# Comprehensive Comparison of Different Integrated Thermal Protection Systems with Ablative Materials for Load-Bearing Components of Reusable Launch Vehicles

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

#### 1.1. Context

#### 1.2. Ablative and Integrated Thermal Protection Systems

#### Use of Phase Change Materials for Integrated Thermal Protection Systems

- A phenolic impregnated carbon ablator (PICA) ablative TPSis analysed by means of a solver based on the one-dimensional finite volume method. The thermal mass is optimised via a root finding algorithm.
- The CMC-based ITPS is composed of a corrugated core sandwich structure made of C/SiC and Saffil
^{®}insulation. The aforementioned solver (with ablation terms deactivated) is used to analyse it. A constrained optimisation algorithm based on sequential least squares programming (SLSQP) implemented in Python^{®}is used to optimise the core and face sheets geometry for minimal thermal mass. - The solution based on lattice core-PCM sandwich structures is analysed via implementing a homogenisation technique based on the semi-analytical model proposed by Hubert et al. [22] and on the application of mixture rules, as reported in [23]. The PCM behaviour is modelled with use of the apparent heat capacity method, implemented in COMSOL
^{®}Multiphysics.

## 2. Governing Equations

#### 2.1. Ablation

- The hot boundary layer gases of the flow,
- The surface (mostly charred) solid material,
- The pyrolysis gas emerging from the depths of the decomposing layer.

#### 2.2. Energy Equation for ITPS

#### 2.3. Material Properties

#### 2.3.1. Ablative Material

#### 2.3.2. Corrugated Core ITPS

#### 2.3.3. Lattice Core ITPS with Embedded PCM

## 3. Solver for Ablative TPS and Corrugated Core ITPS

^{®}. The implementation in Python

^{®}allows a simplified connection of the realised solver with different optimisation packages, which allow one to optimise the thermal mass. This way, an easy and accurate comparison of the two options is achievable. For this reason, the tool is named

**Hot-St**ructure and

**A**blative

**R**eaction

**Shi**eld

**P**rogram (Hot-STARSHIP). Figure 5 shows a flowchart of the program.

#### 3.1. Verification

#### 3.2. Optimisation

^{®}, the Hot-STARSHIP solver can be easily connected to a constrained optimisation algorithm. The constraints are given via maximum achievable temperatures at the boundaries.

## 4. Parametric Study of the Lattice Core-PCM ITPS

^{®}718. The center face sheet, lattice core of the insulation layer, and bottom face sheet are, for simplicity of treatment, considered to be made of Inconel 718 as well.

_{2}ccz cell is consistently the unit cell that exhibits the highest out-of plane thermal conductivity for a given porosity. This is not true for the bcc unit cell, which shows the lowest thermal conductivity. Thus, the chosen unit cell topologies are trivially f

_{2}ccz for the PCM core and bcc for the insulation core. Similarly, the porosity of the lattice structure for the insulation core should be as high as possible to reduce both mass and effective thermal conductivity, thus leading to a trivial choice. The same is not true for the PCM core. The effective thermal conductivity should be high enough to improve the thermal energy storage of the PCM, but, as the conductivity increases with diminishing porosity, it should be kept as low as possible to keep mass at a minimum. For this reason, the porosity is varied in a range, as reported in Table 1. Similarly, the PCM core thickness defines the PCM mass available and thus the thermal response. This is therefore also a parameter to be varied in the study. The insulation core thickness is varied as well, as it influences the effective thermal resistance. The geometric parameters and their range are summarised in Table 1.

^{®}Multiphysics is used, which is based on the finite element method (FEM). It implements the apparent heat capacity method described in Section 2.2. The homogenisation approaches described in Section 2.3.3 are used for both the PCM and insulation core.

## 5. Results and Discussion

#### 5.1. Boundary Conditions

^{2}. In a similar way, the pressure distribution is obtained and is reported in Figure 8b.

^{2}at the longitudinal position of 2.2 m on the ADD is considered. The pressure distribution is applied on the whole component.

