certainly limit the use of a promising system in term of performance - nuclear propulsion system - to non Earth orbit applications at least. Here above we set up one of the most important requirements that must be deal with for an OTV selection/design and roughly it leads to the following classification ; type 1. missions are dedicated to liquid storable propulsion system type 2. missions are dedicated to liquid cryogenic propulsion system (or very high performance storable system) type 3. mission is out of interest with the current technologies type 4. missions are dedicated to nuclear (if allowed) or electric propulsion system. OTV Cost Estimating the price of Orbital Transfer Vehicles is a very tough job, mainly because it is difficult to fix assumptions and the level of technology used Therefore figures hereafter are only order of magnitude estimates. The average cost per OTV unit (using chemical propulsion) is currently between 1 and 20 M$ (US$). Conclusion - OTV in the Near Future (> 1997) We would like to put emphasis on the fact that the market analysis result is confirmed by the numerous and various studies on OTV and their propulsion which have been conducted up to now (mainly in the U.S.A, but also in Europe). For example ESA is currently performing a study on a high performance OTV-engine using space storable propellant under the name Advanced Technology Engine (which could be used on Ariane 5 OTV concept -cf. fig. 8.3) and some developments for the propulsion systems are underway in the US (USAF XLR 132 engine). Consequently, coming with the next generation of Space Transportation System (after 1997) there will a commercially available OTV and Space Solar Power Program should take into account the use of such vehicles if needed Figure 8.3 OTV Concept for a Three Satellite LEO to GEO Bus Stop Mission (from Ariane 5) 8.2.4 Future In-Orbit Vehicles Electric Propulsion Orbital Transfer Vehicles The technology of electric propulsion will allow reductions in the cost of space transportation. It enables this through: • The reduction of the mass launched into orbit • The reduction of the size of the propulsion system • The reduction of the size of the launch vehicle required to place a payload into orbit.
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