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.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.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.the stage was powered by 3 rs-56 rocket engines (derived from the rs-27 main engine of the delta ii rocket) burning 156 t (344,000 lb) 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 the atlas i (and all previous atlas models) were replaced by a hydrazine-fueled roll control system on atlas ii.this system, mounted on the interstage between the first and second stages, utilized small thrusters to control the vehicle's roll.compared to atlas i, the atlas ii first stage was 2.7 m (8 ft 10 in) taller.the atlas ii was the last atlas rocket to use the 'stage-and-a-half' technique, where it ignited all 3 rs-56 engines at liftoff and then jettisoned the 2 rs-56-oba side engines and their support structure during ascent.the two rs-56-oba engines were integrated into a single unit called the ma-5a and shared a common gas generator.they burned for approximately 164 seconds before being jettisoned, when acceleration reached approximately 5.0-5.5 g. the central sustainer engine on the first stage, an rs-56-osa, would burn for an additional 125 seconds after their jettison.it featured better efficiency at high altitudes than the rs-56-obas.the first stage also had the option to be fitted with 4 castor 4a solid rocket boosters as part of the iias version, each providing an additional 478.3 kn (107,500 lbf) of thrust for 56 seconds.the first two boosters were ignited at liftoff, and the other two were ignited after the first two burnt out.both pairs of boosters were jettisoned shortly after their respective burns.centaur ii featured 2 rl-10a-3-3a engines, burning liquid hydrogen and liquid oxygen.it featured propellant tanks 0.9 meters longer than its predecessor, centaur i, giving the stage more propellant and therefore higher performance.due to the super cold propellants inside centaur, foam insulation was installed onto the outer metal skin on the stage to help mitigate propellant boiloff inside the tank.
atlas ii | finalflight | 2004-08-31
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