Non-Linear Viscoelastic Analysis and the Design of Super- Pressure Balloons: Stress, Strain and Stability

The application of geometric and materially non-linear finite element analysis techniques to the NASA Super-Pressure Balloon Program has been driven by the need to understand and overcome deployment and stability problems that have shadowed the chosen ‘pumpkin’ design. Early iterations of the super-pressure balloon designs showed problems of shape instability, characterized by improper deployment and the potential for overall geometric instability once deployed. The latter has been reproduced numerically using inTENS, and the former are better understood following a series of large-scale hangar tests simulating launch and ascent. In both cases the solution lies in minimizing the film lobing between the tendons. These tendons, which span between base and apex end fittings, cause the characteristic pumpkin shape of the balloons and also provide valuable constraint against excessive film deformation. There is also the requirement to generate a biaxial stress field in order to mobilize in-plane shear stiffness. Achieving this will test the structural performance of the film to its limits and make full use of the strain arresting feature of the much stiffer tendons. A full numerical model that takes account of the non-linear viscoelastic response of the film is required to properly judge these design issues over the whole duration of a flight. Stress concentrations must be considered, along with the influence of shape change though film creep on the maintenance of stability. This paper summarizes the current numerical approach, and describes the implementation of ‘whole flight ’ analyses of balloon stresses and stability. I