Fig. 11. Space freighter launch intact recovery of the vehicle. Landing weight of the second stage, the Orbiter, is about 386 kg (850 lb), twice the landing weight of a Boeing 747. Fuel for the second stage is liquid hydrogen. To reduce propellant costs, the first-stage fuel is methane rather than hydrogen. Both stages use liquid oxygen as the oxidizer. Each Solar Power Satellite launch-vehicle flight, as shown in Fig. 12, will deliver 425-t payloads consisting of satellite components, building materials, construction equipment, and expendable supplies to a low-Earth-orbit staging base. Both stages return to an Earth landing strip to load cargo for the next flight. Although a new launch vehicle will probably be required to achieve minimum costs, vehicles evolved from the present Shuttle design are being studied as an alternative for use during the development and early operational stages of the programs. The low Earth orbit to geosynchronous orbit transfer employs electric propulsion with an independent ion-drive transfer stage (Fig. 13). The ion engines use argon as the propellant. The solar array to produce the necessary electrical power would remain a part of the vehicle and be returned to low Earth orbit for reuse. This approach combines the opportunity to perform construction at the final geostationary orbit with the propellant economy of electric propulsion. Repeated flights of a solar array through the inner Van Allen Belt will constitute a severe technology development problem. The silicon solar cell suffers significant reduction of electrical output from each passage through the trapped radiation belt. Exploratory work now underway on localized thermal annealing of silicon solar cells using a laser beam may demonstrate that this radiation damage can be reversed between missions. Gallium
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