versions, Atlas used the Centaur IIA variant which featured 2 RL-10A-4 engines, providing higher thrust and efficiency over the RL-10A-3-3A. The two engines could be fitted with extendable nozzles, which would provide an increase in efficiency and therefore performance. Centaur II was further refined to create the Centaur III, which flew on the Atlas III and continues to fly today on the Atlas V. Atlas II was the final Atlas rocket that only had a dual-engine Centaur available, future rockets had the option for one or two RL-10 engines on Centaur. However, the Centaur V flying on the Vulcan rocket will only utilize two RL-10 engines. 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: Medium, with a diameter of , a height of , and a mass of Large, with a diameter of , a height of , and a mass of Extended, with a diameter of , a height of , and a mass of The Medium variant was not commonly used for Atlas II but was often used in earlier Atlas rockets. The Large and Extended fairing options were also later used on the Atlas III and Atlas V rockets. For the Atlas V, these fairings were part of the 400-series of that rocket, and a further extended option ("Extra Extended") was available. 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 Atlas II was developed from the Atlas I and was available in 3