Flight 94: Released from Alamogordo, New Mexico, 1208 MST, September 3, 1948 Recovered At Villa Ahumada, Chihuahua, Mexico
On this flight, a fourth attempt was made to sustain a Seyfang, neoprene-coated nylon balloon. On Flight 79, a previous Seyfang flight, no ballast equipment had been in operation, and so a careful record of ballast flow on Flight 94 was desired. This was provided by a ballast meter. In addition to this and the barograph and Olland pressure-measuring instruments, a thermograph was also part of the equipment train.
The height-time curve (Figure 43) shows that the initial buoyancy surplus of this balloon (for the most part due to superpressure held behind
Figure 43
the safety valve) was reduced by diffusion so that after one hour of floating it began to descend at an accelerating rate. After falling about 2000 feet, the automatic ballast valve began to operate, and ballast was discharged at the rate of 20 grams per minute. During the descent, however, the strong superheat which the balloon had acquired was reduced by ventilation.
The adiabatic lapse rate of helium is $ 2^{\circ} \mathrm{C} $ per kilometer, whereas air in the troposphere warms up about $ 6^{\circ} \mathrm{C} $ with each kilometer of descent. This means that with each kilometer of fall, the lifting gas was cooled relative to the air by an additional $ 4^{\circ} \mathrm{C} $ . The combination of inertia, loss of superheat through ventilation, and adiabatic cooling of the gas as it was compressed, proved too great for the limited flow of ballast through the automatic valve, and the balloon fell unchecked to the ground.
From Flight 79, it was determined that superheat of nearly $ 4 0^{\circ} \mathrm{C} $ is built up when Seyfang balloons are flown in the sunshine. If this were lost, the buoyancy of the balloon would be reduced by one-sixth, and no satisfactory control could be achieved by ballast dropping.