payloads can be extrapolated from the previous years. Table 8.4 presents the average results in term of commercial payloads to be launched per year. _____________Table 8.4 Average Commercial Payloads Per Year_____ Performance Requirements The required performance is first characterized by the velocity requirement (AV) and the payload mass. But considering the high level of transport cost in Earth orbit i.e. 20 k$/kg in GTO for Ariane 5 (ESA/CNES cost target), for each mission type a complete mass optimization has to be performed in order to save every kg of payload which has to be delivered in orbit. Another input of such optimization is the possible reusability of OTV (or parts of them). But due to the cost of carrying kg of propellants in high orbit just to return the vehicle, it can be demonstrated that a reusable OTV is cost effective even with the use of braking devices (i.e. aerobraking) only if the propulsion system can provide a specific impulse of more than 800 s, which widely exceeds the current (or near future) engine capacities. Thus all Space Power System projects have to be aware that economically speaking for the near future only an expendable vehicle has to be considered except if for technical purposes it is necessary to bring back payload in Earth orbit from higher orbits or from the Moon. Also, important means for OTV propulsion system selection/optimization is staging. For given design and performance parameter (Isp) the number of stages can be derived by theory. Obviously multistage OTV's is much more complicated than single-stage ones. Thus, • solid rocket systems are limited to AV< 3000 - 3500m/s • storable liquid systems are limited to AV< 4000 - 4500m/s • cryogenic liquid systems are limited to AV< 5500 - 6000m/s • nuclear or electric propulsion systems have higher limitation (respectively in the range of 15,000 & 30,000 m/s) Moreover, when for a certain mission a propulsion system is selected it is very important to enhance as much as possible the specific impulse of its engine because of mass savings. For example for an Ariane 5 LEO-GEO transfer (18000 kg in LEO) one more second of Isp allows to gain from 15 to 35 kg on payload (i.e. from 0.3 to 0.7% of the GEO mass capacity) At least several factors apply to the thrust level selection of OTV main engine, • gravity losses make the use of small thrust engine (< few kN) out of interest, • high thrust level is limited because of payload constraints (acceleration), so the best level of thrust is in the range of several tens of kN. Beside all those performance limitations there are also boundary conditions which influence or will influence the real worth of OTV : Handling and Environment limitations. This last aspect would Looking at those figures, with an available market for OTV's between 15 to 30 units per year after 1997, it becomes clear that there is a commercial slot for those vehicles even without Space Solar Power Program needs.
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