roswell

~G = G X ~X (1 • K) K
T
where t::.. G = loss of lift
G =gross load (balloon weight plus
equipment load)
AT : mean temperature difference in
lifting gas before and after sunset
T =free air temperature
K = specific gravity of lifting gas,
relative to air
The specific gravity of 98,% helium, diluted with air, and with
respect to air, is 0.157. It maybe noted that with a lower specific
gravity of a gas, lower ballast corrections are required. Hydrogen, for
example, requires half the ballast Which helium requires for the same
temperature differential. At high altitudes, a difference of 40°C may
be expected in the temperature of the lifting helium from day to night.
This would correspond to a loss of lift ut sunset, on a General Mills 20-
foot balloon, or about 550 grams.
F. Internal Pressure
The maximum internal pressure which can be held within a
spherical container is given by Timoshenko8 :
P • 28 u x t
I"
where Su is the ultimate strength of the material
in tension, t is thickness or the material and r is the radius or the spherical
shape. Applying this equation to a polyethylene film, such as used in the
General :Mills 20-foot balloons, Su at room temperature : 1900 psi., t =0.001",
and r : 10 ft., giving the maximum pressure, P =0.032 psi. This pressure is·
equivalent to about 1.1 inches of water, or 2.5 mb. This small bursting pres-
sure necessitates proper inflation and load values to prevent the balloon's
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