There were several, but the two that stand out[1]: 1) The payload bay grew in size from 12 ft wide to 15 ft wide, in order to accommodate future unspecified mi

There were several, but the two that stand out[1]: 1) The payload bay grew in size from 12 ft wide to 15 ft wide, in order to accommodate future unspecified military needs. 2) There were new cross-range requirements specified for reentry.[2] Basically, the military wanted to be able to launch satellites into polar orbits (useful for surveillance) and retain the capability to land after one orbit of the Earth. However, this meant that the Shuttle also needed to be able to -reach- the landing sites, which are normally not in the direct orbit path, but are divert 1000 miles away laterally (the cross part of cross-range). [1] http://en.wikipedia.org/wiki/Space_Shuttle_design_process#Air_Force_involvement http://en.wikipedia.org/wiki/Space_Shuttle_design_process#Ai... [2] http://yarchive.net/space/shuttle/shuttle_crossrange.html http://yarchive.net/space/shuttle/shuttle_crossrange.html