Large Payload HIAD Systems: Structural Investigation and Optimization

The HIAD structure, under development by NASA researchers, has the potential to provide a decelerator with a significantly decreased mass when compared to traditional, rigid decelerators, and provide the required frontal area to effectively decelerate the sized payloads that will be required for future, human-scale missions. The HIAD system consists of a series of stacked, concentric, inflatable tori that form a cone shape, as shown in Figure 1. The tori consist of a flexible, braided fabric shell and axial cords braided into the shell. The shell and cords are pre-stressed by the internal inflation pressure. The individual tori are strapped together and strapped to the rigid center-body of the structure that accommodates the payload while the outside of the cone is covered by a thermal protection system (TPS). The HIAD system can be deflated and packed within the launch vehicle shroud for deployment on the way to the destination planet. Upon deployment the major diameter of the structure would be significantly greater than the diameter of the launch vehicle, an advantage over a comparable rigid decelerator that is constrained by the launch vehicle size. An important consideration with these inflatable, relatively compliant systems is the structural response during atmospheric reentry. A HIAD configuration that is too compliant may experience a global buckling failure as the structure is loaded, while a system that is overdesigned may have more mass than is necessary. While ground development and testing of structural components and small-scale HIAD systems are important for understanding the structural response, human-scale HIAD systems may be larger than can be physically tested in an economic manner. Numerical modeling techniques for the HIAD system are therefore important for understanding the structural response of full-scale HIAD systems. Others have developed continuum-based FE modeling methodologies for analyzing the HIAD system. These modeling tools, although able to capture the response of the HIAD system, are time consuming to develop, difficult to parameterize and computationally demanding to run because of the relatively compliant nature of the HIAD system, and the added complexities that arise when working with an inflatable system. In the current research a simplified, beam-based FE model approach for the analysis of inflatable members and the HIAD system was developed. The beam-based FE analysis tools that were developed for the inflatable system are three dimensional, accommodate large deformations with a corotational element formulation, accommodate material nonlinearities with the use of a flexibility- or force-based element formulation and account for the effects of pressure volume change work that occurs during certain deformation modes. The beam-based modeling approach incorporates the challenges that are encountered in analyzing an inflatable, textile system, such as the influence of internal inflation pressure, nonlinear material response, the loss of pretension due to inflation pressure, coupling between deformation modes, and the large deformations that occur as a result of having relatively compliant system. The beam-based FE modeling tools were validated by comparing predicted results to results of experimental work conducted by the University of Maine and NASA researchers at the material, component and structural levels. With confidence that the beam-based modeling methodology could be used to make reasonable predictions on the response of the HIAD, it was applied to the analysis of human-scale HIAD structures, including non-axisymmetric configurations and loading regimes. The analysis tools were then further extended to be coupled with structural optimization tools. The beam-based HIAD structural analysis methodologies that have been developed were shown to be capable to capturing the structural response of the HIAD system with significantly decreased analysis time when compared to continuum or shell-based FE modeling schemes. The modeling tools will complement the high fidelity, shell-based FE modeling tools under development by others. While the beam-based modeling tool can be used to efficiently explore the HIAD design space, explore various configurations and parameters, and perform trade, sensitivity and optimization studies, the shell-based modeling tools can be utilized for refined analyses once a final configuration has been down-selected.