The Roswell Report: Fact vs. Fiction in the New Mexico Desert

was controlled by a baroswitch arrangement which dropped a bag by igniting a fuse when the altitude fell below any one of four different levels between 25,000 and 5000 ft. In addition, a delay mechanism consisting of a two-minute fuse was arranged between successive switches so that after ballast was dropped, two minutes would be allowed for the balloon to regain its altitude; if it did not regain in this time another bag of ballast would be dropped. The system was inefficient because if any one of the thirty-six fuse arrangements failed, no more ballast was dropped.

The second type of Japanese balloon was similar, in general, but slightly larger; it was made of oiled silk and therefore would stand a greater internal pressure (approximately six inches of water). The higher the internal pressure that the balloon can stand, the less gas need be valved under conditions of superheating or altitude fluctuations. The Japanese released many balloons of these types from their islands and estimated five to seven per cent of those released reached the west coast of this country. The balloons floated between the surface and 30,000 ft above sea level; those which reached the west coast must have remained aloft from four to ten days. While the altitude maintained was not constant, these balloons were highly successful for the time they remained in the air.

An attempt in this country was made in 1943 by the Dewey and Almy Company, to obtain constant-level balloons which would float at altitudes up to 15,000 ft. An ordinary 350-gram meteorological balloon was used but its volume was controlled by a nonextensible shroud around it. With this method a flight at about 5000 ft was obtained at fairly constant altitude for about an hour and a half.

## 3. Design of controlled-altitude balloons

As a result of the Japanese and other experiments, the use of a nonextensible envelope for the balloons was indicated. If a perfectly nonextensible balloon could be built with no diffusion through the walls, and which could withstand a high internal pressure, it would automatically stay at a constant density where the buoyancy of the full balloon equaled the load. In practice, control devices are needed to offset the leakage and diffusion of gas, to compensate for vertical currents in the atmosphere, to correct for the motion of the balloon due to diurnal changes of the balloon's temperature, and to compensate for the valving of gas which is necessary to prevent rupture of the envelope. It was decided to use a plastic as the balloon fabric, as some modern plastics are quite transparent to radiation, strong, easily fabricated, and relatively inexpensive as compared with coated fabrics.

A. Choice of plastics. In the selection of a plastic material of which to make the balloons, the desirable

properties are: (a) low brittle temperature, (b) low permeability, (c) high tensile strength, (d) high tear resistance, (e) chemical stability, (f) high radiation transmission or reflection. Polyethylene soon recommended itself for use, with its brittle temperature of below $ - 80\mathrm{F} $ . It is apparently unaffected by ultraviolet and ozone. The permeability through one mil of thickness and one square meter of area for 24 hours is ten liters for hydrogen and seven liters for helium, at normal atmospheric temperature and pressure.

FIG. 1. Polyethylene balloon, 20-ft diameter.

Polyethylene is also relatively easy to fabricate. It has an ultimate tensile strength of 1,900 pounds per square inch at 25C, which, in a 15-ft balloon made out of four-mil fabric, represents a working pressure of about 2.3 inches of water. The tensile strength at the temperatures at which the balloon flies at high altitude may be more than three times the value quoted above.

Fig.1 shows a polyethylene balloon $ ^{2} $ flown successfully in Flight 26 described below. Another film investigated is Saran, which has ten times the tensile strength of polyethylene—three times the strength across the seams. Saran has a higher transparency and one-thirtieth the permeability of polyethylene. The effective brittle temperature of Saran for this work is not known reliably.

B. Ballast valve. The altitude control is an automatic ballast-dropping device $ ^{3} $ consisting essentially of