A forming method for large-sized ultra-lightweight cryogenic storage tanks

Abstract

The present invention belongs to the field of aerospace technology, and provides a forming method for a large-size ultra-lightweight cryogenic storage tank. For the forming of the cylindrical section of the metal inner liner of the storage tank, a metal thin strip is wound into a spiral structure. During forming, by simply changing the forming angle, cylindrical sections with different diameters can be produced using metal thin strips of the same width. A structure combining a metal inner liner and a composite outer shell is adopted. Since the composite outer shell has a relatively light mass, compared with traditional metal storage tanks, the use of a composite outer shell can significantly reduce the mass of the storage tank and improve its operating efficiency. The method of the present invention aims to solve the problems of large equivalent surface density, overweight structure, insufficient pressure-bearing capacity, difficulty in manufacturing large-size metal storage tanks in existing metal storage tanks, as well as liquid leakage, connection with other structures, and sealing difficulties in all-composite storage tanks, thereby providing a basic guarantee for the design and manufacture of advanced aircraft such as the new generation of space planes and heavy launch vehicles.

Description

Technical Field

[0001] The present invention relates to the field of aerospace technology, and particularly to a forming method for a large-sized ultra-lightweight cryogenic storage tank. Background Art

[0002] Cryogenic storage tanks are important components of aerospace vehicles and are used to store cryogenic fuels such as liquid hydrogen, liquid oxygen, and kerosene. The overall structure of cryogenic storage tanks is large in size and accounts for a large proportion of the mass. Therefore, optimizing the structure and mass of cryogenic storage tanks is the key to achieving the lightweight of the vehicle, improving the structural efficiency, and the carrying capacity. As pointed out in "Chen Zhenguo, Jiao Weicheng, Yan Meiling, et al. Research Progress on the Leakage Resistance of Carbon Fiber Reinforced Resin Matrix Composite Cryogenic Storage Tanks [J]. Fiberglass / Composite Materials, 2018(11): 109-116.", taking a launch vehicle as an example, the structural weight of the cryogenic storage tank accounts for more than 50% of the total weight of the rocket body structure. The lightweight of the storage tank has always been one of the important ways to improve the launch vehicle efficiency. For the new generation of aerospace planes, the need for the lightweight of the storage tank is more urgent, and the lightweight index is also more stringent. For example, such storage tanks are required to have an equivalent surface density not greater than 9 kg / m². When using aluminum alloy and stainless steel to prepare the tank body, the equivalent thickness of the tank body cannot exceed 3.3 mm and 1.15 mm, which is obviously unreasonable. More importantly, the strength requirement for the tank body material of large-sized storage tanks is getting higher and higher. For example, for a storage tank with a diameter-to-thickness ratio (outer diameter to wall thickness ratio D / t) greater than 1000 and a pressure resistance not less than 0.5 MPa, the yield strength of the tank body material is required to be not less than 350 MPa (taking a safety factor of 1.5). The strength of existing aluminum alloy plates and stainless steel plates can no longer meet the requirements. At the same time, the manufacturing of the tank body of traditional metal material storage tanks mainly includes stamping forming, welding forming, etc. Repeated stamping forming of raw material plates not only consumes more raw materials and has low efficiency, but also is prone to work hardening during repeated stamping and stretching, resulting in cracking. Welding forming is a process of spinning and welding metal plates using methods such as resistance welding and laser welding. It is difficult and costly to manufacture large-diameter storage tanks. For this reason, traditional metal structural materials such as aluminum alloy and stainless steel, as well as existing forming technologies and equipment, can no longer be used for the manufacturing of new large-sized lightweight storage tanks.

[0003] Adopting composite materials is one of the important ways to achieve the lightweight of storage tanks, and relevant research has been carried out at home and abroad with important progress. Using continuous winding of carbon fiber composite materials, NASA and Boeing in the United States have fabricated composite storage tanks with a wall thickness of about 2.0 mm and a diameter of up to 5 m or even larger. However, due to the compatibility problem between the composite material and liquid oxygen, the resin matrix in the composite material will develop cracks or pores at low temperatures, and the fabricated liquid oxygen storage tank has the risk of micro-cracks and liquid leakage, which greatly limits the practical application of composite cryogenic storage tanks. At the same time, the following problems also exist in the all-composite cryogenic storage tank: (1) The processability of the composite material is poor, and it is very difficult to directly connect and seal the shell of the all-composite storage tank with other structures such as fuel delivery pipelines and measuring instruments; (2) There is discontinuity in the composite winding, and the all-composite storage tank has low structural stiffness and poor anti-compression buckling ability, and additional strengthening frames or ribs, etc. need to be added inside and outside the shell, resulting in a complex structure; (3) The cryogenic storage tank needs to adopt complex heat insulation structures, anti-slosh structures, etc., and it is very difficult to set or connect such structures on the all-composite storage tank.

