What are the current status and development trends of fiber composite materials in launch vehicle fairings?

The satellite fairing is one of the most important components of a launch vehicle, possessing a good aerodynamic shape, light structural mass, and reliable separation capability. Its main function is to maintain the aerodynamic shape of the launch vehicle during pre-launch and flight, protecting the payload inside from the influence of the external natural environment and aerodynamic erosion, and reliably separating from the rocket after exiting the atmosphere. Figure 1 shows the launch vehicle fairing in the separation state. The fairing is located at the front of the launch vehicle, protecting the satellite and other precision instruments inside the launch vehicle from heat corrosion during atmospheric exit. Once the launch vehicle has exited the atmosphere, the fairing no longer serves its purpose, and at this point, to reduce the rocket's weight, the fairing separates from the launch vehicle. To ensure data transmission between the payload inside the fairing and the outside world before separation, the fairing generally has electromagnetic wave transmission capabilities. Currently, satellite fairings both domestically and internationally mainly fall into two categories: metal riveted structures and composite material structures. Metal riveted structures commonly use aluminum alloys, while composite material structures use resin-based fiber composite materials with wave-transmitting capabilities. Fairings designed and manufactured from resin-based fiber composite materials can effectively reduce structural weight by 20%-30%, increasing rocket payload capacity and reducing engine fuel consumption under the same payload, thereby lowering rocket launch costs. Furthermore, resin-based fiber composite materials offer greater design flexibility than metallic materials, allowing for integrated design and manufacturing, reducing the number of structural components and improving structural reliability. The excellent corrosion resistance, damp heat resistance, radiation resistance, and fatigue resistance of resin-based fiber composite materials extend the service life of the fairing structure, thus enabling fairing recovery.

2.1 Panel Materials The panel structure provides the aerodynamic shape of the fairing. The panel material is a resin-based fiber composite material, composed of a matrix material and reinforcing materials. Resin-based fiber composite materials are lightweight, high-strength, and heat-resistant, with high design flexibility, and can achieve wave-transmitting functionality over a wide range, making them ideal materials for fairings. This chapter reviews commonly used fiber and resin-based fairing materials both domestically and internationally.

2.1.1 Fiber Materials Resin-based fiber composites possess advantages such as high specific strength and high specific modulus. High-performance reinforcing materials commonly used in fairing panels both domestically and internationally include carbon fiber, glass fiber, and aramid fiber.

Carbon fiber possesses many excellent properties, including high axial strength and modulus, low density, no creep, good fatigue resistance, low coefficient of thermal expansion, and good electrical conductivity and electromagnetic shielding performance. It has been widely used in the aerospace field. fiberglass is the most commonly used reinforcing material for wave-transparent fairing skins, possessing advantages such as high strength, excellent dielectric properties, low moisture absorption, and dimensional stability. Quartz fiberglass possesses the best dielectric properties of all glass fibers, remaining essentially unchanged over a wide frequency band, enabling broadband wave transmission in fairings. Furthermore, quartz fiber exhibits excellent wave transmission performance at high temperatures, and most advanced fairings abroad currently utilize quartz fiber as a reinforcing material. Aramid fiber, a high-tech specialty fiber, possesses high strength and high modulus, excellent heat resistance and flame retardancy, and is used in aerospace applications such as aramid bulletproof vests and helmets. However, the hygroscopic nature of aramid fiber affects its dielectric properties, limiting its application in wave-transparent fairings.

2.1.2 Resin Materials Fairings generally use high-performance thermosetting resins as the panel matrix material, including epoxy resin (EP), bismaleimide resin (BMI), and cyanate ester resin (CE), etc. Epoxy resin is the most commonly used resin material. It boasts excellent thermodynamic properties, strong processability, and superior electrical properties, making it a staple in composite resin matrices. However, epoxy resin suffers from poor impact resistance and a significant decline in mechanical properties under humid and hot conditions. In recent years, researchers have focused on adding flexible groups to epoxy systems to improve their toughness and heat resistance. Bismaleimide (BMI) resin, as an aromatic polyimide material, possesses excellent high and low temperature resistance, high strength and modulus, low coefficient of thermal expansion, low dielectric constant and loss, low vacuum volatiles, and low volatile condensables. It also shares similar advantages with epoxy resin, such as ease of processing, and is widely used in the aerospace field. However, its disadvantages include a high melting point, poor solubility, and high brittleness. Cyanate ester resin (CE) is a new type of resin developed in the 1980s. It features low dielectric constant, high temperature resistance, and resistance to humid heat, and is mainly used in high-performance printed circuit boards and high-performance transparent structural materials. Its disadvantages include high brittleness and poor toughness after curing. To increase the toughness of the resin system, BMI and CE generally use toughened modified mixed resin systems as the composite material matrix. The modified BMI and CE resin systems, while ensuring excellent resistance to damp heat and dielectric properties, can improve the impact strength of the structure.

2.2 Sandwich Materials The fairing sandwich structure plays a supporting role for the panel material, and its performance parameters greatly affect the mechanical properties of the satellite fairing's wave-transmitting section. The sandwich material should meet the following requirements: low density, high compressive modulus, high shear modulus, high flexural strength, and excellent dielectric properties. The main fairing sandwich materials that meet these requirements are: fiberglass honeycomb, aluminum honeycomb, Nomex paper honeycomb, and PMI (polymethyl methacrylate) foam honeycomb.

