When the spacecraft re-enter/enter the atmosphere, serious aerodynamic heating is generated. The thermal protection system is one of the key subsystems that must be relied on to ensure the normal operation of the electronic components inside the aircraft and the manned space. The thermal protection material is the most important part of the thermal protection system. The important part. With the gradual development of my country’s manned spaceflight, lunar exploration project, deep space exploration, manned lunar landing and other projects, as well as the rapid development of new aerospace vehicles, the speed of reentry is getting higher and higher, and the thermal environment it faces is becoming more severe and aerodynamic. The heating time is more than a thousand seconds, the heating capacity is greatly increased, a large area faces the high temperature (≥1000 ℃) of a long-term aerobic environment, and the instantaneous temperature of key parts can reach more than 2000 ℃.
Since the 1950s, aircraft thermal protection material systems and thermal protection methods have continued to develop, especially advanced ultra-lightweight and reusable thermal protection materials represented by ceramic tiles and TUFROC, which are used in advanced spacecraft such as space shuttles and X-37B The successful application on the Internet has attracted wide attention from scientists and engineers. However, in general, this kind of material has insufficient reliability, high price, complicated assembly process, and high maintenance cost, which limits its large number of applications in aerospace vehicles to a certain extent. Resin-based ablation and heat protection materials are based on organic polymers. Through a series of chemical and physical changes, the mass of the material itself is sacrificed to take away a lot of aerodynamic heat, so as to achieve the purpose of heat protection. Due to its high reliability, high cost performance, and simple assembly process, it is still considered to be the most effective, reliable, mature and economical thermal protection method. It is used in the thermal protection system of spacecraft, returnable satellites, missiles, etc. Used extensively.
Driven by the demand for national defense equipment, China’s resin-based ablation and heat protection materials have undergone three milestone developments: glass/phenolic, quartz/phenolic and carbon/phenolic, and the material system is gradually established and improved. Since the beginning of the 21st century, with the gradual development of the national aerospace industry, ablation and heat protection materials are facing an urgent need for weight reduction. Represented by the honeycomb reinforced low-density material system, two representative low-density resin bases H88 and H96 have been developed. Ablative heat protection material (LAC).
1. Low-density resin-based ablation and heat protection material
1.1 Honeycomb reinforced low-density heat protection material
Under the traction of the manned space project, China has developed two typical honeycomb reinforced LAC materials, H88 and H96, based on the ballistic characteristics of the return capsule of the Shenzhou spacecraft. The H88 and H96 materials are supported by fiberglass honeycomb lattices and phenyl silicone rubber as the resin matrix. , Doped with light-weight functional fillers such as short silica fibers, phenolic microspheres, glass microspheres, etc. by physical blending, and quickly filled into the glass fiber reinforced plastic honeycomb lattice through the integral molding process. The addition of lightweight functional fillers is mainly used to reduce material density and thermal conductivity, while ensuring the ability of the material surface to resist airflow.
Shenzhou spacecraft and its heat-proof materials
On the basis of H88 and H96, the microstructure and chemical composition of the material were optimized, and FG4 and FG5 materials with densities of 0.4 g/cm3 and 0.5 g/cm3 were developed, and the room temperature thermal conductivity was ≤0.1 W/ (m·K), used in the low and medium heat flow area on the leeward side of the side wall of the returner In the high heat flow area of the heat-proof outsole of the returner, HC5 and FG7 materials with densities of 0.5 g/cm3 and 0.7 g/cm3 have been developed, and the room temperature thermal conductivity is maintained at between 0.10 ~ 0.12 W/(m·K) In order to increase the high stagnation point ablation performance and shear resistance of the heat protection material.
1.2 SPQ series materials
Driven by the lunar exploration project and new aerospace vehicles, China has developed the SPQ series of low- and medium-density quartz/phenolic and glass/phenolic heat-proof materials based on the oblique winding molding process. The main feature of SPQ material is the addition of a large number of lightweight functional fillers such as phenolic microspheres, glass microspheres, and ceramic powder to the phenolic resin matrix. The two-dimensional fabric woven from quartz fibers and functional fibers is used as the reinforcing phase to make light High-quality hybrid prepreg, by adjusting the reinforcement and resin matrix formula, prepare SPQ series materials that meet different heat protection requirements. The introduction of hollow beads and micropores can significantly reduce the thermal conductivity of the material while reducing the density of the material. Figure 3 shows the optical microscope photo and SEM photo of SPQ9 material after ablation. It can be observed that a large number of lightweight functional filler balls are uniformly dispersed in the SPQ9 resin matrix. After ablation, the lightweight functional filler forms micron-sized closed cells in situ structure.
1.3 Integrated low-density resin-based heat-proof material
In the 1990s, in order to meet the thermal and thermal insulation requirements of advanced spacecraft such as deep-space exploration and aerospace shuttles, and to further reduce the proportion of the thermal protection system in the total system mass, the United States-based aerospace powers successively developed new models Ultra-low-density ablation and heat-proof materials with integrated heat insulation, typical representatives are PhenCarb, BLA, SCRAM, PICA and SIRCA materials. The common features of these materials are ultra-low density (≤0.35 g/cm3), ultra-low thermal conductivity, and compatibility with heat and heat insulation. In the design of the lunar-ground high-speed reentry returner and the new spacecraft thermal protection system for the lunar exploration project, a number of research units have focused on the thermal environment characteristics of the reentry orbit high peak heat flow, high enthalpy, long-term heating and oxygen atmosphere In parallel with universities (such as the Institute of Aerospace Materials and Technology, Harbin Institute of Technology, East China University of Science and Technology, etc.), the research and development of new low-density integrated materials for heat insulation have been carried out.
