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Atlas II
Launch of an Atlas II rocket
FunctionMedium-lift launch vehicle
ManufacturerLockheed Martin
Country of originUnited States
Size
Height47.54 m (156.0 ft)
Diameter3.04 m (10.0 ft)
Mass204,300 kg (450,400 lb)
Stages2.5 (3.5 with IABS)
Capacity
Payload to LEO
Mass
  • Atlas II: 6,580 kg (14,510 lb)
  • Atlas IIA: 7,280 kg (16,050 lb)
  • Atlas IIAS: 8,610 kg (18,980 lb)[1]
Payload to GTO
Mass
  • Atlas II: 2,810 kg (6,190 lb)
  • Atlas IIA: 3,039 kg (6,700 lb)
  • Atlas IIAS: 3,630 kg (8,000 lb)[1]
Associated rockets
FamilyAtlas
Launch history
StatusRetired
Launch sitesCape Canaveral, LC-36
Vandenberg, SLC-3
Total launches63 (II: 10, IIA: 23, IIAS: 30)
Success(es)63
First flightII: December 7, 1991
IIA: June 10, 1992
IIAS: December 16, 1993
Last flightII: March 16, 1998
IIA: December 5, 2002
IIAS: August 31, 2004[2]
Carries passengers or cargoSOHO (Atlas IIAS)
TDRS (Atlas IIA)
First stage
Powered by1 × RS-56-OSA
Maximum thrust386 kN (87,000 lbf)
Specific impulse316 s (3.10 km/s)
Burn time283 seconds
PropellantRP-1 / LOX
Boosters – MA-5A
No. boosters2
Powered by1 × RS-56-OBA
Total thrust2,093.3 kN (470,600 lbf)
Specific impulse299 s (2.93 km/s)
Burn time172 seconds
PropellantRP-1 / LOX
Boosters (Atlas IIAS only) – Castor 4A
No. boosters4
Maximum thrust433.7 kN (97,500 lbf)
Total thrust1,734.8 kN (390,000 lbf)
Specific impulse237.8 s (2.332 km/s)
Burn time56 seconds
PropellantHTPB[3]
Second stage – Centaur II
Powered by2 × RL-10A
Maximum thrust147 kN (33,000 lbf)
Specific impulse449 s (4.40 km/s)
Burn time392 seconds
PropellantLH2 / LOX
Third stage (optional) – IABS
Powered by2 × R-4D
Maximum thrust980 N (220 lbf)
Specific impulse312 s (3.06 km/s)
Burn time60 seconds
PropellantN2O4 / MMH
[edit on Wikidata]

Atlas II was a member of the Atlas family of launch vehicles, which evolved from the successful Atlas missile program of the 1950s. The Atlas II was a direct evolution of the Atlas 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 II, IIA and IIAS models were carried out between 1991 and 2004; all sixty-three launches were successes, making the Atlas II a highly reliable space launch system. The Atlas line was continued by the Atlas III, used between 2000 and 2005, and the Atlas V, which is still in use as of 2025.

Background

[edit]

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.[4] 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.[5]

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.

Design

[edit]

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.[5]

Atlas II also featured lower-cost electronics, an improved flight computer, and longer propellant tanks than its predecessor, Atlas I.[6]

Atlas II first stage

[edit]
Workers at Cape Canaveral Air Force Station prepare to erect the first stage of an Atlas IIA rocket in the launch gantry on Pad 36A ahead of the GOES-L launch. Visible are the RS-56 rocket engines.

The Atlas II first stage was 3.05 m (10.0 ft) in diameter and 24.90 m (81.7 ft) long. The stage was powered by 3 RS-56 rocket engines (derived from the RS-27 main engine of the Delta II rocket) burning 156 t (344,000 lb) 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.[1][7][8] Compared to Atlas I, the Atlas II first stage was 2.7 m (8 ft 10 in) taller.[9]

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.[10][7]

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 478.3 kN (107,500 lbf) 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.[11]

Centaur II upper stage

[edit]
Main article: 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.[1]

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 10.1 m (33 ft) long, carrying almost 17 t (37,000 lb) of fuel. The stage also featured 12 27 N (6.1 lbf) hydrazine thrusters to orient the stage and settle the propellants prior to engine ignition.[7]

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.[12]

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.[13] However, the Centaur V flying on the Vulcan rocket will only utilize two RL-10 engines.[14]