#### 5.2. Thermal Response of the Ablative TPS

^{2}.

#### 5.3. Thermal Response of the Corrugated Core ITPS

^{®}fibrous insulation felt.

^{2}. Figure 11 shows the temperature evolution under the same boundary conditions previously analysed. It can be noticed that a thermal gradient of 1100 K is present between the top face sheet and the bottom face sheet. This indicates that the optimisation reached its goals, achieving a component with a very low effective thermal diffusivity. This allows the re-radiation of a wide amount of the convective heat input. This design is beneficial from the thermal protection design point of view. However, due to the combination of high stiffness of C/SiC and high thermal gradient, thermo-mechanical stresses can become a concern, given the low specific strength of CMCs.

#### 5.4. Thermal Response of the Lattice Core-PCM ITPS

_{2}CO

_{3}(22%)-Na

_{2}CO

_{3}(16%)-K

_{2}CO

_{3}, which, however, exhibits a much higher melting point. This indicates that the thermal behaviour is ascribed to only sensible heat storage. This indicates that the material is not suitable for lightweight latent heat thermal energy storage, as its thermal behaviour is only related to the high thermal mass.

_{2}mixture exhibits a comparably high latent heat, which is shown via the flattening of the temperature curve around its melting point. However, the melting point is higher than that of the LiCl-LiOH mixture, which also shows the highest latent heat of fusion. Thus, the material choice for further consideration in the geometric parametric study falls on the LiCl-LiOH mixture.

#### 5.5. Preliminary Structural Design

- For the corrugated core ITPS solution, no modification of the design is made, and the final geometrical configuration obtained from the thermal optimization (see Section 5.3) is analysed under mechanical and thermal loads.
- The considered configuration of the lattice core-PCM ITPS is the one on the higher end of the geometrical ranges considered in Section 5.4 (i.e., ${t}_{{c}_{PCM}}=10\phantom{\rule{5pt}{0ex}}\mathrm{mm}$, $\u03f5=0.9$, ${t}_{{c}_{ins}}=40\phantom{\rule{5pt}{0ex}}\mathrm{mm}$).
- The mechanical analysis of the load-bearing structure for the ablative PICA TPS analysed in Section 5.2 is used to iteratively optimise the CFRP laminate. The goal of the optimization is to obtain a layup that does not exhibit material failure under the mechanical loads.

#### 5.5.1. Load-Bearing Structure Carrying the Ablative TPS

^{®}2020 on a mesh consisting of a linear shell and beam elements. Typical material data for unidirectional (UD) T700 prepreg material and aluminium honeycomb are used (see Appendix B). Due to the optimised ablative TPS, virtually no thermal loads act on the load-bearing structure underneath. Therefore, only dynamic pressure loads (see Figure 8b) are considered. In the laminate, the number and orientation of the UD layers was iterated, as well as the cross-section of the X-shaped reinforcements and the frame along the ADD’s perimeter.

#### 5.5.2. Corrugated Core ITPS

^{®}APDL is used. To take into account the thermal deformation, a coupled thermo-mechanical analysis is performed. The thermal solver is used to obtain the temperature field on the whole structure for the time point at which the maximum outer facesheet temperature is reached, which also corresponds to the maximum thermal gradient. The analysis leads to the results shown in Figure 18. The material properties are reported in Table A5. Due to the lack of established failure criteria for CMCs, the Von Mises equivalent stress on the component is reported. Widespread failure in several parts of the component is detected. The material tensile strength of 260 MPa is exceeded in several points of the structure, even far from the constraints where a local, artificial increase in stress is observed. This is mainly due to the high thermal gradient acting on the structure. The sandwich structure offers a high bending stiffness, which, although advantageous for the mechanical loading of the component, leads to high thermally induced stresses. Although the material exhibits a low coefficient of thermal expansion (CTE), the thermal stresses still exceed the allowable values in several sections of the component. Future design involving different fibre orientations that achieve a three-dimensional tailoring of the CTE might mitigate the incurred failures while retaining high bending stiffness.

#### 5.5.3. Lattice Core-PCM ITPS

^{®}2020.

