Table of Contents
Introduction: Re-entry as an extreme thermal scenario
The return of a spacecraft to the Earth’s atmosphere is one of the most thermally demanding phases of a mission. During atmospheric re-entry, temperatures of over 1500°C occur on the outside of the vehicle. °C, caused by shock waves, frictional heat and plasma effects in the high atmosphere. At the same time, strong mechanical stresses act on the structure. The thermal protection shield (thermal protection system, TPS) has the task of protecting the spacecraft and its internal components from these extreme conditions – ideally multiple times. The requirement for reusability is increasingly the focus of current space programs, both from government agencies such as NASA and ESA as well as private sponsors.
While earlier systems relied on ablative or ceramic materials, a class of materials is increasingly coming into focus that combines the following two properties: high mechanical strength and good thermal conductivity – metal matrix reinforced composites, or MMCs for short. These materials consist of a metallic matrix (e. g. B. aluminum, titanium or nickel) with embedded ceramic particles or fibers (e. g.B. SiC or Al₂O₃), which give the material specifically desired properties. Their potential lies in particular in the structural integration of thermal protection functions, which can significantly reduce weight, complexity and costs (Oluseyi et al., 2021).
However, the decision as to whether such a material can withstand the extreme demands of re-entry is not based solely on theoretical model assumptions or classic material tests. The Precise knowledge of the thermophysical properties under realistic conditions is crucial – in particular the thermal diffusivity, conductivity and heat capacity over a wide temperature range. This is where a method comes into play that has established itself in material characterization for high-temperature applications: Laser Flash Analysis (LFA).
The Laser Flash Analyzer has proven itself as a precise, non-contact method for measuring thermal diffusivity and forms the basis for determining the thermal conductivity of complex materials such as MMCs. The method is particularly useful for anisotropic or porous samples – such as those found in real TPS configurations. It enables a meaningful evaluation of the thermal conductivity in the axial and radial direction and can be used over large temperature ranges, which is essential for the evaluation of TPS materials.
This article therefore examines how MMCs for thermal protection systems can be evaluated using laser flash analysis. Current research work is used, including the NASA development of reusable metallic TPS concepts (NASA LaRC, 2004) and recent materials science studies on the high-temperature characterization of MMCs (Oluseyi et al., 2021). The focus is not only on the material properties themselves, but also on the metrological requirements and the interpretation of the LFA data in the context of real application scenarios.
The aim is to provide a well-founded insight into the thermophysical evaluation of metallic composite materials for space applications and to demonstrate the contribution of modern analytical methods to the development of reusable heat shields.
Materials technology basis: metal matrix composites as next-generation TPS materials
The selection of suitable materials is a key criterion for thermal protection systems (TPS) that need to be reusable and at the same time remain reliable under extreme conditions. In the aerospace industry, a tension between thermal insulation effect, mechanical integrity and mass saving has dominated for decades. In this respect, metal matrix composites (MMCs) offer an attractive alternative to traditional TPS materials such as ceramics or ablative polymer composites.
MMCs consist of a metallic matrix – often aluminum, titanium or nickel – into which a reinforcing phase of ceramic particles (e.g. e.g. silicon carbide, aluminum oxide) or short fibers. The targeted combination of both phases allows properties such as thermal conductivity, oxidation stability, strength at high temperatures and resistance to thermal shocks to be optimized at system level (Oluseyi et al., 2021).
A key argument for the use of MMCs in TPS components is the possibility of structurally integrating thermal functions. While conventional TPS layers often have to be applied additionally to a load-bearing structure – for example as tiles or panels – MMCs can serve as a load-bearing, heat-conducting and thermally damping system at the same time. This not only reduces the overall weight, but also increases reusability by reducing the tendency to delamination or cracking after repeated thermal cycling.
