Stage 2 docking provisions are required at both the forward and aft ends. The forward docking station uses an international type unit for attaching payloads. In addition, this unit accommodates tankers or is used to connect the stage to a space station for basing. The aft docking provisions are used in conjunction with those in the forward section of stage 1 and enable the stages to be connected. Provisions are also included on stage 2 to allow servicing of stage 1 when the two stages are connected, and the tanker is docked at the forward end of stage 2. Mission Requirements - The requirements of all three mission types - crew rotation, resupply, and GEO sortie - may be satisfied by the different operating modes of one size of common stage POTV. The fully reusable to LEO operating mode satisfies the manned geosynchronous sortie mission requirements which will appear in the program before a GEO base is available. After a GEO base is available, crew rotation and resupply mission requirements may be satisfied by a common stage whose stage 2 is refueled at the GEO base. Sizing Method - The common stage vehicle is sized for the GEO sortie mission requirements. The 4-man crew module described earlier (Figure VI-E-1) is flown as the GEO round trip payload with both stages refueled at LEO. Vehicle Description - The vehicle inboard profile is shown on Figure VI-E-6. The OTV startburn mass is 123 metric tons with a main propellant loading of 106 metric tons. Each stage is 4.42 meters in diameter and 17.06 meters in length (stage 2 length is 15.61 meters with engine nozzles retracted) and are Shuttle compatible and require on-orbit fueling and refueling. The first stage employes four 66,720 newtons (15,000 Ibf) thrust engines and the second stage employs two of the same engines. Mass properties are given in Table VI-E-5 for the fully loaded vehicle sized for the sortie mission described earlier. The vehicle inboard profile for the crew rotation mission is seen on Figure VI-E-7. As described earlier, the crew rotation mission requires stage 2 to be refueled at GEO. A two-man version of the sortie crew module described earlier is assumed to be utilized for manned control of stage 2 and is described by the weights tabulation in Table VI-E-6. On the outbound trip to GEO, the common stage POTV accommodates approximately 20 metric tons of high priority cargo in addition to the 75-man crew rotation passenger module and the 2-man crew control compartment. On the inbound trip, stage 2 returns the crew rotation passenger module and the crew control compartment to LEO. Single Stage Option - Either stage of the common stage vehicle may be flown autonomously in a single stage mode without the crew module. Payload performance to GEO from LEO has been calculated for stage 2 and is listed below:
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