#### 5.6. Final Mass Estimation

## 6. Conclusions

- Ablative TPS solution
- -
- The separation of thermal and structural functions allows one to use efficient materials and construction methods for each absolved function, namely PICA for thermal protection and CFRP-aluminium honeycomb sandwich for load-bearing functionality.
- -
- The solution delivers the lowest overall mass.
- -
- It is easier to obtain a feasible solution because of the two high-TRL solutions used in this concept.
- -
- Reusability is a concern. Indeed, after-flight maintenance operations should include either a check of the receded amount of ablative material or a re-application. Alternatively, a fast-swap concept can be considered, directly removing and substituting both the structural element and the thermal protection system.

- ITPS-CMC corrugated core sandwich
- -
- The concept represents a lightweight, reusable solution for thermal protection purposes.
- -
- However, the thermally optimised solution does not withstand the thermo-mechanical loads.
- -
- Although ceramic matrix composites exhibit a low coefficient of thermal expansion, the high thermal gradients and the high stiffness lead to high thermal stresses compared to the low tensile strength of the material. Improvements in this direction are needed to allow a load bearing functionality of CMC-based TPS. Three-dimensional CTE tailoring via appropriate fibre orientation can be considered in future work.

- ITPS-Lattice core/PCM
- -
- The integration of a PCM drastically reduces outer wall (top face sheet) temperatures and therefore allows use of materials with high specific mechanical properties, i.e., Inconel.
- -
- However, thermal stresses above the yield strength of the respective materials in the different layers are identified. These can be caused by mismatch in the CTE of the different materials and high bending stiffness. Additionally, the use of copper alloy, although beneficial to improving the thermal conductivity of the PCM, has the drawback of a low specific yield strength.
- -
- Different material combinations can be considered in the future. In particular, given the obtained operative temperatures, titanium based alloys are good candidates for the face sheets and for the insulation core. High temperature aluminium alloys, which retain their strength up to 300℃, could be considered for the PCM core. This way, a higher lightweight potential can be obtained.
- -
- Additive manufacturing allows for local adaptation of the structure. Local optimization of lattice unit cell parameters can allow further mass reduction with improved thermo-mechanical behaviour.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

ADD | Aerodynamic drag device |

BFS | Bottom face sheet |

CFD | Computational fluid dynamic |

CFRP | Carbon fibre-reinforced polymer |

CMC | Ceramic matrix composite |

CTE | Coefficient of thermal expansion |

DOF | Degree of freedom |

FEM | Finite element method |

FVM | Finite volume method |

ITPS | Integrated thermal protection system |

PCM | Phase change material |

PICA | Phenolic impregnated carbon ablator |

RLV | Reusable launch vehicle |

SLSQP | Sequential least squares programming |

TACOT | Theoretical ablative composite for open testing |

TFS | Top face sheet |

TPS | Thermal protection system |

TRL | Technology readiness level |

UD | Unidirectional |

## Appendix A. Verification of the Hot-STARSHIP Solver

Property | Symbol | Value |
---|---|---|

Initial length | ${l}_{0}$ | 50 mm |

Initial temperature | ${T}_{ini}$ | 300 K |

Pressure | p | 101,325 Pa |

Turbulent factor | $\lambda $ | 0.5 |

## Appendix B. Material Data

^{®}database for a T700 CFRP composite material and for an aluminium honeycomb.

E1 | E2 | Nu12 | G12 | G13 | G23 | ${\mathit{\alpha}}_{11}$ | ${\mathit{\alpha}}_{22}$ | ${\mathit{\alpha}}_{33}$ |
---|---|---|---|---|---|---|---|---|

121,000 | 8600 | 0.27 | 4700 | 3100 | 4700 | $-4.7\times {10}^{-7}$ | $3\times {10}^{-5}$ | $3\times {10}^{-5}$ |

Tensile X | Compression X | Tensile XY | Compression XY | Shear Strength XY |
---|---|---|---|---|