  • Centaur 2A pictures:
  • Centaur IIA arrives at Launch Complex 36A for the launch of GOES-L.
    Centaur IIA arrives at Launch Complex 36A for the launch of GOES-L.
  • Centaur IIA before mating with Atlas II booster.
    Centaur IIA before mating with Atlas II booster.
  • A close-up view of the RL-10 engines of Centaur IIA.
    A close-up view of the RL-10 engines of Centaur IIA.
  • Centaur IIA, to be used to launch TDRS-I, is lifted for integration.
    Centaur IIA, to be used to launch TDRS-I, is lifted for integration.
  • Centaur IIA for the TDRS-I mission is inspected in Hangar J at Cape Canaveral.
    Centaur IIA for the TDRS-I mission is inspected in Hangar J at Cape Canaveral.

Integrated Apogee Boost Stage

[edit]
Main article: 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

[edit]

Three fairing models were available for the Atlas II:[7]

  • Medium, with a diameter of 3.3 m (11 ft), a height of 10.4 m (34 ft), and a mass of 1,409 kg (3,106 lb)
  • Large, with a diameter of 4.2 m (14 ft), a height of 12.2 m (40 ft), and a mass of 2,087 kg (4,601 lb)
  • Extended, with a diameter of 4.2 m (14 ft), a height of 13.1 m (43 ft), and a mass of 2,255 kg (4,971 lb)

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.[15] The 4-meter Atlas fairing last flew in 2022.[16]

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

[edit]
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

[edit]

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.[1]

Atlas IIA

[edit]

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.[7] The IIA version flew between 1992 and 2002.[12]

Atlas IIAS

[edit]

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.[7] IIAS was used between 1993 and 2004, concurrently with IIA.[11]

Specifications

[edit]
Atlas launch vehicle evolution, including the unflown Atlas V HLV (far right)
  • Primary contractor: Lockheed Martin (airframe, assembly, avionics, test and systems integration)[17]
  • Principal subcontractors: Rocketdyne (first stage engines), Pratt & Whitney (second stage engines), Honeywell & Teledyne (avionics), and Thiokol (solid rocket boosters)[7]
  • 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)[7]
  • Models: II, IIA, and IIAS
  • Launch Sites: Cape Canaveral, LC-36 and Vandenberg, SLC-3E

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Atlas II

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from Grokipedia
The Atlas II was an American expendable launch vehicle developed as an enhanced version of the earlier system, featuring a stretched first stage booster powered by uprated Rocketdyne MA-5A engines producing 474,000 pounds of and paired with the Centaur upper stage for orbital insertions. Development of the Atlas II began in June 1988 under contract with the U.S. , primarily to support the launch of (DSCS) satellites, with production leading to its inaugural flight on December 7, 1991, from Station. The Atlas II family encompassed several variants to accommodate diverse payloads and mission requirements, including the baseline Atlas II with a capacity of approximately 2,800 kg to geosynchronous transfer orbit (GTO), the Atlas IIA with an upgraded upper stage featuring higher-performance RL10A-4 engines for increased performance, and the Atlas IIAS augmented by up to four solid rocket boosters for heavier lifts up to 8,610 kg to (LEO). Over its operational lifespan from 1991 to 2004, the Atlas II conducted 63 missions with a perfect success rate of 100%, launching a wide array of commercial, scientific, and military payloads such as NASA's (), the II F3 , and classified National Reconnaissance Office . The program's final flight occurred on August 31, 2004, with the Atlas IIAS vehicle deploying the NROL-1 reconnaissance satellite, marking the end of the Atlas II era as it transitioned to successors like the for continued U.S. space access.

