Atlas II

The original Atlas II was based on the Atlas I and its predecessors. This version flew between 1991 and 1998.[4]

Atlas IIA was a derivative designed to service the commercial launch market. The main improvement was the switch from the RL10A-3-3A to RL10A-4 engine on the Centaur upper stage.[6] The IIA version flew between 1992 and 2002.[7]

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.[6] IIAS was used between 1993 and 2004, concurrently with IIA.[8]

Workers at Cape Canaveral Air Force Station prepare to erect the first stage of an Atlas II/Centaur rocket in the launch gantry on pad 36A. Shown are the RS-56 rocket engines.

The Atlas II first stage is a 3.04 m (10.0 ft) in diameter and 28.89 m (94.8 ft), the stage is powered by 3 RS-56 rocket engines burning 156 t (344,000 lb) of both RP-1 and Liquid oxygen. The Atlas II was the last Atlas rocket to use the "1.5 staging" technique, with this technique Atlas II ignite its 3 RS-56 engines at liftoff and then jettison the 2 RS-56-OBA side engines during ascent to only use the RS-56-OSA as this engine is more efficient in high altitude.

Compared to Atlas I the Atlas II booster stage is more higher[10] of 2.7 m (8 ft 10 in), it also support up to 4 Castor 4A solid boosters, each providing an additional 478.3 kN (107,500 lb_f) of thrust for 56 seconds.

The Centaur II upper stage uses a pressure-stabilized propellant-tank design and cryogenic propellants. This Centaur is 9.06 m (29.7 ft) long, carrying 16 t (35,000 lb) of fuel, it use 2 RL-10A-3-3A engines.

For the 2A and 2AS version, Atlas uses the Centaur IIA variant which is 1 m (3 ft 3 in) longer than the Centaur II and use 2 RL-10A-4 engines.

• Centaur 2A pictures:
• 
  Centaur IIA arrives at Launch Complex 36A for the launch of GOES-L.
• 
  Centaur IIA before mating with Atlas II booster.
• 
  A close up view of RL-10 engines of Centaur IIA.
• 
  Centaur IIA of TDRS-I is lifted up.

The Integrated Apogee Boost Stage was an optional upper stage, used 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) onboard the Atlas II and, later, 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 1303kg of propellant with a dry mass of 275 kg.