Important distinguishing characteristics of each concept are summarized below: The following considerations led to the selection of the two-stage winged, vertical takeoff, horizontal landing, configuration as the reference system: 1. In both ballistic and winged concepts a two stage configuration has fewer technology development requirements and is less sensitive to design and operations constraints which result in lower operational costs. 2. Winged vehicles are preferred to ballistic vehicles because of the operational simplicity and reduced recovery time of horizontal land landings adjacent to the launch site compared to vertical water landings and the recovery of first stages downrange in the sea. 3. Hydrocarbon fuel is preferred to hydrogen for first stages because of its lesser volume and cost. The relatively simple production of methane by coal gasification guarantees its availability in the time period involved. For these reasons methane was selected for the first stage
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