Editing Atlas II (section)

== Design ==
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&nbsp;kg) into geostationary transfer orbit (GTO), or more on later Atlas II variants.<ref name=a2dsgn>{{cite news|last=Harwood|first=William|title=Atlas-2 rocket launched on maiden launch|url=https://www.upi.com/Archives/1991/12/07/Atlas-2-rocket-launched-on-maiden-launch/8554692082000/|date=December 7, 1991|access-date=September 27, 2022|work=UPI.com}}</ref>

Atlas II also featured lower-cost electronics, an improved flight computer, and longer propellant tanks than its predecessor, [[Atlas I]].<ref name="li">{{cite web|url=http://www.au.af.mil/au/awc/space/factsheets/atlas_ii.htm|title=Atlas II Factsheet|website=au.af.mil|access-date=September 20, 2018|archive-date=May 1, 2017|archive-url=https://web.archive.org/web/20170501044825/http://www.au.af.mil/au/awc/space/factsheets/atlas_ii.htm|url-status=dead}}</ref>

=== Atlas II first stage ===
[[File:KSC-00pp0413.jpg|left|thumb|330px|Workers at Cape Canaveral Air Force Station prepare to erect the first stage of an Atlas&nbsp;IIA rocket in the launch gantry on Pad&nbsp;36A ahead of the GOES-L launch. Visible are the [[RS-56]] rocket engines.]]
The Atlas II first stage was {{cvt|3.05|m||abbr=}} in diameter and {{cvt|24.90|m||abbr=}} long. The stage was powered by 3 [[RS-56]] rocket engines (derived from the [[RS-27]] main engine of the [[Delta&nbsp;II]] rocket) burning {{cvt|156|t|lb|abbr=}} 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&nbsp;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.<ref name=astronautixAtlasII>{{cite web| title=Atlas II |url=http://www.astronautix.com/lvs/atlasii.htm|archive-url=https://web.archive.org/web/20021015095840/http://www.astronautix.com/lvs/atlasii.htm |url-status=dead |archive-date=October 15, 2002 |publisher=Astronautix |access-date=January 9, 2016}}</ref><ref name=ppg /><ref>{{cite web |url=https://rocket.com/sites/default/files/documents/Capabilities/PDFs/Propulsion%20System%20Data%20Sheets.pdf |title=Atlas Roll Control Module (ARCM) |publisher=Aerojet |access-date=November 5, 2022}}</ref> Compared to [[Atlas&nbsp;I]], the Atlas II&nbsp;first stage was {{cvt|2.7|m||abbr=}} taller.<ref>{{Cite web |title=Le lanceur Atlas 2 |lang=fr |url=http://www.capcomespace.net/dossiers/espace_US/lanceurs_US/atlas/atlas_2.htm |access-date=2021-02-14 |website=www.capcomespace.net}}</ref>

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&nbsp;[[g-force|''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.<ref>{{cite web |title=Atlas IIA(S) Data Sheet |url=http://www.spacelaunchreport.com/atlas2a.html |archive-url=https://archive.today/20160813221340/http://www.spacelaunchreport.com/atlas2a.html |url-status=usurped |archive-date=August 13, 2016 |publisher=Space Launch Report |access-date=January 9, 2016}}</ref><ref name=ppg />

The first stage also had the option to be fitted with 4 [[Castor&nbsp;4A]] solid rocket boosters as part of the IIAS version, each providing an additional {{cvt|478.3|kN}} 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.<ref name=astronautixAtlasIIAS />

=== 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.
</gallery>

=== Integrated Apogee Boost Stage ===
{{Main|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&nbsp;kg of propellant with a dry mass of 275&nbsp;kg.

=== Payload fairing ===
Three fairing models were available for the Atlas II:<ref name=ppg />
* '''Medium''', with a diameter of {{convert|3.3|m|ft|abbr=on}}, a height of {{convert|10.4|m|ft|abbr=on}}, and a mass of {{convert|1,409|kg|lb|abbr=on}}
* '''Large''', with a diameter of {{convert|4.2|m|ft|abbr=on}}, a height of {{convert|12.2|m|ft|abbr=on}}, and a mass of {{convert|2,087|kg|lb|abbr=on}}
* '''Extended''', with a diameter of {{convert|4.2|m|ft|abbr=on}}, a height of {{convert|13.1|m|ft|abbr=on}}, and a mass of {{convert|2,255|kg|lb|abbr=on}}

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.<ref>{{cite web | url=https://www.ulalaunch.com/rockets/atlas-v | title=Atlas V }}</ref> The 4-meter Atlas fairing last flew in 2022.<ref>{{cite web | url=https://www.nasaspaceflight.com/2022/11/jpss-2-launch/ | title=last-4m | date=November 10, 2022 }}</ref>

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.