Study On Design And Integral Manufacture Of Composite Thrust Cylinder

Thrust structure serves as a primary load-bearing component which connects the rocket engine and launch vehicle body. It can transfer the concentrated thrust force from the engine into evenly distributed loads on the primary structure of the launch vehicle which is then pushed forward. Design and fabrication of a carbon/epoxy composite thrust cylinder are investigated. An improved fabricating process which is combined the traditional thermal expansion molding with resin transfer molding, called the TEMARTM in this paper, is developed. The key issues related to the composite thrust cylinder and the TEMARTM are investigated. Attentions are focused on structural design, ply lay-up design and fabrication of the composite thrust cylinder, inner dimension accuracy control method, local fiber overlap strengthening method on the interface, permeability test method and filling mould time calculation for the prepreg of the thrust cylinder, mould design and fabrication for integrally fabricating of the composite thrust cylinder with complex structure and axial compression test of the composite thrust cylinder in this paper.Firstly, structural design and lay-up design were determined. Based on the finite element analysis of the composite thrust cylinder, an optimum configuration of the composite thrust cylinder was obtained. The thrust cylinder is composed of a big flange, a small flange, a circumferential rib, twelve primary load-bearing ribs and the cylinder wall with 24 big holes. The diameter of the big flange was 1500mm and that of the small flange was 950mm. The height of the thrust cylinder was 590mm. The thrust cylinder is able to bearing 1000kN axial compressive loads combined with 87kN transverse loads.According to the configuration and function of the composite thrust cylinder, 12k T700 carbon unidirectional tapes were chosen and plied along the axial direction of the thrust cylinder, and 3k T300 cloth along the circumferential direction. The ratio of the tapes to the cloth was investigated by a finite element method and the results shown that the thrust cylinder had best load bearing capacity when the volume ratio of the T700 tapes to T300 cloth is 4:1.Secondly, mould filling time was evaluated. Permeability test method was established, the effects on the permeability were investigated systematically and the permeability of the prepregs was obtained which was applied to calculate the mould filling time. In order to meet the requirement of the TEMARTM process, the bisphenol-F epoxy resin which has excellent mechanical properties and very low viscosity at room temperature was chosen. The viscosity of the bisphol-F resin is only 1/7～1/4 of that of the bisphol-A resin. A mixture of curing agents which consists of a high active DETA and a low active DEPA was obtained. Different pot life can be obtained by adjusting the proportion of DETA to DEPA. Weight proportion of 2:4 of DETA to DEPA was used to cure the bisphol-F epoxy resin. The tensile strength and bending strength of the bisphol-F/DETA/DEPA resin system are 66.0MPa and 102.0MPa, respectively. Initial reaction temperature, the highest heat releasing peak temperature and the highest curing temperature of the bisphol-F/DETA/DEPA resin system are 22℃, 75℃and 116℃, respectively.Thirdly, the dimension accuracy of the fabricated composite thrust cylinder was discussed. The inner dimension of the composite thrust cylinder is determined by the thermal expansion mold. Since the thermal expansion mold is flexure and deformable by heating in nature, the inner dimension of the fabricated composite thrust cylinder is one of the key parameters for the fabricating process. Considering the space between the thermal expansion mould and the wall of the thrust cylinder, a governing equation was developed based on the thermal expansion pressure equation of the thermal expansion mould. The inner dimension of the thrust cylinder can be determined by adjusting the thermal expansion mould size, the expansion pressure and the expansion volume according to the derived governing equation. The specimens fabricated by TEMRTM and traditional RTM process were implemented to analyze the effects of the thermal expansion mould on the thickness, fiber volume content and mechanical properties of the composite specimens. The experimental results show that the dimension calculated by the governing equation reaches the accuracy requirement for the thrust cylinder design. Higher fiber volume and higher mechanical properties of the specimens were obtained from TEMRTM process. Micro morphology observation of the specimens shows that the thermal expansion mould is helpful to reduce the defects and obtain tighter fiber plies.Fourthly, in order to reduce the interface shear stress caused by external loads on the thrust cylinder, a method for local fiber overlap strengthening on the interface was developed. The calculated results and experimental data show that the interface shear stress has been decreased significantly by plying the carbon fiber cloth on the interfaces between the primary load bearing rib, the circumferential ribs and the cylinder wall. The shear stress decreased to 20MPa from 28MPa when the overlap dimension is over 10mm.Fifthly, the mould fabrication and thrust cylinder fabrication were investigated. The half-scaled thrust cylinder mould which consists of the GFRP female mould and the assembled male mould was designed and fabricated. The outer dimension of the thrust cylinder is determined by the rigid female mould while the inner dimension of the cylinder is dominated by the assembled male mould. The assembled male mould is composed of a thermal expansion mould and a rigid metal structure. On one hand, the rigid metal structure supports the thermal expansion mould, on the other hand, the metal structure contains electricity-heating device to provide the temperature for the thermal expansion mould expanding. A half-scaled thrust cylinder was successfully fabricated via the designed TEMRTM mould. The fabricated composite cylinder by TEMRTM process exhibits much better apparent quality and higher dimension accuracy than those fabricated by traditional RTM process.Lastly, the mechanical properties of the thrust cylinder were investigated. Axial compression test for the half-scaled thrust cylinder was conducted. The experiment results show that no crush was happened in the thrust cylinder when the axial compression loads reached 500kN. Curves of axial load on the thrust cylinder to axial displacement of the thrust cylinder showed that the structure stiffness at tested position were different. Curves of axial load on the thrust cylinder to longitudinal deformation of the primary load-bearing rib and circumferential deformation of the circumferential ribs at the same level of loading show only compressive deformation occurred on the primary load-bearing rib while the deformation of the circumferential ribs was quite complex. It is believed that the un-uniform stiffness of the cylinder structure is due to the uneven fiber ply in the primary load-bearing ribs.