plus interest charges, development costs, and smaller additive terms for the ground antenna and the land it would occupy. Of the four critical factors, the most serious is the lift cost, $/kgsy, and it causes problems with two others, $/kw and kg/kw, the cost and mass requirements for the satellite. In an earth-launched system, because the lift cost would be high, we would be forced to make the powerplant mass, kg/kw, as low as possible. In a turbogenerator system this would require raising the turbine operating temperature to a point well beyond present-day practice.Alternatively, we could use solar cells (photovoltaic converters). There too, big improvements both in mass and in cost would have to be achieved in order to obtain an economically reasonable system. Glaser1 has estimated that for an earth-launched SSPS the specific mass for solar panel arrays will have to be reduced to about 0.88 kg/kw. For comparison, the value for photovoltaic solar cell arrays in operational satellites of the last decade has ranged from 78 to 107 kg/kw; one experimental g satellite designed as a short-life test vehicle achieved 29 kg/kw. One way to assess the likelihood of achieving very low mass for long-lifetime solar cells is to see what NASA is planning on for satellites to which it is already committed. For the Solar Electric Propulsion System space probe q scheduled to fly in 1984, the specific mass is intended to be 13 kg/kw, which is still about 15 times too heavy to satisfy the
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