Superpressure Balloon Design Using Nonlinear Viscoelasticity

Stratospheric balloon platforms are used extensively by scientists for a variety of purposes. The typical balloon used today is the zero pressure natural shape fabricated from a thin film of linear low density polyethylene. This material has been found to possess a variety of desirable characteristics suitable to this environment. This film will remain ductile at very low temperatures which will permit it to develop large strains if necessary to satisfy equilibrium considerations. However, in order to achieve long duration flight without significant changes in altitude, the balloon should be pressurized to the extent necessary to maintain constant volume during typical variations in temperature. In the past, pressurized balloons were fabricated from other materials in order to achieve significant increases in strength. Thin films of polyester or polyimide have been used to make relatively small spheres capable of long duration flight. Unfortunately, these materials do not have the ductility of polyethylene at low temperature and are somewhat more fragile and subject to damage. In recent years various organizations have attempted to use the characteristic shape of a pumpkin to limit the stresses in a balloon envelope to that which can be accommodated by laminated fabric materials. While developing the design, analysis and construction techniques for this type of system, the use of polyethylene has been successfully demonstrated to provide a reliable envelope. This shape is achieved by using high strength members in the meridional direction to carry the very high loads generated by the pressure. These so called "tendons" have very low elongation and serve to limit the deformation of the film in that direction. However, earlier designs attempted to limit the stresses in the circumferential direction by using a lobe angle to control the stress. Unfortunately this has led to a number of stability problems with this type of balloon. In order to control the stability of both the deployment and pressurization of a pumpkin shaped balloon, excess material should be removed. However, the stresses in the circumferential direction increase as the gore width is reduced which has led to the fear that "tertiary" creep may occur. The concept of "strain arrest" is now being introduced into the design procedure which will permit the use polyethylene film in a biaxial state of stress with confidence. This concept is based on the observation that the deformation of the material is time dependent and nonlinear. As the envelope material creeps in the circumferential direction, the film will elongate and form a lobe with a smaller radius of curvature. This will cause the stress to decrease and achieve a stable state of equilibrium. This paper will demonstrate the need for an accurate constitutive relation for the material which includes both the nonlinear and time dependent nature of the film in a biaxial stress state. In addition, analysis techniques must be able to describe the response of the system to the time dependent changes in temperature and pressure. Both of these requirements have now been accomplished and will be demonstrated.