Fellowship for Kristen Price: Spallation Effects in TPS Models

During atmospheric entry, space vehicles need to be significantly slowed down from their hyper- sonic velocities in order to achieve a safe landing. The shape of the vehicle generates drag that can help this process, but the large kinetic energy is inevitably transformed into thermal energy, creat- ing a large heat load on the vehicle (see Figure 1). Although most of the energy is convected away around the vehicle, a thermal protection system (TPS) is needed to prevent the remaining heat from reaching inside the space vehicle [2]. One approach is to use an ablative material as a TPS. Commonly, these ablative materials take the form of a carbon ber matrix preform impregnated with a phenolic resin, as shown in Figure 2. The process of ablation absorbs heat through sublimation and mass removal of the material. The bers in this material are laid out to reduce the conductivity of heat through the material, protecting the vehicle from extreme temperatures [2]. However, the brous nature of these materials makes them very brittle, and mechanical erosion makes these bers more susceptible to failure. The process known as spallation occurs when pieces of these bers break o of the matrix and enter the ow before fully ablating. This process has many unwanted eects, including an increase in surface roughness, recession rates, and heat transfer resulting from radiation and turbulence from the particles [3]. As a result, being able to model the process of spallation and accurately account for it in design is crucial to creating a successful TPS. However, the lack of experimental data and knowledge about the details of this process makes it dicult to model accurately. Recent arc-jet test results and analysis produced a data set which provides guidance for modeling the spallation of the TPS and the production of these particles. Utilizing analysis from arc- jet experiments conducted by the applicant and recently submitted to the journal Experimental Thermal and Fluid Science [4], the proposed research seeks to develop and implement a spallation model within a coupled computational uid dynamics and material response ow solver in order to more accurately represent the eects of spallation on thermal protection systems. 3 2 Problem Statement Figure 2: Carbon bers that make up the ablative material (Image taken from Ref. [6]). Currently, the Gas Surface Interactions Lab at the University of Kentucky is working on modeling the behavior of individual spalled par- ticles once they enter the ow. This is achieved using a Lagrangian particle code, tightly cou- pled to the hypersonic Computational Fluid Dy- namics (CFD) module of the KATS framework [3, 5]. This approach allows for a model of the chemical interactions of the spalled particle with the surrounding ow eld, as shown in Figure 3 [3]. It can also be used to understand how indi- vidual spalled particles aect the heating rates on the heat shield. However, this does not take into account the production rate and volume of these particles produced for the given sample type and ow conditions, or the impact on the material response of particles detaching from the surface. Figure 3: Spallation particle code results of CO production (Image taken from Ref. [9]). Previous analysis of experimental results provided insight about the size of these particles, the number of particles pro- duced, and how the production of these particles are aected by various factors [7, 8]. Thus, coupling this analysis with the existing spallation code will provide a model of particles being produced for given sample and ow conditions, as well as the eects of the entire spallation process on the TPS material. 3 Approaches to Solutions The existing experimental data resulted from arc-jet tests in the Hypersonic Materials Environmental Test System (HyMETS) at NASA Langley Research Center. Various ow environments, sample materials, and sample geometries were tested to determine the eects of oxidation, surface shear stress, pyrolysis gas formation, surface heat ux, and, more importantly, spallation and particle generation. In these ex- periments, cameras were utilized to capture high-speed images of these spalled particles being produced, as shown in Figure 4. Utilizing this high-speed imagery, particle tracking velocime- try was used to determine the position, velocity, and accel- eration of each particle along its trajectory [10]. From here, statistical analysis was conducted to determine how particle production changed with the various ow and sample condi- tions. It was found that oxidation, surface shear stress, and heat ux caused an increase in particle production, and therefore provide a basis for the ini- tiation of spalled particles [7]. Additionally, analyzing the acceleration of the particles once removed from the surface allowed for the determination of a time constant, from which an 4 estimation of particle size could be found. Finding this estimation for particle size across a wide range of tracked trajectories allowed for the development of a particle size distribution, in which a range of particle diameters was determined [8]. Figure 4: A spalled particle captured from high-speed imagery of TPS arc-jet tests. Contemporary TPS models use a sim- ple empirical parameter to account for spal- lation, in which a failure parameter (B0f ) is used to predict the amount of mass loss that occurs as a percentage of ablation rates [11]. The proposed work aims to replace this parametric approach with one that better represents the physical process. By conduct- ing material response analysis of the TPS - brous microstructure (using available micro- tomography data) with conditions matching prior