or sunset cuts off the superheat is so large that it is not possible to carry enough ballast to sustain the system under these conditions. On the other hand, the loss of buoyancy through a sealed-off Seyfang balloon at 4100 feet MSL is of the order of 50 grams per hour which is significantly less than the loss expected from a 20-foot, 1 mil polyethylene cell in flight conditions. (With the appendix aperture sealed, such a cell shows a loss of lift of about 40 grams per hour when one-fifth inflated at sea level).
One other type of balloon which has been used as a super-pressure balloon is the neoprene J2000 balloon of Dewey and Almy, surrounded by nylon cloth shroud. The rubber balloon normally would expand until it reached bursting diameter, but when enshrouded, it is limited to the volume of the shroud. The difficulties in launching and flying this balloon are not unusually great, but on each of the several tests which have been made to date improper handling has been a possible cause of the early rupture of the balloon. It is believed, however, that such a balloon is not especially suitable for long flights because of the deterioration which occurs in the neoprene in the presence of sunlight. Perhaps a shroud of material which would filter out the ultraviolet rays would protect and lengthen the life of such a balloon.
Despite the success of the Japanese silk or rice-paper balloons, which were constructed on a super-pressure principle, it is not believed practical at this time to develop a balloon of such strength that it would successfully withstand and retain pressure increases corresponding to the temperature changes from night to day as the superheat of absorbed sunlight is gained. The super-pressure with a neoprene-coated nylon balloon, for example, would be approximately 0.5 psi. That such a balloon could be built is unquestioned. The cost of production, however, appears at this time to be unwarranted.
D. Altitude Controls
Beginning with the arrays of rubber balloons which were first used, various systems of dropping ballast, both solid and liquid, have been attempted with the aim of exactly compensating for the loss of buoyancy which is occasioned as the lifting gas diffuses or leaks through the balloon. On the early rubber balloons only rough incremental ballast dropping was employed. At that time it was decided not to use sand as ballast since most sand contains some water which may freeze while aloft. Further, it is easier to control the flow of a liquid ballast than it is to control sand particles. In the investigations for a suitable liquid ballast the petroleum product known commercially as Mobil Aero compass fluid was finally settled upon. These investigations included tests of cloud point, freezing point, and also density and viscosity over a large range of temperatures. The compass fluid is especially suitable for ballast work
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