[0004] For large-size cryogenic storage tanks of aircraft such as the new generation of spaceplanes and heavy-lift launch vehicles, the requirements for structural lightweight are extremely stringent. Traditional metal material storage tanks cannot meet the equivalent surface density requirements, and the low yield strength cannot meet the pressure-bearing requirements. The forming of large-size metal material storage tanks is difficult and costly, while the all-composite storage tank has the risks of liquid leakage and difficulties in connecting, sealing, and assembling with other structures. Therefore, a new forming method for ultra-lightweight large-size cryogenic storage tanks needs to be proposed. Summary of the Invention

[0005] In order to improve the launch efficiency of space vehicles and meet the stringent lightweight requirements while ensuring safety, the present invention provides a forming method for large-size ultra-lightweight cryogenic storage tanks to solve the problems of large equivalent surface density, overweight structure, insufficient pressure-bearing capacity of existing metal storage tanks, difficulty in manufacturing large-size metal storage tanks, and liquid leakage and difficulties in connecting and sealing with other structures of all-composite storage tanks, so as to provide a basic guarantee for the design and manufacture of advanced aircraft such as the new generation of spaceplanes and heavy-lift launch vehicles.

[0006] To achieve the above object, the technical solution of the present invention is as follows: A forming method for large-size ultra-lightweight cryogenic storage tanks includes the following steps:

[0007] Step 1. Determination and preparation of the cylindrical section of the metal inner tank: Analyze the characteristics of the metal inner tank component to determine the size of the metal strip required for preparation and the diameter of the cylindrical section 4 of the metal inner tank. The cylindrical section is formed by spirally winding a metal strip with a certain thickness and width. The two sides of the strip of each metal cylindrical section are rolled to form a curved seam. According to the two side edges of the strip, one side is curled upward and the other side is curled downward to form the upper seam 10 and the lower seam 11 respectively. During the winding process, the edges of the upper seam 10 and the lower seam 11 are buckled with each other, and then the buckled seams are tightly pressed and fastened by the upper and lower relatively rolling seam wheels in the winding equipment.

[0008] Step 2. Installation of auxiliary structures: Install four vertical sliding grooves on the inner wall of the metal inner tank obtained in Step 1 to support the cylindrical section 4 of the metal inner tank and install auxiliary anti-slosh structures. At the same time, reserve through holes on the inner wall of the metal inner tank for installing instruments and meters.

[0009] Step 3. Preparation and connection of the bottom 3 of the metal inner tank: Prepare the bottom of the tank according to the cylindrical section of the metal inner tank obtained in Step 1. Cut out a structure corresponding to and matching the cylindrical section of the metal inner tank from the prepared bottom 3 of the metal inner tank, and connect it to the cylindrical section 4 of the metal inner tank to form a complete metal inner tank. At the same time, reserve through holes on the bottom 3 of the metal inner tank for installing instruments and meters.

[0010] Step 4. Pretreatment of the metal inner tank: Pretreat the surface of the metal inner tank to improve the bonding performance when connecting with the composite material fiber. A resin film or a flexible layer is added between the metal inner tank and the composite material outer shell to adapt to the thermal expansion and contraction deformation caused by filling and discharging cryogenic propellants and improve the impact resistance.

[0011] Step 5. Preparation of the composite material outer shell 6: Fix and position the metal inner tank obtained in Step 4, and use the winding forming process to evenly wind the composite material fiber around the outer wall of the metal inner tank at a certain angle to obtain a composite material outer shell that fits tightly on the surface of the metal inner tank and has no wrinkles on the surface.

[0012] Step 6. Bonding and curing of the composite material outer shell: After surface treatment of the metal inner tank and the composite material outer shell, bond the metal inner tank and the composite material outer shell together, and finally cure the fiber winding layer by heating or at room temperature.

[0013] Step 7. Installation of instruments and meters: Install the instruments and meters required during the service of the cryogenic storage tank through the through holes reserved on the metal inner tank before to obtain the final large-size ultra-lightweight cryogenic storage tank unit.

