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Aerospace

Date:2023-01-03 

1. aviation applications

The application of carbon fiber reinforced polymer matrix composites (CFRP) in military aviation can be broadly divided into three stages (and also four stages, with little difference). Civil aircraft have higher requirements for safety, economy and reliability than military aircraft, so they are more conservative and delayed in application, but they have generally followed the pace of military aircraft. It is introduced here.

The first stage - non-load-bearing structure. In the 1960s and 1970s, because 1kg CFRP can replace 3kg aluminum alloy roughly, the performance meets the requirements, so it began to be used for non-load-bearing structure, such as hatch, leading edge, mouth cover, fairing and other smaller components. For civil aircraft, in addition to the above applications, a large number of cabin interiors will also use composite materials, but many of these are aramid or fiberglass composites.

Domestic: In terms of difficulty, non-load-bearing structure is a small case of aviation cladding, but it is the most widely used. Domestic technology has no big obstacles, basically reached the foreign similar level, need is large-scale popularization. It is believed that large platforms such as ARJ21, C919 and Yun20 and many small UAV platforms will be able to provide broad application space for this purpose.

These general applications, most of the use of cheap large strands of products is enough; The products above T300 are mostly used in load-bearing structures.

The second stage - secondary bearing structure. In the 1970s and 1980s, with the improvement of mechanical properties and the effect of early application to improve people's confidence, CFRP gradually expanded to aircraft secondary bearing structure, namely, vertical tail, flat tail, canard wings, secondary flap rudder surface and other large force, large size parts.

Among them, in 1971, the United States F-14 fighter jet successfully applied the fiber reinforced epoxy resin composite material on the flat tail, which is a milestone event in the history of composite materials. The Boeing B777 also applies CFRP to multiple parts, including vertical and flat tails, and uses 9.9 tons of composite material, accounting for 11 percent of the total weight of the structure.

Back to this country, China's use of CFRP for the rudder and wing surfaces of military aircraft is also starting to mature.

According to public reports in Glass Steel and other magazines, as early as during the Sixth Five-Year Plan period, Shenyang Aircraft Design Institute, aeronautical Materials Research Institute and Shenyang Aircraft Factory jointly developed composite tail wall panels of fighter aircraft, which were 21kg lighter and 30% lighter than the original aluminum alloy structure. QY8911/HT3 bismaleimide unidirectional carbon fiber prepreg and its composite materials developed and produced by Beijing Aeronautical Technology Research Institute have been used in the front body of aircraft, vertical stabilizer surface, wing outer wing, resistance plate, rectifier wall plate and other components. The J-7A has a CFRP flat tail.

In 2009, at the National Defense Achievement Exhibition marking the 60th anniversary of the founding of the People's Republic of China, it was reported that the J-10 adopted CFRP materials on all seven rudder surfaces and fins, including canard wings, vertical tail, flaperon and ventral fins, which is basically equivalent to the development level of foreign countries at this stage.

It was revealed at the 2011 General Aviation Conference that the Falcon L-15 advanced Education aircraft also has a composite nose hood, rudder and vertical tail, with a CFRP rudder.

On the civilian side, the composite technology for the new ARJ21 regional jet is roughly at that level, a start, but it will take some time for large-scale applications.

▲ Figure 1 CFRP vertical stabilizer based on the structure of "π" shaped joint box segment of a domestic model.

▲ Figure 2 Falcon L-15 adopts T300CFRP tail rudder surface.

The extensive application of CFRP subload-bearing components in China is closely related to the production process of T300. The localization of materials, the expansion of output and the low price, respectively, provide the possibility, applicability and economy for the application of CFRP sub-load-bearing components. Finally, the CFRP subload-bearing component is promoted to become the standard of domestic military and civil aircraft.

At this stage of the material and technology, are T300 and manual stacking process can be achieved, so the future development is relatively assured. But if the part is larger, the bearing capacity is larger, it will involve the main bearing structure.

Since the 1980s, we develop to the third stage. With the maturity of high-performance carbon fiber and prepregate-autoclave molding process, CFRP has gradually entered the main bearing structures with large forces and large sizes, such as wings and fuselages.

