various cluster arrangements. None of these have been too satisfactory
but further investigationwill be made in the field of shrouded or ooated
films.
B. Altitude Controls
Given a balloon capable of caJTying the instruments to a desired
altitude (the theory and computations involved are discussed in Section 3),
there remains the problem of maintaining the cell at a constant level. The
buoyancy of a gas-filled cell will decrease as the cas leaks or diffuses
through the balloon wall. To hold an absolutely om.stant altitude, the
volume of lifting gas entrapped must be maintained in an atmosphere of
unvarying horizontal density, with no change in the total weight supported
by the balloon and with no fluctuations of the temperature of the gas with
respect to the air. The best approximation to these conditions may possibly
be achieved through the use of liquified hydrogen, which would be permitted
to evaporate at a rate in excess of gas leakage. The weight of equipment
required to control this evaporation rate appears to be prohibitive. Liquid
hydrogen, also, is not safe to handle.
Two practical methods of keeping a balloon at nominally constant
altitude have been devised, both using the liquid ballast dropping technique.
(Solid ballast, such as sand, does not flow well and is liable to absorb
moisture whiohwill freeze at the temperatures experienced at high altitudes.
Although a few preliminary flights were made with desiccated sand, a highly
refined water-free kerosene-type petroleum product,campass fluid,was found
to be more satisfactory) ,
In the simpler control system, ballast is dropped at a pre-determined
rate, aimed to slightly exceed the loss of lift of the balloon du~ to leakage
and diffusion. If this method is successfully used, the balloon stays full
because the remaining gas in the balloon has less load to support; the ref ore,
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