ION PROPULSION SYSTEM TECHNOLOGY Characteristic Current Value SSPS Value Overall specific weight, lb m/kW power system thruster system Overall system efficiency Propulsion Time (thruster life), hr Specific impulse, sec Thruster design diameter, cm tnrust, lb The ion propulsion system for a space tug can be designed so it will interface with payloads delivered to LEO by the space shuttle. The payloads would be transferred to a space tug which, over a period of 6 to 12 months, would follow a spiral trajectory to synchronous orbit. The development of a light-weight solar collector array for the SSPS can also provide power for the ion propulsion stage. During transit, between LEO and synchronous orbit, the space plasma and Van Allen belt radiation environment must be traversed. An ion stage system with a constantthrust spiral trajectory would spend about 100 days in the Van Allen belt. Exposure to both these environments would cause some degradation in the solar cell array power output. In the plasma, the degradation would be temporary but in the radiation belt it would be cumulative and permanent. Deployment of the ion stage to an altitude of about 300 miles obviates the effect of the space plasma environments. There are two alternatives to minimize these effects: (1) trajectory shaping to minimize the time spent in the Van Allen radiation belt, and (2) the use of auxiliary propulsion fast-trip sources (such as chemical stages). Cost projections were previously developed for a number of candidate propulsion stages applicable for transportation from Earth-to-LEO and LEO-to- geosynchronous orbit. Preliminary flight profiles and propulsion vehicle combinations have been investigated to expose the type of environmental and gross operational factors that must be considered in evolving a space transportation system for an SSPS. It is evident that both orbital assembly considerations and alternative transportation modes are involved in the definition and development of a low-cost, high-volume, Earth-to-synchronous orbit advanced space transportation system. This definition
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