The original McDonnell Douglas Aircraft Corporation of the United States was the first to develop the composite wing of the F/A-18 in 1976, which increased the amount of composite material to 13%, becoming another important milestone in the history of composite materials. In the later stage, automatic wire laying technology was used to manufacture 12 fuselage skins, 10 inlet pipe skins and 4 horizontal tail skins for FA-18E/F. The F-16 BLOCK50 was followed by CRPR composite wings. The F-22's composite use has increased to 22 percent of the structure's weight. At present, the amount of composite materials in Western military aircraft accounts for about 20%~50% of the total structural weight of the aircraft.

Civil aircraft, Boeing 777 adopts full composite tail, its wing surface and wing box components, are manufactured by automatic tape laying technology. The Airbus A330/A340 aircraft is 9m long, 2m wide and weighs 200kg. The rear fuselage of the A380 has 19 sections of all skin panels, and 22% of the fuselage weight is CFRP. In particular, the 8*7* 2.4-meter center wing box of the A380 weighs 8.8 tons, while the CFRP uses 5.5 tons, 1.5 tons less than the metal material, and its fuel economy is considerable.

The pioneer is Boeing's B787 Dreamliner, which uses 50% composite materials. CFRP is widely used in the wing, fuselage, vertical tail, flat tail, fuselage floor beam, rear pressure frame and other parts, at the same time is the first large commercial airliner to use CFRP composite materials at the same time of the wing and fuselage, 23% of the fuselage are using automatic wire laying mechanism of CFRP material.

The most noteworthy is the fuselage: 787 fuselage process adopts a diameter of 5.8m molding mold installed on a rotating fixture along the long axis rotation, first spread long stringer and then spread skin, forming a smooth appearance of variable thickness of the shell and the fuselage segment composed of common curing stringer, after autoclave curing, remove the mold. This process can replace the fuselage made up of hundreds of skin panels, stiffeners and stringers, and thousands of fasteners, as shown in the figure below.

▲ Figure 3 Boeing 787 diameter 5.8 meters integral forming CFRP frame segment.


In terms of research aircraft, Boeing's X-45 series aircraft use more than 90% composite material, Northrop Grumman's X-47 series aircraft are basically full composite aircraft.

In domestic front:

In December of 2012, AVic West Flight delivered the center wing, flap and motion mechanism section of the C919 to the Commercial Aircraft Corporation of China (COMAC), the two most difficult and workload sections of the seven large sections of the C919, according to a published report on CNR.cn. These two parts are large in size, complex in structure, and require high in contour tolerance. In particular, it is very difficult to process flap flaps of the longest size of domestic civil aircraft, which is as long as 15 meters. Xifei has broken through a number of technical difficulties such as design and manufacturing technology of large-scale composite molding die, preassembly deformation control technology of composite component, etc. The whole development process adopts advanced three-dimensional digital design, transmission and manufacturing. The central wing section adopts advanced medium mode high-strength carbon fiber/toughened epoxy resin composite material except the No. 1 rib which is metal. This is the first time composite materials have been used on the most important main bearing structural parts of fixed-wing aircraft in China, representing the highest level of application of carbon fiber aviation composite materials made in China.

▲ Figure 4 Box segment parts developed in China based on T-joint co-curing/bonding integrated forming process.

▲ Figure 5 The domestic use of CFRP production of a type of longitudinal and longitudinal stiffened fuselage panel.


The product in Figure 5 is still small in area and needs to be machined to form a large panel. On the other hand, the Boeing 787 can be molded as a whole with ultra-long and ultra-wide panels, covering between two large process separation surfaces (the core main frame segment), such as the 47 segment of 5.8m×7m and the 48 segment of CFRP panel with 4.3m×4.6m.

Can we make siding the size of a 787? The answer is yes. In fact, the domestic C919 large aircraft in the beginning, also had the ambition to do similar to the Boeing 787 such as large integral panel. But our technology level is not mature, although we can make, but can not control the stability of batch quality. The rejection rate is high, the cost naturally can not come down. C919 is a commercial aircraft, not a technical verification aircraft, safety and economy are a vote down, so after thinking for a long time, or give up. Let's stick with the block forming.

