1164. Weight Estimation of Composite and Monolithic Spherical Pressure Vessels

Paper

Abstract

This paper presents the principal results of a weight estimation methodology for spherical filament wound composite and conventional monolithic pressure vessels. The estimating technique used was one of first determination of the weight of an ‘Ideal’ closed spherical shell by the classical hoop tension method. This weight determination was then modified by applying ‘Non-Ideal’ factors for bosses, weld lands, tolerances and material build-up at specified locations for fatigue. These non-ideal factors were derived from empirical and actual weight data obtained directly from an actual verification test program. These data were then used in establishing the Shuttle Orbiter pressure vessel design and weight. The result is what is called the vessel ‘Real’ weight and represents the vessel usable weight estimate. The non-ideal factors were equated mathematically by a computer program using a regressive polynomial curve-fit technique. This was done so that the whole pressure vessel weight estimating equation might be adapted to computer aided design programming. In addition to weight estimating equations and curves for the composite pressure vessel, curves are given comparing the weight of composit vessels to the monolithic vessel. By the analysis of the composite and monolithic vessel weight, and pressure/ volume zone selective graph was developed showing when one should choose a composite over a monolithic vessel based on the pressure/volume relationship. In general, for design burst pressures above 3000 psi (2068 N/cm2), the ‘composite vessel is lighter in weight than the monolithic vessel at volumes above approximately 350 cubic inches (5735 cu cm). As the design burst pressure is lowered below 3000 psi (2068 N/cm ), larger vessel volumes are necessary for the weight to favor the composite vessel. After obtaining the actual weight on three types of the Orbiter qualified vessels, the accuracy of the weight estimating technique was found to 3 to 5 percent on the conservative side. As a result of the composite pressure vessel analysis and test program, they were chosen for al17 gaseous pressure-vessels in the Orbiter propulsion pressurization and life support systems. The total weight being 1346 pounds (610.4 Kg) representing a weight reduction 4o2f6 pounds (193.2 Kg) over the more conventional monolithic vessel design, or approximately 30 percent.