Atlas II was a member of the Atlas family of launch vehicles, which evolved from the successful Atlas missile program of the 1950s. The Atlas II was a direct evolution of the Atlas I, featuring longer first stage tanks, higher-performing engines, and the option for strap-on solid rocket boosters. It was designed to launch payloads into low earth orbit, geosynchronous transfer orbit or geosynchronous orbit. Sixty-three launches of the Atlas II, IIA and IIAS models were carried out between 1991 and 2004; all sixty-three launches were successes, making the Atlas II a highly reliable space launch system. The Atlas line was continued by the Atlas III, used between 2000 and 2005, and the Atlas V which is still in use. Background In May 1988, the US Air Force chose General Dynamics (now Lockheed Martin) to develop the Atlas II vehicle, primarily to launch Defense Satellite Communications System payloads under the Medium Launch Vehicle II (MLV-II) program. Additional commercial and U.S. Government sales resulted in production increases leading to greater than 60 vehicles being produced and launched. Atlas II was developed from the Atlas I and featured numerous upgrades over that vehicle. Atlas II was launched from Launch Complex 36 at Cape Canaveral Space Force Station in Florida as well as Space Launch Complex 3E at Vandenberg Space Force Base in California. All launches were successful. Design Atlas II provided higher performance than the earlier Atlas I by using engines with greater thrust and longer propellant tanks for both stages. The increased thrust, engine efficiency, and propellant capacity enabled the vehicle to lift payloads of 6,100 pounds (2,767 kg) into geostationary transfer orbit (GTO), or more on later Atlas II variants. Atlas II also featured lower-cost electronics, an improved flight computer, and longer propellant tanks than its predecessor, Atlas I. Atlas II first stage The Atlas II first stage was in diameter and long. The stage was powered by 3 RS-56 rocket engines (derived from the RS-27 main engine of the Delta II rocket) burning of RP-1 and liquid oxygen. The two booster engines were the RS-56-OBA variants (the complete assembly of both engines and the aft skirt was referred to as the MA-5A), with high thrust but moderate efficiency. The sustainer (center) engine was the RS-56-OSA variant, featuring much less thrust but higher efficiency at high altitudes than the booster engines. The vernier engines used on the first stage of