In order to learn new technologies and processes for the integral forming of large CFRP components, Hafei Composite Materials Co., LTD worked with foreign partners to develop components for the C919. The figure 6 below shows the C919 tail frame segment that Hafei Compounding Company participated in manufacturing. Within the length of 2.4 meters, the diameter of the C919 smoothly transitioned from 2 meters to 1.2 meters. It was formed in one piece, which is the largest volume of the CFRP cooperatively produced in China.

▲ Figure 6C919 tail frame 76-81 CFRP integral forming frame segment.


CFRP main bearing structural components, T700, T800 and other high performance military carbon fiber production, as well as large composite integral molding technology put forward higher demand. Domestic in these two aspects and there are short board even blank. So most applications are exploratory, collaborative, and phased. In the short term, we can not achieve the large-scale application of principal bearing structure CFRP.

Helicopters, rotorcraft, fan blades and other aspects

Advanced composite materials, including CFRP, are used even more. For example, V-22 Osprey tilt-rotor aircraft, 50% of its structure is made of composite materials, including fuselage, wing, tail, rotating mechanism, etc., sharing more than 3,000 kilograms of composite materials, a large part of which is CFRP. The integral rear fuselage of the V-22, which originally consisted of nine hand-laid panels, was converted to an automatic wire-laying process that reduced fasteners by 34%, man-hours by 53%, and scrap rates by 90%. Automatic wire laying technology is also applied to oil storage tank, rotor fairing and main landing gear door. The defunct RAH-66 uses 50 per cent composite, while the latest batch of Tiger gunships in Europe use 80 per cent composite for structural components, which is close to full composite construction.

In domestic front:

In 2011, the International Aviation conference disclosed that the structure of the EC120, developed by our country in cooperation with France and Singapore, such as fuselage, vertical tail, horizontal stability plane, tail fin and forward cabin, is made of CFRP and other composites. In the aspect of military aircraft, in recent years, all the domestic helicopter rotors are multi-dimensional CFRP composite blades, metal rotor blades have been completely eliminated. Newspaper: Composite blade and advanced rotor mechanism, has become a rare advantage of the overall weakness of Chinese helicopter, the level is basically on a level with foreign countries -- J-20, Wuzhi-10, Liaoning these platform breakthroughs are gratifying, and helicopter blade such long-term difficulties in the bit by bit progress, also touching.

And while we're on the subject of blades, let's talk a little bit more about aircraft turbofans. Aero fan blades, mostly made of titanium alloy. The metal blade has a weak point, that is, the vibration damping performance is poor, and it is easy to shake when rotating at high speed, and not easy to attenuate. And if the blade itself has a tiny crack, it will cause the crack to expand rapidly from inside to outside in this continuous tremor, causing the blade to break in a very short time. This is a vibration phenomenon more dangerous than resonance.

For this reason, some fans simply add a boss on either side of each blade, known in technical terms as a "shoulder". It was revealed at the Air Force Achievement Exhibition on the 60th anniversary of the foundation of the People's Republic of China that the J-11 series AL31FN and WS-10A engine air intakes have such a shoulder (see 7 below). In this way, when all the blades rotate at high speed, each shoulder shape is connected to form a strengthening ring, increasing the blade stiffness. Moreover, the blades are stacked in turn, with each shoulder "topping" the preceding blade, effectively reducing damping tremor. The consequence, however, is that the shoulders increase blade thickness and weight, while increasing the number of blades and reducing the thrust-to-weight ratio of the engine.

   ▲ Figure 7 J-10 engine air intake shoulder (red circle).


And the fan blade made of CFRP material, due to the fiber multilayer cross paste, the material itself "anisotropy" performance is superior, slow crack growth, coupled with the vibration attenuation rate is 5-6 times faster than titanium alloy, so you can cancel the blade shoulder. In 2010, it was revealed at the Zhuhai Airshow that the LEAP-X, an engine jointly developed by GE and France's Sinekma for the C919, uses CFRP three-dimensional carbon fiber braided integral fan blades, which not only reduces the weight by 50%, but also reduces the number of blades by half.

