The Shuttle was designed for military missions, and launch technology crossover between military and civilian systems was common in early years. The first manned launchers were versions of nuclear missiles.

From the National Aeronautics and Space Act:

102 (b) The Congress declares that the general welfare and security of the United States require that adequate provision be made for aeronautical and space activities.
The Congress further declares that such activities shall be the responsibility of, and shall be directed by, a civilian agency exercising control over aeronautical and space activities sponsored by the United States, except that activities peculiar to or primarily associated with the development of weapons systems, military operations, or the defense of the United States (including the research and development necessary to make effective provision for the defense of the United States) shall be the responsibility of, and shall be directed by, the Department of Defense; and that determination as to which such agency has responsibility for and direction of any such activity shall be made by the President in conformity with section 2471(e).