experiments, the formation of individ- ual particles will be investigated in greater detail and validated using the experimental results. In particular, a probabilistic model for the creation of particles will be derived and introduced into the macroscopic Material Response (MR) module of the KATS framework [12, 13, 14, 15, 16]. In addition, the proba- bility density of the particle size determined from the previous experiments will allow for the introduction of spallation at dierent volumes and frequencies, thus representing the actual statistical equivalent of the particles being produced during the process. The corresponding mass loss produced by particle generation will be modeled within the MR model, includ- ing the impact on surface structure and heat transfer rates. To model the impact of the particle generation on the ow eld, the particles will be introduced into the CFD model using the Lagrangian/Eulerian coupling approach already implemented. Executing this new model within the CFD and MR codes will result in a more detailed and precise approach of modeling the process of spallation, as well as the repercussions that it has on the TPS. Finally, pending arc-jet facility availability, further experimental analysis could be con- ducted to validate previous results. Material samples have already been developed with aerogel slots designed to physically capture spalled particles. Similar to Stardust, which used aerogel slabs to capture comet dust, the aerogel is intended to trap spalled particles from the sample to be later analyzed [17]. This would allow for direct measurement of these particles and for better understanding of their structure and geometry, conrming previously determined size distributions and providing further information on their formation. Although not necessary for the proposed research, improved measurements of particle dimensions and density could provide even tighter ranges of particle size and mass loss resulting from this process, further improving the accuracy of the proposed spallation model. 4 Specic Goals for Funded Period As detailed in the Approaches to Solution section, the main goals of the proposed research are listed below: 1. Utilize experimental results to determine probabilistic model for particle creation, par- 5 ticularly size and frequency of particles produced. 2. Integrate this model into the existing spallation code. 3. If facility availability allows, conduct additional arc-jet tests to conrm particle size using direct measurement. Figure 5: Expected tasks and milestone completions. 5 Relevance to Space Technology Since thermal protection systems are critically important, they are one of the key areas of study for NASA's future technology. They are impactful in so many dierent areas of research that they are discussed in 7 out of the 14 NASA technology roadmaps (TA08-TA14) [18]. Therefore, any development regarding TPS technology can have dramatic impacts across NASA's technological goals. The proposed research specically focuses on TA14.3, as it aims to integrate the process of spallation into TPS modeling, which is a crucial and growing technology for the rapidly expanding space economy [19]. The existing experimental results will provide new insight into this process that is not re ected in existing TPS models. The inability to test these models under actual re-entry conditions, as well as the lack of experimental data on the process of spallation, makes this research dicult to achieve without this previously conducted arc-jet test data and analysis. Additionally, the coupling of CFD and the response of the material under complex ow environments achieved by KATS is crucial to accurately modeling this process and its eects on the ow and material. Thus, the tools and data utilized in this proposed work provide a unique combination to achieve more accurate modeling and improve TPS design so that mass, size and cost are conserved. This analysis can also be extended to many other areas and applications in the future. In particular, dusty ows are a very critical area of research since they are a common prob- lem found in planetary exploration, such as on Mars. Experimentally, similar analysis can be done with arc-jet facilities modied to inject dust particles into the ow. Variation in sample type, ow conditions, and dust particles can provide an analysis of the eects of these various factors on heat ux and erosion rates of the samples. Numerically, a similar method to the spallation model can be utilized to model these dusty ows by inserting dust particles upstream of the sample surface and varying their eect on the samples based on the experimental results. 6 Relevance to Previous Work The research being conducted for my graduate work thus far will provide an excellent skill set for conducting the proposed research. As discussed above, my previous research involves conducting the analysis on the existing arc-jet experiment data to determine a 6 characterization of the behavior of these spalled particles. A familiarity with this data and the analysis conducted will provide a crucial understanding of this process in order to better implement it into the model. Additionally, an understanding of previous arc-jet tests and what is important for analysis will be very helpful in future testing. The results obtained from the research conducted thus far, as well as the model development, implementation, and validation resulting from the coupling with the existing spallation code, will be the focus of my dissertation. 