Using porous hybrid phenolic resin as the matrix, and by changing the fiber structure of the reinforcement, the DMC, DMS, and ZMS series of integrated composite materials for heat and insulation with adjustable and controllable densities between 0.25 and 1.3 g/cm3 have been developed. The typical feature of this type of material is that the micro-nano open pore structure of the aerogel material is introduced into the internal structure of the composite material, which greatly reduces the thermal conductivity of the material and significantly improves its thermal insulation performance. Figure 5 shows the microscopic morphology photos of LAC integrated insulation materials prepared from different fiber structure reinforcements. Utilizing the "morphological replication effect" after phenolic aerogel ablation and carbonization, the carbon layer structure maintains the aerogel's loose and porous form, which not only effectively reduces the thermal conductivity of the carbon layer, but also improves the surface radiation heat dissipation. The nano-functional components in the porous hybrid resin improve the ablation resistance, shear resistance, oxidation resistance and mechanical properties of the resin matrix and the carbonized layer, and further reduce the thermal conductivity of the material.
1.4 Multifunctional integrated lightweight heat-proof material
At present, in the field of multifunctional integrated resin-based ablation and heat protection materials, my country is basically in the ranks of the international frontiers. In recent years, the research team has successively developed lightweight heat protection/insulation/dimension, heat protection/insulation/stealth, heat protection/insulation/bearing, heat protection/insulation/flame retardant suitable for multiple complex thermal environments And other multifunctional integrated materials.
To meet the needs of new aerospace vehicles for weight reduction, heat protection, heat insulation and radar stealth, a lightweight heat protection/heat insulation/stealth integrated material (HRC) has been developed. In order to meet the thermal load requirements of key thermal protection parts of aerospace vehicles at high temperatures, a heat-preventing/insulating/bearing integrated composite material (HIS) with a density of ≤1.2 g/cm3 has been developed. Aiming at the phenomenon of open flame burning when the heat-proof material of the skirt of the launch vehicle engine is ignited, the integrated material of the multi-functional fusion of heat protection/heat insulation/flame retardant has been invented, which effectively solves the problem of the open flame of the heat-proof skirt when the rocket engine is ignited phenomenon.
With the continuous development of advanced aerospace vehicles and flight control technology, the trajectory of the aircraft and the flight thermal environment are becoming more and more diversified and complicated. More and more stringent requirements are put forward for the functionality of heat protection materials, and the multifunctional integration of heat protection materials Modification is an important technical means to solve the advanced thermal protection system of spacecraft in the future.
2 Research prospects of low-density resin-based ablation and heat protection materials
2.1 Multifunctional compatibility and integration
Lightweight thermal protection structure is the continuous pursuit of aerospace vehicles, and the multifunctional and functional integration of thermal protection systems is an inevitable requirement for the development of advanced aerospace vehicles. With the emergence of the concept of smart aerospace vehicles, in the future, the thermal protection parts of aerospace vehicles will be finely designed and manufactured by regions, materials, functions, thicknesses, and shapes. New high-performance resin matrix, continuous functionally graded composite materials and Flexible conformal heat-proof materials will be the key development direction of low-density resin-based heat-proof materials.
The heat-proof material is a barrier against the aerodynamic thermal environment of the reentry/enter spacecraft, and constitutes the outermost structure. In addition to satisfying the aerodynamic and heat-proof requirements, it must also have heat insulation, erosion resistance, thermal load bearing, aerodynamic shape, etc. Multiple functions. With the increasingly harsh thermal environment of spacecraft, the continuous extension of heating time, the strict quality constraints of thermal protection systems, and the development trend of low-density resin-based ablation and heat protection materials, the development trend of low-density resin-based ablation and heat protection materials must be to achieve heat protection under the premise of lightweight The compatibility and integration of multiple functions such as, heat insulation, aerodynamic shape, thermal bearing, stealth, flame retardant, etc., simplifying the design, assembly and maintenance costs of thermal protection systems are the inevitable development of low-density resin-based ablation and heat-resistant composite materials in the future trend.
2.2 Synergy of multiple thermal protection mechanisms
Ablation and heat protection materials make a series of complex physical and chemical responses in aerodynamic thermal environment. The material and the thermal environment are highly coupled, and the material body undergoes an unsteady mass and heat transfer process. Related to the prevention of resin-based ablation materials The study of thermal mechanism has always been regarded as the most scientifically challenging subject. During the ablation process of the material, a large amount of aerodynamic heat heats the surface of the material in the form of convection and radiation, and then a complex chemical reaction occurs. The resin matrix decomposes and takes away a large amount of incoming heat. The pyrolysis product forms a porous carbonized layer. As shown in Figure 6. Clarify the action mechanism of various heat protection mechanisms in the resin-based ablation heat protection material in the entire thermal protection process and their proportions in the total thermal effect, reveal the mutual coupling between various heat protection mechanisms, and detect unsteady burning During the corrosion process, the material body heat and mass transfer process, and the relationship between the microstructure and performance of the material is grasped, which is helpful to realize the collaborative design and cooperation between the heat protection, heat insulation and other functions of the low-density resin-based ablation heat protection material system Matching has very important scientific guiding significance for the structural design, development, preparation and engineering application of new resin-based LAC materials.