ROUTES FOR IMPLEMENTATION Even if the final economic solution proves to be a very large vehicle, interim development steps are required for several reasons: - technical and economic concept verification; - gain of operational experience; - adaptation to initially lower transportation requirements (build-up phase). One specific European route could be to develop a reusable second stage within the ARIANE Program (Fig. 7). This 50 ton vehicle could be made compatible with the ARIANE 4 first stage and would represent an essential economic and operational improvement: 1. Capability to recover payloads from Earth orbit (material processing payloads and/or even a manned module). 2. Launch cost reduction by multiple reuse of the second stage. 3. Gain of technical and operational experience for the next generation of fully reusable launch vehicles. The last item certainly would give Europe a competitive edge over the US. The early concept verification together with the advantage of the equatorial Kourou launch site could put Europe in a position to offer the world's lowest cost space transportation system at the end of the decade. Besides the SPS possibility there exists the more and more attractive alternative of nuclear waste disposal into space, requiring the same type and size of launch vehicle. The reusable ARIANE second stage requires a new structure and heat shield development, however, not a new engine. Eight of the existing HM 7 engines would constitute the propulsion system. The alternative to a European route would be cooperation with the US, developing jointly a new economic cargo launch vehicle. This cooperation would make sense for a larger size SSTO where the Shuttle engines could be used (avoiding a
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