An uncrewed side-mounted concept of the Space Shuttle named Shuttle-C was investigated between 1984 and 1995. The Shuttle-C cargo only option was not funded in the 1980s and 1990s due to NASA's budgetary constraints. After the Space Shuttle "Columbia" disaster, a two-year industry study was prepared in 2004 and 2005 to further investigate the concept as a Shuttle replacement. The Exploration Systems Architecture Study (ESAS) in 2005 also investigated a Shuttle-C option for Project Constellation, again only in an uncrewed version. All these concepts intended the side-mounted carrier to be an autonomous spacecraft which would detach from the External Tank after main engine cut-off, similarly to the Space Shuttle. Some of the studies included the reuse of the Space Shuttle Main Engines on this side-mounted carrier. None of the concepts involved in-ascent fairing separation.