7 Research Team Collaboration Conducting this research as part of the GSIL at the University of Kentucky under Dr. Alexandre Martin will provide excellent support for accomplishing this research. This re- search group is well-known for their knowledge and extensive work with ablation processes and modeling of thermal protection systems. The KATS software has been proven to provide reliable, high-delity material response results. This will be benecial when implementing the results of the experiments into the model. Particularly, previous work done both with spallation and with the HYMETS facility provides multiple resources for integrating the experimental results with the spallation code [20, 21]. Note that although this research will be conducted with the assistance of this research group and utilizing the KATS model, it is a unique and separate development to the model that is not a part of any existing project. The additional support of Dr. Sean Bailey will also provide valuable guidance for this re- search, particularly with his expertise in experimental design, ow eld diagnostics, and data analysis, [22, 23] as well as his experience testing at HYMETS [10, 24]. 8 Visiting Technologist Experience Although the support of this research team and its resources are incredibly important, the facilities, knowledge, and research being conducted at NASA research centers provide invaluable support. As I learned from my previous summer internship experience at Ames Research Center, being present in the heart of research and among the top scientists in their elds allowed for an incomparable experience and opportunity to learn. Working with the people and projects I had researched and read about, having access to various facilities, and being surrounded by multiple experts who share their input and ideas allowed me to grow signicantly in my knowledge and provided me countless opportunities for collaboration. I know these experts and resources will be vital to developing my research. Therefore, the visiting technologist experience is undoubtedly an important part of the NSTGRO experience that will allow me to make the most out of my research. In particular, being present with other researchers who are working on TPS modeling and understanding the spallation process can aid in implementing these results to obtain the most eective model. Additionally, the visiting technologist experience would be extremely helpful with the possibility of conducting further experimental analysis and potentially utilizing NASA facilities. Thus, this experience will provide incomparable resources for conducting the proposed research and working with experts in the eld to obtain the best results. 7 References [1] Camillo, Jim. \Resin Shields Orion Craft and Crew During Re-Entry." Assembly, 8 Sept. 2016, www.assemblymag.com/articles/93551-resin-shields-orion-craft-and-crew- during-re-entry. [2] Bowman, W. H. and Lawrence, R. M., \Ablative materials for high-temperature thermal protection of space vehicles," Journal of Chemical Education, Vol. 48, No. 10, October 1971, pp. 690{691. [3] Davuluri, R. S. C., Zhang, H., and Martin, A., \Numerical study of spallation phe- nomenon in an arc-jet environment," Journal of Thermophysics and Heat Transfer, Vol. 30, No. 1, Jan. 2016, pp. 32{41. doi:10.2514/1.T4586 [4] Price, K. J., Borchetta, C. G., Hardy, J. M., Panerai, F., Bailey, S. C. C., and Martin, A., \Arc-Jet Measurements of Low Density Ablator Spallation," Experimental Thermal and Fluid Science, 2020, In review. [5] Duzel, U., Schroeder, O. M., Zhang, H., and Martin, A., \Numerical Simulation of an Arc Jet Test Section," Journal of Thermophysics and Heat Transfer, Vol. 34, No. 2, April 2020, pp. 393{403. [6] Panerai, F., Martin, A., Mansour, N.N., Sepka, S.A., and Lachaud, Jean., \Flow-Tube Oxidation Experiments on the Carbon Preform of a Phenolic-Impregnated Carbon Ab- lator," Journal of Thermophysics and Heat Transfer, Vol. 28, No. 2, April{June 2014, pp. 181-190. doi:10.2514/1.T4265. [7] Price, K. J., Hardy, J. M., Borchetta, C. G., Panerai, F., Bailey, S. C., and Mar- tin, A., \Analysis of spallation products using arc-jet experiments," AIAA Scitech 2020 Forum, Orlando, Florida, 6{10 January 2020. doi:10.2514/6.2020-1707, URL https://doi.org/10.2514/6.2020-1707. [8] Price, K. J., Bailey, S. C.C., Panerai, F., Hardy, J. M., Borchetta, C. G., and Mar- tin, A., \Spallation particle size analysis resulting from arc-jet experiments," AIAA Aviation 2020 Forum, Virtual Event, 15{19 June 2020. doi:10.2514/6.2020-3279, URL https://doi.org/10.2514/6.2020-3279. [9] Davuluri, R. S. C., Zhang, H., and Martin, A., \Eects of spalled particles thermal degradation on a hypersonic ow eld environment," AIAA Scitech 2016 Forum, San Diego, California, 4-8 January 2016. doi:10.2514/6.2016-0248, URL https://doi.org/10.2514/6.2016-0248. [10] Bailey, S. C., Bauer, D., Panerai, F., Splinter, S. C., Danehy, P. M., Hardy, J. M., and Martin, A., \Experimental analysis of spallation particle trajectories in an arc-jet envi- ronment," International Journal of Experimental Heat Transfer, Thermodynamics, and Fluid Mechanics, Vol. 93, 2018, pp. 319{325. doi:10.1016/j.exptherm usci.2018.01.005. [11] Milos, F.S., and Chen, Y.-K., \Ablation, Thermal Response, and Chemistry Program for Analysis of Thermal Protection Systems," Journal of Spacecraft and Rockets, Vol. 50, No. 1, January{February 2013. doi:10.2514/1.A32302. 8 [12] Weng, H., Bailey, S. C. C., and Martin, A., \Numerical study of iso-Q sample geometric eects on charring ablative materials," International Journal of Heat and Mass Transfer, Vol. 80, Jan. 2015, pp. 570{596. doi:10.1016/j.ijheatmasstransfer.2014.09.040 [13] Weng, H. and Martin, A., \Multidimensional modeling of pyrolysis gas transport inside charring ablative materials," Journal of Thermophysics and Heat Transfer, Vol. 28, No. 4, Oct.{Dec. 2014, pp. 583{597. doi:10.2514/1.T4434 [14] Fu, R., Weng, H., Wenk,