Strap-on Boosters
The Titan III was developed from the Titan II launch vehicle by adding a pair of solid-fueled strap-ons and used by the United States Air Force as an unmanned heavy-lift vehicle. It was also planned to be used for the Manned Orbital Laboratory program, which was cancelled in 1969.
NASA's Space Shuttle was the first manned vehicle to use solid-fueled boosters as strap-ons. The solid boosters consisted of stacked segments, and were recovered and reused multiple times.
Read more about this topic: Booster (rocketry)