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, the tanks, prior to integration into the Atlas or Centaur rocket body, are inflated with nitrogen to give the tanks their shape and strength. While other rockets, including the Titan and Saturn rockets (and even the Space Shuttle's external tank) can remain rigid due to the internal framework, balloon tanks must always remain pressurized, as any drop in pressurization will result in the failure of the vehicle.
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 former 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.
After its initial development in the Atlas rocket, Bossart used the same technology with the high-energy Centaur upper stage. The Centaur rocket, fueled with liquid hydrogen and powered by an RL10 engine, was originally planned to be used with the Saturn V rocket for high-energy missions to the Solar System, but was later adapted for use as part of the Atlas and Titan rockets.
In May 1963 at Vandenberg Air Force Base an Atlas/Agena vehicle under static testing suffered a pressurisation failure leading to the total collapse of the vehicle. Fortunately, only a dummy payload was installed.
With the introduction of the Atlas V EELV, balloon tanks and half-staging are no longer used on the Atlas rocket. Centaur, however, retains this feature and it has not undergone a single catastrophic failure since the Atlas V's first flight.