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

# CONTROLLED-ALTITUDE FREE BALLOONS

By Athelstan F. Spilhaus, C. S. Schneider, and C. B. Moore

College of Engineering, New York University

(Manuscript received 4 December 1947)

## ABSTRACT

The results of an experimental program to develop balloons with associated control devices, which will float at constant pressure in the atmosphere, are given. Newly developed plastic balloons and automatic ballast equipment are described. Examples of successful controlled-altitude flights are shown, together with a preliminary analysis of their trajectories. The constant-level balloon may provide data not obtainable from an ordinary pilot-balloon network. Future possibilities and plans for its use are indicated.

## 1. Purpose

Drift bottles have been used for many years in the study of ocean currents and have provided interesting data. In meteorology, no corresponding device has been available. It is evident, however, that a balloon which is free to move with the air currents, and yet whose altitude can be controlled, has many important applications in meteorology, as well as in other fields, where it may be desired to keep instruments at altitude for considerable lengths of time. An example is in the investigation of cosmic rays; here, clusters of ordinary extensible meteorological balloons have been used, but the constancy of altitude obtained is not sufficient for many meteorological applications. The purpose of the present investigation was to develop a balloon with a control system which would fly at a predetermined constant level for periods of many hours. Such a balloon has wider application than the ocean drift bottle, because, whereas the latter is limited to surface (or near surface) currents, controlled free balloons may be set to drift at any pressure elevation desired, or along other thermodynamically defined surfaces, as long as the element defining the surface changes in a monotone fashion in the vertical.

In addition to the uses for maintaining instruments at high elevations, there are numerous potential applications of these balloons. Direct measurements of air trajectories and of lateral diffusion become possible. The balloons may also be used as vehicles to convey and drop radiosondes over ocean areas. One problem in this application is to obtain an absolute altitude tie-in point, as it will be difficult to identify the point at which the radiosonde reaches the sea surface.

## 2. Earlier attempts

There have been numerous attempts for various purposes to get a balloon or group of balloons to stay at a fairly constant altitude. Meisinger was interested

in the meteorological aspects of this, using a manned balloon. In the investigation of cosmic rays, as for example, by Clarke and Korff (1941), clusters of ordinary meteorological balloons, 350-gram or 700-gram size, numbering anywhere from twenty to nearly seventy, were utilized. No altitude-control devices were used; the balloons were merely given different amounts of inflation. Thus the whole train ascended to an altitude where certain of the more highly inflated balloons burst until the remainder just balanced the load; thereafter, the assembly descended slowly due to loss of lift by the diffusion of gas. The only provision for having the system regain altitude if it descended too low was by arranging the launching before dawn, so that after the bursting of the first balloon and the subsequent descent, superheating of the balloons by the rising sun would cause the whole assembly to rise again, thereby increasing the duration of the flight. The system does not have sufficient control for many purposes.

The much-publicized use of balloons by the Japanese in the last war represents an attempt which must be considered highly successful from the point of view of the length of time which the balloons stayed in the air. Here the objective was not to obtain any critical altitude control, but rather to insure that the balloons remained floating. The Japanese nonextensible balloons were of two types. One type was of heavy paper, coated to minimize diffusion, of spherical shape, about 25 to 30 ft in diameter, and containing about 19,000 cubic feet of gas. A solid-ballast control system was utilized and gas was valved at a low internal pressure (about two inches of water) to prevent the balloons from rupturing due to the increase of the internal pressure by altitude fluctuations or radiation changes. Such a valve tends to conserve the lifting gas but acts as a safety device to prevent damage of the envelope due to too great an internal pressure.

The solid-ballast system was complex; approximately 900 pounds of sand was used on each balloon, distributed in thirty-six bags. The dropping of ballast