versions. Atlas II The original Atlas II was based on the Atlas I and its predecessors. Its lengthened propellant tanks and improved electronics over the Atlas I offered better performance. It was designed to work as part of the US Air Force's Medium Launch Vehicle II program. This version flew between 1991 and 1998. Atlas IIA Atlas IIA was a derivative of the Atlas II 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, increasing the stage's performance and the vehicle's payload capability. The IIA version flew between 1992 and 2002. Atlas IIAS Atlas IIAS was largely identical to IIA, but added four Castor 4A solid 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. Specifications Primary contractor: Lockheed Martin – airframe, assembly, avionics, test and systems integration Principal subcontractors: Rocketdyne (First stage engines), Pratt & Whitney (Centaur RL-10 engines), Honeywell & Teledyne (avionics), and Thiokol (Castor 4A SRBs) Power Plant: Two Rocketdyne RS-56-OBA engines (making up the MA-5A), one RS-56-OSA, two Pratt & Whitney RL-10A-3-3A or RL10A-4 Centaur engines Thrust: 494,500 lbf (2,200 kN) Length: Up to 156 ft (47.54 m); 16 ft (4.87 m) high engine cluster Core Diameter: 10 feet (3.04 m) Gross Liftoff Weight: 414,000 lb (204,300 kg) Fairing options: 3 (Medium, Large, Extended) Models: II, IIA, and IIAS Launch Sites: Cape Canaveral SFS Launch Complex 36, Florida and Vandenberg Space Force Base Space Launch Complex 3E, California See also Comparison of orbital launchers families Atlas I Atlas III Atlas (rocket family) References USAF Atlas II Fact Sheet External links Animation of the Atlas-IIAS launch and Terra satellite deployment Atlas (rocket family) Lockheed Martin space launch vehicles