Long endurance high altitude balloon

A high altitude, high pressure balloon comprised of a number of gores. The gores are made from a material and have a size, that are the results of an analysis of the anticipated stresses to which the balloon will be subjected. In particular, the gores are made from nylon and have dimensions that have been determined to result in stable creep characteristics. An alternate embodiment of the balloon uses multiple spheres.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to high altitude balloons, and more particularly to 
a high altitude balloon designed to remain aloft for extended time 
periods. 
BACKGROUND OF THE INVENTION 
Although balloons have been used for useful applications for over 200 
years, for over 160 of these 200 years, balloons were made from rubber and 
fabric, and were too heavy to escape the lower atmosphere. It is only in 
recent years that the development of plastic films has made high altitude 
balloons possible. These films, first developed in the 1940's, have 
permitted the manufacture of balloons that can reach the stratosphere and 
near space. 
High altitude balloons are designed to carry a payload into altitudes as 
high as 120,000 feet or higher. These balloons stay aloft in the air by 
being filled with lighter than air lift gases. Typical uses of high 
altitude balloons are communications and observation platforms and 
geophysical and astrophysical research. 
Many high altitude balloons are "low pressure" balloons and are vented so 
that the lift gas inside the balloon may escape. This helps preserve the 
integrity of the balloon material, but encourages the balloon to change 
volume, which causes its altitude to change. Ballast is used to help 
maintain a constant altitude. However, venting and ballast are effective 
only to the extent that low pressure balloons are useful for short 
duration flights in the order of several days. 
High pressure balloons are another approach to maintaining constant volume. 
A problem with high pressure balloons, however, is that the balloon must 
not only withstand stresses due to the payload, but also those produced by 
pressurization. These stresses affect the characteristics of the material 
used to construct the balloon, such as by stretching or deterioration, and 
cause the volume of the balloon to change or cause destruction of the 
balloon. 
One characteristic of potential balloon materials, that determines their 
suitability for high pressure balloons is known as "creep". Creep is a 
mechanical behavior of materials that continue to strain with time when 
subjected to a constant stress even at a constant temperature. More 
technically, creep is the time-dependent portion of strain. For 
creep-susceptible materials, increasing either stress or temperature 
increases creep. When creep is present, material failure may occur at 
stresses or temperature that are below those present during short duration 
uses. Certain materials, notably nylon, have been rejected in the past due 
to their susceptibility to creep. 
Creep is only one property of potential balloon materials that affects the 
success of the balloon. Also, good properties in one area often detract 
from the properties in another area. Thus, selection of materials is an 
important decision in the design process. A common material used for 
existing high pressure balloons is polyester, such as Mylar, which has 
good strength and modulus characteristics. Also used are layers of 
different plastic films, with each layer selected for certain desired 
properties. 
A problem with previous attempts to maintain long duration flights of high 
pressure balloons is failure of the balloon material. These failures are 
attributable to a number of factors, especially including temperature 
extremes and high gas pressures. Furthermore, the failure rate increases 
as the payload and therefore the balloon size and pressure increase. The 
high failure rate of high altitude, high pressure balloons, combined with 
the expense of trial and error balloon testing, has led to a reluctance to 
experiment with new materials. A need exists for a balloon that can be 
especially designed to withstand the pressures and temperatures of high 
altitude, long duration flights. 
SUMMARY OF THE INVENTION 
The invention comprises a high pressure balloon manufactured from multiple 
gores, which are made from a polyamide material such as nylon. The overall 
shape of the gores result in a generally spherical shaped balloon. The 
dimensions of the gores are determined according to a mechanical analysis 
that accounts for material creep by determining an appropriate gas fill 
pressure for a desired balloon volume. An additional feature of the 
invention is that the gore dimensions may be selected according to a 
thermal analysis. 
An alternative embodiment of the invention comprises a multiple sphere, 
high pressure, high altitude balloon. This embodiment has the general 
shape of at least two intersecting spheres, and can be referred to as a 
"multicell" balloon. A belt constrains the spheres at their intersection. 
A technical advantage of the invention is that a high altitude balloon is 
provided that is useful for flights of long durations. For example, using 
balloon designed according to the invention, a flight supporting a 50 
pound payload for one year is believed to be feasible. Various methods of 
material and thermal analysis make it possible to predict the balloon's 
behavior at the anticipated stresses. Without such an analysis, the 
expense of constructing test balloons greatly limits experimentation with 
new materials. 
