Thermo-Structural Analysis of Thermal Protection System for Re-Entry Module of Human Space Flight

Thermo-Structural Analysis of Thermal Protection System for Re-Entry Module of Human Space Flight Manu. Jˡ, G. Vinod 2, Dr. Roy N Mathews 3 Abstract Advanced Space Transportation systems involve the reusable vehicles or modules which can be recovered from orbits of Earth or outer planets. The major challenge in the development of such modules is the design of the thermal protection system (TPS) which should withstand the high aerodynamic heating levels encountered during the atmospheric re-entry. The objective of the study was to conduct a thermo-structural analysis of the TPS of a re-entry module called Crew module developed by Indian Space Research Organisation (ISRO). Thermo-structural analysis consists of heat transfer analysis to obtain temperature distribution with its variation for the entire duration of the operation. It is followed by structural analysis for thermal and mechanical load to obtain structural deformations and stresses. Index Terms Aerodynamic heating, Re-entry vehicle, Thermal protection system, Thermo-structural analysis. I INTRODUCTION The development of atmospheric re-entry vehicles began in the late 1950's. Atmospheric re-entry refers to the movement of human made objects as they enter the atmosphere of a planet from outer space. The major challenge in the development of re-entry vehicle is the design of Thermal Protection System (TPS) which should withstand the high aerodynamic heating levels encountered during the atmospheric re-entry. Aerodynamic heating refers to the heating of a body produced by the passage of air or other gases over its surface. It is caused by friction and compression process and significant chiefly at high speeds. Due to aerodynamic heating external surfaces of the re-entry vehicle gets heated. Thermal Protection Systems are necessary in order to protect the internal structure of the vehicle from the elevated heat fluxes occurring on the external surfaces. The design of a Thermal Protection System is based on the principle that the energy released by the aerodynamic heating must be absorbed or rejected by the Thermal Protection System. Crew Module Atmospheric Re-entry Experiment (CARE) is an experimental test vehicle for the Indian Space Research Organisations future orbital vehicle. The TPS of crew module is made up of carbon-phenolic tiles. Schematic sketch of TPS for crew module is shown in Figure 1.1. The forward heat shield will be the leading edge during the re-entry; hence it will be subjected to maximum heat flux. Manu J PG Scholar, Department of Mechanical Engg, Mar Athanasius College of Engineering, Kothamangalam 9447706304 Figure 1.1: TPS of crew module Finite element model of the TPS was designed using ANSYS WORKBENCH. Transient thermal analysis has been carried out up to 200s. The temperature distribution corresponding 100s (which is the maximum heat flux condition), 180s (which is the maximum temperature condition) and 200s are obtained for thermo-structural analysis. Thermo-structural analysis has been carried out at all the above mentioned time instants by applying temperature and pressure as loads. II FINITE ELEMENT MODEL Three dimensional CAD geometry was generated and imported to the finite element software. Finite element modelling is done in ANSYS WORKBENCH. Hexahedral type of meshing is used for the forward heat shield, flare and conical heat shield which contains 3459351 numbers of nodes and 843069 numbers of elements. Tetrahedral type of meshing is used for the inner metallic structure which contains 2433560 numbers of nodes and 1283451 numbers of elements Figure 2.1: Finite element model of forward heat shield and flare region ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 125

