The Atlas I was an Americanexpendable launch system, used in the 1990s to launch a variety of different satellites. The "I" in "Atlas I" can cause confusion, as all previous Atlas rockets were designated using letters, ending with the Atlas H, however subsequent rockets were designated using roman numerals, starting with the Atlas II. Officially, the "I" is the roman numeral "1".
The first stage of the Atlas I was derived from Atlas G, and the second stage was a Centaur. The first stage had an improved MA-5A propulsion system, consisting of the LR-89 booster engine with dual thrust chambers and a more powerful RS-27 replacing the traditional LR-105 sustainer engine. With the new RS-27 sustainer engine, all three of the large Atlas thrust chambers produced equal thrust. Booster engine jettison occurred prior to sustainer engine cut-off for stage-and-a-half staging of the Atlas. (Used in conjunction with the Centaur, this gave the Atlas I 2.5 stages).