2.3.3 Size and Shape
The overall shape of the SPB is an oblate spheroid (like a sphere, only squashed
on the top and bottom) with the height about 60% of the diameter. It consists of
many panels called gores that run from the top to the bottom. At the edge of each
gore is a very strong and lightweight tendon that also runs from top to bottom to
maintain the structure of polyethylene super-pressure balloon. These tendons are
often made of braided Zylon strands. This is the material of choice due to its low
weight, high tensile strength, and thermal properties. A gore has a slightly curved
lobed shape when under pressure, which is why this type of envelope is known as
a “pumpkin” balloon.
Even with very small internal pressures, the forces are quite large because of
the size of the balloon. The strength required for the film materials is a function of
the internal pressure times the radius divided by the film thickness. If the balloon
were spherical in shape, this very large radius would lead to a very high strength
requirement for the film. But higher strength requirements often require stronger
materials and those are often heavier. The films used for the NASA balloons are
very thin and lightweight. See Chapter 3 for details.
Fig. 2.5 Artistic rendering of a “pumpkin” balloon. Courtesy of NASA
18 Stratospheric Balloon Descriptions
The overall shape of the SPB is an oblate spheroid (like a sphere, only squashed
on the top and bottom) with the height about 60% of the diameter. It consists of
many panels called gores that run from the top to the bottom. At the edge of each
gore is a very strong and lightweight tendon that also runs from top to bottom to
maintain the structure of polyethylene super-pressure balloon. These tendons are
often made of braided Zylon strands. This is the material of choice due to its low
weight, high tensile strength, and thermal properties. A gore has a slightly curved
lobed shape when under pressure, which is why this type of envelope is known as
a “pumpkin” balloon.
Even with very small internal pressures, the forces are quite large because of
the size of the balloon. The strength required for the film materials is a function of
the internal pressure times the radius divided by the film thickness. If the balloon
were spherical in shape, this very large radius would lead to a very high strength
requirement for the film. But higher strength requirements often require stronger
materials and those are often heavier. The films used for the NASA balloons are
very thin and lightweight. See Chapter 3 for details.
Fig. 2.5 Artistic rendering of a “pumpkin” balloon. Courtesy of NASA
18 Stratospheric Balloon Descriptions