Carrier rocket fatigue load assignment system and method based on 3D model

Abstract

A fatigue load assignment system and method for a launch vehicle based on a 3D model, including: a 3D modeling module for the launch vehicle, an aerodynamic fatigue load conversion module, and a fatigue load post-processing module; the 3D modeling module for the launch vehicle establishes a refined 3D finite element model of the launch vehicle; the aerodynamic fatigue load conversion module obtains the aerodynamic fatigue load input from the superior, and uses the minimum deformation energy conversion algorithm for aerodynamic fatigue load applicable to the launch vehicle to obtain the minimum deformation energy; according to the minimum deformation energy, the force and moment of the fatigue load relative to the centroid in the 3D finite element model are obtained to complete the assignment process; the fatigue load post-processing module determines whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the force and moment of the fatigue load relative to the centroid in the 3D finite element model. The present invention solves the problem of refined assignment of the fatigue load of the launch vehicle, makes the load design of the launch vehicle more reasonable, effectively reduces the design margin, and improves the structural safety and reliability.

Description

Technical Field

[0001] The present invention relates to a payload design method for a launch vehicle, and in particular to a refined fatigue load assignment system and method for a launch vehicle. Background Art

[0002] At present, the traditional launch vehicle structure static load calculation in China adopts a particle beam model. This model does not consider structural differences and mostly gives results by adding a safety factor to the static load, lacking the research on the regularity of load excitation sources. The load design conditions tend to be conservative, and the refined design level of the obtained load results is not high, which is not conducive to structural weight estimation and stiffness design. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a new load design system and method for a launch vehicle, establish a more accurate three-dimensional load calculation model for the launch vehicle; study the three-dimensional fluid-structure fatigue load conversion technology, which can truly reflect the continuous distribution characteristics of structural mass and aerodynamic fatigue loads, and realize the conversion between aerodynamic grid node loads and structural finite element grid node loads; conduct research on the post-processing technology of fatigue load assignment results to provide more detailed load input for strength design; realize the load reduction conditions of the launch vehicle, based on the weight reduction of the launch vehicle structure, thereby improving the carrying capacity.

[0004] The technical solution of the present invention:

[0005] A fatigue load assignment system for a launch vehicle based on a three-dimensional model, including: a three-dimensional modeling module for the launch vehicle, an aerodynamic fatigue load conversion module, and a fatigue load post-processing module;

[0006] The three-dimensional modeling module for the launch vehicle: establish a refined three-dimensional finite element model for the launch vehicle; the three-dimensional finite element model can simulate the structural form, layout, center of mass, moment of inertia, and connection stiffness of the launch vehicle.

[0007] The aerodynamic fatigue load conversion module: obtain the aerodynamic fatigue load input from the superior, adopt an aerodynamic fatigue load minimum deformation energy conversion algorithm applicable to the launch vehicle to obtain the minimum deformation energy; complete the assignment process according to the minimum deformation energy to obtain the force and moment of the fatigue load relative to the center of mass in the three-dimensional finite element model;

[0008] The method for the aerodynamic fatigue load conversion module to obtain the minimum deformation energy U is specifically as follows:

[0009]

[0010]

[0011]

[0012]

[0013] Among them, U j is the deformation energy of finite element node j; EJ is the flexural rigidity of the assumed beam; P j is the force of finite element node j; L j is the distance from finite element node j to the nearest aerodynamic fatigue load node, that is, the assumed beam length; P A is the resultant force of the aerodynamic fatigue model; x j , y j , z j are the three-direction coordinates of finite element node j; x A , y A , z A are the three-direction coordinates of the centroid of the aerodynamic fatigue load model; λ, λ x , λ y , λ z are the multipliers in the extreme value function established by the Lagrange multiplier method, and n is the number of nodes in the finite element model.

[0014] Fatigue load post-processing module: Determine whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model.

