To use the first of these principles it is possible to maintain a condition of buoyancy by at least the following two methods: (1) dropping a part of the load, as ballast, to match the loss of lifting gas which occurs as a result of diffusion and leakage; (2) replacement of the lifting gas by evaporation from a reservoir of liquified helium or hydrogen. Of these two methods, ballast dropping is most satisfactory from the consideration of simplicity of control and safety of personnel. While the use of liquid helium is theoretically more efficient, the amount and complexity of control equipment adds much to the cost and also the weight of air-borne equipment.
The development of non-elastic balloons which can withstand high internal pressure was investigated. Two designs which compromise extreme cost (required for balloons of high internal pressure) with small wall strength, hence small super pressure, were tested.
At first, attempts were made to control balloon performance by using buoyancy-load balance techniques with elastic balloons, but the difficulties which were experienced resulted in the development of a third principle of operation combining a non-extensible balloon with a system of controls which can be applied either to a freely expanding balloon or to a balloon of fixed volume.
III. METHODS OF ATTACK
The work on the development of controlled-altitude balloons may be divided into three phases, each one identified by the type of balloon which was used. Concurrent with the balloon development was the design and testing of control equipment required to maintain the balloon at specific altitudes. Some of the equipment instrumentation was used on more than one kind of balloon, but in general the problems and methods of attack are identified with one of the three types of balloons.
A. Rubber Balloons
Following the example of Clarke and Korff, assemblies of neoprene rubber balloons were first considered. Using these freely expanding balloons it was necessary to balance the load to be lifted with the buoyancy given by an integral number of balloons. One or more accessory balloons were attached to the assembly to provide lifting force to carry the train aloft. With the gear at a predetermined altitude, the lifting balloons were cut loose from the train by a pressure-activated switch, leaving the equipment at floating level, more or less exactly balanced. Since there is no inherent stability in an extensible balloon, any existing unbalance will cause the train to rise or fall indefinitely until the balloon reaches
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