from wiki: Atlas was unusual 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 would collapse under its own weight if not kept pressurized, and had to have 5 psi (34 kPa) nitrogen in the tank even when not fuelled. The only other known use of balloon tanks at the time of writing is the Centaur high-energy upper stage, although some rockets (such as the Falcon series) use partially pressure-supported tanks.
Atlas also had a staging system different from most multistage rockets, which drop both engines and fuel tanks simultaneously, before firing the next stage's engines. Atlas ignites all three of its engines at launch; the booster engines would be discarded, while the sustainer continued to burn.
Rockets using this technique are sometimes called "stage-and-a-half" boosters. This 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.
Atlas D weighed 255,950 lb (without payload) and had an empty weight of only 11,894 lb, the other 95.35% was propellant. Dropping the 6,720 lb booster engine and fairing reduced the dry weight to 5,174 lb, a mere 2.02% of the initial gross weight of the vehicle (still excluding payload). This very low dry weight allowed Atlas D to send its thermonuclear warhead to ranges as great as 9,000 miles (14,500 km) or orbit payloads without an upper stage.
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