Development

Origins and Evolution

The Atlas II launch vehicle traces its origins to the SM-65 Atlas intercontinental ballistic missile program initiated in the 1950s by Convair, a predecessor company to General Dynamics, as the first successful U.S. ICBM designed for rapid deployment during the Cold War. This missile evolved into a space launch vehicle in the late 1950s and early 1960s, pairing with the Centaur upper stage—developed by General Dynamics/Astronautics and adopted by NASA in 1959—to enable high-energy missions requiring cryogenic propulsion. The Atlas-Centaur combination first achieved operational success in 1963 and supported key scientific endeavors, including the Pioneer program for interplanetary probes to Jupiter and beyond in the 1970s, as well as the Surveyor lunar lander missions starting with Surveyor 1 in 1966, which provided critical data for the Apollo program. By the late 1980s, following the Challenger disaster and growing demand for reliable commercial satellite launches, General Dynamics—later acquired by Lockheed Martin in 1993—shifted focus toward commercial applications of the Atlas family. In 1987, the company announced plans to produce 18 commercial Atlas-Centaur rockets. Development of the Atlas II specifically began in June 1988 under a U.S. Air Force contract for the Medium Launch Vehicle II program, aiming to enhance performance for geostationary transfer orbit (GTO) payloads such as defense and commercial communications satellites. The Atlas II represented a commercial evolution of earlier Atlas models, with its occurring on December 7, 1991, from Cape Canaveral's Complex 36B, successfully deploying a for the . The family, encompassing Atlas II, IIA, and IIAS variants, conducted a total of 63 launches between 1991 and 2004 before retirement, achieving full success and solidifying its role in the medium-lift sector. This development was driven by the need to increase capacity to GTO from approximately 2,300 kg on the to 2,810 kg on the baseline Atlas II, enabling competition with international vehicles like Europe's and Russia's Proton in the burgeoning commercial market. The upper stage retained its heritage from designs originally intended for lunar and planetary missions, providing the high-efficiency propulsion essential for these orbits.

Design Enhancements

The Atlas II introduced several key design enhancements over the baseline configuration, primarily aimed at increasing payload capacity and reliability through structural and modifications. Evolving from the 's , the Atlas II featured a stretched core stage, extending the propellant tanks by 108 inches (approximately 2.7 meters) to accommodate greater volumes of and (LOX) in its stage-and-a-half setup, which utilized three Rocketdyne MA-5A engines—two booster engines and one sustainer—for improved thrust and efficiency. Later variants incorporated uprated solid rocket boosters (SRBs) for additional initial thrust augmentation; specifically, the Atlas IIAS model added four Castor IVA SRBs, each providing significant boost during the ascent phase to support heavier payloads. The attitude control system was upgraded by replacing the vernier engines of the with hydrazine-fueled thruster modules, which enhanced reliability by eliminating handling complexities and providing precise roll control and propellant settling functions. These modifications resulted in an overall vehicle height of 47.4 meters for the base Atlas II model, reflecting the integrated changes to the core stage and upper structure. The modular architecture of the Atlas II family facilitated adaptations across variants, such as engine upratings in the Atlas IIA and SRB integration in the Atlas IIAS, enabling a range of mission profiles without fundamental redesigns.

Design

Booster and Core Stage

The Atlas II booster and core stage utilizes a stage-and-a-half configuration, featuring three Rocketdyne/ MA-5A engines fueled by and (). This setup includes two gimbaled booster engines flanking a central sustainer engine, all igniting simultaneously at liftoff to provide continuous propulsion without interruption during the initial ascent phase. The booster engines each produce approximately 1,047 kN of vacuum , operating for about seconds to accelerate the vehicle before their package is jettisoned, while the sustainer engine delivers around 386 kN of vacuum and burns for roughly 280 seconds to achieve the required for upper stage separation and orbital insertion. The combined system generates a total sea-level of approximately 2,150 kN, enabling efficient ascent performance for medium-lift missions. Propellant is stored in stretched, integrally stiffened thin-wall tanks arranged in a common bulkhead configuration, with the LOX tank above the RP-1 tank to minimize overall length and mass. These balloon tanks, maintained at structural integrity through internal pressurization via helium spheres and autogenous gas, hold a total of about 156,000 kg of cryogenic propellants at a mixture ratio of approximately 2.25:1 (oxidizer to fuel). In the Atlas IIAS variant, four Thiokol Castor 120 solid rocket motors are integrated as strap-on boosters to the core stage, augmenting initial thrust for heavier payloads, though detailed performance variations are addressed in variant-specific descriptions.