2321 | −1082 | 29 | −100 | 60 |

E1 | E2 | E3 | Nu12 | Nu13 | Nu23 | G12 | G13 | G23 |
---|---|---|---|---|---|---|---|---|

1 | 1 | 255 | 0.49 | 0.01 | 0.01 | $1\times {10}^{-6}$ | 37 | 70 |

Density [g/cm^{3}] | Tensile Str. [MPa] | Compressive Str. [MPa] | Young Modulus [GPa] |
---|---|---|---|

1.8 | 260 | 590 | 90 |

## References

- Niederstrasser, C.G. The small launch vehicle survey a 2021 update (The rockets are flying). J. Space Saf. Eng.
**2022**, 9, 341–354. [Google Scholar] [CrossRef] - Governale, G.; Rimani, J.; Viola, N.; Villace, V.F. A trade-off methodology for micro-launchers. Aerosp. Syst.
**2021**, 4, 209–226. [Google Scholar] [CrossRef] - Medici, G.; Bergström, R.; Martí, L.; Palumbo, N.; Hove, B.; Viladegut, A.; Paris, S.; Soepper, M.; Bhardwaj, P.; Rellakis, D.; et al. A Novel Design Approach for a Reusable VTOL Micro Launch Vehicle. In Proceedings of the 72nd International Astronautical Congress, Dubai, United Arab Emirates, 25–29 October 2021; Available online: https://www.researchgate.net/publication/360008193_A_novel_design_approach_for_a_reusable_VTOL_Micro_Launch_Vehicle (accessed on 21 March 2023).
- Marwege, A.; Gülhan, A.; Klevanski, J.; Hantz, C.; Karl, S.; Laureti, M.; De Zaiacomo, G.; Vos, J.; Jevons, M.; Thies, C.; et al. RETALT: Review of technologies and overview of design changes. CEAS Space J.
**2022**, 14, 433–445. [Google Scholar] [CrossRef] [PubMed] - Natali, M.; Kenny, J.M.; Torre, L. Science and technology of polymeric ablative materials for thermal protection systems and propulsion devices: A review. Prog. Mater. Sci.
**2016**, 84, 192–275. [Google Scholar] [CrossRef] - Uyanna, O.; Najafi, H. Thermal protection systems for space vehicles: A review on technology development, current challenges and future prospects. Acta Astronaut.
**2020**, 176, 341–356. [Google Scholar] [CrossRef] - Dorsey, J.T.; Poteet, C.C.; Wurster, K.E.; Chen, R.R. Metallic Thermal Protection System Requirements, Environments, and Integrated Concepts. J. Spacecr. Rockets
**2004**, 41, 162–172. [Google Scholar] [CrossRef] - Le, V.T.; Goo, N.S. Design, Fabrication, and Testing of Metallic Thermal Protection Systems for Spaceplane Vehicles. J. Spacecr. Rockets
**2021**, 58, 1043–1060. [Google Scholar] [CrossRef] - Le, V.T.; Goo, N.S. Thermomechanical Performance of Bio-Inspired Corrugated-Core Sandwich Structure for a Thermal Protection System Panel. Appl. Sci.
**2019**, 9, 5541. [Google Scholar] [CrossRef][Green Version] - Heidenreich, B. C/SiC and C/C-SiC Composites. In Ceramic Matrix Composites; Bansal, N.P., Lamon, J., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, USA; pp. 147–216. [CrossRef]
- Glass, D.E. Ceramic matrix composite (CMC) thermal protection systems (TPS) and hot structures for hypersonic vehicles. In Proceedings of the 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, Dayton, OH, USA, 28 April–1 May 2008; pp. 1–36. [Google Scholar] [CrossRef][Green Version]
- Ferraiuolo, M.; Scigliano, R.; Riccio, A.; Bottone, E.; Rennella, M. Thermo-structural design of a Ceramic Matrix Composite wing leading edge for a re-entry vehicle. Compos. Struct.
