In the process of human technological development, hypersonic aircraft are regarded by many countries as a new high ground for future technological competition. However, when the aircraft breaks through its limits, it is also accompanied by an ultimate challenge that can be called a "fiery purgatory". Imagine a series of adverse phenomena, known as "thermal barriers," caused by the rapid increase in surface temperature of the aircraft due to high-speed airflow, as the aircraft rushes at the edge or interior of the atmosphere at supersonic speeds. At this year's annual meeting of the China Association for Science and Technology, relevant experts conducted in-depth discussions on this "new thermal barrier" that has evolved into a geometric surge in strength, breadth, and complexity in the hypersonic era. This "new thermal barrier" is not only a "roadblock" in the development of hypersonic vehicles, but also a constant search for new solutions by scientists and engineers to unleash more potential of hypersonic vehicles. Pushing aside the fog of the "thermal barrier", looking back and looking forward to this peak showdown between human intelligence and invisible flames. Dancing with Flames - Faster speed brings greater challenges. Ultrasonic flight will make the air emit a piercing 'scream'. Hypersonic flight is even more astonishing, with a minimum speed threshold of 5 Mach, which is 5 times the speed of sound. However, while the speed soared, it also brought about the problem of "thermal barrier". When the aircraft breaks through the air and rushes, intense friction and compression convert tremendous kinetic energy into scorching heat energy, which is called "aerodynamic heating". Ordinary metals will soften, melt, or even burn out in a furnace. On the aircraft, the consequences of this temperature rise are even more deadly: material strength collapse, structural distortion and deformation, and internal precision instruments being baked into scrap metal... During a test launch of the Russian "Vanguard" missile, local thermal stress exceeded the material fatigue limit, causing erosion damage to the thermal protection layer and affecting the integrity of the missile structure. During hypersonic flight, the surface temperature of the aircraft can soar to 1000-2000 ℃. What's even more severe is that missions often require hypersonic aircraft to endure for minutes or even hours in such extreme high temperature environments, and the cumulative effect of heat tests the durability limits of materials and structures. The heat generated by supersonic flight mainly comes from the friction between the air and the fuselage. Hypersonic flight is different. When a hypersonic aircraft breaks through the air, the airflow becomes entangled with the surface of the aircraft, resulting in three key phenomena that shape heat flux: shock waves, boundary layer separation, and turbulence. They are like three "flame sculptors" with different techniques, jointly determining the distribution and burning intensity of heat on the aircraft body. When the sharp leading edge of a hypersonic aircraft collides head-on with the airflow, it seems to hit an invisible "high-pressure air wall", which is a shock wave. Passing through this shock wave, the temperature, pressure, and density of the air skyrocket in a straight line, shooting heat towards the surface of the aircraft like a high-pressure water gun. In some areas, the low-speed airflow layer closely attached to the surface of the aircraft will "peel off" from the surface, which is called boundary layer separation. This kind of detachment will completely disrupt the relatively regular distribution of heat flow, forming unexpected "high-temperature islands" in some places, which can easily cause local overheating and erosion. Turbulence makes heat flow even more unpredictable. Its characteristics are extremely sensitive, and even a slight roughness or geometric change on the surface of the aircraft can cause a significant change in its temperament, making heat flow prediction like looking at flowers in fog. The high temperature generated by hypersonic flight is comprehensive, covering almost the entire surface of the aircraft. Long term roasting can cause the overall structure of the aircraft to soften and deform, material properties to degrade rapidly, and even its internal electronic devices to malfunction due to high temperatures. At this point, the "heat-resistant pad" in a single part is completely ineffective. With the repeated breakthroughs in the speed limit of hypersonic aircraft, a comprehensive "fire revolution" from the overall configuration, thermal protection system to internal thermal management of the aircraft is urgent. Entangled with airflow - multiple solutions require collaboration from multiple fields. In the extreme furnace of hypersonic, aerodynamic heating, material properties, and structural mechanics are not fighting on their own, but are deeply intertwined and cause and effect each other, forming a dangerous "death triangle" closed loop: aerodynamic heating heats the surface of the aircraft red, and the strength and stiffness of the materials decrease in flames, even leading to erosion or microstructural degradation. So the structure begins to deform, twist, or produce unstable vibrations. The deformation of the structure in turn changes the airflow pattern around it, thereby affecting the distribution and intensity of aerodynamic heating again, forming a new thermal shock. The second test flight of the American HTV-2 "Falcon" hypersonic aircraft failed due to the burning of multiple layers of carbon cloth on the leading edge of the aircraft's wings caused by high heat flow, which affected aerodynamic performance and ultimately led to the aircraft losing control. This dangerous interaction is more alarming at the material level. For example, ceramic based composite materials, under sustained high temperatures, their internal microstructure will quietly change and their performance will gradually decline. At the same time, the enormous thermal stress caused by pneumatic heating