[0014] Advantages of the present invention: (1) In the forming method of the large-sized ultra-lightweight cryogenic storage tank of the present invention, since the cylindrical section of the metal inner liner of the storage tank is formed by winding a metal thin strip into a spiral structure, during forming, only by changing the forming angle, metal thin strips of the same width can be used to produce cylindrical sections of different diameters. Because it is a continuous bending forming, the length of the cylindrical section is not limited, which can effectively meet the production requirements of large-sized storage tanks. At the same time, since the spiral structure adopted by the cylindrical section of the metal inner liner of the storage tank has certain axial and circumferential extensibility, it can absorb the thermal expansion and contraction deformation caused by the filling and discharging of cryogenic propellants to a certain extent, improve the impact resistance, and make the structure more stable.

[0015] (2) In the forming method of the large-sized ultra-lightweight cryogenic storage tank of the present invention, compared with the connection auxiliary structure on the composite material storage tank, the anti-slosh structure arranged inside the metal inner liner is simpler and can effectively suppress the sloshing of the liquid. At the same time, the sealing lap joint between the wound metal inner liner and the composite material outer shell adopts laser welding or polymer bonding process. If a flexible layer is added between the two, it can adapt to the thermal expansion and contraction of the internal metal inner liner, thereby reducing the stress between the inner and outer layers. In addition, the flexible layer can further improve the sealing performance and durability of the cryogenic storage tank.

[0016] (3) In the forming method of the large-sized ultra-lightweight cryogenic storage tank of the present invention, a structure combining a metal inner liner and a composite material outer shell is adopted. Since the composite material outer shell is lighter in mass, compared with the traditional metal storage tank, using a composite material outer shell can significantly reduce the mass of the storage tank and improve the operating efficiency of the storage tank. The composite material outer shell has excellent properties such as high strength, high stiffness, and low density, and also has good fatigue resistance. Compared with the traditional metal storage tank, using a composite material outer shell can improve the structural strength of the storage tank and its load-bearing capacity.

[0017] (4) In the forming method of the large-sized ultra-lightweight cryogenic storage tank of the present invention, a structure combining a metal inner liner and a composite material outer shell is adopted. On the one hand, the metal inner liner has good sealing performance. By adopting a perfect sealing design at the joint between the metal inner liner and the composite material outer shell, liquid leakage can be effectively prevented. At the same time, the metal inner liner is compatible with liquid oxygen and has good corrosion resistance, which can ensure the sealing performance and protection performance of the storage tank. The metal inner liner is easy to process, and other structures such as fuel delivery pipelines and measuring instruments can be directly connected to the inner wall of the metal inner liner. At the same time, an adhesive is used to connect the metal inner liner and the composite material outer shell, which can ensure the connection performance of the delivery pipelines and measuring instruments while also ensuring the sealing performance. Description of the Drawings

[0018] Figure 1 is the schematic diagram of the forming method of the large-sized ultra-lightweight cryogenic storage tank in the present invention;

[0019] Figure 2(a) is the schematic diagram of the connection of the thin strip of the cylindrical section of the metal inner liner in the present invention;

[0020] Figure 2(b) is a schematic diagram of the cylindrical section of the metal inner tank of the present invention;

[0021] Figure 3 is a schematic diagram of the first connection scheme between the bottom of the metal inner tank and the cylindrical section of the present invention;

[0022] Figure 4 is a schematic diagram of the second connection scheme between the bottom of the metal inner tank and the cylindrical section of the present invention;

[0023] Figure 5 is a schematic diagram of the connection between the composite material outer shell and the metal inner tank of the present invention;

[0024] Figure 6 is a schematic diagram of the winding of the composite material outer shell of the present invention;

[0025] Figure 7 is a composition diagram of the large-size ultra-lightweight cryogenic storage tank of the present invention.

[0026] In the figure, 1 is a pressure regulating valve, 2 is an exhaust pipe, 3 is the bottom of the metal inner tank, 4 is the cylindrical section of the metal inner tank, 5 is an adiabatic flexible layer, 6 is a composite material outer shell, 7 is a liquid filling pipe, 8 is a liquid level gauge, 9 is a pressure gauge, the upper seam 10, and the lower seam 11. Specific Embodiments

[0027] The following further illustrates the specific embodiments of the present invention in conjunction with the drawings and technical solutions.

