Design and Evaluation of a Reinforced Advanced-Grid Stiffened Composite Structure
Abstract : A composite grid-stiffened structure concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to previous designs, this concept is lighter weight and requires reduced manufacturing costs. Various failure mechanisms were examined for the composite grid-stiffened structure. The controlling criterion for this design was determined to be joint peel-off failure. The identification of this failure mechanism and the assessment of bounding strains to control it, required extensive test and analysis effort. The final fairing design incorporated an undesirably thick skin to reduce the strain between the skin and ribs. This project investigated a means of controlling joint failure of the un-reinforced grid-stiffened structure in an effort to reduce the skin thickness. Implementing lightweight foam inserts between the stiffeners on the interior of the fairing delayed failure to higher loads. A foam reinforced test panel, designed with equal mass to the current structure, withstood a substantially greater compressive force. The alternative concept also demonstrated an improved response, failing in global buckling instead of experiencing early failure due to joint strain.
DEVELOPMENT OF LOW- COST LIGHTWEIGHT TOOLING FOR LARGE COMPOSITE STRUCTURES
[Abstract] The Access to Space initiative is providing the incentive for launching increasingly large payloads. Composites are an excellent choice for the launch vehicle structure to carry these large payloads, as a result of low mass density and high strength and stiffness. However, very large composite structures present special difficulties for fabrication. Tools can become very large, heavy and expensive, and autoclaves sufficiently large for curing the polymer matrix are very expensive. Therefore, there are very few available. Furthermore, many very large composites are used in low production quantity applications, such as space launch vehicle structures. This paper describes a group of research projects directed at reducing the cost and complexity of fabrication of tooling for large composite structures. Multiple tooling methods for various applications are under development. This paper is focused on tooling used for the lay-up and cure of composite structures. It will discuss the issues and challenges of fabricating large cure tooling, potential solutions to those issues, plans to evaluate and optimize those solutions, and progress to date on those plans. The program management and funding are being provided by the Air Force Research Laboratory (AFRL), Space Vehicles Directorate (Kirtland AFB), and includes SBIR partnerships with ATA Engineering and Nielsen Engineering and Research (NEAR) and contracts with ATK. Research presented is limited to joint efforts of AFRL and ATK applied to AFRL programs and complementary, but independent, ATK research.
The HyperSizing Method for Structures
A practical structural optimization system specifically designed for effective engineering solutions is presented. The system, called HyperSizer [1], is coupled with finite element analysis (FEA). The system is based primarily on accurate engineering analyses and secondarily on discrete optimization. Its underlying method is a departure from typical finite element design sensitivity and optimization that emphasize numeric optimizers, and model based user defined constraints on strength and stability failure analyses. HyperSizer's built-in detailed analysis capabilities, and its ease of use makes it suitable as a tool for performing automated structural analyses of any general structure. Indeed, this is the fundamental premise of the HyperSizing method. HyperSizer's ability to predict structural failure will be first presented, and then the benefits of coupling the closed form analytical capabilities with those provided with FEA. The example application is a space launch vehicle, which containing 7 assemblies, 21 optimization groups, and 203 structural components. It demonstrates how an engineer is able to provide ‘real-world’ expertise in the optimization process by interacting with HyperSizer for designs on the fly. © January 1999 by Collier Research Corporation. Published with permission by NAFEMS World Congress '99. NAFEMS is the International Association for the Engineering Analysis Community.