1959, the Atlas missile was the US's 1st liquid fueled ICBM. Its use of balloon tanks for fuel, made of very thin stainless steel with no rigid support, pressure in the tanks provides the rigidity required for flight.
Atlas (and its 'bigger brother', the Titan) were phased out in favor of the LGM-30 Minuteman, a solid-fuelled rocket which could be stored for long periods and launched, without fueling, at the turn of a key.
Atlas also had 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 launchâ€â€ÂÂlater, 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 extremely 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 less than the technical and weight penalty required to throw half of them away mid-flight.
