The geometry, fiber materials, and coating materials of composite high-pressure vessels directly affect product performance; therefore, evaluation should be based on comprehensive criteria.
Spherical. For all-metal high-pressure vessels, the spherical structure is optimal. From a strength perspective, a spherical shell is isotropic, a property that aligns well with the inherent isotropy of metal materials. Compared to other shapes, a spherical vessel has a smaller surface area for a given volume, resulting in a lighter vessel.
Furthermore, a sphere's load-bearing capacity is double that of a cylinder; consequently, most all-metal high-pressure vessels utilize a spherical design to achieve the best overall performance metrics.
However, for composite high-pressure vessels, isotropic layup is a primary consideration. Filament winding of spherical vessels is restricted to "great circles"-meaning the filament must be wound along the circumference of the maximum diameter to prevent slippage. The angle between the fibers and the meridians varies across different latitudes: it is smallest at the equator and reaches 90° at the polar opening, where the fiber is tangent to the opening's edge. Because the meridional and circumferential components of the winding force differ across winding paths, the vessel-despite its spherical shape (which implies uniform loading in all directions)-cannot effectively meet uniform pressure requirements due to the anisotropy of the wound fibers. While using multiple circular windings with enlarged polar openings can improve the stress state, achieving an equal-strength design remains difficult.
Meeting equal-strength requirements necessitates isotropic layup, yet the winding processes and equipment required to achieve this are complex. Under current conditions, a cylindrical shape with domed ends is more practical. Significant breakthroughs have been made in isotropic wrapping technology for spherical high-pressure vessels, and specialized research has been conducted on winding patterns and equipment for these structures. From a long-term development perspective, however, the spherical composite vessel may not be the shape of choice for future applications.
Cylindrical. Cylindrical composite high-pressure vessels consist of a cylindrical body and two end closures (heads). The heads typically employ designs such as isotensoid geodesic winding, isotensoid winding, or ellipsoidal shapes. This type of high-pressure vessel features a rational structural design and offers convenience in manufacturing, handling, and installation.
Stress analysis of the geometry reveals that the circumferential stress in a simplified vessel body is twice the axial stress. In the case of an all-metal vessel, the axial strength margin is substantial when the design satisfies circumferential stress requirements. Conversely, composite high-pressure vessels can achieve an equal-strength design across the cylinder's cross-section-balancing circumferential and axial strength-through specific winding patterns. Axial tension is borne by the axial component of longitudinally wound fibers, while circumferential tension is borne by the hoop-wound fibers and the circumferential component of the longitudinal (axial) fibers. The fiber quantities in both directions are calculated based on biaxial stress equilibrium, allowing for optimal fiber utilization and high structural efficiency. Consequently, the simple cylindrical shape is currently the most widely used configuration for composite high-pressure vessels, both domestically and internationally.
Quasi-spherical composite high-pressure vessels-essentially shortened cylindrical vessels with an aspect ratio of approximately 1-serve as cost-effective alternatives to all-metal or composite spherical vessels. They are suitable for installations originally designed for spherical vessels while effectively leveraging the unidirectional strength characteristics of fiber materials. The liner features integrally formed (barrel-shaped) ellipsoidal ends and is fully reinforced on the exterior with high-strength carbon fibers wound in both helical and hoop patterns. The vessel's external shape makes it suitable for installation in a wide range of applications.
Continuous fibers commonly used in filament-wound composite high-pressure vessels include E-glass, S-glass, Kevlar 49, and carbon fibers. Due to their lower performance characteristics, glass fibers are currently utilized primarily in ground-based equipment, whereas Kevlar 49 is employed in the majority of spacecraft. Modern spacecraft predominantly utilize high-performance carbon fibers. Fiber selection requires a comprehensive evaluation of factors such as strength, elastic modulus, and linear density.