At present, only composite blades of turbofan engines can be seen in China, and CFRP has not been reported in actual turbofan engines. CJ-1000A engine developed at Zhuhai Airshow in 2012 is the first commercial FRP aero-engine developed in our country, which is reported to have adopted CFRP with wide FRP strings composite with large curved swept fan blades. Let's wait and see.

In 2011 China International General Aviation Conference, "Tiancrossbow", "Wind blade" and other UAVs adopted all-aircraft structure CFRP material, V750 unmanned helicopter, small two-seat general aviation aircraft, also widely adopted CFPR skin, which can be regarded as the domestic carbon fiber composite material in the field of general aviation beneficial attempt.

2. the application of CFRP in aerospace

Nose cones and wings: intercontinental missiles and spacecraft reenter the atmosphere at high speed, due to the resistance of adiabatic compressed air, the surface temperature of the aircraft is very high. The highest temperature on the surface of the command module of the Apollo spacecraft reached 2,740 ℃. Using the branch of CFRP series -- carbon fiber carbon reinforced composite material CFRC(also known as carbon/carbon composite material) to make ablative material, excellent thermodynamic performance, good heat protection effect. For example, the American carbon/carbon composite material in 3837℃ high temperature for 255 seconds, the line ablation rate is only 0.005 mm/second, to ensure that the space shuttle in the environment of 1650℃ continuous working 40 minutes safe and sound. In addition, carbon/carbon composite materials used to manufacture the nose cone and wing tip of intercontinental ballistic missiles have low ablation rate, uniform ablation and symmetrical ablation during the ablation process. This maintains a good aerodynamic profile of the aircraft and helps reduce unguided errors, such as the carbon/carbon composite nose cone used by the US Minuteman III missile.

Nozzle throat lining: The high temperature, high pressure and high energy particles produced during the combustion of solid rocket motor propellant are ejected from the nozzle at the supersonic speed of 3.0~4.5 Mach. The nozzle is subjected to 3500℃ high temperature, 5~15MPa pressure and high temperature scour. The Minuteman III missile, the third pole rocket nozzle throat of the United States, uses carbon cloth impregnated resin to meet the requirements of 3260℃ for 60 seconds. The key parts of the nozzle of the third-stage engine of the MX ballistic missile, such as the front section of the outer head cap, the entrance section of the integral throat lining and the downstream section of the throat, adopted the CFRC. Carbon fiber filled EPDM (EPOM) is used as the insulation layer of fixed body and flexible joint. The Navy Trident Type II (D-5) uses CFRCS for its first and second stage engines.

Engine housing: The weight reduction of the missile engine housing is conducive to improving the missile range. The solid engine shell of the US' Polaris' missile is made of metal to CFRP material, which increases its range by about twice. For example, the two stage shells of the "Polaris" A-I model are made of steel, with a range of only 2200km; The first stage of Type A II is made of steel, and the second stage is made of GFRP, increasing the range to 2800km; Both A and III stages use GFRP, increasing the range to 4600km. The Trident II (D-5), with its solid engine casing using CFRP, has an increased range from 7400km to 12000 km and a hit accuracy of 90m, making it the main model of submarine launched intercontinental ballistic missiles. Moreover, the current new types of rockets in the United States are mostly made of CFRP composite materials, which are lightweight, small in size, and have a long range.


Reentry warhead: The large area heat resistant material for the head of intercontinental ballistic missiles is mostly made of viscose based carbon fiber reinforced phenolic resin. Amoco, Hitco in the United States, and Switlangsk in Belarus( СВЕТЛОГОРСК) It is a major manufacturer of viscose based carbon fibers in the world. Not only does it have good heat protection effect, but the purity of viscose based carbon fiber and phenolic resin is high, and the content of alkali and alkaline earth metals is relatively low. The ablation wake formed during re-entry into the atmosphere contains less metal ions, making it difficult to track, enhancing the missile's penetration and survival ability.