experienced by the TPS during re-entry. Heat flux at any point on outer surface of TPS follows a parabolic curve with respect to time, being zero at the start and attaining a maximum at time 100sec. Figure 2.2: Finite element model of conical heat shield 3.1 Material Properties The material properties required for the thermal analysis are thermal conductivity, specific heat and density. Carbon-phenolic tiles used for TPS are orthotropic in nature and the material properties are temperature dependent. Table 3.1: Variation of thermal conductivity with temperature Temperature(T) Thermal Conductivity [K] [W/mK] X-dierction Y-dierction Z-dierction Figure 2.3: Finite element model of inner metallic backup structure Fig. 2.1 shows the finite element model of forward heat shield and flare region, Fig. 2.2 shows the finite element model of conical heat shield and Fig. 2.3 shows the finite element model of inner metallic backup structure. 300 0.67 0.67 0.26 600 1.36 1.36 0.62 900 2.09 2.09 1.02 1200 3.12 3.12 1.47 1500 4.69 4.69 2.01 1800 7.05 7.05 2.66 2100 10.45 10.45 3.46 2200 11.5 11.5 3.76 3000 11.5 11.5 3.76 III TRANSIENT THERMAL ANALYSIS Transient thermal analysis determines temperatures and other thermal quantities that vary over time. Engineers commonly use temperatures that a transient thermal analysis calculates as input to structural analysis for thermal stress evaluations. A transient thermal analysis follows basically the same procedures as a steady state thermal analysis. The main difference is that most applied loads in a transient thermal analysis are functions of time. The transient temperature distribution T (x; y; z; t) throughout the domain is obtained by solving the three-dimensional heat conduction equation shown below in the substrate along with appropriate initial and boundary conditions. ρc p( T/ t) = / x (k x T/ x) + / y (k y T/ y) + / z(k z T/ z) (1) Where ρ Density. C p Specific Heat. k x, k y, k z Thermal Conductivities in x, y, z directions. All material properties are considered temperature dependent. Initial conditions applied to solve Eq.(1) is T (x; y; z; 0) = T 0 (2) Where T 0 is the ambient temperature. In the analysis, T 0 is set as 300K.Boundary condition applied is the heat flux Fig. 3.1: Temperature Vs Thermal Conductivity The properties are evaluated at room temperature as well as elevated temperatures. Table 3.1 gives the thermal conductivity at elevated temperature. The variation of thermal conductivity with temperature is shown in Fig.3.1. It can be seen that the thermal conductivity of carbon-phenolic composite increases with increase in temperature. Table 3.2 gives the specific heat at elevated temperature. The variation of specific heat with temperature is shown in Fig.3.2. It can be seen that specific heat initially increases with increase in temperature and maintain a maximum value in the temperature range of 700K to 1000K, after that specific heat decreases with increase in temperature. ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 126

Table 3.2: Variation of Specific Heat with temperature Temperature(T) Specific Heat(Cp) [K] [J/kgK] 300 1074.31 500 1386.74 600 1520.5 700 18694 1000 18694 1010 1999.74 1100 1999.74 1400 2167.16 1700 2273.38 Fig. 3.3: Temperature distribution on TPS at 100s. Fig. 3.2: Temperature Vs Specific heat Fig. 3.4: Temperature distribution on inner metallic backup structure at 100s 3.3 Transient Thermal Analysis Results Transient thermal analysis has been carried out up to 200s. Temperature distribution corresponding to 100s (which is the maximum heat flux condition), 180s (which is the maximum temperature condition) and 200s are considered for structural analysis. Fig. 3.5: Temperature distribution on TPS at 180s ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 127

Fig. 3.6: Temperature distribution on inner metallic backup structure at 180s Fig. 3.3 shows the temperature distribution on TPS at 100s and the maximum temperature is 1842.9K, which is below the maximum allowable design temperature. Fig. 3.4 shows the temperature distribution on the inner metallic backup structure at 100s, which is at room temperature throughout 100s. Therefore the function of TPS is satisfied. Fig. 3.5 shows the temperature distribution on TPS at 180s and the maximum temperature is found to be 2303.4K, which is below the maximum allowable design temperature. Fig. 3.6 shows the temperature distribution on the inner metallic backup structure at 180s, which is at room temperature throughout 180s. Therefore the function of TPS is satisfied. Fig. 3.8: Temperature distribution on inner metallic backup structure at 200s Fig. 3.7 shows the temperature distribution on TPS at 200s and the maximum temperature is found to be 2278.7K, which is below the maximum allowable design temperature. Fig. 3.8 shows the temperature distribution on the inner metallic structure at 200s, which is at room temperature throughout 200s.. Therefore the function of TPS is satisfied. IV THERMO-STRUCTURAL ANALYSIS Structural analysis is probably the most common application of the finite element method. It is the science, which ensures safety of structures and fulfils the functions for which they have been built. Here the temperature values obtained from the transient thermal analysis is imported and applied as load along with the pressure for carrying out thermo-structural analysis at 100s,180s and 200s.The governing equation for the analysis are given below Fig. 3.7: Temperature distribution TPS at 200s Thermal-mechanical strain x = α x T + σ x / E x μ xy σ y / E x μ xz σ z / E x (3) y = α y T + μ xy σ x / E x σ y / E y μ yz σ z / E y (4) z = α z T + μ xz σ x / E x μ yz σ y / E y σ z / E z (5) (6) xy = σ xy / G xy ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 128

