View source for Atlas II

{{Short description|American rocket}}
{{Use mdy dates|date=June 2019}}
{{Infobox rocket
|image = DF-SC-99-00074 cropped and rotated.jpeg
|caption = Launch of an Atlas II rocket
|name = Atlas II
|function = [[Medium-lift launch vehicle]]
|manufacturer = [[Lockheed Martin]]
|country-origin = United States
|height = {{convert|47.54|m|ft|abbr=on}}
|diameter = {{convert|3.04|m|ft|abbr=on}}
|mass = {{convert|204,300|kg|lb|abbr=on}}
|stages = 2.5 (3.5 with [[Integrated Apogee Boost Stage|IABS]])
<!----Payloads (optional)---->
|capacities = 
  {{Infobox rocket/payload
  |location =  [[Low Earth orbit|LEO]]
  |kilos    =  {{plainlist|
* '''Atlas II''': {{cvt |6,580|kg|lb}}
* '''Atlas IIA''': {{cvt|7,280|kg|lb}}
* '''Atlas IIAS''': {{cvt|8,610|kg|lb}}<ref name=astronautixAtlasII />
    }}
  }}
  {{Infobox rocket/payload
  |location =  [[Geostationary transfer orbit|GTO]]
  |kilos    =  {{plainlist|
* '''Atlas II''': {{cvt |2,810|kg|lb}}
* '''Atlas IIA''': {{cvt|3,039|kg|lb}}
* '''Atlas IIAS''': {{cvt|3,630|kg|lb}}<ref name=astronautixAtlasII />
    }}
  }}
|family = [[Atlas (rocket family)|Atlas]]
|status = Retired
|sites = [[Cape Canaveral Space Force Station|Cape Canaveral]], [[Cape Canaveral Launch Complex 36|LC-36]]<br /> [[Vandenberg Space Force Base|Vandenberg]], [[Vandenberg Space Launch Complex 3|SLC-3]]
|launches = 63 ('''II:'''&nbsp;10, '''IIA:'''&nbsp;23, '''IIAS:'''&nbsp;30)
|success = 63
|fail =
|partial =
|first='''II:''' December 7, 1991<br />'''IIA:''' June 10, 1992<br />'''IIAS:''' December 16, 1993
|last='''II:''' March 16, 1998<br />'''IIA:''' December 5, 2002<br />'''IIAS:''' August 31, 2004<ref name="final">Tariq Malik [http://www.space.com/296-final-atlas-2-rocket-orbits-classified-satellite.html "Final Atlas 2 Rocket Orbits Classified U.S. Satellite"], ''Space News,'' August 31, 2004 (Accessed September 24, 2014)</ref>
|payloads = [[Solar and Heliospheric Observatory|SOHO]] (Atlas IIAS)<br />[[Tracking and Data Relay Satellite|TDRS]] (Atlas IIA)
| stagedata= 
{{Infobox rocket/stage
  |type       = stage
  |stageno    = First
  |engines    = 1 × [[RS-56]]-OSA
  |thrust     = {{cvt|386|kN}}
  |SI         = {{cvt|316|isp}}
  |burntime   = 283 seconds
  |fuel       = [[RP-1]] / [[LOX]]
  }}
{{Infobox rocket/stage
  |type       =  booster
  |name       =  [[MA-5A]]
  |number     =  2
  |engines    =  1 × [[RS-56]]-OBA
  |thrust     =  
  |total      =  {{cvt|2,093.3|kN}}
  |SI         =  {{cvt|299|isp}}
  |burntime   =  172 seconds
  |fuel       =  [[RP-1]] / [[LOX]]
  }}
{{Infobox rocket/stage
  |type       =  booster
  |diff       =  Atlas IIAS only
  |name       =  [[Castor (rocket stage)|Castor 4A]]
  |number     =  4
  |solid      =  yes
  |thrust     =  {{cvt|433.7|kN}}
  |total      =  {{cvt|1734.8|kN}}
  |SI         =  {{cvt|237.8|isp}}
  |burntime   =  56 seconds
  |fuel       =  [[Hydroxyl-terminated polybutadiene|HTPB]]<ref>{{cite web |last1=Wade |first1=Mark |title=Castor 4A engine |url=http://www.astronautix.com/c/castor4aengine.html |archive-url=https://web.archive.org/web/20161228152826/http://astronautix.com/c/castor4aengine.html |url-status=dead |archive-date=December 28, 2016 |website=astronautix.com}}</ref>
  }}
{{Infobox rocket/stage
  |type       = stage
  |stageno    = Second
  |name       = [[Centaur (rocket stage)|Centaur II]]
  |engines    = 2 × [[RL-10]]A
  |thrust     = {{cvt|147|kN}}
  |SI         = {{cvt|449|isp}}
  |burntime   = 392 seconds
  |fuel       = [[Liquid hydrogen|LH{{sub|2}}]] / [[LOX]]
  }}
{{Infobox rocket/stage
  |type       = stage
  |stageno    = Third
  |name       = [[Integrated Apogee Boost Stage|IABS]] 
  |diff       = optional
  |engines    = 2 × [[R-4D]]
  |thrust     = {{cvt|980|N}}
  |SI         = {{cvt|312|isp}}
  |burntime   = 60 seconds
  |fuel       = [[N2O4|N{{sub|2}}O{{sub|4}}]] / [[Monomethylhydrazine|MMH]]
  }}
}}

