Editing Atlas II (section)

=== Centaur II upper stage ===
{{Main|Centaur (rocket stage)}}
The second stage of Atlas II, the Centaur II, was the result of over 3 decades of flights and enhancements of the Centaur upper stage. 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. Centaur II's foam insulation was permanently attached to the side of the stage, whereas previous versions of the stage (including Centaur I) jettisoned their insulation panels during flight.<ref name=astronautixAtlasII />

The Centaur II upper stage (along with all other Centaur variants) used a pressure-stabilized propellant tank design and [[Cryogenic fuel|cryogenic propellants]]. The two stainless steel propellant tanks were separated by a common bulkhead, which helped keep mass down. Centaur II was {{cvt|10.1|m||abbr=}} long, carrying almost {{cvt|17|t|lb|abbr=}} of fuel. The stage also featured 12 {{convert|27|N|lbf|abbr=on}} [[hydrazine]] thrusters to orient the stage and settle the propellants prior to engine ignition.<ref name=ppg />

For the IIA and IIAS 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.<ref name=astronautixAtlasIIA />

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.<ref>{{cite web | url=http://www.astronautix.com/a/atlas3a.html | title=Atlas 3A }}</ref> However, the Centaur V flying on the [[Vulcan Centaur|Vulcan rocket]] will only utilize two RL-10 engines.<ref>{{cite web | url=https://www.ulalaunch.com/rockets/vulcan-centaur | title=Vulcan }}</ref><gallery mode="packed" caption="Centaur 2A pictures:" heights=170>
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.
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