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AuGusT 1948 ATHELSTAN F. SPILHAUS, C. S. SCHNEIDER, AND C. B. MOORE 131
was controlled by a baroswitch arrangement which
dropped a bag by igniting a fuse when the altitude fell
below any one of four different levels between 25,000
and 5000 ft. In addition, a delay mechanism consisting
of a two-minute fuse was arranged between successive
switches so that after ballast was dropped, two minutes
would be allowed for the balloon to regain its altitude;
if it did not regain in this time another bag of ballast
would be dropped. The system was inefficient because
if any one of the thirty-six fuse arrangements failed,
no more ballast was dropped.
The second type of Japanese balloon was similar,
in general, but slightly larger; it was made of oiled
silk and therefore would stand a greater internal
pressure (approximately six inches of water). The
higher the internal pressure that the balloon can
stand, the less gas need be valved under conditions of
superheating or altitude fluctuations. The Japanese
released many balloons of these types from their
islands and estimated five to seven per cent of those
released reached the west coast of this country. The
balloons floated between the surface and 30,000 ft
above sea level; those which reached the west coast
must have remained aloft from four to ten days. While
the altitude maintained was not constant, these bal-
loons were highly successful for the time they remained
in the air.
An attempt in this country was made in 1943 by the
Dewey and Almy Company, to obtain constant-level
balloons which would float at altitudes up to 15.,000
ft. An ordinary 350-gram meteorological balloon was
used but its volume was controlled by a nonextensible
shroud around it. With this method a flight at about
5000 ft was obtained at fairly constant altitude for
about an hour and a half.
3. Design of controlled-altitude balloons
As a result of the Japanese and other expeiiments,
the use of a nonextensible envelope for the balloons
was indicated. If a perfectly nonextensible balloon
could be built with no diffusion through the walls, and
which could withstand a high internal pressure, it
would automatically stay at a constant density where
the buoyancy of the full balloon equaled the load. In
practice, control devices are needed to offset the leak-
age and diffusion of gas, to compensate for vertical
currents in the atmosphere, to correct for the motion
of the balloon due to diurnal changes of the balloon's
temperature, and to compensate for the valving of gas
which is necessary to prevent rupture of the envelope.
It was decided to use a plastic as the balloon fabric,
as some modern plastics are quite transparent to radia-
tion, strong, easily fabricated, and relatively inexpen-
sive as compared with coated fabrics.
A. Choice of plastics.-ln the selection of a plastic
material of which to make the balloons, the desirable
properties are: (a) low brittle temperature, (b) low
permeability, (c) high tensile strength, (d) high tear
resistance, (e) chemical stability, (f) high radiation
transmission or reflection. Polyethylene soon recom-
mended itself for use, with its brittle temperature of
below -80F. It is apparently unaffected by ultraviolet
and ozone. The permeability through one mil of thick-
ness and one square meter of area for 24 hours is ten
liters for hydrogen and seven liters for helium, at
normal atmospheric temperature and pressure.
FIG. I. Polyethylene balloon, 20-ft diameter.
Polyethylene is also relatively easy to fabricate. It
has an ultimate tensile strength of 1,900 pounds per
square inch at 2SC, which, in a 15-ft balloon made out
of four-mil fabric, represents a working pressure of
about 2.3 inches of water. The tensile strength at the
temperatures at which the balloon flies at high altitude
may be more than three times the value quoted above.
Fig. 1 shows a polyethylene balloon2 flown success-
fully in Flight 26 described below. Another film in-
vestigated is Saran, which has ten times the tensile
strength of polyethylene-three times the strength
across the seams. Saran has a higher transparency and
one-thirtieth the permeability of polyethylene. The
effective brittle temperature of Saran for this work is
not known reliably.
B. Ballast valve.-The altitude control is an auto-
matic ballast-dropping device3 consisting essentially of
2 Made by General Mills, Inc.
1 Made by Kollsman Instrument Division of Square D Com-
pany.