drifts, and possibly microwave recoil. Attitude control propulsion requirements include counteracting the effects of gravity gradient torques (in both the LEO and the GEO) and, possibly, microwave recoil and antenna angular accelerations. This subsection is devoted primarily to discussing the propulsion requirements for orbit transfer. The propulsion requirements for attitude control are discussed in subsection 7.1. 6.2. However, the orbit transfer propulsion system(s) can be used (at a reduced allocation) to aid in station keeping and controlling the attitude of the SPS. The major portion of the cost of the SPS propulsion system involves transporting the propellant required for the LEO to GEO transfer from the Earth's surface to LEO. To minimize this amount of propellant, and subsequently the cost, the SPS propulsion system must utilize an engine capable of producing a specific impulse much greater than conventional chemical engines. The high specific impulse achievable with electric propulsion is attractive, particularly when considering the availability of a large amount of onboard power. In designing a propulsion system to transfer a photovoltaic SPS from the assembly LEO to the operational GEO, numerous design variables must be considered, such as follows: 1. A low SPS acceleration level will be required (< 0. 001 g). 2. Continuous tangential thrusting to maximize propulsion use efficiency is desirable. 3. Plume impingement avoidance seems necessaiy. 4. Earth occultation, if unavoidable, will allow electric propulsion only during the sunlit portion of each orbit, thereby requiring on-off operation of electric thrusters with subsequent thermal cycling; the effective shadow time is increased by any thruster start delays.
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