The Tubular Gore Tetroon (TGT) is my solution to the “Upson Ring” (UR) problem. When a large balloon is first launched for a very high altitude flight the gas at sea level pressure is only a small bubble. All the weight of balloon fabric and payload hangs on the circumference of that bubble. This is the UR. One could compare the load at the UR to the breaking length of a piece of cord. A 100 pound breaking strength cord that weighs one pound per foot can only be hanging 100 feet.. Any additional length paid out will break the cord.
If one tied one pound lift balloons to the cord at each one foot point on the cord, the breaking length would be unlimited.
A tetrahedral balloon can be fabricated with one web of fabric that is spiral formed in a short tube. (Like one ply of a toilet paper cardboard core.) If the ends are sealed off skewed at 90 degrees to each other and the two lines from one end of one end seal to the two ends of the opposite seal are the same length as the end seal, an equilateral tetrahedral surface is formed . It is made up of four equilateral triangles.
Such a tetrahedron is termed a “Tetroon”.
The payload is attached to one vertex. The center of the opposite triangle becomes the top center of the balloon. The web of the tetroon’s skin arbitrarily starts at the load point corner and goes to a second corner at the other end of the end seal and spirals around to finally end up along the horizontal seal between the opposite corners.
For the TGT the web is a tube of balloon material. Its diameter is such that the unit lineal volume has an aerostatic buoyancy slightly greater than its weight. The tube is open to the central volume of the tetrahedron only near the end of the tube that is at the loading point (the bottom).
At initial inflation, only the tube is inflated with just enough gas for payload and free lift discharged into the central volume. The bubble in the central volume is very small and has comparatively little buoyancy against its small UR. As the balloon rises, gas discharges (by expansion) into the central volume. In effect, the central volume is being inflated at altitudes where the atmospheric pressure is lower. With the lower density the lifting gas has lower unit lift and so puts a lower unit stress on the skin.
In the case of an ultra high altitude balloon where the payload is only a very small percentage of the total load this economy can be dramatic.