Atlas II was a member of the Atlas family of launch vehicles, which evolved from the successful Atlas missile program of the 1950s. 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 the most reliable launch system in history. The Atlas line was continued by the Atlas III, used between 2000 and 2005, and the Atlas V which is still in use.
Design
Atlas II provided higher performance than the earlier Atlas I by using engines with greater thrust and longer fuel tanks for both stages. LR-89 and LR-105 were replaced by the RS-56, derived from the RS-27. The total thrust capability of the Atlas II of 490,000 pounds force enabled the booster to lift payloads of 6,100 pounds into geosynchronous transfer orbit of 22,000 miles or more. Atlas II was the last Atlas to use a three engine, "stage-and-a-half" design: two of its three engines were jettisoned during ascent, but its fuel tanks and other structural elements were retained. The two booster engines, RS-56-OBAs, were integrated into a single unit called the MA-5A and shared a common gas generator. They burned for 164 seconds before being jettisoned. The central sustainer engine, an RS-56-OSA, would burn for an additional 125 seconds. The Vernier engines on the first stage of the Atlas I were replaced by a hydrazine fueled roll control system. This series used an improved Centaur upper stage, the world's first cryogenic propellant stage, to increase its payload capability. Atlas II also had lower-cost electronics, an improved flight computer and longer propellant tanks than its predecessor, Atlas I.
Versions
Atlas II
The original Atlas II was based on the Atlas I and its predecessors. This version flew between 1991 and 1998.
Atlas IIA
Atlas IIA was a derivative designed to service the commercial launch market. The main improvement was the switch from the RL10A-3-3A to RL10A-4 engine on the Centaur upper stage. The IIA version flew between 1992 and 2002.
Atlas IIAS
Atlas IIAS was largely identical to IIA, but added four Castor 4Asolid rocket boosters to increase performance. These boosters were ignited in pairs, with one pair igniting on the ground, and the second igniting in the air shortly after the first pair separated. The half-stage booster section would then drop off as usual. IIAS was used between 1993 and 2004, concurrently with IIA.
