8.2 Review and Analysis of Upper Stages/Orbital Transfer Vehicles 8.2.1 Definitions Orbital Transfer Vehicles (OTV's) are important parts of the Space Transportation System. Their missions start from a stable Earth orbit—typically a Low Earth Orbit (LEO)—-established by a launch vehicle, and their task is the transportation of payload(s) into other orbits and/or back -mainly higher energy orbits as : • Sun Synchronous Orbit (SSO) • Geostationary Transfer Orbit (GTO) • Geostationary Earth Orbit (GEO) • Interplanetary orbit Due to their different task and the resulting type of vehicle it is useful to distinguish OTV's and OMV's (Orbital Maneuvering Vehicles). These OMV's are also dedicated to payload transfer but only in a zone near an orbiting system, i.e. Space Station Freedom or MIR. Only small orbit changes are part of their task and generally a AV of 250 m/s is considered as the upper limit for this kind of vehicles. Figure 8.1 shows typical mission profiles for each vehicle type. Figure 8.1 OMV/OTV Missions Now the discussion will focus on OTV. We are going to classify the missions it could achieve. In fact the most important parameters to design those vehicles and to select the most efficient concept are the AV (Velocity Increment) they have to provide to the payload and the orbit they have to reach. Thus the classification is generally realized in term of AV and mission profile. Type 1. To deliver payload from LEO or Space Station to GTO AV = 2500 m/s Type 2. To deliver several payloads (in one mission) from LEO to higher orbits (GTO, SSO, GEO) -Bus stop mission2500 m/s < AV < 5000 m/s Type 3. To perform servicing missions in GEO with return to LEO for example: LEO-GTO-GEO-GTO-Aerobraking-LEO AV > 6500 m/s
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