Design and structural analysis of a superpressure hangar test balloon

Abstract

The design and structural analysis of a hangar size test balloon is described and results are presented. As part of NASA's Long Duration Balloon Vehicle (LDBV) development effort, laminated scrims and fabrics arc being considered as candidate superpressure balloon materials. Prior to flying a vehicle made from any of these materials, hangar tests of small structures need to be conducted to verify solutions to several design questions. Optimum loaded balloon shape, method of load transfer, and distribution of stresses. The process begins with finite element analysis of spheres and shapes derived from the basic sphere. These principally include sphere, sphere on cone and sphere with reflex base. The effects of stiffened seams arc discussed. Various uniform and hydrostatic loadings arc used to investigate stress distributions. Simplified material properties are used for determining the stress trends at this stage of the analysis. As material selections narrow, characteristics of specific materials will be incorporated in the model. Background further gains in altitude stability with the goal of eliminating the need for ballast. This should result in the capability of large scientific balloons to fly for significantly longer duration than ever before. Material Design and Selection Process The Phase II effort began with analysis performed to determine the strength requirements for a material for the next generation of long duration balloon vehicles. For fully pressurized systems, this requires the material to withstand pressure loadings that have the trajectory performance result of requiring no ballast to maintain altitude through normal diurnal cycles and through storm situations that subject the balloon system to extreme radiation environments. Initial analysis assumed that the new material would have radiative properties equal to or better than current polyethylene films. Several composite materials have been identified out of hundreds that were researched as candidate materials for these new super pressure balloons. The selection process includes measurements of radiative properties in the solar and infrared wavelengths, strengths at temperatures ranging from -80C to 25C, the capacity to be seamed, tear strength, toughness, permeability, and resistance to pinholing among others. The Long Duration Balloon Vehicle As part of the ongoing NASA Balloon Program Research and Development effort, NASA's Balloon Programs Branch initiated a project to increase the duration capability of large scientific balloons. The Long Duration Balloon Vehicle (LDBV) effort has been an attempt to address the specific design issues related to the balloon structure. Results of early work on Over pressurized Zero-Pressure balloons1 lead to a significant number of successful tests ranging from small hangar sized pressurized balloons to 0.8 MCM pressurized test flights.2 The first phase of the LDBV project was aimed at maximizing the altitude stability using approved NASA polyethylene films, primarily through pressurization. Phase I resulted in the development of several control features for ballooning, including autoballasting and autovalving of closed helium balloons safely into float. Pressurized natural shaped ductless polyethylene balloons were proven to have the capability of improved diurnal altitude stability using significantly less ballast. Phase II of the LDBV project is focused on employing new materials to show even