Centaur Upper Stage

The Centaur II upper stage, exclusive to the Atlas II family of launch vehicles, represents a stretched variant of the legacy Centaur design, extended by approximately 1.0 meter compared to earlier configurations to accommodate greater propellant loads. This modification allowed the stage to carry roughly 16,900 kg of cryogenic propellants—comprising approximately 2,800 kg of (LH2) and 14,100 kg of (LOX)—supporting dual-burn profiles for missions to geosynchronous transfer orbit (GTO). Propulsion is provided by two engines mounted at the aft end of the stage, with the RL10A-3 variant used on the Atlas II and early IIA configurations, and the higher-performance RL10A-4 employed on later IIA and all IIAS vehicles. Each RL10A-3 delivers 73.4 kN (16,500 lbf) of vacuum thrust with a specific impulse of 444 seconds, while the RL10A-4 increases this to 99.2 kN (22,300 lbf) at 451 seconds; both models feature restart capability, enabling a primary burn after separation followed by an apogee kick for payload circularization. The stage maintains attitude control using a monopropellant (RCS) with multiple thrusters for three-axis stabilization, roll maneuvers, and propellant settling, supplemented by a helium-based pressurant to maintain tank pressures during flight. Total fueled mass for the stands at approximately 19,000 kg, including structural elements and subsystems. Following core stage burnout, the separates via pyrotechnic devices in the interstage adapter, initiating a coast phase before upper stage ignition to optimize trajectory insertion.

Payload Accommodation

The payload accommodation system features a protective fairing designed to shield satellites and other during atmospheric ascent. The fairing is available in two primary configurations: a medium version with a 3.30 m and 10.36 m , weighing 1,375 kg, or a large version with a 4.19 m and 12.22 m , weighing 2,005 kg. Constructed as a two-half-shell aluminum structure with skin/stringer/frame assembly, the fairing provides an internal usable of up to 3.65 m for the large variant, accommodating payloads within a maximum of 4.57 m and supporting mass limits up to 4,500 kg depending on mission volume requirements. The fairing is jettisoned at approximately 100 km altitude, once and diminish sufficiently to expose the payload without risk, typically prior to upper stage engine cutoff for orbital insertion. An optional augmentation for geosynchronous missions is the Integrated Apogee Boost Stage (IABS), which integrates a PAM-D solid rocket motor to enable direct insertion into from a supersynchronous transfer trajectory provided by stage. The IABS adds approximately 500 kg dry mass to the vehicle and 1.8 m to its length, delivering a delta-V of about 1.4 km/s for circularization at GEO altitudes. This subsystem mounts atop the adapter, compatible with standard interfaces, and supports missions requiring precise apogee kicks without relying on onboard satellite propulsion. Payload integration occurs via standardized adapter fittings, such as the Type A or B classes with 1.57 m (62-inch) diameters, which secure satellites using pyrotechnic V-band clamps for reliable separation. These s, rated for payloads in the 1,570 kg class and beyond, include electrical interfaces for command destruct systems and real-time transmission during ascent, ensuring compliance with launch site safety protocols. The overall accommodation emphasizes , allowing for composite or aluminum fairing options to minimize and acoustic loads on sensitive payloads.

Variants

Atlas II

The Atlas II served as the baseline variant of the Atlas II family of expendable launch vehicles, debuting with its first flight on December 7, 1991, as part of the U.S. Air Force's Medium Launch Vehicle II program. Unlike later iterations, it incorporated no strap-on solid rocket boosters (SRBs) or propulsion enhancements, relying instead on an extended version of the Atlas I's liquid-fueled core stage paired with the Centaur upper stage. This configuration enabled a geosynchronous transfer orbit (GTO) payload capacity of 2,810 kg, making it suitable for medium-class missions without the added complexity or cost of auxiliary boosters. Propulsion for the baseline Atlas II utilized the Rocketdyne MA-5A engine system on the core booster , delivering approximately 2,093.7 kN of , alongside two RL10A-3-3A engines on , each producing 73.4 kN in vacuum. The absence of steel-cased SRBs distinguished it from upgraded models, resulting in comparatively lower liftoff and limiting its application to payloads that did not demand higher performance envelopes. The core design, with its lengthened propellant tanks, remained consistent across the Atlas II series. The variant completed 10 successful launches through its retirement in 1998, with a perfect success rate that underscored its reliability for targeted applications. These flights focused on commercial satellites, exemplified by the inaugural mission carrying the II F3 satellite to , alongside payloads. Due to its inherent constraints, the Atlas II excelled with lighter payloads, offering a cost-effective option for operators prioritizing over in contrast to the more powerful Atlas IIA and IIAS.