[0015] The method for the fatigue load post-processing module to determine whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements is specifically as follows:

[0016] 21) Calculate the resultant force F and resultant moment M of the aerodynamic fatigue load relative to the centroid of the launch vehicle;

[0017] 22) Calculate the resultant force and resultant moment of the finite element load based on the centroid according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model;

[0018] 23) Obtain the difference x1 between the resultant force of the aerodynamic fatigue load and the resultant force of the finite element load, and obtain the difference x2 between the resultant moment of the aerodynamic fatigue load and the resultant moment of the finite element load;

[0019] 24) If x1 < 0.5%F and x2 < 0.5%M; then it is determined that the aerodynamic fatigue load conversion algorithm meets the usage requirements; otherwise, it is determined that the aerodynamic fatigue load conversion algorithm does not meet the usage requirements.

[0020] A method for assigning fatigue loads to a launch vehicle based on a three-dimensional model includes the following steps:

[0021] 1) Establish a three-dimensional finite element model of the launch vehicle and perform mesh division; in step 1), the three-dimensional finite element model simulates the real structural form, layout, centroid, moment of inertia and connection stiffness of the launch vehicle;

[0022] 2) Obtain the pneumatic fatigue load input from the superior, and use the minimum strain energy conversion algorithm for pneumatic fatigue load applicable to launch vehicles to obtain the minimum strain energy;

[0023] 3) Obtain the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model according to the minimum strain energy, and complete the assignment process;

[0024] 4) Determine whether the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model; if not, re-mesh and return to step 1) until the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements.

[0025] The advantages of the present invention compared with the prior art are as follows:

[0026] (1) In the load design of reusable launch vehicles, an innovative fatigue load assignment method based on a three-dimensional model is adopted. The fatigue load assignment position is more refined, providing a new design method for the load design of launch vehicles and more accurate input conditions for structural design.

[0027] (2) For the complex structure of reusable launch vehicles, a three-dimensional conversion assignment method of fatigue load with minimum strain energy is adopted, and the conversion deviation of the fatigue load is <0.5%. The fatigue load assignment accuracy is higher, providing a favorable basis for the weight reduction design of the launch vehicle structure.

[0028] (3) By adopting the method of circularly searching for the finite element element closest to the pneumatic point, the design iteration process can be accelerated and the calculation efficiency can be improved. Description of the Drawings

[0029] Figure 1 It is the general assembly drawing of the second stage of the launch vehicle.

[0030] Figure 2 It is the geometric model of the second stage of the launch vehicle.

[0031] Figure 3 It is the finite element mesh of the second stage of the launch vehicle.

[0032] Figure 4 It is the pneumatic mesh.

[0033] Figure 5 It is the schematic diagram of the fatigue load of the converted finite element nodes.

[0034] Figure 6 It is the flow chart of the method of the present invention. Detailed Embodiment

[0035] The fatigue load assignment system of the launch vehicle based on the three-dimensional model of the present invention includes: a three-dimensional modeling module of the launch vehicle, a pneumatic fatigue load conversion module, and a fatigue load post-processing module.

[0036] Three-dimensional modeling module of the launch vehicle: Establish a refined three-dimensional finite element model of the launch vehicle; the three-dimensional finite element model can simulate the structural form, layout, center of mass, moment of inertia, and connection stiffness of the launch vehicle.

[0037] Pneumatic fatigue load conversion module: Obtain the pneumatic fatigue load input from the superior. Since the nodes of the pneumatic calculation model are inconsistent with the nodes of the refined three-dimensional finite element model, use the pneumatic fatigue load minimum strain energy conversion algorithm applicable to the launch vehicle to obtain the minimum strain energy; according to the minimum strain energy, obtain the force and moment of the fatigue load relative to the center of mass in the three-dimensional finite element model to complete the assignment process;

[0038] The method for the pneumatic fatigue load conversion module to obtain the minimum strain energy U is specifically as follows:

[0039]

[0040]

[0041]

[0042]

[0043] Among them: U j is the strain energy of finite element node j; EJ is the flexural rigidity of the assumed beam; P j is the force of finite element node j; L j is the distance from finite element node j to the nearest pneumatic fatigue load node, that is, the assumed beam length; P A is the resultant force of the pneumatic fatigue model; x j , y j , z j are the three-direction coordinates of finite element node j; x A , y A , z A are the three-direction coordinates of the center of mass of the pneumatic fatigue load model; λ, λ x , λ y , λ z are the multipliers in the extreme value function established by the Lagrange multiplier method, and n is the number of nodes in the finite element model.

