Authors: Arianna Matcuk, Aryan Wadhwani, and Michael Wen
Mentor: Samuel Baker. Samuel is a doctoral candidate at the University of Oxford Centre of Excellence for Hybrid Thermal Propulsion Systems.
Abstract
In this work, recent progress in the development of materials for hypersonic aircraft is reviewed. Firstly, an introduction to the challenges of material design for hypersonic applications is presented, and the main mechanisms of material failure are discussed. This is followed by a literature review of recent works investigating the use of materials for the development of primary aerostructures, thermal protection systems, and air-breathing propulsion systems. Finally, a summary of the key developments is provided, along with an overall outlook on the state of the field.
Introduction
With recent developments in hypersonic air-breathing propulsion systems, such as ramjets or scramjets, as well as a growing market around the world for civilian purposes, more emphasis has been put on the reusability of propulsion systems to optimize cost-efficiency, and more importantly, reliability [1], [2]. Flying through the atmosphere at hypersonic speeds, defined as exceeding five times the speed of sound or Mach 5 [3], as a result of friction, compression, and shock waves, the stagnation temperature within the shock layer of supersonic aircraft can reach temperatures over 10,000°C [4]. This introduces several issues that harm the material’s lifespan, notably microcracking due to thermal gradients, thermal shocks, and oxidation. Currently, no materials are advanced enough to resist physical and chemical changes at this temperature [5], [6]. This necessitates the development of new, more advanced materials to ensure reusability and reliability. Current research has yielded metallic alloys, ceramics, and carbon-based composites, which all have high-temperature resistance, oxidation resistance, high specific heats, and strong and stable structures. Two of the main mechanisms for hypersonic material failure, oxidation and microcracking, are discussed below.
Oxidation
Temperatures over 3000°C will have enough energy to separate the bonds of O2 & N2 molecules and disassociate them into free radicals. These free radicals are highly reactive, which significantly accelerates chemical reactions, rapidly accelerating material oxidation [7]. This introduces stresses on the structure and strength of the aircraft, damaging material properties, and reducing material lifespans. Furthermore, materials such as titanium and ceramics can become brittle [8], [9], degrading their structural integrity and damaging their strength.
Microcracking
Uneven heating and thermal gradients can cause the surface to develop microcracking due to physical and chemical changes. Due to high cyclic loads, mechanical stresses, and thermal stresses, these microcracks can grow in size and further accelerate oxidation. When oxidation occurs, since the expansion coefficients of materials and the oxides that form on the surface are different, more ablation and microcracks form, creating a positive feedback loop [7]. Reusing air-breathing propulsion systems will fatigue the system after every use and the cyclic contraction and ablation from running tests then cooling down to room temperature can also form microcracks [10]. Furthermore, surface microcracks can lead to internal cracks, which propagate exponentially like a domino effect, eventually leading to fracture [9], [11]. The porosity of the material also plays a factor in the severity of microcracking, as having some pores will help with compensating for volume expansion as a result of ablation [7].
Literature Review
Primary Aerostructure
Overview
Aerostructure and the materials that compose of its components are critical to lifting body or ballistic structure design. Lifting bodies, aircraft or spacecraft configurations that generate lift, are incorporated into hypersonic vehicle design. Aeroshells and airframes are examples of lightweight primary structures within lifting bodies [7]. Aeroshells use solid or sandwich constructions in order to minimize weight while enabling cooling strategies and preserving rigidity. They are typically made of a heat shield and a backshell. An airframe is the basic structure or body of an aircraft, excluding the engine. As previously mentioned, extremely high temperatures are a major concern in the design of hypersonic aircraft materials. To illustrate the locations of an aerostructure that might be subjected to the largest temperature changes, an image of the surface heat flux colormap is shown for the X-43 hypersonic aircraft in Ref [7].
Carbon and ceramic composites are candidates for components of aerostructure. Carbon-carbon composites can be used for thermal protection systems due to their thermal conductivity over a wide range of temperatures and can be implemented in aeroshells [12]. Ceramic composites can also be used for thermal protection as they possess high mass-specific properties, high thermal stability, low thermal expansion and good tribological behavior [13]. Additionally, carbon fiber/ZrOC (C/ZrOC) has demonstrated potential in surpassing challenges with hypersonic aeroshells due to its high-temperature, dimensionally stable structural insulator material [7].