[0028] A forming method for a large-size ultra-lightweight cryogenic storage tank comprises the following steps:

[0029] Example 1: In combination with Figure 1 , Figure 2, Figure 3 , Figure 4 , Figure 5 it is described that the forming method for the large-size ultra-lightweight cryogenic storage tank proposed by the present invention is carried out according to the following steps:

[0030] The present invention provides a forming method for a large-size ultra-lightweight cryogenic storage tank, comprising the following steps:

[0031] Step 1. Determination and preparation of the cylindrical section of the metal inner tank: Analyze the characteristics of the metal inner tank component, determine the size of the metal strip required for preparation and the diameter of the cylindrical section 4 of the metal inner tank. The cylindrical section 4 of the metal inner tank is formed by spiral winding of a metal strip with a certain thickness and width. The two sides of the strip of each metal cylinder section are rolled to form curved seams. According to one side curling upward and the other side curling downward, they are divided into the upper seam 10 and the lower seam 11. During the winding process, the edges of the upper seam 10 and the lower seam 11 are buckled with each other, and then the buckled seams are tightly pressed by the upper and lower relatively rolling seaming wheels, and then fastened by welding;

[0032] Step 2. Installation of auxiliary structure: Four vertical sliding grooves are installed on the inner wall of the metal inner container obtained in Step 1 by welding to support the cylindrical section 4 of the metal inner container and install the auxiliary anti-slosh structure. At the same time, through holes are reserved on the inner wall of the metal inner container for subsequent installation of instruments and meters.

[0033] Step 3. Preparation and connection of the bottom of the metal inner container: Select a suitable plate according to the cylindrical section of the metal inner container obtained in Step 1, and prepare the bottom of the container by stamping or drawing. Cut out the structure corresponding to the cylindrical section of the metal inner container on the prepared bottom of the metal inner container, and connect it to the cylindrical section by welding to form a complete metal inner container. At the same time, through holes are reserved on the bottom of the metal inner container for subsequent installation of instruments and meters.

[0034] Step 4. Pretreatment of the metal inner container: Pretreat the surface of the metal inner container to improve the bonding performance when connecting with the composite material fiber. Considering the mismatch of thermal expansion between the metal inner container and the composite material shell, a resin film or a flexible layer is added between the two to adapt to the thermal expansion and contraction deformation caused by the filling and discharging of cryogenic propellants and improve the impact resistance.

[0035] Step 5. Preparation of the composite material shell: Fix and position the metal inner container obtained in Step 4, and evenly wind the composite material fiber around the outer wall of the metal inner container at a certain angle by using the winding forming process to obtain a composite material shell that fits tightly on the surface of the metal inner container and has no wrinkles on the surface.

[0036] Step 6. Bonding and curing of the composite material shell: After surface treatment of the metal inner container and the composite material shell, use an adhesive to bond the metal inner container and the composite material shell together, and finally cure the fiber winding layer under heating or at room temperature.

[0037] Step 7. Installation of instruments and meters: Install the instruments and meters required during the service of the cryogenic storage tank through the through holes reserved on the metal inner container before to obtain the final large-size ultra-lightweight cryogenic storage tank monomer.

[0038] In the forming method of the large-size ultra-lightweight cryogenic storage tank in this embodiment, since the cylindrical section of the metal inner liner of the storage tank is formed by winding a metal thin strip into a spiral structure, during forming, only by changing the forming angle, metal thin strips of the same width can be used to produce cylindrical sections of different diameters. Because it is a continuous bending forming process, the length of the cylindrical section is not limited, which can effectively meet the production requirements of large-size storage tanks. At the same time, since the spiral structure adopted by the cylindrical section of the metal inner liner of the storage tank has certain axial and circumferential extensibility, it can absorb the thermal expansion and contraction deformation caused by filling and discharging cryogenic propellants to a certain extent, improve the impact resistance, and make the structure more stable; compared with the connection auxiliary structure on the composite material storage tank, it is simpler to set up an anti-slosh structure inside the metal inner liner, and it can effectively suppress the sloshing of the liquid. At the same time, the sealed lap joint between the wound metal inner liner and the composite material outer shell adopts laser welding or polymer bonding technology. If a flexible layer is added between the two, it can adapt to the thermal expansion and contraction of the internal metal inner liner, thereby reducing the stress between the inner and outer layers. In addition, the flexible layer can further improve the sealing performance and durability of the cryogenic storage tank; because the composite material outer shell is lighter in weight, compared with the traditional metal storage tank, using the composite material outer shell can greatly reduce the mass of the storage tank and improve the operating efficiency of the storage tank. The composite material outer shell has excellent properties such as high strength, high stiffness, and low density, and also has good fatigue resistance. Compared with the traditional metal storage tank, using the composite material outer shell can improve the structural strength of the storage tank and its load capacity; on the one hand, the metal inner liner has good sealing performance. By adopting a perfect sealing design at the joint between the metal inner liner and the composite material outer shell, liquid leakage can be effectively prevented. At the same time, the metal inner liner is compatible with liquid oxygen and has good corrosion resistance, which can ensure the sealing performance and protection performance of the storage tank. The metal inner liner has good machinability, and other structures such as fuel delivery pipelines and measuring instruments can be directly connected to the inner wall of the metal inner liner. At the same time, the metal inner liner and the composite material outer shell are connected by an adhesive, which can ensure the connection performance of the delivery pipelines and measuring instruments while also ensuring the sealing performance.

