Cargo Orbital Transfer Vehicle (COTV) - The COTV will be utilized for transport of all SPS hardware, GEO bases and consumables, and GEO personnel consumables from LEO to GEO. Again, several candidates have been identified in two basic categories of independent power and dependent power and then in three categories of high thrust chemical propulsion, low thrust electrical propulsion, and combined chemical/ electrical propulsion. For SPS geosynchronous construction, independent power is required of the COTV and based on studies to date, conventional Op/H? high thrust propulsion is selected with a 2-1/2 stage option for vehicle sizing purposes. The range of COTV~ (subscript "G" denoting SPS GEO construction) is described as follows: For SPS LEO construction, dependent power vehicles utilizing electrical propulsion for most of the transfer from LEO to GEO have been selected. Conventional Op/H? chemical propulsion will be required for occultation periods or for transporting the payload plus electrical propulsion module to sunlit altitudes. The extent of this supplemental propulsion is not yet well defined, as departure altitude, data, inclination, desired launch window and thrust/mass ratio all interact to determine the minimum sunlit "handover" altitudes. One SPS concept, the planar photovoltaic array design, may be separated into 30 modules, each of which can provide electrical power to the electrical propulsion stages to avoid excessive structural loads on the array modules, for propulsion units are assumed to be mounted to the corners of each module. Studies of the relative merits of the propulsion options have not indicated a clear preference. Therefore, for vehicle sizing purposes for this scenario, an expendable MPD thermal arcjet propulsion unit with reusable chemical propulsion to 3500 km altitude is assumed for the COTV. (subscript "L" denoting SPS LEO construction). The range of COTV^ isL given as follows:
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