Although it is not impossible, it is difficult to envision practical human exploration missions beyond Earth orbit without the ability to launch payloads much larger than the capacity of the Space Shuttle and other existing launch systems. Assembling massive spacecraft from twenty-ton segments involves coordination of multiple launches from different sites, long-term orbital storage, and complex integration activities in space. The only hope for this approach is to dramatically lower the cost per launch in order to balance the high integration cost and operational risk. Achieving lower launch costs with existing systems has proven to be an elusive goal.
If it were possible to launch much heavier payloads at a cost per flight which is comparable to that of existing systems then there might be an alternative to the multiple-launch approach. But heavier payloads imply a lower launch rate which makes the investment in a new vehicle and associated infrastructure difficult to justify. One solution to this problem may be to devise a heavy-lift launcher which shares the same infrastructure as an existing system
