B. Altitude Control Equipment
Flights of 20-foot General Mills balloons, using no control equipment, have been sent to altitudes of about 50,000 feet. After reaching maximum altitude, the balloons all exhibit a tendency to float then descend at an increasing rate for periods of from 2 to 6 hours. In stable layers of air such as the stratosphere the descent of a balloon is retarded by the helium, on compression, getting warmer than the surrounding air. This results in much longer duration flights requiring no external control though, strictly speaking, the altitude is not constant. This concept is in good general agreement with the observed data; balloons have remained in a semi-floating state much longer (up to 30 hours) when in the stratospheric inversion than when in less stable lower atmospheric layers.
When it is desired to maintain a balloon at constant level for a guaranteed period of time in excess of two hours, a ballast system of altitude control should be added to the flight gear. The level at which the balloon is to float must be the maximum altitude to which it can carry the payload. To compensate for loss in buoyancy occasioned by loss of lifting gas through diffusion and leakage, a continual lightening of the load is required. To effect this in a simple fashion, liquid ballast is permitted to flow through an orifice at a predetermined rate which exceeds the expected loss of lift. (See Section IV, D) The reservoir and ballast assembly which has been developed for this use is shown in Figure 6. A detail sketch of the orifice in its mounting is shown as Figure 7, and Figure 8 shows a suitable filter which must be used to protect the orifice from clogging. The liquid ballast must (1) not freeze, but flow well at cold temperature (-80°C); (2) not absorb water, which would freeze; and (3) be relatively inexpensive. A recommended liquid is Aeromobil Compass Fluid, made by Socony-Vacuum Co. (Air Force Spec. AN-C-116).
There are three possible objections to the use of this simple control system. First, a continued lessening of the total weight on the balloon—with no change in volume—must result in a constantly rising ceiling. For a 20-foot balloon at 45,000 feet, this change is approximately 1000 feet with each kilogram of ballast dropped (see Section IV, E). Second, only a prefixed ballast flow is permitted, and excessive loss of lift, as might come when the gas is cooled at sunset (when the balloon loses superheat),
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