Conical-cylindrical shells are common geometries in launch-vehicle structures as stage adapters and payload adapters, and they are susceptible to buckling due to their large radius-to-thickness ratios. Buckling design guidance is available but it is limited for conical and cylindrical shells. There is no available buckling design guidance for conical-cylindrical shells. This paper presents the validation of two finite element models used to successfully predict the buckling behavior of a composite conical-cylindrical shell with and without reinforcement tested in two separate campaigns. The laminate design for the first test campaign consisted of a quasi-isotropic layup. For the second test campaign, additional composite plies were applied to reinforce the transition region of the original laminate. The work presented demonstrates the ability to predict the buckling behavior of a composite conical-cylindrical shells with two different designs, which may aid in creating buckling design guidance for conical-cylindrical shells. Additionally, this paper shows that there is no appreciable benefit to adding reinforcement to the transition region if the intent is to increase the buckling load, due to the fact reinforcement brings increased buckling imperfection sensitivity to the shell.@en

Launch-vehicle shell structures, which can be comprised of both cylindrical and conical sections, are known to be susceptible to buckling due to their large radius-to-thickness ratios. The advancements in composite manufacturing and numerical methods have enabled designers to consider more nontraditional shapes, such as connecting the conical and cylindrical sections with a toroidal transition to create a single-piece conical-cylindrical shell. This single-piece construction eliminates the need for a heavy interface ring between sections and has the potential to save mass. To better understand the buckling behavior, a composite conical-cylindrical shell was designed, fabricated, and tested. Prior to test, a finite element model that included thickness variations and radial imperfections was created. The test article buckled elastically at 251.8 kN, approximately 8.8% higher than the predicted buckling load of 231.4 kN Continued research in conical-cylindrical structures has the potential to expand the design space for launch-vehicle structures and lead to improved designs and reduced weight.@en

Launch vehicle structures, such as payload adapters and interstages, are increasingly designed and constructed using composite materials due to their high stiffness- and strength-to-weight ratios. Therefore, it is important to develop a validated finite element modeling methodology for designing and analyzing composite launch-vehicle shell structures. This can be achieved, in part, by correlating high-fidelity numerical models with test data. Buckling is often an important failure mode for cylindrical shells, and the buckling response of such structures is also often quite sensitive to imperfections in geometry and loading. Hence, it is crucial to understand the model parameters and details required to accurately predict the buckling load and behavior of composite cylindrical shells, especially if the shell is buckling critical. The inclusion of as-built features, such as radial imperfections, thickness variations, and loading imperfections can help improve the correlation between test and analysis. To demonstrate such an approach, a validated modeling methodology that was used to predict the buckling behavior of a scaled component for a launch-vehicle-like structure is presented, and results from the model are compared with experimental results. The modeling approach presented herein was used to successfully predict the buckling behavior.

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A subscale solid laminate composite cylinder with a 31.5-in. diameter, a 48.2-in. length, and a [23/0/-23]4S layup was designated NDL-1 and was designed to fail in buckling after a series of subcritical tests. NDL-1 was experimentally loaded in axial compression until buckling at 466.3 kips. Visual inspection of the test article after the test revealed that a shallow area of delamination occurred near the buckling initiation site. After a successful first buckling test, there was an opportunity to test NDL-1 in axial compression to failure a second time despite that the test article was not designed or analyzed for multiple buckling tests. During the second test to failure, NDL-1 reached a peak load of 390.4 kips, 16.3% lower than the first test. Buckling did not initiate in the same location during the second test to failure as the first test to failure. The second buckling event caused a large V-shaped crack that penetrated through the thickness of the barrel. Though buckling initiated at different locations, a similar radial deformation pattern was present just prior to buckling during the both tests. In the end, NDL-1 maintained a significant amount of its structural integrity and a similar prebuckling radial displacement pattern, even after the first buckling test. @en