Yoshitaka Saito

A super-pressure balloon (SPB) is expected to provide cost-effective platforms for long-duration experiments in the stratosphere. The authors propose a new method to produce a light SPB, where the entire balloon envelope is covered by a diamond-shaped net; thereby effectively suppressing the stresses developed in the balloon envelope. The feasibility of the method was validated in 2010 through a ground inflation test using a prototype balloon. The development of the proposed SPB has progressed since then, involving the ground inflation test of a ∼6,000 m³ model balloon and the test flight of a ∼2,000 m³ balloon. Meanwhile, it has been realized that the inflated shape of the proposed SPB varies largely depending on the geometrical parameters of the cover net. The unique feature of the proposed SPB suggests that the optimal geometrical configuration of the cover net exists in that it maximizes the structural efficiency of the balloon. In this paper, the authors investigate the variation of the volume-to-weight ratio of the proposed SPB against the geometrical parameters of the cover net. The results show that the balloon volume will increase by a factor of three compared to that achieved by the current design practice without changing the balloon weight.

<p>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.</p>

The tandem balloon system with a super-pressure(SP)balloon and a zero-pressure(ZP)balloon, which can fly a long duration flight, has been developed since 2009. The fabrication procedure of a SP balloon covered by a net was studied and the first SP balloon was made in Nov. 2010, and the second balloon with an improved design was made in Apr. 2011 to show the resist pressure of 9,600 Pa which is comparable to the theoretical prediction. Then, the development to enlarge the balloon was continued and, in parallel, the flight termination mechanism of the SP balloon was investigated, and the launching procedure was studied. In May, 2012, a 20-m φballoon for the fright test was checked its deployment and gas leakage through the ground in.ation test to show excellent results. On Jun. 9, 2012, the 20-m φ balloon was launched with a 15,000 m3 ZP balloon in the tandem configuration. The system was launched without problem with a procedure in which farst filling a Helium gas in the ZP balloon and then filling the top part of the SP balloon to make the balloon in the elliptical shape. The tandem balloon system ascended to reach the level fright altitude of 29.2 km. The differential pressure between the inner pressure of the SP balloon and the atmospheric pressure increased as ascend. Although a small hole was made in the SP balloon at the differential pressure of 400 to 500 Pa, the differential pressure reached the highest value of 814 Pa and kept positive through the level fright lasting for 25 minutes due to its slow leakage. The perfect deployment of the SP balloon was monitored by the on-board ITV camera and its diameter was confirmed as designed. The altitude variation of the tandem balloon system was measured for the first time and the deviation was evaluated to be 11.2 m as the sigma of the Gauss distribution. It was well smaller than those of the ZP balloons as theoretically predicted. At the last of the fright test, the termination mechanism dropping a weight was also tested and it functioned as expected. Development to make a larger balloon for single balloon frights and a small tandem balloon system for atmospheric observation will be continued.

A light super-pressure balloon of which weight will be comparable to the weight of the zero-pressure balloon has been developed using a method to cover a balloon with a diamond-shaped net of high-tensil fibers. To solve a stress concentration problem found through the fright test of B12-02 in 2012, a new 3-m balloon setting the meridian length of the balloon gore film equal to the length of the net was developed. Through the ground inflation tests at room temperature and at . 30 ℃ , it was confirmed that the problem was solved and the balloon had sufficient capacity for the resist pressure. On May 25, 2013, a super-pressure balloon of the same model was launched in the tandem balloon configuration with 2 kg rubber balloons. It stayed at a level altitude before and after the sunrise. It was con.rmed that the balloon could withstand the maximum differential pressure of 6,280 Pa, could withstand the differential pressure of 5,600 Pa for 2 hours, and there was a small gas leak through a hole with an area of 0.4 mm2 which was also found in the ground leakage test. It was also found that the ascending speed after the sunrise was gentle with a low speed of 0.4 m/sec, and the gas temperature roze by 20 ℃ with a time constant of 3,000 sec. These results indicated that the improvement was adequate and there was no problem for the super-pressure balloon to fly in the environment of the stratosphere except for the problem of the small gas leak.

The tandem balloon system with a super-pressure balloon and a zero-pressure balloon is a vehicle which can fly a long duration flight changing the level flight altitudes. This system requires a super-pressure balloon with good capacity to endure high pressure. A pumpkin shaped 3-m balloon with 20 μm polyethylene films covered by a Vectran net with diamond shapes was developed and showed the excellent capacity to endure the pressure of 9,600 Pa on the ground test. A flight test of a tandem balloon system with a super-pressure balloon of the same model and a 2-kg rubber balloon was performed on June 1, 2011, from the Taiki Aerospace Research Field. It was launched in the early morning and the flight lasted crossing the sunrise. It was found that the temperature of the skin of the super-pressure balloon changed by 30 degrees before and after the sunrise and the lift of a rubber balloon at night flight decreased by 5 % after the launch. These parameters are quite useful to evaluate the required capability for the pressure resistance of super-pressure balloons, and to determine the lift of rubber balloons for future tandem systems. Development to enlarge the super-pressure balloon will be continued to provide the system for future scientific experiments.

The lobed-pumpkin shaped super-pressure balloon is able to withstand high pressure due to its small local curvature of the film being independent of the balloon size. It yields the small local curvature as a bulge among adjacent load ropes connected between the top and the bottom fittings. The small curvature is also expected if the balloon is covered by a diamond-shaped net with vertically elongated shape. In addition to the merit of the small curvature, the method using the daimond-shaped net has following merits; 1. the weight of the film is reduced since a weak but light film is able to be used by covering the balloon with a fine mesh net which mesh size is determined without depending on the gore width, 2. the deployment problem known for the lobed-pumpkin balloon can be solved due to its lack of additional films, 3. the capacity of resist pressure is not reduced due to the manufacturing error, since the local distortion of the mesh size does not affect the global balloon shape. We made a small balloon with a 3 m diameter using a 20 μm polyethylene film and a Kevlar net, and performed an inflation test which showed an expected burst pressure. We are going to make larger balloons for the ground inflation tests and launch a larger super-pressure balloon as a part of a tandem balloon system in 2011.