'''Atlas II''' was a member of the [[Atlas (rocket family)|Atlas]] family of [[launch vehicle]]s, which evolved from the successful [[Atlas missile]] program of the 1950s. The Atlas&nbsp;II was a direct evolution of the [[Atlas&nbsp;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&nbsp;II, IIA and IIAS models were carried out between 1991 and 2004; all sixty-three launches were successes, making the Atlas&nbsp;II a highly reliable space launch system. The Atlas line was continued by the [[Atlas&nbsp;III]], used between 2000 and 2005, and the [[Atlas&nbsp;V]], which is still in use {{as of|2024|lc=y}}.

== Background ==
In May 1988, the US Air Force chose [[General Dynamics]] (now [[Lockheed Martin]]) to develop the Atlas II vehicle, primarily to launch [[Defense Satellite Communications System]] payloads under the Medium Launch Vehicle II (MLV-II) program.<ref>{{cite news|title=General Dynamics will build a half-billion-dollar rocket fleet|url=https://www.upi.com/Archives/1988/05/03/General-Dynamics-will-build-a-half-billion-dollar-rocket-fleet-to/2468578635200/|date=May 3, 1988|access-date=September 27, 2022|work=UPI.com}}</ref> 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.<ref name=a2dsgn />

Atlas II was launched from [[Launch Complex 36]] at [[Cape Canaveral Space Force Station]] in Florida as well as [[Vandenberg Space Launch Complex 3|Space Launch Complex 3E]] at [[Vandenberg Space Force Base]] in California. All launches were successful.

== 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.

== Versions ==
[[File:LC-36-Atlas-2.jpg|right|thumb|The first stage of an Atlas IIA rocket is erected vertical at Cape Canaveral Pad 36A.]]
Atlas II was developed into three versions.

=== Atlas II ===
The original Atlas II was based on the [[Atlas I]] and [[Atlas (rocket family)|its predecessors]]. Its lengthened propellant tanks and improved electronics over the Atlas I offered better performance. It was designed to work as part of the US Air Force's Medium Launch Vehicle II program. This version flew between 1991 and 1998.<ref name=astronautixAtlasII />

