Stability of Cyclically Symmetric Strained Pumpkin Balloons and the Formation of Undesired Equilibria

Abstract

NASA's effort to develop a large payload, high altitude, long-duration balloon, the ultralong duration balloon, focuses on a pumpkin shape superpressure design. It has been observed that a pumpkin balloon may be unable to pressurize into the desired cyclically symmetric equilibrium configuration, settling into a distorted, undesired state instead. Hoop stress considerations in the pumpkin design lead to choosing the lowest possible bulge radius, whereas robust deployment is favored by a large bulge radius. Mechanical locking may be a contributing factor in the formation of undesired equilibria. Long term success of the pumpkin balloon for NASA requires a thorough understanding of the phenomenon of multiple stable equilibria. This paper uses the notion of stability to classify balloon designs. When we applied our finite element model to a balloon based on the NASA Phase IV-A pumpkin design, we found the fully inflated/fully deployed strained equilibrium float configuration was unstable. To demonstrate our approach for exploring the stability of constant bulge radius designs and their sensitivity to parameter changes we carry out a number of parametric studies. We focus on analytical studies, but we also compare our results with flight data whenever possible.