[0039] Example 2: As shown in combination with Figure 1 Figure 2, in Step 1, the material of the selected metal thin strip is 5A02 aluminum alloy, with a thickness of 0.5 mm. It is connected and continuously wound into a cylindrical section with a diameter of 2000 mm and a length of 2000 mm. During the winding process, transition sections are reserved at both ends of the cylindrical section for subsequent connection to the tank bottom. Other steps are the same as in Example 1.

[0040] In this embodiment, the cylindrical tube section formed by rolling 5A02 aluminum alloy thin strips is stronger than the traditional straight seam tube section. After the inner wall of the cylindrical tube section is compressed, the principal stress is circumferential stress. The spiral weld is at a 45-degree angle with the axis to avoid the principal stress, while the principal stress of the straight seam tube is exactly perpendicular to the weld, and the strength is generally lower than that of the spiral structure. When the cylindrical tube section is formed, the impact toughness along the rolling direction is the largest, while the impact toughness in the straight rolling direction is the smallest, and the difference between the two is several times. The spiral line shape of the spiral structure can effectively avoid the effect of the principal stress along the rolling direction during the rolling of the traditional straight seam tube, reduce the impact on the weak links of the spiral structure's toughness, and improve durability.

[0041] Example 3: Combination Figure 3 Note: In step three, the bottom of the metal liner box is prepared from 5A02 aluminum alloy sheet by stamping or stretching. According to the final rolled shape of the cylindrical section of the metal liner, plasma cutting, laser cutting and other methods are used to cut out a shape that matches the cylindrical section so as to be connected to the cylindrical section. The other steps are the same as in Example 1.

[0042] In this embodiment, the metal liner box bottom is cut into a shape that matches the metal liner tube section. During the transition connection process, the cylindrical tube section can gradually transition to a wide main body structure, which can disperse the force and avoid weak locations such as welds.

[0043] Example 4: Combination Figure 4 Note: In step three, the metal liner box bottom is prepared from 5A02 aluminum alloy sheet by stamping or stretching. During the preparation of the metal liner box bottom and the rolling of the metal liner tube segment, plasma cutting, laser cutting and other methods are used to cut out a circular ring shape in cross section. After grinding and polishing, the metal liner box bottom is connected by welding. The other steps are the same as in Example 1.

[0044] In this embodiment, due to the consistent connection cross-section, the connection between the metal liner bottom and the metal liner is relatively stable, which can reduce the displacement of the joint and also alleviate stress concentration, thereby improving the durability of the storage tank.

[0045] Example 5: Combination Figure 5 Note: In step 4, use sodium hydroxide (NaOH) solution or sulfuric acid / chromic acid ( ) solution corrodes the aluminum strip, polyether triamine T5000 is dissolved in acetone and fully stirred, and then the aluminum strip pretreated with NaOH solution is immersed in the polyether triamine T5000 solution at 25°C for 1 hour. Finally, the treated aluminum strip is dried at 50°C for 1 hour to prepare an insulating flexible layer 5, which is evenly laid on the outer wall of the metal liner for subsequent connection with the composite material shell. The other steps are the same as those in Example 1.

[0046] In this embodiment, when the composite material and the metal inner tank are filled and discharged with cryogenic propellant, the inner and outer walls are separated due to different thermal expansions. Adding a flexible layer between them can effectively improve the situation. At the same time, polyether triamine is a high-performance insulating material with excellent heat resistance, cold resistance and electrical insulation properties, thus improving the heat insulation performance of the storage tank.

[0047] Example 6: Combining Figure 6 It is illustrated that in step five, the composite material fiber is selected as carbon fiber reinforced composite material. When winding the composite material fiber, the longitudinal winding method is adopted. The composite material fiber makes a circular motion along a fixed plane, while the metal inner tank rotates around its own axis. Each time the composite material fiber rotates one week, the metal inner tank rotates through a corresponding angle. The angle between the composite material fiber and the axis of the metal inner tank is 0° - 25°. The fiber winding angle should be controlled within a certain range to relieve the accumulation of fibers at the bottom of the end box of the metal inner tank. When winding the fibers, it is ensured that there is no overlap or cross between adjacent fibers, and the phenomenon of fiber bridging caused by too small diameter of the end polar hole should also be prevented.

