Balloon tank

A balloon tank is a style of fuel tank used in the Atlas ICBM and Centaur upper stage that does not use an internal framework, but instead relies on a constant internal pressurization to keep its shape.

Balloon tanks are very light, and this permits a good propellant mass fraction.

One disadvantage of balloon tanks is that if pressurisation fails, the vehicle collapses.

Balloon tanks get their name from the cylindrical party balloons which inspired Karel Bossart, who designed both the Atlas and Centaur rockets for the U.S. Air Force and NASA. Constructed of very thin stainless steel, prior to integration into the Atlas or Centaur rocket body the tanks are inflated with nitrogen to give them their shape and strength. As such, balloon tanks must always remain pressurized, as any drop in pressurization will result in failure. By contrast, non-balloon tanks in other liquid-propelled rockets remain rigid while empty due to an internal framework, although they do also depend on internal pressurization to support thrust and launch loads.

The use of balloon tanks in the Atlas ICBM was the brainchild of rocket designer Karel Bossart. Instead of the traditional internal framework used on the rockets available at that time (especially the Redstone IRBM, a direct descendant of the V-2 rocket), the Atlas ICBM designers used the balloon tank concept to lighten the rocket enough to hurl a 5 megaton thermonuclear warhead onto a target in the Soviet Union from a launch pad in continental USA. This tank technology made feasible a relatively simple stage-and-a-half design for Atlas instead of the more complicated staging used in later Titan ICBMs.

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