Launch Vehicles An advanced, chemically propelled "space tug" could bring from low earth orbit to geosynchronous orbit, as payload, about one-third of the total payload delivered to low earth orbit from the earth's surface. When the cost of space-tug operations is included, the cost of transport ($/kgsy) from the earth to geosynchronous orbit can then be taken as roughly four times the cost of transport to low earth orbit. For simplicity, lift cost figures in the following discussion will refer to the over-all transport from the surface of the earth to geosynchronous orbit ($/kgsy) and will be taken as four times the cost to low orbit. An additional uncertainty of about - 30 percent is a consequence of this simplification. The performance target figure10 for the space shuttle is $1400/kgsy, not including development costs of several billion dollars. A heavy-lift launch vehicle (70-ton payload) using the same kind of engines that are already being developed for the shuttle, and therefore obtainable without large additional expense for development, is estimated to be capable11 of achieving $600/kgsy to $1000/kgsy. To summarize, for an economically viable earth-launched photovoltaic SSPS the specific mass (kg/kw) must be reduced by about a factor of 30 to 60 below the corresponding figure for satellites of the 1970's; the lift cost to geosynchronous orbit must be reduced by about a factor of 4 below figures estimated to be attainable in the 1980's without large additional development costs; the capital cost ($/kw) must be reduced by a factor of about 30.
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