The Shuttle-C concept would theoretically cut development costs for a heavy launch vehicle by re-using technology developed for the shuttle program. End-of-life and space Shuttle hardware would also have been used. One proposal even involved converting "Columbia" or "Enterprise" into a single-use cargo launcher. Before the loss of Space Shuttle "Challenger", NASA had expected about 14 shuttle flights a year. In the aftermath of the "Challenger" incident, it became clear that this launch rate was not feasible for a variety of reasons. With the Shuttle-C, it was thought that the lower maintenance and safety requirements for the uncrewed vehicle would allow a higher flight rate.