Atlas IIA

The Atlas IIA, introduced in 1992, served as an enhanced mid-tier variant of the Atlas II family, building on the baseline configuration through targeted improvements to upper stage for greater efficiency and payload performance. The principal upgrade involved replacing the Centaur's two RL10A-3-3A engines with RL10A-4 engines, each producing 92.5 kN of vacuum and offering higher via optional extendible nozzles, thereby boosting overall upper stage by approximately 26% compared to the original setup. This enhancement stemmed directly from the baseline Atlas II heritage but focused on optimizing cryogenic without adding strap-on solids. These engine modifications enabled the Atlas IIA to achieve a payload capacity of 3,180 kg to geosynchronous transfer orbit (GTO), a notable increase over the base model's capabilities while maintaining a streamlined two-stage design. The variant's total height, including fairing, measured 50.5 m, supporting a range of commercial and government missions. Over its operational lifespan, the Atlas IIA completed 23 successful flights from Cape Canaveral, including key deployments of Eutelsat and Inmarsat communications satellites that expanded global telecommunications coverage. A critical design change in the Atlas IIA was the improved restart capability of the Centaur stage, facilitated by the RL10A-4 engines' advanced and systems, which reduced propellant boil-off rates during coast phases and enabled more reliable multiple-burn profiles for extended-duration missions. This reliability feature proved essential for precise orbital insertions in geosynchronous applications.

Atlas IIAS

The Atlas IIAS was the most advanced variant in the Atlas II family, introduced in 1993 to support heavier payloads through the addition of solid rocket boosters and an optional extended . The first launch occurred on December 16, 1993, from Cape Canaveral's Launch Complex 36B, successfully deploying the Telstar 401 communications satellite. This configuration retained the core structure of the Atlas IIA, including its MA-5A engines providing a baseline of approximately 2,110 kN at liftoff, but augmented performance for geosynchronous transfer orbit (GTO) missions requiring greater initial acceleration. Key enhancements included four steel-cased Castor IVA solid rocket boosters (SRBs) strapped to the first stage, with two ignited at liftoff and the remaining two air-lit approximately 10 seconds after the initial pair's burnout to maintain profile. Each SRB delivered an average sea-level of about 445 kN and burned for roughly 52 seconds, contributing to a total liftoff of approximately 3,900 kN across the vehicle. The SRBs separated sequentially, with the first pair jettisoned around 54 seconds after launch, optimizing structural loads and enabling the vehicle to achieve its highest performance levels. An extended fairing option, measuring up to 4.2 meters in diameter and 11.6 meters in length, accommodated larger satellites, expanding payload volume for missions like those in the series. The Atlas IIAS offered the family's peak capabilities, delivering up to 8,610 kg to (LEO) at 185 km altitude and 28.5° inclination, or approximately 3,800 kg to GTO, making it ideal for demanding commercial and government missions such as the 1997 launch of the DirecTV-6 (Tempo 2) satellite. Over its operational life, the variant completed 30 successful flights from and Vandenberg Air Force Base, achieving a 100% success rate. The final mission, designated NROL-1 and carrying a classified payload, lifted off on August 31, 2004, marking the end of Atlas IIAS operations.

Operational History

Launch Sites and Campaigns

The Atlas II family of launch vehicles was exclusively launched from Cape Canaveral Space Launch Complex 36 (SLC-36), utilizing pads A and B at the Cape Canaveral Space Force Station in Florida. A total of 63 launches occurred between December 1991 and August 2004, marking the primary operational site for the program's duration. No launches took place from Vandenberg Space Force Base due to logistical and safety challenges in handling the liquid hydrogen propellants required by the Centaur upper stage in that facility. Launch campaigns for the Atlas II were coordinated by Astronautics, with commercial missions marketed and supported through Lockheed Martin Commercial Launch Services, which evolved into International Launch Services (ILS) following its formation in 1995 as a for Atlas and Proton vehicles. U.S. government missions, including those for payloads, were managed in coordination with the Space Command, leveraging the infrastructure of the 45th Wing at for and support. Vehicle processing typically began with horizontal integration of the booster, core stage, and Centaur upper stage at the Astrotech Space Operations facility in , where payloads were also prepared and mated in a controlled environment. Following integration, the fully assembled stack was transported horizontally and then erected vertically at SLC-36 via a mobile launch platform, allowing for final systems checks and fueling. This workflow minimized on-pad time and enhanced safety. A standard Atlas II launch campaign spanned 30 to 45 days, incorporating phases for payload arrival and processing, vehicle assembly, integrated testing, static fire rehearsals, and countdown operations, with the goal of optimizing turnaround efficiency for the . The Atlas II demonstrated exceptional reliability, achieving a 100% success rate over its 63 flights, which was credited to stringent pre-launch testing protocols, allowing for component-level verifications, and from prior Atlas iterations.

