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National Aeronautics and Space Administration

Goddard Space Flight Center

Scientific Ballooning.

Balloon Program Office

Super Pressure Balloon Design Approach

A balloon that always maintains a positive internal pressure in relationship to the environment it is floating in is called a Super Pressure Balloon. The NASA Super Pressure Balloon is a sealed structure that is filled with a measured and specific amount of Helium lifting gas. The balloon rises after launch, and the Helium expands as the ambient atmospheric pressure goes down. The NASA Super Pressure Balloon is designed to fly at a specific pressure altitude with a known mass of payload hanging from the balloon.

When the balloon reaches the desired float altitude, the extra Helium is not vented off, but fills out the shape and pressurizes the balloon. The amount of Helium first put into the balloon is determined by how much is needed to lift the entire flight system plus some extra Helium to provide an upward force. This extra Helium is enough to pressurize the balloon when it reaches the float altitude, but too much to over pressurize the balloon. The Super Pressure Balloon is designed to fly with a positive internal pressure at all times. When the sun heats the balloon during the day it has a higher internal pressure (also called differential pressure since this represents the pressure difference above the atmospheric pressure it is flying in), and at night when the balloon cools down, the differential pressure is much lower, but still above ambient. The differential pressure range of the NASA Super Pressure Balloon is up to 180 Pa (0.0261 psi). This is a very small internal pressure, but it is enough to keep this balloon flying through the night!

Depiction of the shape of a Super Pressure Balloon from the side at float
Super Pressure Balloon from the side at float

The overall shape of the NASA Super Pressure Balloon is an oblate spheroid (like a sphere only squashed on the top and bottom). The height is about 60% of the diameter. The NASA Super Pressure Balloon is made up of many separate panels or gores that run from top to bottom on the balloon. At the edge of each of these gores is a very strong and light weight tendon or rope that runs from top to bottom on the balloon. Each of these gores are shaped that, while under pressure, have a slightly curved lobed shape. This is why this type of balloon has been called a pumpkin shaped Super Pressure Balloon.

Up looking view of the Super Pressure Balloon at float
Up looking view of the Super Pressure Balloon at float

Even with very small internal pressures, because of the size of the balloon, the forces are quite large. The strength required for the film materials is a function of the internal pressure times the radius divided by the film thickness (Pr/t or Pressure times the radius divided by the film thickness). If the balloon was spherical in shape, this very large radius would lead to a very high strength requirement for the film. High strength requirements often require stronger materials which are often heavier. The films used for the NASA balloons are very thin and light weight.

The Super Pressure Balloon is basically made up of two elements; very thin film and the tendons that run top to bottom. There are two methods NASA chose to reduce these very high strength requirements. The first was to make each of the gore sections slightly lobed and reducing the local radius of the film. This reduced the radius on the film from over 50 m to around 1 m. Reduced radius translates to reduced strength requirements in the horizontal or “hoop” direction for the film. Second, the tendons that run top to bottom in the balloon are used to resolve the forces in that direction. Each tendon can have approximately 7,000 N (~1,600 lbs) per tendon, which means the fittings on the top and bottom of the balloon must resolve a total force of ~1,960,000 N (~448,000 lbs) for the 280 tendons that are used in the balloon.

Lobing in the Super Pressure Balloon gores is visible
Lobing in the Super Pressure Balloon gores is visible

Close up view of balloon lobing
Close up view of balloon lobing

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