In practice, however, the properties of MMCs are highly dependent on the respective material system, the manufacturing route and the microstructure. Aluminium-SiC composites, for example, are characterized by high thermal conductivity and low density, but have limited oxidation stability above 600 °C. Titanium-based MMCs, on the other hand, offer excellent high-temperature stability up to over 1000 °C. °C, but present greater challenges in terms of processing and fiber-matrix bonding.
An in-depth understanding of the thermophysical properties – in particular the temperature-dependent thermal diffusivity and thermal conductivity – is therefore essential in order to qualify these materials specifically for TPS applications.
Another feature of modern MMCs is their increasing manufacturability through additive manufacturing, in particular through processes such as laser powder bed fusion (LPBF) or directed energy deposition (DED). These enable targeted tuning of the local microstructure and the integration of graduated material transitions that can better compensate for thermomechanical stresses. In combination with methods such as laser flash analysis, these material systems can not only be developed, but also precisely tested and evaluated.
The next section therefore presents the metrological methodology of laser flash analysis (LFA) – and explains how it can be used to precisely determine the decisive thermophysical characteristics of MMCs for the high-temperature range.
Measurement technology: Laser flash analysis as the key to thermal characterization of MMCs
The thermal performance of a material under high-temperature conditions depends largely on three parameters: the thermal conductivity (λ)the thermal diffusivity (α) and the specific heat capacity (cp). For metal matrix reinforced composites (MMCs) that are used at temperatures above 1000 °C are to function as thermal protection systems (TPS), a precise and material-specific determination of these properties is essential. Laser flash analysis (LFA) has established itself as the standard method for determining thermal diffusivity and is particularly suitable for high-temperature applications.
The LFA is based on a transient, non-contact measuring principleA flat sample plate is bombarded on its reverse side with a short, high-energy laser pulse. The resulting temperature increase on the opposite side is measured with an infrared sensor. The thermal diffusivity can be determined from the time course of this temperature response. α directly. The thermal conductivity λ results from the relationship:
\lambda = \alpha \cdot \rho \cdot c_p
\quad \text{mit} \quad
\begin{cases}
\lambda : \text{thermal conductivity (W/m-K)} \\
\alpha : \text{thermal diffusivity (m$^2$/s)} \\
\rho : \text{density (kg/m$^3$)} \\
c_p : \text{Specific heat capacity (J/kg-K)}
\end{cases}
\)
Whereby ρ is the density and cp is the specific heat capacity of the material. These two values can usually be determined separately or used from literature values or supplementary measurement methods such as DSC (Differential Scanning Calorimetry).
A key advantage of the LFA is that the method can also be used for complex, inhomogeneous or anisotropic materials. as is typically the case with MMCs. The targeted selection of sample thickness, laser energy and detection time allows both materials with high and very low thermal conductivity to be examined. This is particularly relevant for TPS components with a layered structure or directional microstructure, where heat propagation can be highly directional.
In addition, LFA measurements can be carried out in a wide temperature range – temperatures of up to 2800 °C are possible, depending on the sample material and the sensor technology. This enables a continuous analysis of the temperature behavior of TPS materials during different phases of a re-entry, from heating by friction to cooling in the final flight phase.
In addition to the classic individual measurement, the LFA can also be used to time- and temperature-dependent curvescyclic loads and targeted ageing tests. This is particularly valuable in the context of the reusability of space components: thermal damage such as micro-cracking, delamination or oxidation attacks often manifest themselves in measurable changes in thermal diffusivity – long before mechanical tests detect failures.
In the practical application of TPS developments, LFA is therefore not only used for material evaluation, but increasingly also for validation of numerical models (z.FEM or CFD), for process control during production (e. g.e.g. after additive manufacturing) and for the series release of highly stressed components.
Case study: NASA-X-33 and the development of metallic TPS with MMCs
As part of the development of reusable space systems, at the end of the 1990s NASA launched the X-33 technology demonstrator new standards. The unmanned test vehicle was part of the larger Reusable Launch Vehicle (RLV) programs and was intended to test technologies that would enable economical, fully reusable access to space. One of the biggest challenges in this project was the development of a robust, lightweight and reusable spacecraft. thermal protection system (TPS) – and here the focus was on the focus was on metallic conceptswhich differed significantly from earlier ablative systems (NASA LaRC, 2004).