Notable Missions and Achievements

The Atlas II family achieved its first commercial launch on December 7, 1991, successfully deploying the telecommunications satellite into geosynchronous transfer orbit (GTO) from Cape Canaveral's Space Launch Complex 36B. This mission marked a pivotal step in transitioning the vehicle from military applications to the burgeoning commercial sector, demonstrating its reliability for private payloads. Subsequent commercial successes included multiple launches of the Galaxy series satellites for PanAmSat, such as Galaxy 3R in December 1995 and Galaxy VIII-i in December 1997, both placed into GTO to expand global broadcasting capabilities. The Atlas II family ultimately supported over 20 satellites to GTO, including high-profile missions like JCSAT-4 in 1997 and 1C in 2000, facilitating the rapid expansion of international networks for voice, data, and video services. On the government side, the Atlas II enabled critical national security and scientific missions, including deployments for the U.S. Navy's UHF Follow-On (UHF F/O) series and the (DSCS). A notable example was the final Atlas IIAS launch on August 31, 2004, which carried the classified NRO payload (also known as NROL-1 or Quasar-15) into orbit, supporting U.S. intelligence and reconnaissance operations. This mission also represented the vehicle's support for and Department of Defense payloads throughout its operational life. The Atlas II family's perfect record of 63 successful launches from to —without a single failure—established it as the most reliable U.S. expendable series in history, significantly contributing to the growth of global satellite constellations by providing dependable access to GTO for commercial operators. This reliability helped bolster the U.S. commercial launch market during the , as domestic providers like captured a substantial portion of international contracts amid rising demand for geostationary communications satellites. The vehicle's retirement was triggered by the introduction of the more capable and cost-efficient in 2002, leading to a phase-out of Atlas II operations by 2004. The USA-179 mission not only capped the program's legacy but also marked the end of launches from Complex 36A at .

Specifications

Physical Characteristics

The Atlas II family features a two-stage configuration consisting of a liquid-fueled first stage and upper stage, with optional strap-on solid rocket boosters on the IIA and IIAS variants and a for payload encapsulation. The core first stage and upper stage both have a of 3.05 m, while the optional has a of 4 m. The overall height of the vehicle is 47.5 m in the base Atlas II configuration. Height variations occur across variants due to differences in booster integration and fairing options, with the IIAS equipped with an extended fairing reaching up to 52.8 m. The gross liftoff mass ranges from 187,600 kg for the base Atlas II to 234,000 kg for the IIAS variant, which incorporates four Castor 4A solid rocket boosters. The approximate dry mass of the vehicle, including stages, boosters, and fairing where applicable, is 25,000 kg. The first stage propellant load consists of 156,400 kg of (refined kerosene) and (LOX). The upper stage carries 16,930 kg of liquid hydrogen (LH2) and LOX.

Performance Metrics

The Atlas II core stage utilizes the Rocketdyne MA-5A engine assembly, delivering a liftoff thrust of 2,110 kN for both the base Atlas II and Atlas IIA variants, while the Atlas IIAS configuration augments this with four Castor 4A solid rocket boosters to achieve a total liftoff thrust of 3,546 kN. The core stage operates with a specific impulse of 316 seconds, primarily driven by the sustainer engine, whereas the upper stage employs RL-10 engines with a specific impulse of 444 to 449 seconds across variants, optimizing efficiency for upper-stage burns. Payload capacities reflect these propulsion enhancements: the Atlas II supports up to 6,580 kg to (LEO, 185 km at 28.5°) and 2,810 kg to geosynchronous transfer orbit (GTO), the Atlas IIA increases this to 7,280 kg for LEO and 3,039 kg for GTO, and the Atlas IIAS further boosts performance to 8,610 kg for LEO and 3,630 kg for GTO. Direct insertion to is not supported without an integrated apogee boost subsystem on the payload, as the Centaur stage typically delivers to GTO. The core stage burn provides an initial velocity increment of approximately 4.5 km/s, with upper stage contributing an additional 3.5 km/s to reach GTO profiles, yielding an overall delta-v of about 7.8 km/s sufficient for LEO missions. This performance profile, combined with the vehicle's gross mass of roughly 187,600 kg influencing acceleration, ensured reliable orbital insertion capabilities. The Atlas II family's efficiency is underscored by its 100% mission success rate over 63 launches, facilitating consistent and dependable deployments without failures.

References

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