the balloon can rise slowly until the balloon is again full and the
equilibrium is again reached between the buoyancy and the load. In tb3
General Mills 20-foot balloon, for example, diffusion losses equal about
300 grams per hourJ the balloon at its ceiling of 50,000 feet, with a
30-pound payload, risea about 900 feet with each kilogr~ of ballast
dropped. This means that a balloon, using the simple ballast-dropping
technique, will float at a ceiling·which rises at the rate of about 360
feet per hour. An idealized flight of this type is shown in the llolid
curve of Fig. 7., neglecting the oscillation shown at sunset.
The "manual ballast valve" Which was developed for this simple
control system is shown in Fig. a. This valve can be adjusted prior to
balloon release to allow any predetermined flow of compass fluid up to
2000 grams per hour. The filter housing and ballast reservoir used
with this valve are shown in Figures 9 and 10. This method is good where
l) a slowly rising ceiling can be tolerated, and 2) the flight tloes not
have to go thrcugh a sunset while at its ceiling.
For economy of ballast, hence longer flight duration, it is desirable
to keep the constant flow as close as possible to the total loss of buoyancy
resulting from diffusion and leakage. This means that whenever rapid loss
of buoyancy occurs, due to changes in solar radiation, the manual ballast
valve alone will not sustain the balloon. When the balloon is suddenly
cooled, due to sunset or clouds cutting off in§olation (loss of superheat),
the heavy loss will start the balloon downward and only a rapid expenditure
of ballast will check its fall and restore its stability.
The second type of ballast dropping control has been devised to
operate on a demand basis, when such a descent occurs. This control is
called the automatic ballast valve. Figures 11, 12 and 13 shaw the ap-
pearance and design of this pressure-actuated needle valve.
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