Analytical Design of Surface Porosity in 2D C/C to Delay Boundary Layer Transition for Hypersonic Aeroshell Applications

One of the critical aspects of the aeroshell materials in current hypersonic vehicles is the ability to manage the integrated heat loads that result from the pull-up and glide portions of anticipated flight trajectories. The boundary layer transition (BLT) from laminar to turbulent results in increased thermal loading which requires additional insulation to maintain acceptable internal temperatures, resulting in a heavier vehicle. The ability to delay the boundary layer transition has advantages from both a weight and cost perspective, as well as allowing for a more controlled flight. Hypersonic BLT delay strategies have been investigated and previous research has shown that surface porosity can effectively damp the second mode instability to delay BLT.  Under the Phase I effort, Materials, Research & Design, Inc. (MR&D) successfully demonstrated the ability to fabricate a standard 2D C-C material with prescribed surface porosity perpendicular to the outer ply of the material. In addition, a modeling-based approach was developed for evaluating the effect of surface porosity on attenuating the 2nd mode instability using Computational Fluid Dynamics (CFD) and stability analyses. This methodology was used to evaluate a 7-degree half angle cone geometry at two different Reynolds numbers. The results of the CFD analysis were validated using published temperature and streamwise velocity profiles and then stability analysis was performed using a porous wall boundary condition. The stability analysis showed that the material fabricated during the program would successfully attenuate the second mode frequency and delay turbulence. The Phase II program aims to expand upon this effort and raise the TRL of this technology by validating the component in a relevant environment and demonstrating a subsystem prototype.  The main objective for the Phase II effort is to design, fabricate, and wind tunnel test a C/C aeroshell material with prescribed surface porosity that is capable of delaying boundary layer transition due to the second mode instability under relevant hypersonic boost-glide conditions.  The technical approach is a combined analytical and experimental approach, which includes analysis and design work to determine suitable surface porosity for a representative trajectory, fabrication of coupons with surface porosity and testing of the absorption coefficients as well as the recession behavior of the material, and finally the design, fabrication, and testing of a wind tunnel model using both a smooth and porous surface to demonstrate the ability to delay boundary layer transition.