Recent Developments on the Super-Pressure Balloon with a Diamond Shaped Net — Ground Inflation Tests of Two 2,000 m³ Balloons —

Abstract

A super-pressure balloon with a diamond shaped net is considered to be a vehicle which satisfies scientist requirements of a long duration balloon flight at high altitude. The development of the balloon was started in 2011, and ground inflation tests of scaled models and some flight tests have been performed. In 2016 and 2017, ground inflation tests of two 2,000 m³ balloons, NPB2-1 and NPB2-2, were performed. The meridian lengths and radii of the balloons were measured simultaneously as a function of the differential pressure between inside and outside the balloon to derive their ratio for the first time. Tension along the circumferential direction is resolved in the net balloon, the results show that within the error of a few percent, the ratio is same as that of the pumpkin shape, which withstands the differential pressure only by tension along the meridian direction. The time variation of the differential pressure between inside and outside the balloon was also measured. Based on a simple model, assuming that the balloon volume is a linear function of the differential pressure, the variation of the differential pressure due to the variation of the room temperature and the atmospheric pressure are corrected. Considering the residuals are all due to the gas leakage, the upper limit of a hole size of 0.3 cm² is derived for the NPB2-2 balloon.