(Parametric stage mass fraction data were derived from the Boeing FSTSA study.) Results of the sizing study are plotted on Figure VI-E-4. The single stage and common stage are seen to yield similar ignition weights as are the two 1-1/2 stage drop tank options. On the basis of minimum ignition weight, and hence POTV delivery requirements to LEO, the 1-1/2 stage appears to be the most attractive. However, a comprehensive costing and program analysis is required of each option to determine the most cost efficient option. In addition to the POTV options reusable to LEO, POTV options that are refuelable in GEO are viable options during the operational phase when a GEO base exists, provided an economic COTV is available to deliver propellants. Of particular interest is the common stage option sized for stage 2 refueling at GEO. These results are plotted on Figure VI-E-5 with the previous common stage option refueled at LEO. A factor of over four decrease in vehicle ignition weight is seen over the range of GEO payload capability between the two common stage options. The single stage and 1-1/2 stage options refuelable at GEO were also sized and compared to common stage option with stage 2 refuelable at GEO. Common stage vehicle ignition weights at LEO were seen to be lower than the other two options and with the assumption of stage GEO refueling capability, the common stage is the most desirable option for the operational program phase. In addition, prior to this phase, manned GEO sortie flights probably will be required from LEO. The same common stage vehicle could accommodate the smaller sortie crew module (Figure VI-E-1) round trip from LEO to GEO and return. POTV REFERENCE CONFIGURATION - COMMON STAGE LO?/LH? On the basis of mission versatility described above, the common stage LO2/LH2 option is selected as the POTV reference configuration. General Vehicle Description - This concept consists of two nearly identical stages used in tandem that provide the required mission delta-V. The first of these stages is unmanned and is used to provide approximately 85% of the delta-V required for departure from LEO on a crew rotation flight. Stage 2 provides the remainder of the boost delta-V as well as the impulse required for injection into the destination orbit and for the return to LEO. Following separation from stage 2, stage 1 is returned unmanned to LEO. Splitting the delta-V as described above, results in the stages having identical propellant capacities. Subsystems design approaches are also common between the stages including the size of the main engine. Taken individually, each of these stages is similar to the single stage concept in terms of subsystem selection and location. At the forward end of the stage 1 are two types of docking provisions. One of these systems is used to connect with stage 2 while the center mounted unit is an international type design that allows docking with systems other than stage 2; examples of these other systems include a tanker for independent servicing or a space station if basing is required while awaiting the return of stage 2.
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