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 rocket engines. 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 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 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. However, the Centaur V flying on the
Vulcan rocket will only utilize two RL-10 engines. File:Centaur stage arrives at pad 36A for GOES-L mission (KSC-00PP-0421).jpg|Centaur IIA arrives at Launch Complex 36A for the launch of
GOES-L. File:Centaur stage KSC-00PP-0424.jpg|Centaur IIA before mating with Atlas II booster. File:KSC-00PP-0425.jpg|A close-up view of the RL-10 engines of Centaur IIA. File:CentaurIIA-TDRS-I-LIFT.jpg|Centaur IIA, to be used to launch TDRS-I, is lifted for integration. File:KSC-00PP-0665.jpg|Centaur IIA for the TDRS-I mission is inspected in Hangar J at Cape Canaveral.
Integrated Apogee Boost Stage The Integrated Apogee Boost Stage was an optional upper stage, used only as an apogee kick stage when launching
Defense Satellite Communications System III satellites (which were designed to be delivered directly to
geostationary orbit using the
Transtage or
Inertial Upper Stage, and so were not capable of performing their own circularization burn at the apogee of their
geostationary transfer orbit) on board the Atlas II and, later, the
Delta IV. It was powered by two
R-4D engines and could operate on-orbit for up to twelve days before deploying its payload, allowing additional flexibility in mission planning. The IABS measured 2.9 m in diameter, and 0.68 m in length, carrying 1303 kg of propellant with a dry mass of 275 kg.
Payload fairing Three fairing models were available for the Atlas II: The 4-meter Atlas fairing last flew in 2022. Atlas II rockets flying with a Medium fairing could move the most payload to orbit, as that fairing was the lightest. Similarly, rockets with Large or Extended fairings suffered slight hits to their payload capacity. == Versions ==