Interstage connection: The 2.34m high coupling designed by GE for the Atlas missile is made of carbon fiber epoxy resin composite material except the mouth cover, which is 44% lighter than aluminum alloy.

Satellite structural materials: Cornwell Corporation of the United States has produced four CFRP beam structures for the dual element "OV-I" satellite, reducing weight by 68%. The CFRP connection bracket of the Earth Observation Module of the American "ATS" satellite is 4.4 meters long, weighs only 3.6 kilograms, and can withstand a load of 9 tons. Reduce weight by more than 50% compared to the best metal bracket, and have minimal deformation at high and low temperatures.

In view of this, after analyzing the public reports of the Indian Agni 5 missile (with a length of 17.5 meters, a weight of 50 tons, a 1 ton warhead, a slender and sharp warhead shape...), it is estimated that it does not yet have a rocket engine CFRP shell or a rocket CFRP shell, and lacks the independent production capacity of adhesive based carbon fiber required for long-range intercontinental missiles to re-enter the atmosphere with high ballistic trajectory. If that's the case, then facing the flaunting of its aerospace and intercontinental missile powers, it can only be said that India's progress is significant, and the gap is equally significant.


Domestic aspect:

According to public reports in magazines such as Synthetic Fiber and online, China's application of carbon fiber in strategic weapons is as follows:

Rocket engine casing: China's GFRP solid engine casing began in the 1980s and has achieved success. The casing of the "Dongfanghong-2" communication satellite operation site engine, the "Fengyun-2" meteorological satellite operation site engine, and the "Long March-2E" engine are all manufactured using GFRP. The successfully developed large SPTM-14 engine (with a shell diameter of 1402mm and a length of 2058mm) in China, paired with the Chang'er-Bale rocket, successfully launched a simulated satellite into orbit, marking the practical stage of China's large GFRP shell. Afterwards, China successfully developed the EPKM-17 upper stage engine casing (1700mm in diameter and 1874mm in length), which was paired with the Chang'ei High Thrust Rocket. At the end of 1995, the "Asia 2" and "Exesta 1" satellites were successfully sent into 36000km of space.

Rocket missile shell: China has also made significant progress in developing CFRP shells. In the late 1990s, the foundation test, shell structural strength test, ignition test, and other comprehensive assessments of the T300 solid rocket engine shell were conducted, and the 12K T700 CFRP shell structural strength test was completed. The first one used in the model was the fourth stage (diameter 640mm) of the "Pioneer 1" solid small carrier engine, which was successfully flown in September 2003. Achieved a historic leap in the CFRP shell. At present, the pre research test of T800 CFRP shell has been carried out.

Nozzle throat lining: The C/CFRP nozzle developed in China was successfully ignited in 1989, with a large size of only 0.9mm at the thinnest exit wall thickness( Ф The nozzle (around 500-2000mm) exhibits excellent comprehensive performance.

Reentry warhead: According to the report of the "Scientific Pioneer of Creating National Defense Advanced Materials - model worker of Shanghai Professor Pan Ding" in Donghua Alumni, Professor Pan Ding, a material science professor and doctoral supervisor of Donghua University, presided over the national major military scientific research project of "300Kg/year viscose based carbon fiber test line" in 2001-2003, A high-purity aerospace grade viscose based carbon fiber, which fills the domestic gap and meets the international advanced level of product quality, was made from domestically produced cotton cellulose precursor fibers different from foreign raw materials. The results were transferred to Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences for large-scale production without compensation. The research group has also formulated the "GJB3839-2000" national standard, forming a unique technology and application equipment with independent intellectual property rights for preparing carbon fibers from cotton cellulose viscose cord. This technology and product won the second prize of the National Science and Technology Progress Award in 2003, solving the problem of DF-31 missile design and making China the third largest country in the world, apart from the United States and Russia, to independently master this product and its production technology.


