LEO-MEO Relay: In-Space Transportation The AV requirements for LEO-to-MEO transfers are of the order 2100 - 2800 m/sec, depending on the MEO altitude in the range 6500 - 10,000 km. A highly mass-efficient and affordable OTV is very important to this concept. Solar electric propulsion (SEP) operating at 4000 - 5000 sec specific impulse is an excellent candidate for this function, providing propellant mass fractions of only 4 to 7% compared to 38 to 47% for cryogenic chemical propulsion. Low-thrust SEP transfer time is about 3 months. As an example of payload capability, a 100 metric ton relay satellite can be transported to MEO by a lightweight SEP system (20 kg/kW specific mass) operating at 1.4 MW power with an initial mass in LEO of 140 metric tons. Figure 3-6 The LEO Sunsynchronous Sun Tower SSP Space Segment Concept (not modified for relay use) LEO-MEO Relay: Space Segment A description of the basic sun-synchronous Sun Tower concept is given in section 3.3 above. When used with MEO relay satellites, its design must be modified to provide a power transmission link to the relays. A physically steerable, be, rotating transmitter at LEO altitudes would be susceptible to micro-meteoroid damage and increased drag. Electronic beam steering would be used to avoid these issues. If ReflectArrays are not used, the Sun Tower would require a mm-wave (nominally 245 GHz) EM
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