**1. Introduction**

*1.1. Context*

Microlaunchers are considered a strategic asset to achieve high frequency, tailored access to space for small satellites [1]. The addition of first stage reusability lowers costs and improves business sustainability. However, the structural mass does not scale linearly with the launchers' size, leading to microlaunchers having comparatively higher structural coefficients with respect to conventional launchers, such as Ariane 5, Soyuz, and Falcon

**Citation:** 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

Academic Editors: Spiros Pantelakis, Andreas Strohmayer and Jordi Pons-Prats

Received: 31 January 2023 Revised: 18 March 2023 Accepted: 20 March 2023 Published: 22 March 2023

**Copyright:** © 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/).

9 [2]. This affects the launchers' performance and thus the economic viability. To address this challenge, novel lightweight solutions are required for primary structural components. Additionally, to reduce wet and dry mass as well as complexity, passive re-entry and landing concepts can be considered. This, among others, is the aim of the feasibility studies led within the framework of the research project "Recovery and Return To Base" (RRTB) funded through the European Horizon 2020 programme [3]. In particular, the project aims to perform a re-entry flight phase without the use of retro-propulsion to achieve deceleration. Four mechanically actuated aerodynamic drag devices (ADDs), as shown in Figure 1, are extended from the rocket body to achieve the desired ballistic coefficient and obtain a sufficient deceleration during the re-entry and descent flight phase.

**Figure 1.** Architectural CAD sketch of the aerodynamic drag devices in open configuration.

The ADDs are extended before the beginning of the re-entry flight phase and are then exposed to high convective heat fluxes and high dynamic pressures. Similar drag devices were investigated in the European project RETALT for a conventional size reusable launcher. As described by Marwege et al. [4], the design was judged unfeasible because of excessive reaction forces and moments, which would have led to high structural mass of the components. Although similar issues are faced for the design of ADDs for the mission considered in the RRTB project, different sizes and different design methodologies allow for not excluding the concept a priori. The design of such components requires a holistic approach and is mainly influenced by the thermal protection system (TPS) design, the structural design, and the design of the extension mechanisms. Each element intrinsically influences the design of the others. This work concentrates on the design of the TPS and offers a consideration on lightweight design potential of different thermal protection solutions and their effect on the overall structural mass. In particular, ablative materials as well as integrated TPS (ITPS) solutions are considered. ITPSs are load bearing structures made of materials with a high operative temperature and a high thermal insulation capability. The geometry is designed to increase the thermal resistance and have a high specific stiffness, thus aiming to obtain holistic mass reductions.

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

Ablative materials represent a high TRL solution for medium to high heat fluxes, typical of a ballistic re-entry [5]. Due to the endothermic reactions that take place during ablation, such materials offer high mass-specific thermal protection properties when compared to TPS based on sensible heat storage, i.e., ceramic tiles. However, the obvious disadvantage is that ablative materials are intrinsically expendable. Thus, re-application or replacement of the entire structure is often necessary to achieve reusability of the component. Furthermore the ablative TPS represents a non load-bearing add-on to the structural mass.

Passive TPS based on tiles, often made of high temperature metallic alloys or ceramic matrix composites (CMCs), are commonly employed for reusable launch vehicles (RLVs) [6]. Their application is limited by the maximum operative temperature of the material, thermally induced stresses, and outward thermal deflection. As indicated by Dorsey et al. in [7] as well as by Le and Goo in [8,9], excessive deflection can cause transition from a laminar flow on the outer wall of the TPS to a turbulent one. This is correlated with an increase of the heat transfer coefficient between the wall and hot gas, which thus causes an increase in the convective heat flux that the TPS outer wall experiences. Such an increase is coupled to even higher deformations and stresses, which can lead to failure. In particular, as evidenced by Heidenreich et al. [10], the higher the maximum operative temperature is, the lower the specific tensile strength of the material. CMCs received increasing attention due to the relatively constant mechanical properties at different temperatures and for retaining the highest specific tensile strength at temperatures above 1000 °C ([10–12]). In parallel to such material development, several efforts were made by authors in the literature to obtain passive thermal protection systems with high thermal and mechanical load bearing capability. With this goal, Blosser et al. [13] and Fischer et al. [14] developed structures consisting of a metallic honeycomb sandwich (based on nickel-superalloys and gamma titanium-aluminide, respectively) on the outer surface and a fibrous insulation encapsulated between the sandwich and the vehicle interior.

Bapanapalli et al. [15] and Gogu et al. [16] first proposed the concept of an ITPS. It is based on a corrugated core sandwich panel, hosting a fibrous insulation within the webs. In recent years, several authors investigated the use of CMCs as a structural material for ITPS, with different core topologies [17]. Le et al. [18] indicated that the choice of a particular core topology and material combination is not trivial and depends on the thermal and mechanical load profiles, the specific mechanical properties of the material as a function of expected temperature, and the obtainable effective thermal conductivity.

The specific stiffness and strength of most materials decrease with increasing temperature. Although CMCs are suitable for operation at high temperature (above 1000 °C), the specific mechanical properties are, in absolute terms, lower than the ones of high temperature alloys, i.e., Ni-based superalloys or titanium aluminides. Additionally, operation at high temperature, i.e., approaching the radiative equilibrium temperature at the outer face sheet, while reducing the sensibly stored thermal energy, is cause for high thermal gradients in the out-of-plane direction of the sandwich structure. These gradients, in turn, increase the thermally induced stresses with respect to operation at lower temperatures. Therefore, when considering load-bearing components that require a lightweight, reusable TPS, the choice of material and configuration is not trivial. For a load-bearing component, a reduction of the wall temperature can be beneficial by allowing the use of materials with high specific mechanical properties.

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

In this perspective, latent heat storage, i.e., melting of a so-called phase change material (PCM), is more efficient than sensible storage in mass-specific terms. Indeed, the thermal mass required to store heat via phase change is lower than the one needed to sensibly store the same amount of energy. However, only few authors ([19,20]) have investigated the use of PCMs for TPS. As explained by Nazir et al. [21], encapsulation and

thermal conductivity improvement of PCMs proved to be the main challenge hindering the widespread application of such materials.

Recent literature ([22,23]) introduced the use of additively manufactured lattice structures to address both issues. These cellular solids were found to deliver high effective thermal conductivity, improving the thermal energy storage capability of the material. Additionally, sandwich structures built with such lattice cores exhibit attractive specific mechanical properties, simplify the junction of the core with the face sheets, and allow for the reduction of issues related to delamination [24]. Due to these favorable thermal and mechanical properties, lattice core sandwich structures with embedded PCMs are attractive for use in load-bearing lightweight TPS.

To address the aforementioned challenges in ITPS design, considering both the thermal and the mechanical behaviour is of fundamental importance. Thus, in this work, we perform a comparison between the thermal response of the three introduced TPS concepts for use on the ADD of the RLV considered in the RRTB project:


After treatment of each concept's thermal design, a preliminary structural design for each considered solution is described. In the end, a comprehensive evaluation of both the thermal response results and the associated mechanical design is given. The results are compared in terms of total mass.

#### **2. Governing Equations**