**2019**, 207, 264–272. [Google Scholar] [CrossRef] - Blosser, M.L.; Chen, R.R.; Schmidt, I.H.; Dorsey, J.T.; Poteet, C.C.; Bird, R.K.; Wurster, K.E. Development of advanced metallic thermal-protection-system prototype hardware. J. Spacecr. Rockets
**2004**, 41, 183–194. [Google Scholar] [CrossRef] - Fischer, W.; Bolz, J. ULTIMATE: Metallic TPS for Future RLV’s. In Proceedings of the 9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, San Francisco, CA, USA, 5–8 June 2006. [Google Scholar] [CrossRef]
- Bapanapalli, S.; Martinez, O.; Gogu, C.; Sankar, B.; Haftka, R.; Blosser, M. (Student Paper) Analysis and Design of Corrugated-Core Sandwich Panels for Thermal Protection Systems of Space Vehicles. In Proceedings of the 47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference; 14th AIAA/ASME/AHS Adaptive Structures Conference, Newport, RI, USA, 1–4 May 2006. [Google Scholar] [CrossRef][Green Version]
- Gogu, C.; Bapanapalli, S.K.; Haftka, R.T.; Sankar, B.V. Comparison of materials for an integrated thermal protection system for spacecraft reentry. J. Spacecr. Rockets
**2009**, 46, 501–513. [Google Scholar] [CrossRef] - Li, Y.; Zhang, L.; He, R.; Ma, Y.; Zhang, K.; Bai, X.; Xu, B.; Chen, Y. Integrated thermal protection system based on C/SiC composite corrugated core sandwich plane structure. Aerosp. Sci. Technol.
**2019**, 91, 607–616. [Google Scholar] [CrossRef] - Le, V.T.; Ha, N.S.; Goo, N.S. Advanced sandwich structures for thermal protection systems in hypersonic vehicles: A review. Compos. Part B Eng.
**2021**, 226, 109301. [Google Scholar] [CrossRef] - Yendler, B.; Dang, K.; Forrest, M. A Reusable Heat Shield Using Phase Change Materials. In Proceedings of the 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, USA, 9–12 January 2006. [Google Scholar] [CrossRef]
- Cao, C.; Wang, R.; Xing, X.; Liu, W.; Song, H.; Huang, C. Performance improvement of integrated thermal protection system using shaped-stabilized composite phase change material. Appl. Therm. Eng.
**2020**, 164, 114529. [Google Scholar] [CrossRef] - Nazir, H.; Batool, M.; Osorio, F.J.B.; Isaza-Ruiz, M.; Xu, X.; Vignarooban, K.; Phelan, P.; Inamuddin; Kannan, A.M. Recent developments in phase change materials for energy storage applications: A review. Int. J. Heat Mass Transf.
**2019**, 129, 491–523. [Google Scholar] [CrossRef] - Hubert, R.; Bou Matar, O.; Foncin, J.; Coquet, P.; Tan, D.; Li, H.; Teo, E.H.T.; Merlet, T.; Pernod, P. An effective thermal conductivity model for architected phase change material enhancer: Theoretical and experimental investigations. Int. J. Heat Mass Transf.
**2021**, 176, 121364. [Google Scholar] [CrossRef] - Piacquadio, S.; Schirp-Schoenen, M.; Mameli, M.; Filippeschi, S.; Schröder, K.U. Experimental Analysis of the Thermal Energy Storage Potential of a Phase Change Material embedded in Additively Manufactured Lattice Structures. Appl. Therm. Eng.
**2022**, 153, 119091. [Google Scholar] [CrossRef] - Bühring, J.; Nuño, M.; Schröder, K.U. Additive manufactured sandwich structures: Mechanical characterization and usage potential in small aircraft. Aerosp. Sci. Technol.
**2021**, 111, 106548. [Google Scholar] [CrossRef] - Chen, Y.K.; Milos, F.S. Multidimensional finite volume fully implicit ablation and thermal response code. J. Spacecr. Rockets
**2018**, 55, 914–927. [Google Scholar] [CrossRef] - Chen, Y.K.; Milos, F.S. Two-Dimensional Implicit Thermal Response and Ablation Program for Charring Materials. J. Spacecr. Rockets
**2001**, 38, 473–481. [Google Scholar] [CrossRef] - Scoggins, J.B.; Leroy, V.; Bellas-Chatzigeorgis, G.; Dias, B.; Magin, T.E. Mutation++: MUlticomponent Thermodynamic And Transport properties for IONized gases in C++. SoftwareX
**2020**, 12, 100575. [Google Scholar] [CrossRef] - Gordon, S.; McBride, B.J. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications. Part 1: Analysis. NASA Technical Report, Document ID 19950013764; NASA Lewis Research Center: Cleveland, OH, USA, 1994. [Google Scholar]
- De Mûelenaere, J.; Lachaud, J.; Mansour, N.N.; Magin, T.E. Stagnation line approximation for ablation thermochemistry. In Proceedings of the 42nd AIAA Thermophysics Conference, Honolulu, HI, USA, 27–30 June 2011; pp. 1–14. [Google Scholar] [CrossRef]
- Bonacina, C.; Comini, G.; Fasano, A.; Primicerio, M. Numerical solution of phase-change problems. Int. J. Heat Mass Transf.
**1973**, 16, 1825–1832. [Google Scholar] [CrossRef] - Amar, A.J. Modeling of One-Dimensional Ablation with Porous Flow Using Finite Control Volume Procedure. Master’s Thesis, North Carolina State University, Raleigh, NC, USA, 2006. [Google Scholar]
- Chen, Y.K.; Milos, F.S. Ablation and thermal response program for spacecraft heatshield analysis. In Proceedings of the 36th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, USA, 12–15 January 1998; Volume 36. [Google Scholar] [CrossRef]
- Lachaud, J.; Martin, A.; Eekelen, T.V.; Cozmuta, I. Ablation test-case series #2-Numerical simulation of ablative-material response: Code and model comparisons. In Proceedings of the 5th Ablation Workshop, Lexington, KY, USA, 28 February–1 March 2012. [Google Scholar]
- The SciPy Community. scipy.optimize.toms748. Available online: https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.toms748.html (accessed on 21 March 2023).
- Alefeld, G.E.; Potra, F.A.; Shi, Y. Algorithm 748: Enclosing Zeros of Continuous Functions. ACM Trans. Math. Softw. TOMS
**1995**, 21, 327–344. [Google Scholar] [CrossRef] - Virtanen, P.; Gommers, R.; Oliphant, T.E.; Haberland, M.; Reddy, T.; Cournapeau, D.; Burovski, E.; Peterson, P.; Weckesser, W.; Bright, J.; et al. SciPy 1.0–Fundamental Algorithms for Scientific Computing in Python. Nat. Methods
**2019**, 17, 261–272. [Google Scholar] [CrossRef][Green Version] - Varelas, K.; Dahito, M.A. Benchmarking multivariate solvers of scipy on the noiseless testbed. In Proceedings of the GECCO 2019 Companion–Proceedings of the 2019 Genetic and Evolutionary Computation Conference Companion, Prague, Czech Republic, 13–17 July 2019; pp. 1946–1954. [Google Scholar] [CrossRef][Green Version]
- Sutton, K.; Graves, R.A.J. A General Stagnation-Point Convective-Heating Equation for Arbitrary Gas Mixtures; Technical Report November; NASA Langley Research Center: Hampton, VA, USA, 1971. [Google Scholar]
- Chen, Y.K.; Milos, F.S. Ablation and Thermal Response Program for Spacecraft Heatshield Analysis. J. Spacecr. Rockets
**1999**, 36, 475–483. [Google Scholar] [CrossRef][Green Version] - Tran, H.; Johnson, C.; Hsu, M.T.; Chem, H.; Dill, H.; Chen-Johnson, A.; Tran, H.; Johnson, C.; Hsu, M.T.; Chem, H.; et al. Qualification of the forebody heatshield of the Stardust’s Sample Return Capsule. In Proceedings of the 32nd Thermophysics Conference, Atlanta, GA, USA, 23–25 June 1997. [Google Scholar] [CrossRef]
- Tsai, S.W.; Wu, E.M. A general theory of strength for anisotropic materials. J. Compos. Mater.
**1971**, 5, 58–80. [Google Scholar] [CrossRef] - Lachaud, J.; Mansour, N.N. Porous-material analysis toolbox based on openfoam and applications. J. Thermophys. Heat Transf.
**2014**, 28, 191–202. [Google Scholar] [CrossRef]