The so-called Metallic Thermal Protection System (METTPS) consisted of multi-layer sandwich structures with oxidation-resistant metallic cover layerstypically made of Inconel or titanium alloys, on a thermally insulating core (e. g.e.g. a honeycomb structure made of stainless steel or Ti). Such systems offer several advantages: they can be structurally integrated, have a high mechanical strength, are impact-resistant and – unlike many ceramic solutions – can be repaired segment by segment if damaged.
However, the performance of these systems depends largely on the on the thermophysical properties of the materials used. from. Precise knowledge of the thermal conductivity and thermal diffusivity is necessary to correctly model temperature distributions within the TPS, predict the thermomechanical behavior and avoid local hot spots.
The program ultimately identified several MMC-based variants with sufficiently high thermal endurance, low delamination tendency and good reusability. These systems combined the advantages of structure-supporting metals with controlled thermal conduction, making them ideal for repeated use in suborbital or orbital spacecraft. Later concepts – such as the TPS system of the Dream Chaser or metallic surface cladding for heat shields of the Starship project – are also based on this material and testing philosophy.
Conclusion and outlook: LFA as the key to the development of reusable space materials
The development of reusable thermal protection systems (TPS) is a key challenge in modern space technology. The focus here is on materials that have both high thermomechanical load-bearing capacity and structural integrability – properties that metal matrix reinforced composites (MMCs) fulfill to a particularly high degree. Their hybrid structure of metallic matrix and ceramic reinforcement allows the targeted coordination of thermal conductivity, strength and temperature resistance over a wide range. However, the selection of suitable MMC systems depends crucially on the reliable characterization of their thermophysical properties – especially under realistic high-temperature conditions.
Laser flash analysis (LFA) has established itself as an indispensable method in this context. It not only allows the precise measurement of thermal diffusivity over large temperature ranges, but also offers the possibility of analyzing anisotropic or complex structured materials. The ability of LFA to detect direction-dependent thermal conductivity behavior, especially in modern, graduated or additively manufactured MMCs, is highly relevant.
The combination of precise thermal analysis and numerical simulationLFA measurement values can be transferred directly into finite element models to predict temperature fields, thermal stresses and structural behavior under real operating conditions. In addition, the method is also suitable for quality monitoring and ageing analysis of reusable TPS components – an aspect that is gaining in importance in view of the increasing cyclical use of space systems such as Starship, Dream Chaser or Space Rider.
Future developments could further expand the role of the LFA. This opens up prospects for the Inline characterization additively manufactured MMCs in industrial processes, for example through miniaturized LFA systems with optical pulse generation and IR detection in the installation space. Coupling with Thermogravimetry (TGA), dilatometer (DIL) and differential scanning calorimetry (DSC) for the simultaneous determination of cp and density values promises greater accuracy in the derivation of thermal conductivity.
In the context of digital material development – for example through the use of digital twins or AI-supported material models – LFA data represents an essential basis for the data-based selection and optimization of future TPS materials. The method therefore not only contributes to the experimental validation of existing designs, but also enables the targeted development of new material concepts in virtual space.
The combination of innovative materials such as MMCs, precise characterization by LFA and intelligent simulation design thus promises sustainable progress in the development of reusable space systems – with direct benefits for the performance, costs and safety of future missions.
List of sources
Oluseyi P. Oladijo et al. (2021). HighTemperature Properties of Metal Matrix Composites. In: Encyclopedia of Materials: Composites. Elsevier. https://doi.org/10.1016/B978-0-12-819724-0.00096-3
NASA Thermal Protection Materials Branch. (2023). Testing and fabrication of TPS materials: use of Laser Flash Analysis (LFA). NASA Website. https://www.nasa.gov/thermal-protection-materials-branch-testing-and-fabrication/?utm_source=chatgpt.com