will continuously accumulate inside the material. When this stress exceeds the material's bearing limit, tiny cracks will form. These cracks are like ant nests on a dam, not only weakening the strength of the material itself, but also becoming channels for high-temperature gases to invade inward, further damaging the thermal protection effect and endangering the overall structural safety. For example, in metal based composite materials, at high temperatures, the metal matrix will soften like butter, and the performance of reinforcing fibers or particles may also change. In addition, the unevenness of aerodynamic heating itself can lead to extremely complex three-dimensional deformation of the structure, posing a nightmare problem for aircraft structural designers and thermal protection engineers. Therefore, unraveling the knot of the "new thermal barrier" is not a solo performance of a single discipline. It requires the deep integration and collaborative operation of top-notch brains in fields such as aerothermodynamics, materials science, and structural mechanics. On the one hand, advanced computer simulations are needed to accurately reproduce the complex "death tango" between airflow, materials, and structures in the virtual world; On the other hand, it is necessary to rigorously test materials and structures in real ground testing equipment by immersing them in flame furnaces that simulate hypersonic environments, in order to provide theoretical foundations and experimental evidence for the design of future aircraft. Playing with thermal barriers - taking multiple measures to solve the problem of heat flow. Faced with the "new thermal barrier", traditional passive defense methods such as insulation tiles used on space shuttles or thermal barrier coatings on engine blade surfaces are gradually becoming inadequate. China, the United States, Russia and other countries have continuously proposed their own solutions to the problem of new thermal barriers in hypersonic vehicles through measures such as material innovation, combination of active and passive protection technologies, and system integration optimization. At present, there are mainly two paths. The first path is to take the initiative. The essence of active thermal protection lies in the word "active". It does not require hard handling of flames, but cleverly adjusts the input or output of energy to reduce the "fire snake" flowing towards the surface of the aircraft. One method is to draw inspiration from the circulatory system of life, allowing the cooling medium to circulate in the internal pipes of the aircraft's shell or critical structures. When the cooling medium flows through a high-temperature area, it carries away the heat generated by pneumatic heating, just like the human blood circulation takes away metabolic heat. Another approach is to deploy powerful 'heat sponges'. By utilizing the enormous heat capacity of the heat sink material itself, a large amount of heat can be stored in a short period of time, delaying the rapid rise in surface temperature. Installing this' heat sponge 'on critical parts of the aircraft can earn valuable response time for other protective measures. The British "Blade" engine provides an innovative solution for active cooling. It can lower the temperature of the inhaled 1000 ℃ air to 140 ℃ within 0.05 seconds through an efficient pre cooler. In 2016, the UK announced the concept of a hypersonic aircraft based on the "Sabre" engine technology. The second path is passive defense. The core of passive defense lies in fully utilizing the performance of the material itself. Scientists have never stopped exploring materials: ceramic matrix composites have always been the main force due to their excellent high-temperature resistance. Researchers are constantly optimizing its formula and preparation process, committed to enhancing its ability to resist high-temperature oxidation and withstand sudden changes in temperature. Carbon carbon composite materials, with their lightweight and high-strength properties, perform well in ultra-high temperature regions, and scientists hope to endow them with stronger antioxidant capabilities. The American X-51A hypersonic aircraft adopts carbon carbon composite materials and ultra-high temperature ceramic coatings, which can maintain structural integrity under hypersonic conditions. At present, intelligent thermal protection materials are a cutting-edge research hotspot in materials science. Their design goal is to automatically adjust their key thermophysical properties based on real-time changes in external heat flow or temperature - such as reducing heat transfer to the interior during sudden temperature increases, or dissipating more heat in the form of infrared radiation, achieving dynamic and adaptive thermal protection effects. Simply achieving perfect thermal protection is not the ultimate goal of researchers. They hope that thermal protection structures can not only block flames, but also fulfill multiple roles, such as bearing loads, transmitting electromagnetic waves, and even stealth. This requires researchers to carefully design the microstructure of materials and cleverly layout functional layers to enhance the overall performance of the aircraft. In November of this year, a research team from the National University of Defense Technology published an article in Nature Communications stating that they had successfully developed a multifunctional composite material based on metasurfaces. This material can absorb ultra wideband radar waves from 2GHz to 12GHz at a high temperature of 1000 ℃. With the continuous deepening of the understanding of the essence of "new thermal barriers", the design concept of aircraft thermal protection systems is also undergoing a profound paradigm revolution - shifting from a single performance priority of "treating the head and foot" to a globally coordinated and multidisciplinary system optimization design. This is a long and tortuous process, and the future still requires continuous innovation that seeks to explore from top to bottom. (New Society)