The Shuttle-C concept would theoretically cut development costs for a heavy launch vehicle by re-using technology developed for the shuttle program. The proposal involved using, at various times, existing spaceframes, Space Shuttle Main Engines that had reached maintenance lifetime limits, and spare navigation computers. One proposal even involved converting the 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 unmanned vehicle would allow a higher flight rate.