Space Launching System
The Space Launcher System (SLS) was a 1960s-era design program of the US Air Force for a family of launch vehicles based around a set of common components. After a series of studies in the late 1950s, the Air Force had concluded that the maximum efficiency would be gained by using only liquid hydrogen fuel for upper stages, which demanded the use of boosters based on segmented solid fuel rockets. By combining one of three upper stages with three different diameters of solids built to any length needed, the SLS provided wide flexibility in launch capability.
The SLS was one of two programs being designed at different divisions within the Air Force, with the ultimate aim of providing the launch services for the X-20 Dyna Soar manned spaceplane. Its competition was an upgraded version of the Titan I with a new upper stage that produced the Titan C concept. In the end, neither SLS or Titan C would be developed, in its place the new Titan III was selected, combining the new missile of the Titan C with the solid boosters of the SLS.