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A balloon tank is a style of propellant tank used in the SM-65 Atlas intercontinental ballistic missile (ICBM) and Centaur upper stage that does not use an internal framework, but instead relies on a positive internal pressurization to keep its shape.
Balloon tanks are very light, and this permits a good propellant mass fraction. A disadvantage of balloon tanks is that if pressurization fails, the vessel collapses.