[0044] Fatigue load post - processing module: Determine whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the forces and moments of the fatigue load relative to the centroid in the three - dimensional finite element model. Perform rapid batch processing on the load calculation result data and achieve visual display and text output in the forms of graphs, charts, and contour maps.

[0045] The method for the fatigue load post - processing module to determine whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements is as follows:

[0046] 21) Calculate the resultant force F and resultant moment M of the aerodynamic fatigue load relative to the centroid of the launch vehicle.

[0047] 22) Calculate the resultant force and resultant moment of the finite - element load based on the centroid according to the forces and moments of the fatigue load relative to the centroid in the three - dimensional finite element model.

[0048] 23) Obtain the difference x1 between the resultant force of the aerodynamic fatigue load and the resultant force of the finite - element load, and obtain the difference x2 between the resultant moment of the aerodynamic fatigue load and the resultant moment of the finite - element load.

[0049] 24) If x1 < 0.5%F and x2 < 0.5%M, then determine that the aerodynamic fatigue load conversion algorithm meets the usage requirements; otherwise, determine that the aerodynamic fatigue load conversion algorithm does not meet the usage requirements.

[0050] As Figure 6 shown, a method for assigning fatigue loads to a launch vehicle based on a three - dimensional model includes the following steps:

[0051] 1) Establish a three - dimensional finite - element model of the launch vehicle and perform mesh division; mainly use shell elements, and try to divide the mesh into regular quadrilaterals; in step 1), the three - dimensional finite - element model simulates the real structural form, layout, centroid, moment of inertia, and connection stiffness of the launch vehicle.

[0052] 2) Obtain the aerodynamic fatigue load input from the superior, and use the minimum - strain - energy conversion algorithm for aerodynamic fatigue loads applicable to the launch vehicle to obtain the minimum strain energy.

[0053] 3) Obtain the forces and moments of the fatigue load relative to the centroid in the three - dimensional finite - element model according to the minimum strain energy, and complete the assignment process.

[0054] 4) Determine whether the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the forces and moments of the fatigue load relative to the centroid in the three - dimensional finite - element model; if not, re - divide the mesh and return to step 1) until the aerodynamic fatigue load conversion algorithm of the launch vehicle meets the usage requirements (in different repeated cycles, the mesh division is changed, that is, the three - direction coordinates of the three - dimensional finite - element nodes, and repeat steps 1) - 3)).

[0055] Example

[0056] (1) Launch vehicle 3D modeling module

[0057] The structure of a launch vehicle is generally composed of a non-sealed shell structure (tail section, transition section, box section, interstage section, instrument cabin, fairing, etc.) and valves, ducts, gas cylinders and instrument cables on the rocket. When establishing a three-dimensional finite element model of a launch vehicle, according to research needs, the instruments and cables on each section are reasonably simplified to form a suitable finite element unit, and the structural stiffness of the launch vehicle is reasonably simulated to facilitate multiple rounds of repeated iterative analysis of the fatigue load assignment of the launch vehicle. For some special parts of the launch vehicle, such as the connection part, when the design needs to focus on the structural strength of the special part, a suitable finite element model can be established as needed to provide a more accurate load distribution of the part, thereby providing a basis for the structural design of the special part. A three-dimensional model is established using commercial three-dimensional modeling software. After the simplified model, a launch vehicle substage consists of an interstage, an oxygen box, a box section, a fuel box, a rear transition section, an engine mounting bracket, a tail section, an engine and a frame. The general assembly drawing is shown in Figure 1.

[0058] Process the 3D model into an intermediate conversion format and import it into CAE to obtain Figure 2 The initial model shown.

[0059] The launch vehicle is mainly a thin-walled structure. Therefore, the shell unit is used to divide the mesh. The mesh is divided into regular quadrilaterals as much as possible. According to the characteristics of the launch vehicle sub-stage structure, the interstage, oxygen tank, fuel tank and other structures are divided and assembled. Figure 3 shown.

[0060] Parameters are set for each section, and the main parameters include equivalent density, equivalent thickness, elastic modulus, Poisson's ratio and shear modulus.

