7. Reaction mass for the mass driver reaction engine is provided by space shuttle external tanks, which are brought into orbit in addition to the normal shuttle payload. Each tank has mass 35 t, since from SP-428, 30 external tanks have mass 1050 t [15]. The tanks are powdered by grinding equipment with mass 25 t [16], 8. Large mass driver reaction engines are built by the space manufacturing facility when it reaches 3x10^ t/yr. throughput to transport the solar power satellites from the space manufacturing facility to geosynchronous orbit, and use waste from manufacturing for reaction mass. The design is assumed here similar enough to that of the mass driver reaction engine that no additional research cost is required, and it is also assumed that both have the same exhaust velocities. The large mass driver reaction engines have a mass empty of 1600 t, and consume 1700 t/mo. of reaction mass [17], Enough large mass driver reaction engines are used to make the ratio of mass of payload plus the mass of the large mass driver reaction engines themselves to the rate of exhaust the same as in the mass driver reaction engine trips, so the large mass driver reaction engines can make any trip in the same time the mass driver reaction engine would require, since both have the same exhaust velocity. Since the ratio of the rate of exhaust to the mass empty for the two types of mass driver reaction engines differs by less than 10%, it will be assumed that the number of large mass driver reaction engines required to complete a trip in the same time as a mass driver reaction engine is proportional to the payload and inversely proportional to the exhaust rate, or where Using appropriate figures,
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