would increase the orbital period and then restore it to nominal over the same elapsed time. A summary of the flight control requirements is presented in Table 1. TABLE 1 FLIGHT CONTROL REQUIREMENTS Gravity Gradient 1.67x10 9 newtons m sec Solar Pressure Ty = 5500 newton-m Tx = 136 newton-m Electromagnetic Field Interactions < 10"5 newton-m Rotary Joint Friction 216 newton-m Microwave Transmission Recoil Pressure 11 newtons Aerodynamic 22x10~6 newton RCS (Ion Thruster, ISP 8000 sec) Propellant Weight to Control SSPS to within 1 Deg 43.6 kg/day 15,500 kg/year An SSPS in synchronous equatorial orbit would pass through the Earth's shadow around the time of equinoxes, at which time it would be eclipsed for a maximum of 72 minutes a day (near midnight at the SSPS longitude). This orbit provides a 6- to 15-fold conversion advantage over solar-energy conversion on Earth. A comparison of the maximum allowable costs of photovoltaic energy-conversion devices indicates that for a terrestrial solar-power application these devices are competitive with other energy-conversion methods if they cost about $2.30 per square meter. Because of the favorable conditions for energy conversion that exist in space, these devices are competitive if they cost about $45 per square meter in an Earth-orbit application (11). Solar Energy Conversion. — The photovoltaic conversion of solar energy into electricity is ideally suited to the purposes of an SSPS. In contrast to any process based on thermodynamic energy conversion, there are no moving parts, fluid does not circulate, no material is consumed, and a photovoltaic solar cell can operate for long periods without maintenance. There has been a substantial development in photovoltaic energy conversion since the first laboratory demonstration of the silicon solar cell in 1953. Today, such cells are a necessary part of the power supply system of nearly every unmannea spacecraft, and considerable experience has been accumulated to achieve long-term and reliable operations under the conditions existing in space. Thus, the “Skylab” spacecraft relies on silicon solar cells to provide about 25 kW of power. As a result of many years of operational experience, a substantial technological base exists on which further developments can be based (12). These developments are directed towards increasing the efficiency of solar cells, reducing their weight and cost, and maintaining their operation over extended periods.
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