**Figure 2.**Schematic of a one-dimensional control volume describing the coordinates and labeling used in Equation (3).

**Figure 5.**Flowchart of the developed FVM solver. The dashed line indicates a shortcut for the non-decomposing case.

**Figure 6.**Schematic of a hierarchical lattice core ITPS, in which the PCM is embedded in the outermost core (orange), and a fibrous high temperature insulation is embedded in the innermost core (blue).

**Figure 7.**Mechanical boundary conditions (in black) and mechanical load (in red) acting on the ADD: (

**a**) in a longitudinal view of the ADD, (

**b**) in a circumferential cut (A–A) view.

**Figure 8.**(

**a**) Heat flux distribution along the ADD longitudinal coordinate as a function of time; (

**b**) dynamic pressure distribution along the ADD longitudinal coordinate as a function of time.

**Figure 9.**Applied thermal boundary conditions for (

**left**) an ablative material and (

**right**) for an homogenised ITPS with different cores.

**Figure 10.**(

**a**) Temperature evolution during re-entry. Wall indicates the receding outer surface, whereas the other temperature curves are at fixed z coordinate; (

**b**) material recession of the ablative PICA TPS during re-entry.

**Figure 11.**Temperature evolution at different points in the out-of-plane direction (z) for the optimised CMC-based corrugated core ITPS.

**Figure 13.**Temperature evolution for different PCM core thicknesses (${t}_{{c}_{PCM}}$) and porosities ($\u03f5$) of the lattice structure.

**Figure 14.**Top and bottom face sheet temperature evolution for different thicknesses of the insulation layer (${t}_{{c}_{ins}}$).

**Figure 15.**Temperature profile of the hierarchical sandwich structure (${t}_{{c}_{PCM}}$ 10 mm, $\u03f5=0.9$, ${t}_{{c}_{ins}}$ 40 mm) with schematic description of the evaluation points considered.

**Figure 16.**Structural design of the load-bearing composite sandwich structure carrying the ablative TPS with reinforcements in form of an X-shaped frame and beams along the outer edges.

**Figure 18.**Von Mises stress [MPa] for the corrugated core ITPS subjected to coupled pressure and thermal loads.

**Figure 19.**Deformation of the outer face sheet in millimetres on the lattice/PCM model for combined thermal and pressure loading. Cutouts represent the areas with local effects around the nodal constraints that were ignored in stress evaluations.

**Table 1.**Geometrical parameters of the lattice structures with reference to Figure 6.

Core | Unit Cell | Porosity [-] | Core Thickness t_{C}[mm] |
---|---|---|---|

PCM (outer) | f_{2}ccz | (0.95–0.8) | (5–20) |

Insulation (inner) | bcc | 0.95 | (10–50) |

Material | Density [kg/m ^{3}] | Specific Heat Capacity [J/(kg K)] | Thermal Conductivity [W/(m K)] | Thermal Diffusivity [mm ^{2}/s] | Melting Point [℃] | Latent Heat of Fusion [kJ/kg] |
---|---|---|---|---|---|---|

Erythritol | 950 | 1900 | 0.4 | 0.22 | 134 | 213 |

LiCl(37%)-LiOH | 1550 | 2400 | 1.1 | 0.29 | 262 | 485 |

KCl(61%)-MgCl_{2} | 2110 | 900 | 0.8 | 0.42 | 435 | 351 |

Li_{2}CO_{3}(22%)-Na_{2}CO_{3}(16%)-K_{2}CO_{3} | 2340 | 2000 | 1.9 | 0.40 | 580 | 288 |

Parameter | Value |
---|---|

${t}_{T}$ | 1.7 mm |

${t}_{C}$ | 35 mm |

${t}_{W}$ | 1 mm |

p | 25 mm |

$\theta $ | 60° |

**Table 4.**Geometrical and physical properties of the lattice core-PCM ITPS layers, with reference to the schematic in Figure 6.