(7) (8) yz = σ yz / G yz xz = σ xz / G xz Where typical terms are: x - direct strain in x direction xy - shear strain in the x-y plane σ x - direct stress in x direction σ xy - shear stress on x-y plane 4.1 Material Properties The material properties needed for thermo-structural analysis are coefficient of thermal expansion, Young's modulus, and Shear modulus. Table 4.1: Variation of Coefficient of thermal expansion with Temperature Temperature(T) Coefficient of thermal expansion(α ) [K] [K -1 ] X-direction Y-direction Z-direction 373 1.97E-05 1.97E-05 1.60E-05 973 3.90E-06 3.90E-06-0.000107 1473-1.10E-06-1.10E-06-7.50E-05 1973 2.60E-06 2.60E-06-5.20E-05 2173 4.90E-06 4.90E-06-4.80E-05 2273 6.00E-06 6.00E-06-4.70E-05 2373 7.20E-06 7.20E-06-4.60E-05 The properties are evaluated at room temperature as well as elevated temperatures. Table 4.1 gives the coefficient of thermal expansion at elevated temperature. The variation of coefficient of thermal expansion with temperature is shown in Fig.4. In x and y direction coefficient of thermal expansion decreases with increase in temperature during initial stages and reaches negative value at temperature 1473K, after that it increases steadily and reaches a positive value. In z direction the coefficient of thermal expansion have positive value only during initial stages. Fig. 4.1: Coefficient of thermal expansion Vs Temperature Table 4.2: Variation of Young's Modulus with Temperature Temperature(T) [K] Young's Modulus(E) [MPa] X-direction Y-direction Z-direction 300 16900 16900 11900 373 16500 16500 10900 473 15000 15000 6580 573 12700 12700 5520 873 14700 14700 5660 1273 11800 11800 2520 1773 11800 11800 2590 2273 10300 10300 2470 Fig. 4.2: Young's Modulus Vs Temperature The properties are evaluated at room temperature as well as elevated temperatures. Table 4.2 gives the Young's Modulus at elevated temperature. The variation of Young's Modulus with temperature is shown in Fig.4.2 Young s modulus in x and y direction goes on decreasing with increase in temperature but there is a sudden rise in the value when the temperature is 873K, after that the value decreases with increase in temperature. In z direction the value of Young s modulus decreases with increase in temperature. Table 4.3: Variation of Shear Modulus with Temperature Temperature(T) [K] Shear Modulus 300 4540 373 4270 473 4270 573 4270 873 4370 1273 4370 1773 4670 2273 4670 ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 129

Figure 4.4: Radial deformation on TPS at 100s Fig. 4.3: Shear Modulus Vs Temperature Table 4.3 gives the Shear Modulus at elevated temperature. The variation of Shear Modulus with temperature is shown in Fig.4.3. Fig.4.3 shows that with the increase in temperature from 300K the value of shear modulus decreases but the value remains constant for a temperature range of 373K to 573K and there after it increases with increase in temperature. 4.1 Thermo-Structural Analysis Results The finite element model is run for 200s with temperature and pressure as loads. Total deformation, directional deformation and equivalent stress are obtained corresponding to 100s (which is the maximum heat flux condition), 180s (which is the maximum temperature condition) and 200s. Fig.4.4 to Fig.4.6 shows the radial deformation, hoop deformation and axial deformation on TPS at 100s respectively. The maximum radial deformation is 0.759mm, where as the maximum hoop deformation is 0.237mm and the maximum axial deformation is 1.7mm.Fig.4.7 shows the radial stress on the forward heat shield at 100s and the maximum radial stress is 147MPa. On the forward heat shield for a thickness of 0.2mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Fig.4.8 shows the hoop stress on the forward heat shield at 100s and the maximum hoop stress is 78MPa. The stress values exceed the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. Fig.4.9 shows the axial stress on the forward heat shield at 100s and the maximum axial stress is 55MPa. The stress values exceeds the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. Figure 4.5: Hoop deformation on TPS at 100s Figure 4.6: Axial deformation on TPS at 100s Figure 4.7: Radial stress on forward heat shield at 100s ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 130

Figure 4.8: Hoop stress on forward heat shield at 100s Figure 4.10: Radial stress on flare region at 100s Figure 4.9: Axial stress on forward heat shield at 100s Fig.4.10 shows the radial stress on flare region at 100s and the maximum radial stress is 90MPa. On the flare region for a thickness of 0.2mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant.fig.4.11 shows the hoop stress on the flare region at 100s and the maximum hoop stress is 300MPa. The stress values exceed the allowable strength of 41MPa locally at some regions. On all other regions the stress values are within the allowable limit. Fig.4.12 shows the axial stress on flare region at 100s and the maximum axial stress is 138MPa. On the flare region for a thickness of 0.2mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Figure 4.11: Hoop stress on flare region at 100s Figure 4.12: Axial stress on flare region at 100s ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 131

