Design
Atlas is almost unique in its use of balloon tanks for fuel, made of very thin stainless steel with minimal or no rigid support structures. Pressure in the tanks provides the structural rigidity required for flight. An Atlas rocket will collapse under its own weight if not kept pressurized. The only other known use of balloon tanks at the time of writing is the Centaur high-energy upper stage.

Atlas also has a unique and somewhat odd staging system. Most rockets stage by dropping both engines and fuel tanks. However, when the Atlas missile was being developed, there were considerable doubts as to whether or not a rocket motor could be ignited in space. Therefore, the decision was made to ignite all three of the Atlas' engines at launch - later, two of the engines would be discarded, while the third continued to burn. Rockets using this technique are sometimes called stage and a half boosters. This technique is made possible by the extreme light weight of the balloon tanks. The tanks make up such a small percentage of the total booster weight that the weight penalty of lifting them to orbit is not offset by the technical and weight penalty required to throw half of them away mid-flight. Depending on how you look at it, this makes Atlas a single-stage-to-orbit booster (though most call it a 1.5 stage to orbit).


Current Atlas Family
Atlas II
Atlas III
Atlas V
GX
The Atlas II series had 63 successful flights with the last launched August 31, 2004, it is considered the most reliable launcher in the world.

The newest version of Atlas, the Atlas V, is an Atlas in name alone as it contains little Atlas technology. It no longer uses balloon tanks nor 1.5 staging, but incorporates a rigid framework for its first stage booster much like the Titan family of vehicles. Ironically, given Atlas's origin as a military weapon, the Atlas III and Atlas V use Russian-designed engines.