[0048] In this embodiment, the carbon fiber reinforced composite material has excellent properties such as high strength, high stiffness and low density. Winding the composite material fiber on the surface of the metal inner tank can effectively improve its structural strength and meet the requirements of lightweight of the cryogenic storage tank at the same time.

[0049] Example 7: Combining Figure 6 It is illustrated that in step six, the adhesives used include epoxy resin adhesives, polyurethane glues, acrylic glues, silicone rubber glues, etc.

[0050] The advantages of using adhesives such as epoxy resin for bonding in this embodiment are its high bonding strength, good corrosion resistance, good low-temperature resistance, and the ability to fill tiny gaps, improving the reliability of bonding. At the same time, the epoxy resin adhesive also has good heat resistance and chemical corrosion resistance, and can meet the usage requirements of the storage tank monomer.

[0051] Example 8: Combining Figure 7 It is illustrated that in step seven, the instrumentation installed through the reserved through holes includes: the liquid filling pipe 7 required when the storage tank is in service and filled and discharged with cryogenic propellant; the pressure regulating valve 1 and the exhaust pipe 2 for adjusting the internal pressure of the storage tank; the liquid level gauge 8 and the pressure gauge 9 for observing the filling and discharging situation of the cryogenic propellant inside the storage tank. Other steps are the same as those in Example 1.

[0052] Installing in this embodiment improves the safety, service performance and maintenance convenience of the equipment. At the same time, it can also reduce the labor cost and improve the work efficiency.

Claims

1. A forming method for a large-sized ultra-lightweight cryogenic storage tank, characterized in that The steps are as follows: Step 1. Determination and preparation of the cylindrical section of the metal inner tank: Analyze the characteristics of the metal inner tank component, determine the dimensions of the metal strip required for preparation and the diameter of the cylindrical section (4) of the metal inner tank. The cylindrical section is formed by spirally winding a metal strip with a certain thickness and width. The two sides of the strip of each metal cylindrical section are rolled to form a bent seam. According to the two side edges of the strip, one side curls upward and the other side curls downward, which are the upper seam (10) and the lower seam (11) respectively. During the winding process, the edges of the upper seam (10) and the lower seam (11) are buckled with each other, and then the buckled seams are tightly pressed and fastened by the upper and lower relatively rolling clamping wheels in the winding equipment; Step 2. Installation of auxiliary structures: Install four vertical sliding grooves on the inner wall of the metal inner tank obtained in Step 1 to support the cylindrical section (4) of the metal inner tank and install auxiliary anti-slosh structures. At the same time, reserve through holes on the inner wall of the metal inner tank for installing instruments; Step 3. Preparation and connection of the bottom (3) of the metal inner tank: Prepare the bottom of the tank according to the cylindrical section of the metal inner tank obtained in Step 1. Cut out a structure corresponding to and matching the cylindrical section of the metal inner tank from the prepared bottom (3) of the metal inner tank, and connect it to the cylindrical section of the metal inner tank to form a complete metal inner tank. At the same time, reserve through holes on the bottom (3) of the metal inner tank for installing instruments; Step 4. Pretreatment of the metal inner tank: Pretreat the surface of the metal inner tank to improve the bonding performance when connecting with the composite material fibers. A resin film or a flexible layer is added between the metal inner tank and the composite material shell to adapt to the thermal expansion and contraction deformation caused by filling and discharging cryogenic propellants and improve the impact resistance; Step 5. Preparation of the composite material shell (6): Fix and position the metal inner tank obtained in Step 4, and evenly wind the composite material fibers around the outer wall of the metal inner tank at a certain angle by using a winding forming process to obtain a composite material shell that fits tightly on the surface of the metal inner tank and has no wrinkles on the surface; Step 6. Bonding and curing of the composite material shell: After surface treatment of the metal inner tank and the composite material shell, bond the metal inner tank and the composite material shell together, and finally cure the fiber winding layer under heating or at room temperature; Step 7. Installation of instruments: Install the instruments required during the service of the cryogenic storage tank through the through holes reserved on the metal inner tank before to obtain the final large-size and ultra-lightweight cryogenic storage tank monomer.

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