Figure 4.13: Radial stress on conical heat shield at 100s Fig.4.13 shows the radial stress on conical heat shield at 100s and the maximum radial stress is 188MPa. The stress values exceed the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. Fig.4.14 shows the hoop stress on the conical heat shield at 100s and the maximum hoop stress is 136MPa. On the conical heat shield for a thickness of 0.2mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Fig.4.15 shows the axial stress on conical heat shield at 100s and the maximum axial stress is 109MPa. The stress values exceed the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit.fig.4.16 shows the equivalent stress on inner metallic backup structure at 100s and the maximum value is found to be 263.03MPa which is below the allowable strength of 375MPa. Figure 4.14: Hoop stress on conical heat shield at 100s Figure 4.15: Axial stress on conical heat shield at 100s Figure 4.16: Equivalent stress on inner metallic backup structure at 100s Fig.4.17 to Fig.4.19 shows the radial deformation, hoop deformation and axial deformation on TPS at 180s respectively. The maximum radial deformation is 1.43mm, where as the maximum hoop deformation is 0.379mm and the maximum axial deformation is 1.96mm. Fig.4.20 shows the radial stress on the forward heat shield at 180s and the maximum radial stress is 110MPa. On the forward heat shield for a thickness of 0.5mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Fig.4.21 shows the hoop stress on the forward heat shield at 180s and the maximum hoop stress is 86MPa. The stress values exceed the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. Fig.4.22 shows the axial stress on the forward heat shield at 180s and the maximum axial stress is 55MPa The stress values exceeds the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 132

Figure 4.17: Radial deformation on TPS at 180s Figure 4.20: Radial stress on forward heat shield at 180s Figure 4.18: Hoop deformation on TPS at 180s Figure 4.21: Hoop stress on forward heat shield at 180s Figure 4.19: Axial deformation on TPS at 180s Fig.4.23 shows the radial stress on flare region at 180s and the maximum radial stress is 178MPa. On the flare region for a thickness of 0.5mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant.fig.4.24 shows the hoop stress on the flare region at 180s and the maximum hoop stress is 384MPa. The stress values exceed the allowable strength of 41MPa locally at some regions. On all other regions the stress values are within the allowable limit. Fig.4.25 shows the axial stress on flare region at 180s and the maximum axial stress is 322MPa. On the flare region for a thickness of 0.5mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Figure 4.22: Axial stress on forward heat shield at 180s Fig.4.26 shows the radial stress on conical heat shield at 180s and the maximum radial stress is 238MPa. The stress values exceed the allowable strength of 41MPa locally at some regions. On all other regions the stress values are within the allowable limit. Fig.4.27 shows the hoop stress on the conical heat shield at 180s and the maximum hoop stress is 395MPa. On the conical heat shield for a thickness of 0.5mm the stress values exceeds the allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Fig.4.28 shows the axial stress on conical heat shield at 180s and the maximum axial stress is 290MPa. On the conical heat shield for a thickness of 0.5mm the stress values exceeds the ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 133

allowable strength of 41MPa and that region will be eroded in the subsequent time instant. Figure 4.26: Radial stress on conical heat shield at 180s Figure 4.23: Radial stress on flare region at 180s Figure 4.27: Hoop stress on conical heat shield at 180s Figure 4.24: Hoop stress on flare region at 180s Figure 4.28: Axial stress on conical heat shield at 180s Figure 4.25: Axial stress on flare region at 180s Figure 4.29: Equivalent stress on inner metallic backup ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 134

structure at 180s Fig.4.29 shows the equivalent stress on inner metallic backup structure at 180s and the maximum value is found to be 316.03MPa which is below the allowable strength of 375MPa. bolt hole. On all other regions the stress values are within the allowable limit. Figure 4.30: Radial deformation on TPS at 200s Figure 4.33: Radial stress on forward heat shield at 200s Figure 4.31: Hoop deformation on TPS at 200s Figure 4.34: Hoop stress on forward heat shield at 200s Figure 4.32: Axial deformation on TPS at 200s Fig.4.30 to Fig.4.32 shows the radial deformation, hoop deformation and axial deformation on TPS at 180s respectively. The maximum radial deformation is 1.57mm, where as the maximum hoop deformation is 0.39mm and the maximum axial deformation is 2.04mm. Fig.4.33 shows the radial stress on the forward heat shield at 200s and the maximum radial stress is 110MPa. On the forward heat shield the stress values exceeds the allowable strength of 41MP for a thickness of 0.75mm, which is only 1.5% of total thickness 50mm. Therefore the forward heat shield configuration is safe. Fig.4.34 shows the hoop stress on the forward heat shield at 200s and the maximum hoop stress is 85MPa. The stress values exceed the allowable strength of 41MPa locally at the bolt hole. On all other regions the stress values are within the allowable limit. Fig.4.35 shows the axial stress on the forward heat shield at 200s and the maximum axial stress is 61MPa The stress values exceeds the allowable strength of 41MPa locally at the Figure 4.35: Axial stress on forward heat shield at 200s Fig.4.36 shows the radial stress on flare region at 200s and the maximum radial stress is 173MPa. On the flare region he stress values exceeds the allowable strength of 41MP for a thickness of 0.75mm, which is only 1.5% of total thickness 50mm. Therefore the flare region is safe. Fig.4.37 shows the hoop stress on the flare region at 200s and the maximum hoop stress is 380MPa. The stress values exceed the allowable strength of 41MPa locally at some regions. On all other regions the stress values are within the allowable limit. Fig.4.38 shows the axial stress on flare region at 200s and the maximum axial stress is 316MPa. On the flare region he stress values exceeds the allowable strength of 41MP for a thickness of 0.75mm, which is only 1.5% of total thickness 50mm. Therefore the flare region is safe. Fig.4.39 shows the radial stress on conical heat shield at 200s and the maximum radial stress is 246MPa. The stress ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 135

