The problem of low-cost access to orbit of large unmanned payloads is more difficult, however. I suppose it is possible that a large cargo version of the SSTO vehicle may be developed, but given the technical difficulties of developing such a vehicle anyway, it seems unlikely that it will ever be able to carry a large payload mass—say, in excess of 10 metric tonnes—into orbit. To solve this, one option is to take the brute force approach of developing large, unmanned cargo carriers, using tried and tested rocket components. The aim would be to develop a reliable heavy-lift workhorse, able to loft payloads of the order of 150 to 200 metric tonnes into low Earth orbit to satisfy the requirements for future manned exploration programs. It is difficult to predict the likely cost per kilogram of payload into orbit, but it would seem reasonable to have a design target of halving the launch costs.
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