Recent Developments
Atomic oxygen causes serious erosion to exterior aerostructures through high-speed collisions. Although protective coatings can protect the structure from these collisions, the diffusion of oxygen through the protective layer can be detrimental to the underlying material [14]. However, reducing porosity can help reduce oxygen diffusion and erosion. Hatta et al. [15] investigated uncoated C/C and reported high in-plane tensile strengths of 165 MPa. Despite this, uncoated C/C and other carbonaceous composites erode rapidly at elevated temperatures with oxidation beginning at about 370°C in air, with dramatic oxidation occurring beyond 500°C [16]. This demonstrates the need for coatings or other materials for resisting oxidation and erosion while exposed to the high temperatures experienced in hypersonic flight.
Recently, MATECH, an institute that provides ceramic products, had been permitted to develop hypersonic aeroshells for flight testing from carbon fiber/ZrOC (C/ZrOC), in which the company was able to manufacture 50 kilograms of the ceramic matrix composite in 2023. C/ZrOC is a low-cost, highly scalable, and easy to manufacture hypersonic material while being tested in multiple government labs under extreme heat and stagnation pressures [17]. MATECH has also developed a C/ZrOC thermal protection system that is ideal for reusable heat shields for commercial flight vehicles. Multiple other composites and alloys have been experimented on, with some showing favorable temperature-resistant properties as detailed inside Ref [7]. C/ZrOC along with other carbon fibers and ceramic matrix composites are promising materials that can be implemented into hypersonic aeroshells.
Thermal Protection Systems
Overview
Thermal protection systems (TPS) are vital for hypersonic vehicles, which regularly face extreme conditions during flights exceeding Mach 5. TPS are a heat shield which prevent hot air from getting inside the hypersonic vehicle and provide protection against collisions with debris in space [18]. These systems are designed in consideration of the intense heat generated by aerodynamic compression and friction, with temperatures rising to above thousands of degrees Celsius [5]. TPS materials therefore need to withstand high heat fluxes, extreme thermal gradients, and oxidative environments [5]. They are generally classified into three types: passive, semi-passive, and active systems, each tailored for specific heat flux scenarios and flight durations. Passive thermal protection systems rely on materials which absorb and dissipate heat without any formal control. Meanwhile, active thermal protection systems involve dynamic mechanisms to control heat such as pumps and valves. Finally, semi-passive thermal protection systems use a combination of both passive materials and active thermal protection technologies in order to control heat [11].
Recent Developments
As a form of passive TPS, blanket tile insulation can typically be used in less critical areas of the hypersonic vehicle aerostructure, whereas in the more important aspects of the vehicle, more expensive and higher performing materials such as coated carbon-carbon panels would typically be used for protection [12]. Heat pipes are a form of semi-passive thermal protection that is growing in popularity [7]. Heat pipes are a self-containing, two-phase heat transfer device, whereby heat is conducted away from critical areas through the container into the wick via the working fluid. The operation of a heat pipe initially works as the heat causes the working fluid to evaporate. Then, the hot vapour then moves to a cooler part of the heat pipe, called the condenser. In the condenser, the working fluid condenses and expels the stored heat through the wick and container at near less critical parts of the vehicle. Steeves et al. [19] tested the use of five different working metals: mercury, caesium, potassium, sodium and lithium. An analysis of their heat transfer limits were analysed and it found that nickel-potassium exhibited the best performance. However, these simulations need to be validated against experimental data, and future experiments are needed to construct more comprehensive databases [13]. Authors also investigated the effect of thermal protection system size on aerothermoelastic stability of the hypersonic panel. They tested seven different case sizes with various thicknesses chosen and compared via structural weight. The results show that the size of the TPS can significantly affect the aerothermoelastic stability limits of the panel.
Nickel-Based Superalloys
Nickel-based superalloys are one of the most commonly used metals in propulsion systems for their unique resistance against oxidation and their good thermal and structural strength. Nickel is mixed with a variety of different metals, such as titanium and chromium, to create these resistant superalloys [20]. Especially in very high temperature environments, nickel-based superalloys are used because they have a very high melting point and they are designed to form aluminum oxide (Al3O3) coatings when they get oxidized on the surface, to protect themselves from oxidation damage [7]. Thus, nickel-based alloys commonly used in the combustion chamber of air-breathing propulsion engines because of their exceptional resistance against oxidation and high melting point. They are resistant against physical and chemical changes, maintaining their mechanical structure well [9].
