geosynchronous orbit, and return to that orbit for subsequent rendezvous with an orbiting craft or support facility for refueling and reuse. An alternative mission sequence for performing personnel interchange between low Earth orbit and geosynchronous operating locale would be to employ chemical propulsion for the outbound journey and make use of the Earth's atmosphere for braking for the return journey. This operating mode has been suggested by studies performed at the MSFC. The concept, known as "AMOOS" (Atmospheric Maneuvering Orbit to Orbit Stage), employs multiple passes through the Earth's atmosphere to acquire low Earth orbit. Significant savings in propellant quantity are thus achieved and translate into additional payload fraction of the outbound ignition weight of the vehicle? The potential improvement is reduced by the weight of the necessary retrobraking and thermal protection devices to provide the deceleration in a reasonable number of passes. An extension and perhaps improvement to this concept is to employ the Earth's atmosphere for braking with the target being atmospheric flight return directly to Earth. In this mission mode, the deorbit burn from geosynchronous altitude is reduced to a minimum, aid a direct single pass entry established for return to a desired location on the surface of the Earth. Either winged or ballistic entry bodies may be employed for this purpose. This mission mode enjoys an increase in payload fraction of the initial mass in low orbit and, in addition, offers the operational advantage of returning the crew and passengers directly to a desired location on Earth rather than requiring the intermediate stop in low Earth orbit and solution of the rendezvous problem. This mission possibility has not yet been subjected to analysis and requires study in the months to come in order to quantify the benefits. For the purpose of the present JSC in-house SPS study, the conservative choice is made to employ conventional chemical rocketry with return of the vehicle and crew to low Earth orbit. Single stage, 1-1/2 stage (outbound propellant tanks left in geosynchronous orbit), 2 stage, and 2-1/2 stage configurations are all candidates for this mission. Additionally, for those cases where economic (electric propulsion) cargo transportation is possible, significant advantages accrue to the orbit transfer vehicle for personnel by storing propellants for the return journey in geosynchronous orbit, having previously been delivered there by the more economic cargo OTV. All of these candidates deserve scrutiny to support the orbit transfer of personnel and occasional high priority freight. 4. Systems Requirements Analysis The Satellite Power Station Program poses a set of space transportation systems requirements which are unprecedented in scale and in the press for economy of operation. The Space Shuttle will serve the purpose of providing orbital test of satellite power station technology and components in the 1980 time frame, of providing the operational and facility structure on which the satellite power station transportation system may be based, and in providing the basis for evolutionary development of an effective personnel and priority vehicle to be used in the
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