=== Atlas IIA ===
Atlas IIA was a derivative of the Atlas II designed to service the commercial launch market. The main improvement was the switch from the [[RL10]]A-4 engine on the Centaur upper stage, increasing the stage's performance and the vehicle's payload capability.<ref name=ppg>{{cite web|title=Atlas Launch System Payload Planner's Guide|url=http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/MiscFiles/atlas.pdf|publisher=Lockheed Martin|access-date=January 9, 2016|url-status=dead|archive-url=https://web.archive.org/web/20150421050205/http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/MiscFiles/atlas.pdf|archive-date=April 21, 2015}}</ref> The IIA version flew between 1992 and 2002.<ref name=astronautixAtlasIIA>{{cite web|title=Atlas IIA|url=http://www.astronautix.com/lvs/atlasiia.htm|archive-url=https://web.archive.org/web/20020319100934/http://www.astronautix.com/lvs/atlasiia.htm|url-status=dead|archive-date=March 19, 2002|publisher=Astronautix|access-date=January 9, 2016}}</ref>

=== Atlas IIAS ===
Atlas IIAS was largely identical to IIA, but added four [[Castor (rocket stage)|Castor 4A]] solid rocket boosters to increase performance. These boosters were ignited in pairs, with one pair igniting on the ground, and the second igniting in the air shortly after the first pair separated. The "half-stage booster section would then drop off as usual.<ref name=ppg /> IIAS was used between 1993 and 2004, concurrently with IIA.<ref name=astronautixAtlasIIAS>{{cite web|title=Atlas IIAS|url=http://www.astronautix.com/lvs/atlsiias.htm|archive-url=https://web.archive.org/web/20020501080034/http://www.astronautix.com/lvs/atlsiias.htm|url-status=dead|archive-date=May 1, 2002|publisher=Astronautix|access-date=January 9, 2016}}</ref>

== Specifications ==
[[File:Atlas evolution.png|thumb|Atlas launch vehicle evolution, including the unflown Atlas V HLV (far right)]]
* '''Primary contractor''': Lockheed Martin (airframe, assembly, avionics, test and systems integration)<ref>Spaceflight Now, [http://www.spaceflightnow.com/atlas/ac162/011007atlas2as.html Atlas IIAS] (accessed September 24, 2014)</ref>
* '''Principal subcontractors''': Rocketdyne (first stage engines), Pratt & Whitney (second stage engines), Honeywell & Teledyne (avionics), and Thiokol (solid rocket boosters)<ref name=ppg />
* '''Engines''':
** '''First stage''': 1 × [[RS-56]]-OSA
** '''Boosters''' (MA-5A): 1 × [[RS-56]]-OBA each (2 total)
** '''Boosters''' (Atlas IIAS): 1 × [[Castor 4A]] each (4 total)
** '''Second stage''': 2 × [[RL10]]A-3-3A or [[RL10]]A-4
* '''Thrust''': 494,500&nbsp;lbf (2,200&nbsp;kN)
* '''Length''': Up to 156&nbsp;ft (47.54 m); 16&nbsp;ft (4.87 m) high engine cluster
* '''Core diameter''': 10 feet (3.04 m)
* '''Gross liftoff weight''': 414,000&nbsp;lb (204,300&nbsp;kg)
* '''Fairing options''': 3 (Medium, Large, Extended)<ref name=ppg />
* '''Models''': II, IIA, and IIAS
* '''Launch Sites''': [[Cape Canaveral Space Force Station|Cape Canaveral]], [[Cape Canaveral Launch Complex 36|LC-36]] and [[Vandenberg Space Force Base|Vandenberg]], [[Vandenberg Space Launch Complex 3|SLC-3E]]

== See also ==
* [[Comparison of orbital launchers families]]
* [[Atlas I]]
* [[Atlas III]]
* [[Atlas (rocket family)]]

== References ==
{{Reflist}}
* [https://web.archive.org/web/20030624105706/http://www.af.mil/factsheets/factsheet.asp?fsID=80 USAF Atlas II Fact Sheet]

== External links ==
* [https://web.archive.org/web/20060103205959/http://terra.nasa.gov/About/SC/launch_composite.html Animation of the Atlas-IIAS launch and Terra satellite deployment]

{{Atlas rockets}}
{{USAF space vehicles}}
{{Expendable launch systems}}
{{US launch systems}}

[[Category:Atlas (rocket family)]]
[[Category:Lockheed Martin space launch vehicles]]