Atlas II#Atlas IIAS

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.{{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}} 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.

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

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.{{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}}

Atlas II also featured lower-cost electronics, an improved flight computer, and longer propellant tanks than its predecessor, Atlas I.{{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}}

File:KSC-00pp0413.jpg 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 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 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.{{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}}{{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}} Compared to Atlas I, the Atlas II first stage was {{cvt|2.7|m||abbr=}} taller.{{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}}

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.{{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}}

The first stage also had the option to be fitted with 4 Castor 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.

{{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.

The Centaur II upper stage (along with all other Centaur variants) used a pressure-stabilized propellant tank design and 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.

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.

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.{{cite web | url=http://www.astronautix.com/a/atlas3a.html | title=Atlas 3A }} However, the Centaur V flying on the Vulcan rocket will only utilize two RL-10 engines.{{cite web | url=https://www.ulalaunch.com/rockets/vulcan-centaur | title=Vulcan }}

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.

{{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 kg of propellant with a dry mass of 275 kg.

Three fairing models were available for the Atlas II:

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

File:LC-36-Atlas-2.jpg

Atlas II was developed into three versions.

The original Atlas II was based on the Atlas I and 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.

Atlas IIA was a derivative of the Atlas II designed to service the commercial launch market. The main improvement was the switch from the RL10A-4 engine on the Centaur upper stage, increasing the stage's performance and the vehicle's payload capability.{{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}} The IIA version flew between 1992 and 2002.{{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}}

Atlas IIAS was largely identical to IIA, but added four 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. IIAS was used between 1993 and 2004, concurrently with IIA.{{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}}

File:Atlas evolution.png

• Primary contractor: Lockheed Martin (airframe, assembly, avionics, test and systems integration)Spaceflight Now, [http://www.spaceflightnow.com/atlas/ac162/011007atlas2as.html Atlas IIAS] (accessed September 24, 2014)
• Principal subcontractors: Rocketdyne (first stage engines), Pratt & Whitney (second stage engines), Honeywell & Teledyne (avionics), and Thiokol (solid rocket boosters)
• 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 × RL10A-3-3A or RL10A-4
• Thrust: 494,500 lbf (2,200 kN)
• Length: Up to 156 ft (47.54 m); 16 ft (4.87 m) high engine cluster
• Core diameter: 10 feet (3.04 m)
• Gross liftoff weight: 414,000 lb (204,300 kg)
• Fairing options: 3 (Medium, Large, Extended)
• Models: II, IIA, and IIAS
• Launch Sites: Cape Canaveral, LC-36 and Vandenberg, SLC-3E

• Comparison of orbital launchers families
• Atlas I
• Atlas III
• Atlas (rocket family)

{{Reflist}}

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

• [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