Friday, April 6, 2012
Clefted Equilibrium Shapes of Superpressure Balloon Structures
Xiaowei Deng, Graduate candidate, Aeronautics, Caltech
Lobed super-pressure balloons have shown a tendency to deploy into unexpected asymmetric shapes. The design of the first balloons had been based purely on stress analysis, but it has been modified to strike a balance between the lower stresses achieved by increasing lobing and the risk of incomplete deployment. Our particular interest in this research is in predicting if any clefts will remain in a balloon when it reaches its float altitude and is fully pressurized. A three-dimensional finite element model of balloon structures incorporating wrinkling and frictionless contact, able to simulate the shapes taken up by lobed super-pressure balloons during the final stages of their ascent has been established. Two different methods have been considered to predict if the deployed balloon will have any residual clefts: (i) a simulation of the deflation & reinflation of the balloon, to find out if it follows the same equilibrium path, and (ii) a perturbation technique that seeds a a clefting imperfection. Compared to Method I, method II provides a much more efficient computational test. The clefting test has been applied successfully to three 27m diameter super-pressure balloons that had been tested indoors by NASA, of which one had remained clefted when it was inflated and the other two had deployed completely. In addition to these detailed numerical simulations, a dimensionless cleft factor can be employed as an indicator of the tendency to S-cleft of a whole family of balloon. The cleft factor is expressed in the form of a power-law relation to a set of dimensionless groups. An example illustrates how to calculate the coefficients of the power law and analyze the sensitivity to clefting of balloons with different design parameters.