Titanium-Based Alloys
Titanium-based alloys are generally less thermally resistant and suffer more against oxidation compared to nickel-based superalloys. However, they have nearly half the density as nickel-based alloys with a good strength-to-weight ratio, making it suitable for sections of the propulsion system that only bear around 800-1000°C [21]. Because of their high strength but high reactivity with oxygen, titanium-based alloys along with coating are commonly used to make diffusers/inlets [21].
Ceramic Matrix Composites
Ceramic matrix composites (CMCs) compared to other metal alloys, like nickel-based alloys, are much more thermally resistant and more resistant against oxidation [8]. CMCs are also manufactured in a way to be reinforced through their shape, being made in fibers and whiskers to increase strength [7]. Compared to nickel-based superalloys (~1500°C) [21], CMCs (~1700°C) [8], [22] are much more suitable for high temperatures. Additionally, CMCs also have a low density, making them favorable for many aerospace applications for weight saving and protection for important components [8]. CMCs also have the benefit of relatively good resistance against corrosion and oxidation. However, the downside of CMCs are their brittleness and poor monolithic shock resistance [7]. Many failures are often catastrophic fractures due to its poor thermal conductivity and shock resistance [8]. Even so, CMCs are very suitable for lining in combustion chambers and coatings, as well as for the nozzles [22].
Carbon-Carbon Composites
Carbon-Carbon Composites (CCCs), similarly to CMCs, are also extremely resistant against temperature, if not even more so. Carbon-carbon composites have a melting temperature of over 3000°C, as well as having low thermal expansion coefficients and high thermal conductivity [23]. However, unlike CMCs, carbon-carbon composites are not as chemically resistant and are susceptible to oxidation [7]. Combining ceramic matrixes and CCCs can help offset their weakness against oxidation, but also damages CCCs mechanical strength at temperatures around 1200°C [7]. For CCCs, reducing porosity becomes a major issue, as pores can introduce oxygen, accelerating oxidation and thus, erosion. High refractory materials often need to be applied on the surface of CCC’s to protect against rapid erosion [7], [23].
Discussion
As more powerful ramjet engines are being researched and perfected to go faster and burn hotter, the necessity of materials that can withstand the high temperatures within the propulsion system are required. Simultaneously, more research from companies such as Hermeus, NASA, and Boeing are being put into turning hypersonic aircraft into a reusable means of transport. Recently, more and more research has been put into ceramic matrix composites, carbon-carbon composites, and their variations. High entropy alloys are being researched as well [7]. CMCs have better oxidation and thermal resistance relative to metals, while CCCs have better thermal resistance and a lower expansion ratio relative to metals. They are also less dense and can provide significant weight loss to aircraft. Developments are currently being put into ultra-high temperature and refractory ceramics [7], as well as methods, such as the aforementioned weaving of carbon-carbon composites, to achieve increased resilience [10]. UHTCs have been noted to be able to withstand temperatures over 3000°C [24]. Sharma and Mahajan investigate the prospect of using CCC composites and CMC’s in ramjet nozzles [25], noting their potential due to their high strength, high thermal conductivity, low coefficient of thermal expansion when lined well with anti-oxidation coating. They show potential in reducing erosion and extending lifetimes. Overall, the field of material science for supersonic air-breathing engines is still a relatively new field. Especially with new emerging technologies in high-grade aerospace materials, new discoveries are being made every day.
Conclusion
Overall, for primary aerostructures, multiple materials such as carbon-carbon composites and ceramic matrix composites have demonstrated promising properties and results. However, there are still drawbacks in developing materials for hypersonic aircraft including erosion from oxygen diffusion and high temperatures. Regarding thermal protection systems, there is a pressing need to expand experimental databases for the verification of simulation results, in order to accelerate the rate of progress in designing complex TPSs. Finally, advancing air-breathing propulsion systems in the future will require more thermally and oxidation resistant materials, such as ceramic matrix composites, carbon-carbon composites, or improved metallic alloys. Currently, research is being pushed towards the research and development of carbon and ceramic based composites, with new refractory composites and ultra-high temperature ceramics being developed. However, more research is needed in order to render materials not only durable and resistant against the extreme environment, but also reusable and reliable over multiple flights, and resistant to fatigue due to cyclic stress or erosion.
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