Fiberglass honeycomb has an insignificant weight reduction effect and poor mechanical properties. In the aerospace field, Nomex paper honeycomb and PMI foam honeycomb have gradually replaced it; the combination of aluminum honeycomb and carbon fiber panels can cause electrochemical corrosion, failing to meet the corrosion resistance requirements of composite materials in the harsh environment of spacecraft; Nomex paper honeycomb is made of aramid impregnated with phenolic resin, and the combination of Nomex honeycomb and carbon fiber panels does not cause electrochemical corrosion, has a higher shear modulus than PMI rigid foam, is lower in cost, has good processability, and is widely used in fuselage, connectors, and other parts, making it widely used in the aerospace field; PMI foam sandwich structure is superior to other honeycomb sandwich structures in many mechanical properties, and is a high-rigidity rigid structure. With a tensile strength exceeding 0.5 MPa, a density below 100 kg/m³, and a heat distortion temperature reaching 240℃, PMI exhibits excellent interfacial bonding strength with thermosetting resins such as EP, BMI, and CE. As a sandwich structure, it is less prone to detachment from the panel interface. Furthermore, PMI structures offer excellent wave transmission performance, enabling broadband wave transmission while meeting load requirements. It is widely used in fairings abroad, and domestically, it is primarily used in structural components of civil aircraft. The new generation of launch vehicles-the Long March 3A series-adopted a domestically produced PMI sandwich structure for its fairing front cone, reducing satellite launch costs and demonstrating broad application prospects.

Current Status of Domestic Fairing Material Development

To meet the future development needs of fairings for my country's launch vehicles and other spacecraft, various universities and research institutes in China have focused on modifying fiber composite materials to design and develop high-performance, low-cost fiber composite materials. This has enhanced the launch vehicle's ability to access space and reduced launch cycles and costs.

In the research of resin-based materials for launch vehicle fairings, Yang Shiyong et al. from the Institute of Chemistry, Chinese Academy of Sciences, have developed a series of polyimide resin matrices, including the first generation resistant to 316℃, the second generation resistant to 371℃, and the third generation resistant to 426℃. They have also developed different molding methods for high-temperature resistant carbon fiber reinforced resin matrix composites, including vacuum autoclave technology, vacuum high-temperature RTM technology, and reactive hot molding technology. He Guowen conducted microscopic analysis studies on the preparation and interfacial bonding strength of polyimide composites, providing a reference for the application of polyimide composites in aircraft fairings. Rao Xianhua from Jilin University has conducted research on high-performance phenylacetylene-terminated polyimide resin matrices and their carbon fiber composites, broadening the horizons for the development of high-temperature resistant polyimide carbon fiber composite fairings. The Aerospace Materials and Processes Research Institute has developed a quartz-reinforced polyimide composite material that can be used long-term at temperatures up to 370℃. This material also exhibits low dielectric constant and dielectric loss, stable performance, and excellent dielectric and mechanical properties, making it a suitable candidate for a wave-transparent/high-load-bearing functional material. Zhang Xing et al. designed a fully wave-transparent satellite fairing using a glass fiber panel-aramid paper honeycomb sandwich structure. Experimental results have verified that it achieves a high-efficiency wave transmission capability with an average 90% omnidirectional wave transmission rate. Even with a weight reduction of over 20%, the load-bearing capacity still meets design requirements, achieving lightweight, high load-bearing capacity, and omnidirectional wave transmission functionality, which has been verified in flight tests. Qu Guangyan of the Harbin FRP Research Institute designed a lightweight, high-strength, and thickness-controllable composite material fairing structure using a high-strength carbon fiber composite material co-curing process. This overcomes the mold design challenges brought about by large-scale integrated molding technology. The high-strength carbon fiber composite material fairing maintains its aerodynamic shape while ensuring large dimensions, protecting the internal payload from external environmental influences.

Current Status of Fairing Material Development Abroad

The United States boasts the most advanced technology in the design and manufacturing of launch vehicle fairing materials. Fairing materials include both inorganic and organic materials, and molding processes include vacuum bagging, molding, and casting. The US possesses comprehensive fairing performance testing and experimental methods, as well as wind tunnel technology for real-world testing. In the 1970s, the US developed the Duroid 5870 composite material and completed tests on fairing thermal loading, high-temperature electrical properties, ablation, and rain erosion resistance. It also developed automated fairing testing equipment for electrical performance testing. Currently, in the high-temperature resistant organic resin-based composite material (HTPMCs) system, the PI resins (PMR-15 and PMR-50) developed by the US have become fundamental materials in the aerospace industry, significantly promoting the application of PI composite materials in large structural components and reducing costs. Quartz fiber's excellent dielectric properties facilitate broadband wave transmission, and most foreign launch vehicle fairings are now made of quartz fiber composite materials. The United States has developed a quartz fiber-reinforced PI resin composite fairing structure with an operating temperature exceeding 538°C. Russia possesses world-leading composite material manufacturing technology. While developing and applying advanced high-temperature resistant materials, it has also developed new composite materials suitable for broadband fairings and electromagnetic/infrared dual-mode fairings. Simultaneously, it is strengthening research on fairing processing and manufacturing technologies, focusing on the structural and electrical performance design of fairings. The modified phenolic resin developed by Russia has achieved very high levels of processing and wave transmission performance, with a maximum operating temperature of 600°C, and has been successfully applied to broadband fairings.