values exceed the allowable strength of 41MPa locally at some regions. On all other regions the stress values are within the allowable limit. Fig.4.40 shows the hoop stress on the conical heat shield at 200s and the maximum hoop stress is 392MPa. On the conical heat shield the stress values exceeds the allowable strength of 41MP for a thickness of 0.75mm, which is only 1.5% of total thickness 50mm. Therefore the conical heat shield configuration is safe. Fig.4.41 shows the axial stress on conical heat shield at 200s and the maximum axial stress is 298MPa. On the conical heat shield the stress values exceeds the allowable strength of 41MP for a thickness of 0.75mm, which is only 1.5% of total thickness 50mm. Therefore the conical heat shield configuration is safe. Figure 4.38: Axial stress on flare region at 200s Figure 4.36: Radial stress on flare region at 200s Figure 4.39: Radial stress on conical heat shield at 200s Figure 4.37: Hoop stress on flare region at 200s Figure 4.40: Hoop stress on conical heat shield at 200s ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 136

Thermo-structural analysis was carried out at 100s (which is the maximum heat flux condition), 180s (which is the maximum temperature condition) and 200s to assess the deformations and stresses. The maximum radial deformation, hoop deformation and axial deformations are 1.57mm, 0.39mm and 2.04mm respectively (at 200s) which are within the desired limit. At 200s the stress values on the TPS exceeds the allowable strength of 41MPa for a thickness of 0.75mm which is only 1.5% of total thickness 50mm.On all other regions the stress values are within the allowable limit. Therefore the carbon-phenolic TPS configuration is safe. Also the equivalent stress on inner metallic backup structure is below the allowable strength of 375MPa for the entire flight duration. Figure 4.41: Axial stress on conical heat shield at 200s Fig.4.42 shows the equivalent stress on inner metallic backup structure at 100s and the maximum value is found to be 353MPa which is below the allowable strength of 375MPa. REFERENCES [1] Santhosh,B., Thermo-Structural Analysis of 3D Composites for Reusable Applications, Proceedings of 2 nd International Conference on Materials for the Future(ICMF),2011. [2] Roberto,Silva., Direct Coupled Thermo- Structural analysis in ANSYS, Proceedings of ESSS Conference, Bourbon Atibaia,2013. [3] Thirapat,Kitinirunkul., Affecting Factors of the Mechanical Properties to Phenolic Fibre Composite, World Academy of Science, Engineering and Technology, International Journal of Chemical, Nuclear, Metallurgical and Materials Engineering Vol : 7 No: 10, 2013. [4] Zinchengo,V. I., Thermal degradation of a carbon phenolic composite in a high-temperature gas flow, January February, 1995, Volume 31, Issue 1,pp 79-86. [5] Suneeth,Sukumaran., Design and Analysis of Metallic Thermal Protection Systems,International Journal of Scientific and Research Publications, Volume 1 ISSN 2250-3, Issue 2, February 2013. Figure 4.42: Equivalent stress on inner metallic backup structure at 200s V CONCLUSION Transient thermal analysis was carried out to obtain temperature distribution in a TPS of re-entry module called Crew module which is developed by ISRO. The flight duration was 200s and temperature distribution corresponding 100s (which is the maximum heat flux condition), 180s (which is the maximum temperature condition) and 200s are obtained for structural analysis. The maximum temperature obtained is 2303.5 (at 180s) and is found to be below maximum allowable design temperature of the carbon-phenolic composite with which the TPS is made, also the inner metallic structure is maintained at room temperature during the entire flight duration. Therefore the function of the TPS is satisfied. ISSN: 2278 7798 All Rights Reserved 2016 IJSETR 137