Table 1. EOTV Sizing Assumptions Table 2. EOTV Solar Array Weight Summary An all-electric thruster system was selected for attitude control during occultation periods to minimize propellant weight requirements (Figure 1). The power storage system was sized to accommodate maximum gravity gradient torques and occultation periods. An excess of thrusters is included in each array to provide for potential failures, to permit higher thrust from active arrays when thrusting is limited or precluded from a specific array due to potential thruster exhaust impingement on the solar array, and to provide thrust differential as required for thrust vector/attitude control. Having established the solar array operating voltage, the maximum thruster screen grid voltage is established, which in turn fixes propellant ion specific impulse. To assure adequate grid life for a minimum round-trip capability of approximately 4000 hours, a maximum beam current of 1000 amp/m^ was selected. Based on the available power and a desire to maintain reasonable thruster size, the remaining thruster parameters are established. A rectangular thruster configuration (1 m x 1.5 m) is assumed. Primary thruster characteristics are summarized in Table 3. Conventional power conditioners for ion bombardment thrusters regulate all supplies, serving as an interface between the power source (solar array) and the thrusters. Various direct-drive concepts have been proposed in which the primary (beam power)-thruster supply is obtained directly from the solar array power bus. This approach reduces power conditioner mass, power loss, and cost and improves system reliability. Solar cell temperature, efficiency, and output voltage variations will cause acceptable transients in beam voltage during thruster operation. Based on the individual thruster power requirements and the available array power, 100 thrusters may be operated simultaneously. An additional 20 thrusters are added to provide the required thrust margin. The thrusters are arranged in 4 arrays of 30 thrusters each. The thruster array mass summary is presented in Table 4. The EOTV performance is based on a 120-day trip time from LEO to GEO (obtained from trade studies). Knowing the propellant consumption rate of the thrusters and the thrusting time, the maximum propellant which can be consumed is determined, which in turn defines the payload capability. The vehicle also is sized to provide for the return to LEO of 10% of the LEO-to-GEO payload. The EOTV weight summary is presented in Table 5. Since the EOTV solar array utilizes the same configuration, materials, and manufacturing processes as the satellite, common technology requirements are evident. The unique technology requirement is in the primary area of ion engine development. The key requirement is in large size (1.0 m x 1.5 m) high current density (1000 amp/m?) thruster demonstration.
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