Layer | Component | Material | Thickness [mm] | Volume Fraction | Density [kg/m ^{3}] | Areal Weight [kg/m ^{2}] |
---|---|---|---|---|---|---|

1 | Top face sheet (TFS) | Inconel 718 | 1 | 1 | 8170 | 8.17 |

2 | PCM lattice core | CuCr1Zr | (5–10) | 0.1 | 8900 | (4.45–8.9) |

2 | PCM | LiCl-LiOH | (5–10) | 0.9 | 1550 | (6.97–13.95) |

3 | Center face sheet | Inconel 718 | 1 | 1 | 8170 | 8.17 |

4 | Insulation lattice core | Inconel 718 | (20–40) | 0.05 | 8170 | (8.17–16.34) |

4 | Insulation | Saffil^{®} | (20–40) | 0.95 | 96 | (1.82–3.64) |

5 | Bottom face sheet (BFS) | Inconel 718 | 1 | 1 | 8170 | 8.17 |

Total | (45.92–67.34) |

**Table 5.**Results of the mechanical simulation for the lattice core/PCM solution (see evaluated area in Figure 19).

Load Case | Max. Displacement [mm] | Outer Face Sheet Max. von Mises Stress [MPa] | Inner Face Sheet Max. von Mises Stress [MPa] | Inner Lattice Core Max. Principal Stress [MPa] | Outer Lattice Core Max. Principal Stress [MPa] |
---|---|---|---|---|---|

Thermal | 35.6 | 1196 | 1197 | 744 | 698 |

+Pressure | 42 | 1195 | 1177 | 752 | 707 |

Allowable | 1035 | 1035 | 1035 | 310 |

Ablative TPS | CMC Corrugated Core | Lattice Core / PCM | ||||||
---|---|---|---|---|---|---|---|---|

Component | Mass [kg] | Areal Density [kg/m^{2}] | Component | Mass [kg] | Areal Density [kg/m^{2}] | Component | Mass [kg] | Areal Density [kg/m^{2}] |

Face sheets | 71 | 23.7 | ||||||

CFRP sandwich | 35 | 11.7 | CMC | 60 | 20 | Lattice core | 99 | 33 |

PICA TPS | 32 | 10.7 | Insulation | 11 | 3.7 | PCM | 42 | 14 |

Σ | 67 | 22.4 | Σ | 71 | 23.7 | Σ | 212 | 70.7 |

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. |

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Piacquadio, S.; Pridöhl, D.; Henkel, N.; Bergström, R.; Zamprotta, A.; Dafnis, A.; Schröder, K.-U.
Comprehensive Comparison of Different Integrated Thermal Protection Systems with Ablative Materials for Load-Bearing Components of Reusable Launch Vehicles. *Aerospace* **2023**, *10*, 319.
https://doi.org/10.3390/aerospace10030319

**AMA Style**

Piacquadio S, Pridöhl D, Henkel N, Bergström R, Zamprotta A, Dafnis A, Schröder K-U.
Comprehensive Comparison of Different Integrated Thermal Protection Systems with Ablative Materials for Load-Bearing Components of Reusable Launch Vehicles. *Aerospace*. 2023; 10(3):319.
https://doi.org/10.3390/aerospace10030319

**Chicago/Turabian Style**

Piacquadio, Stefano, Dominik Pridöhl, Nils Henkel, Rasmus Bergström, Alessandro Zamprotta, Athanasios Dafnis, and Kai-Uwe Schröder.
2023. "Comprehensive Comparison of Different Integrated Thermal Protection Systems with Ablative Materials for Load-Bearing Components of Reusable Launch Vehicles" *Aerospace* 10, no. 3: 319.
https://doi.org/10.3390/aerospace10030319