volume of the 20-foot cell is about 4300 cubic feet and its uninflated length is 38 feet. It is made up of 20 goras, heat sealed together in a butt weld. Along the seams thus formed, a special acetate-fiber scotch type tape (Minnesota Mining and Mfg. Co.,) is laid to reinforce the weld and to carry and distribute the load. These tapes converge to an appendix ring at the balloon bottom, to which the load harness is attached. By using this stressed tape design, much larger loads may be carried than the thin polyethylene alone could hold. To exclude air entering through the bottom, which is left open, an external skirt or appendix is added.
Figure 1 shows a 20-foot balloon ready to be released, with an external appendix in position. As the balloon rises, the lifting gas inside will expand until the balloon is full, whereupon the excess gas which was needed to make the balloon rise will be valved out. The full balloon will then float at a level where the buoyancy just balances the load. It will remain there until buoyancy is lost by diffusion of the lifting gas, or by cooling, as at sunset.
Neglecting minor effects, the amount of gas which is needed to just balance the load at the maximum or floating elevation would also just balance the load at any lower level, including the surface, although the balloon would be less than completely full at such a lower level.
B. Load Limits
For a given lifting gas, the altitude to which a balloon will rise is determined principally by the load it bears. With a 20-foot General Mills balloon, using helium, a payload of 40 pounds will reach approximately 46,000 feet and an 18-pound load will go to about 58,000 feet. Although the manufacturers recommend keeping the payload between 18 and 40 pounds, no trouble has been found in launching loads of as much as 70 pounds (37,000 feet) or as small as 4 pounds (67,000 feet).
C. Appendices
For highest altitudes and smallest sunset effects on a balloon, it is necessary to keep air from diluting the helium. To accomplish this, a check valve is required in order that helium may be valved when the balloon is full, yet air not be permitted to enter at any time. An appendix, consisting of a tube of balloon material, whose length is about 2 to $2\frac{1}{2}$ times its diameter is used for this purpose, and is supplied as part of the General Mills balloon.
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