Multiple sphere embodiments of the invention provide an improved lift to 
weight ratio. This shape takes advantage of the low stresses inherent in 
spherical shapes with a reduction in balloon diameter.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a perspective view of a high altitude balloon 10, which is 
generally spherical in shape. However, because the spherical shape is 
difficult to maintain during use, the shape often departs from spherical, 
although it tends to maintain symmetry. Thus, balloon 10 is equivalently a 
"single cell" balloon 10. As explained below, an alternative embodiment of 
balloon 10 is comprised of multiple spheres, and is equivalently a 
"multi-cell" balloon 10. 
In practice, the spherical shape of balloon 10 is the result of its being 
fabricated from a number of gores 12 cut from flat sheets of material. The 
shape becomes more nearly spherical with an increased number of gores 12. 
A typical balloon 10 has any number of gores 12, depending on its size. 
Balloon 10 is designed to carry aloft a payload 20 secured to balloon 10 
by means of load lines 19 that are attached to the upper apex of balloon 
10 and run down the sides of balloon 10. Any number of load lines 19 may 
be used, as is appropriate for the strength of each load line 19 and the 
weight of the payload 20. The load lines 19 are attached in a manner that 
minimizes stress at any one particular point, by techniques known in the 
art. 
The size of balloon 10 is determined by various parameters, including the 
weight of payload 20 that balloon 10 will carry and the altitude at which 
it is to float. The parameters used in designing balloon 10 are a feature 
of the invention and are discussed in further detail below. 
The edges of each gore 12 are sealed to the edges of adjacent gores 12 by 
any one of a variety of means. The construction of such balloons by 
cutting and sealing gores 12 is known in the art of balloon manufacture. 
One process for constructing a low pressure balloon, which may be used to 
construct balloon 10 is described in U.S. Pat. No. 4,877,205, which is 
incorporated by reference herein. 
Balloon 10 is sealed and not vented, as in low pressure balloons. Thus, at 
the upper end of gores 12, where they meet at the upper apex, an end cap 
14 is sealed in. Also, at the bottom apex, a flange and gasket 16 are used 
to seal that end, with a connection fitting 18 for filling balloon 10 with 
gas. This placement of flange and gasket 16 and fitting 18 is not material 
to the invention and their positions could be reversed. 
FIG. 2 illustrates an additional step in the process described in U.S. Pat. 
No. 4,877,205. This additional step is particularly useful for 
construction of balloon 10, whose shape is more spherical than the shape 
of low pressure balloons. A problem with the wide gores 12 is that the 
workers' ability to reach across table 20 as they fold and shape each gore 
12, is limited. To ease the manufacturing process, the gore material is 
longitudinally folded as it is laid on table 20. Each gore 12(i) is folded 
longitudinally, in half, on table 20 rather than being spread out its 
entire width. Index line 40 is marked on table 20 to indicate a half gore 
width rather than a full gore width. The fold of gore 12(i) is placed 
along index line 40. The edges of the folded gore 12(i) are placed along 
the table edge 28, as bottom and top edges. Each next seal is made to the 
bottom edge of the folded gore 12(i), while the top edge is folded back. 
After the seal is made, the seam is pulled away from table edge 28 to 
leave workspace for sealing the next seam. 
The material used for balloon 10 is an important feature of the invention. 
Balloon 10 is constructed of a film of polyamide plastic, such as nylon 6, 
which is referred to herein as the "gore material". In the preferred 
embodiment, the gore material is a commercially available brand of 
biaxially oriented nylon 6, known as Emblem and manufactured by the Allied 
Signal Corporation. This material has a high modulus, high tensile 
strength, and good thermal properties. 
The thickness of the gore material is another design parameter. For a given 
gore material, a greater thickness may be used to contain gases at higher 
pressures. Of course, increased thickness results in a heavier balloon, 
which must be considered when determining balloon size, i.e., volume. A 
typical thickness that might be used is 0.48 mil. 