Satellite structure: According to a report from China Quality News Network, the Chang'e-2 lunar exploration satellite launched in 2011 in China has an important supporting part for its directional antenna, which is a CFRP composite material developed by Harbin Glass Fiber Reinforced Plastic Research Institute. The total weight is only over 500 grams, which is nearly 300 grams lighter than using aluminum alloy material, but its load-bearing capacity is not inferior.

A friend said, what is 300 grams? Hehe, you should know that the weight loss of satellites is measured in grams. If you lose 1 gram, you can save 500 grams of fuel. Less than 300 grams, the satellite can carry an additional camera or telescope to complete more tasks. Let's take a look at the weight loss ratio: 40%, which is still very effective.

3. Summary

Mr. Shi Changxu, a Chinese material master, commented in 2010 that the current application of CFRT in China is approximately at the level of developed Western countries in the 1980s.

From the above introduction, it can be seen that China's carbon fiber composites are catching up and catching up in the military field, with many highlights. But in the development of civil aviation, it has been significantly lagging behind countries such as the United States, Europe, and Japan. The direct reason is that the cost is too high, much more expensive than the aluminum alloy to be replaced, and even more expensive than titanium alloy.

The indirect reasons for this are multifaceted.

Firstly, strategic military small tow products have benefited from the attention of two generations of "core" leaders, and the complete localization of T300 military carbon fiber has led to rapid development of secondary load-bearing structural military components. However, the policy support in the field of civilian large wire bundles is relatively lagging behind. In fact, the country had limited resources and manpower back then, so it was entirely reasonable to focus on developing military small tow for emergency purposes. However, in the long run, the market space for general and civilian products is greater, and it is a solid foundation for the sustainable development and innovation of the carbon fiber industry. In today's era where military products have opened a breakthrough, economic development, and national strength have been enhanced, not to mention large tow products. Even small tow products should be expanded from a market and civilian perspective to broaden their industry foundation. The military should lead the people, the people should support the military, the seedlings should be grafted, and the branches and leaves should be scattered, forming a positive interaction between military technology and civilian industry. This is a policy level reason.

Secondly, there are more than ten domestic carbon fiber manufacturers, all of which are thriving. Although it may seem lively, a large part of them do not have mastered the core technology. Either the key equipment and materials need to be imported, or the process parameters and quality control are not fully understood. Even now, many enterprises still have to import DMSO solvent from Dongli Company at a high price for PAN raw material production, which belongs to the "independent production" of imitating cat and tiger. Most manufacturers have significant differences in product quality batches, with entanglement and breakage occurring from time to time. The production capacity of qualified PAN raw materials is only 100 tons/year, which cannot reach the basic level of economies of scale. There is great room for improvement in industrial layout and mastery of key technologies. This is the reason for the production level of PAN precursor and carbon fiber.



Thirdly, in the field of prepreg automatic stacking technology and overall molding technology, it has become a mature manufacturing technology in developed countries. However, in the field of carbon fiber composite materials in China's aerospace industry, it remains the largest short board or even blank in industrial production. Even with the introduction of equipment, our research on the physical properties and mechanical properties of composite materials is not thorough, and our understanding of processing parameters is insufficient. Without knowing the reasons, we directly use foreign software to design composite material solutions, resulting in low production, high prices, unstable quality, and low innovation ability of CFRP composite materials. Military components, regardless of cost, are nothing more than a major obstacle to commercial mass production and application. Many manufacturers are hesitant and hesitant to move forward, simply using metal materials that have already been thoroughly researched is more reliable and cost-effective. This is the reason for the production of composite materials.

Fourthly, the design of aerospace spacecraft needs to combine the performance characteristics of composite materials and strengthen the overall design concept, rather than simply replacing original metal components. To give a simple example, after using CFRP composite material for the flat tail of a certain type of military aircraft in China, it was indeed much lighter, but it changed the overall torque balance of the aircraft and required adjustment through counterweights. As a result, the overall weight reduction effect was not ideal. Of course, item by item substitution is also an effective verification step, but there is a concept that needs to be emphasized: local optimization does not represent overall optimization. In today's increasingly widespread application of composite materials, top-level design and global optimization are necessary to maximize the functional and economic benefits of composite materials. This is the reason for the design philosophy.