Description:

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Hypersonics; Nuclear
OBJECTIVE: Formulate, implement, and validate finite-rate ablation models for hypersonic boost-glide vehicle thermal protection systems (TPS) such as 2D and 3D carbon-carbon (C/C) for accurate prediction of vehicle shape change, temperature distributions, and ablated species.
DESCRIPTION: Boost-glide vehicles primarily rely on ablating TPS. Unlike strategic ballistic missiles, boost-glide weapons operate for a long duration in a range of altitudes and velocities where the rates of energy excitation, chemical reactions, ionization, and gas-surface interactions are comparable to the rate of fluid motion. This implies that the accuracy of legacy modeling approaches that assume equilibrium thermo-chemistry and equilibrium ablation are questionable for such systems. Recent numerical studies of graphite ablation at boost-glide relevant flight conditions have shown that equilibrium ablation models (i.e., B’ method) significantly over-predict nose-tip ablation compared to finite-rate ablation models [Ref 1].
Over the last decade, significant improvements have been made in the understanding and prediction of finite-rate processes in hypersonic flows by using ab initio quantum chemistry methods to generate potential energy surfaces (PES) for molecular interactions [Ref 1]. The PES can be used for the direct simulation of collisions between air species to obtain data for relaxation and reaction rates at specified conditions [Ref 2]. These data have enabled improved finite-rate chemistry models for computational fluid dynamics (CFD) simulations that accurately account for the vibrational energy state of the air species in the dissociation process [Ref 3]. Over the same time period, improvements have been made in the development of finite-rate carbon oxidation models based on molecular beam experiments of high-velocity O, and O2 species impacting high-temperature carbon material [Ref 4]. Such experiments enable the characterization of individual reactions and rate parameters as opposed to plasma wind tunnels experiments that measure the combined outcome of many reactions such that multiple combinations of parameters can be used to match the measured recession rate. Different finite-rate ablation models that produce comparable ablation rates can predict large difference in the ablation species such as CO, CO2, and CN. Very recently, molecular beam experiments that included the interaction with N and N2 species have provided the data needed for the development of construction of air-carbon ablation models relevant to hypersonic flight that include nitration in addition to oxidation [Ref 5].
Recent progress in the development of finite-rate carbon ablation models is encouraging, but these models need to be validated under a range of relevant hypersonic conditions and geometries for simple and relevant materials such as 2D and 3D C/C. This is needed as current molecular beam data has been obtained on highly oriented pyrolytic graphite (HOPG) and vitreous carbon as experimental models for the fibers and matrix, respectively, of C/C.
It is also imperative to implement the new finite-rate ablation models in fully coupled CFD / material response codes to predict the TPS shape change, temperature distributions and ablated species. In addition, the assessment of TPS thermo-structural properties, surface roughness, and damage requires the simulation of the material microstructure.
PHASE I: Formulate and implement finite-rate air-carbon ablation models in coupled CFD material response. Perform comparison against existing experimental data for simple materials such as HOPG and vitreous carbon. Quantities of interest include ablation rates (shape change), surface temperature, and ablated species. Criteria for success include the successful model implementation in material response code and validation.
PHASE II: Refine the finite-rate ablation models and their implementation for relevant materials such as 2D and 3D C/C. The model refinement could include fundamental measurements on C/C to understanding the effect of the material microstructure on the reaction rates and parameters. Validation under a range of relevant hypersonic flow conditions in an arc-jet and or inductively coupled plasma torch (ICP) for nose-tip, leading-edge, and acreage are required to assess the accuracy of the ablation models.
PHASE III DUAL USE APPLICATIONS: Improve the models and efficiency of the coupled hypersonic CFD / ablation toolset for prediction of shape change on complex geometries. The ablation toolset will ultimately be demonstrated on a relevant Navy weapons geometry via ground and/or flight test once a sufficient TRL is achieved.
In the near term, this technology is geared toward military applications, but the methodologies for ablation model development and ablation toolset can be commercialized for commercial space vehicles using C/C TPS. Additional development for carbon-phenolic materials are also possible to expand the range of applications.
REFERENCES:
1. G. V. Candler, "Rate effects in hypersonic flows," Annual Review of Fluid Mechanics, vol. 51, pp. 379-402, 2019.
2. R. L. Jaffe, D. W. Schwenke and M. Panesi, "First Principles Calculation of Heavy Particle Rate Coefficients," in Hypersonic Nonequilibrium Flows: Fundamentals and Recent Advances, Reston, American Institute of Aeronautics and Astronautics, Inc., 2015, pp. 103-158.
3. R. S. Chaudhry, J. D. Bender, T. E. Schwartzentruber and G. . V. Candler, "Quasiclassical Trajectory Analysis of Nitrogen for High-Temperature Chemical Kinetics," Journal of Thermophysics and Heat Transfer, vol. 32, no. 4, pp. 833-845, 2018.
4. S. Poovathingal, T. E. Schwartzentruber, V. J. Murray, T. K. Minton and G. V. Candler, "Finite-rate oxidation model for carbon surfaces from molecular beam experiments," AIAA journal, vol. 55, no. 5, pp. 1644-1658, 2017.
5. V. j. Murray, P. Recio, A. Caracciolo, C. Miossec, N. Balucani, P. Casavec and T. K. Minton, "Oxidation and nitridation of vitreous carbon at high temperatures," Carbon, vol. 167, pp. 388-402, 2020.
KEYWORDS: Ablation; carbon-carbon; finite-rate; boost-glide; molecular-beam; oxidation; nitration