Because biaxially oriented nylon tends to lose its orientation if heated 
above its shrink temperature, these films cannot be heat-sealed directly 
to each other to form balloon 10. Thus, referring again to FIG. 1, a 
special tape 13 is used to butt-seal gores 12 to each other. To seal gores 
12, an adhesive is applied to one side of tape 13. A special tape 
dispensing apparatus folds tape 13 longitudinally and presents the 
adhesive-faced tape 13 along the edge of the gores 12 to be sealed. An 
alternative method would use tape 13 on both sides of the seam. The 
preferred adhesive is a polyamide hot melt adhesive, for example, the 
UNI-REZ 2654 adhesive manufactured by the Union Camp Corporation. 
As stated above, the volume of balloon 10 is a design parameter that is 
determined, for the most part, by the user's desired altitude and payload. 
For balloon 10, for which constant volume is desired, there are two 
governing parameters: the maximum and the minimum stress to which the 
balloon 10 will be subjected. 
The stress to which balloon 10 is subjected is a function of maximum and 
minimum temperatures to which balloon 10 will be exposed. The 
supertemperature is a major factor in the superpressure of balloon 10, 
where "supertemperature" is the temperature difference between the lift 
gas and the ambient air and "superpressure" is the pressure above the 
atmospheric pressure at altitude. In general, the maximum and minimum 
supertemperatures correspond to the day and night temperatures of the gas 
inside balloon 10 at float altitude. At night the gas temperature may 
become close to the ambient temperature, but during the day the 
temperature increases and causes an increase in pressure. 
Thus, the maximum supertemperature determines the maximum expected pressure 
in the balloon material, which determines the stress levels for which 
balloon 10 must be designed. The minimum supertemperature determines the 
amount of gas in the balloon needed to prevent loss of volume due to loss 
of pressure. An accurate analysis of these temperature extremes and their 
effect on the gore material is necessary to prevent overpressurization or 
loss in volume. 
An initial step in the design analysis is forecasting the temperatures of 
the balloon gas. This process includes not only a forecast of the ambient 
temperatures during day and night, but also optical and thermal analysis 
of the gore material. The latter analysis includes determining 
characteristics such as transmissivity, reflectivity, and emissivity, at 
both solar and infra red wavelengths. 
FIGS. 3-5 illustrate an analysis used to determine the creep properties of 
the gore material used for the preferred embodiment. As explained below, 
these charts illustrate that for a given material and applied pressure, 
creep can be analyzed at various temperatures to determine where it is 
stable. Stated another way, the creep of the gore material at anticipated 
stresses and temperatures can be analyzed to determine if the gore 
material is suitable and for how long it can be expected to remain 
suitable. 
FIG. 3 is a strain-time chart of a sample of gore material to which 5035 
pounds per square inch (psi) are applied in a uniform direction at room 
temperature. The applied stress is in the "machine direction" of the gore 
material, which is rolled off in strips during manufacture. This machine 
direction is the same as the meridional direction of balloon 10 because of 
the manner in which gores 12 are cut from stripped-off portions of a roll 
of gore material. 
It is seen from FIG. 3 that at first, there is an essentially instantaneous 
deformation. Then, the strain continues to increase with time, but at a 
decreasing rate until the strain becomes nearly constant. This stage is 
often referred to in the art of materials engineering as "second stage 
creep". At that stress and temperature, the creep is considered to be 
stable. After the second stage creep, a point is reached, after which the 
creep rate increases again during what is referred to as third stage 
creep. 
FIG. 4 is a strain-time chart, which charts the same parameters as FIG. 3, 
except that time is on a logarithmic scale. Also, time is charted in 
seconds, across a greater range than in FIG. 3. For example, one day is in 
the order of 10.sup.3 seconds, and one year is in the order of 10.sup.7 
seconds. The linear portion of the strain to time relationship illustrates 
stable creep over a duration of time at the given stress and temperature. 
FIG. 5 is a third strain-time chart, which charts both axial and hoop 
stress. For purposes of the analysis of FIG. 6, the gore material is 
formed into a cylinder, the ends are sealed, and the cylinder is 
pressurized. The axial strain is the result of stress along the axis of 
the cylinder as a result of the pressurization, together with stress 
resulting from an additional applied load so that the behavior of the 
cylinder will approximate that of a pressurized sphere. The hoop strain is 
the result of stress around the circumference of the cylinder as a result 
of the pressure. 
FIGS. 3-5 illustrate the gore material analysis used to construct balloon 
10. For different balloon volumes and pressures, materials, and 
temperatures, the strain and therefore the range of creep stability will 
vary. However, the significance of the analysis is that potential gore 
materials can be successfully analyzed and tested. For a given gore 
material, a range of permissible strain levels at various temperature is 
determined. From these strain levels, corresponding stress levels can be 
determined. These are the stress levels for which balloon 10 is then 
designed. 
FIG. 6 illustrates the steps of an analysis of the preferred gore material, 
for determining acceptable stress levels. The input data are the strain 
levels derived from the analysis of FIGS. 3-5. For example, if it is 
determined that a 5035 psi stress will result in a stable strain rate of 
0.07, such as indicated in FIGS. 3-5, then those parameters can be used to 
determine if the resulting stress is below the yield stress. To perform 
the test, a varying load is applied to a test sample of gore material, 
having a known length (L) in both the machine direction (MD) and 
transverse direction (TD) at a fixed strain rate. The material is 
stretched at a constant velocity (V). For example, in FIG. 6, V=2 inches 
per minute and L=4 inches. The yield stress is determined by finding the 
break in linearity, i.e., the yield stress point, in the stress-strain 
relationship. As indicated, at a maximum expected supertemperature of 23 
degrees centigrade and a minimum expected supertemperature of -30 degrees 
centigrade, the expected stress is below the yield stress. 
The results of the creep analysis of FIGS. 3-5 and the stress-strain 
analysis of FIG. 6 can be used to compile the properties of a potential 
gore material to determine if it is suitable. Referring to FIG. 6, the 
modulus, i.e., stress divided by strain is 360 ksi and 542 ksi for the two 
different temperatures, where ksi is kilopounds per square inch. The 
machine direction proportional limits are 7000 and 15,000 for the two 
temperatures, and the transverse direction proportional limits are 7,500 
and 17,500. In practice, due to the difficulty of determining the 
departure from linearity in the analysis of FIG. 6, an offset line, 
parallel to the first linear portion of the data but offset by a strain 
value of 0.01, is drawn such that it intersects the second linear portion 
of the data. The point where this line intersects the data is referred to 
as the "offset yield stress", and the corresponding value is used rather 
than the proportional limit to determine stress limits. 
Using the above-described analysis, an appropriate size and shape of 
balloon 10, and hence of its gores 12, can now be determined. The stress 
must be sufficiently low to prevent creep and stay below the offset yield 
stress. It must be sufficiently high to maintain a positive pressure at 
night during low temperatures. 
FIGS. 3-6 illustrate only one example of an analysis for a given material 
of a given thickness for a given altitude and payload. If any of these 
parameters are varied, the same type of analysis can be used to predict 
the suitability of a balloon 10 for its intended use and to design its 
optimum size. 
FIG. 7 is a perspective view of an alternative embodiment of the invention, 
a high altitude, high pressure balloon 70 having a vertical column of two 
intersecting spheres 71(a) and 70(1). The number of spheres is not limited 
to two; the advantages of the multi-sphere balloon 70 can be obtained with 
any number of intersecting spheres. 
FIGS. 8A and 8B illustrate the advantages of the multi-sphere balloon 70 
over the single sphere balloon 10. As stated above, the result of using a 
balloon 10 having this shape is that the same overall volume may be 
accomplished with less weight. Although the multi-sphere balloon 70 has a 
poorer volume to surface area ratio when compared to a single sphere, 
because the radius of curvature is reduced, thinner gore material can be 
used. 
The distances between the centers of spheres may be anywhere between zero 
and twice the radius for spheres of equal radius. For spheres of unequal 
radius, r1 and r2, where r1&lt;r2, the distances between the intersecting 
centers may range between r1-r2 and r2+r1. Ideally, the distance between 
the centers is determined so as to maximize the volume to weight ratio. 
An important factor in the design of balloon 70 is controlling the stress 
at the intersection of the spheres 71(a) and 71(b). This intersection area 
tends to develop high circumferential loads, and thus a belt 72 is placed 
at that region that carry those loads. 
Other Embodiments 
Although the invention has been described with reference to specific 
embodiments, this description is not meant to be construed in a limiting 
sense. Various modifications of the disclosed embodiments, as well as 
alternative embodiments will be apparent to persons skilled in the art. It 
is, therefore, contemplated that the appended claims will cover all 
modifications that fall within the true scope of the invention.