[0061] (2) Aerodynamic fatigue load conversion module

[0062] 1) Aerodynamic fatigue load calculation

[0063] According to the characteristic trajectory data, including flight time, altitude, speed, Mach number, dynamic pressure, and separation time, the working condition with the largest overload is selected for aerodynamic fatigue load calculation. The aerodynamic fatigue load is calculated by aerodynamic software, such as Figure 4 As shown, the aerodynamic fatigue load of the launch vehicle is obtained, including the aerodynamic forces and moment coefficients in three directions.

[0064] 2) Aerodynamic fatigue loads converted into structural fatigue loads

[0065] By developing a load conversion program, aerodynamic fatigue loads are equivalently converted into structural fatigue loads.

[0066] Input File

[0067] Input file 1: This file is used to input the calculation results of aerodynamic fatigue load. There are 6 columns of data, the first three columns are the Cartesian coordinates of the aerodynamic nodes, and the last three columns are the components of the aerodynamic nodes in the three directions of the Cartesian coordinates. The launch vehicle sub-stage area is divided into several parts such as the outer shell, tail baffle, and engine. The aerodynamic fatigue load data of these parts are made into input file 1 respectively as aerodynamic fatigue load input.

[0068] Input file 2: This file is used to input the unit and node information in the finite element model. The first part is the unit information, and the second part is the node information. According to the first-level area, it is divided into several parts such as the shell, tail baffle, and engine. The finite element model geometric data of these parts are made into input file 2 respectively as unit and node information input.

[0069] The node information output by the finite element software is in the large domain format. When this program reads the finite element model information, the large domain format of the node information needs to be converted into the small domain format, otherwise the program will report an error.

[0070] Input Parameters

[0071] The various control parameters required for the program to run are divided into two types: mandatory and non-mandatory. The mandatory parameters must be input by the user and have a greater impact on the results. The non-mandatory parameters can use the default values. The input parameters mainly include unit type, unit order, input file format, aerodynamic input fatigue load form and unit, structural output fatigue load form and unit, grid form, moment point coordinates, aerodynamic coordinate origin, structural coordinate origin, and coordinate axis correspondence.

[0072] Find the finite element structural node closest to the aerodynamic node

[0073] Input the fatigue load on the pneumatic node, find the finite element unit structure node closest to the pneumatic node, loop all the pneumatic points, loop the finite element structure unit under each pneumatic point, compare the distance between the pneumatic point and the center of each finite element structure unit, find the finite element unit closest to it (the center coordinates of the finite element unit are the average coordinates of each node on the unit), which is the finite element unit to which the pneumatic node belongs. Then translate the force on the pneumatic node to each node of the finite element unit to which it belongs. The method of cyclically finding the finite element unit closest to the pneumatic point can accelerate the design iteration process and improve the calculation efficiency.

[0074] Conversion Algorithm of Node Load Data between Aerodynamic Grid and Structural Finite Element Grid

[0075] After finding the finite element element closest to the pneumatic point for the pneumatic fatigue load, the pneumatic fatigue load is converted by the minimum strain energy algorithm. When the structure is stressed and deformed, strain energy or deformation energy is stored in the structure. If the loading process is static or quasi-static, the structure does not generate kinetic energy, and assuming that the internal friction in the structure can be ignored, and thus the temperature change of the structure caused by the load can be ignored. According to the law of conservation of energy, the work done by the external force during the deformation process of the structure will all be converted into deformation energy.

[0076] Assume there is an invisible beam between the finite element node and the pneumatic node. This beam is a cantilever beam with one end fixed at the pneumatic node, and the length of the cantilever beam is L j , then the finite element node at the free end of the cantilever beam is assigned a load P j When its deformation energy is:

[0077]

[0078] In the formula: EJ is the flexural rigidity of the imaginary beam.

[0079] The number of finite element nodes in the three-dimensional model of the launch vehicle is n, and the deformation energy is:

[0080]

[0081] According to the principle of minimum strain energy, the load assigned to the finite element node should minimize the deformation energy of the entire system, and at the same time should also meet the conditions of static equivalence (equal resultant force, equal resultant moment), that is, the following four equations should be satisfied:

[0082]

[0083]

[0084] In the formula: n is the number of nodes in the finite element model, n = 1, 2, 3...

[0085] Taking the four equations in formula (3) as the constraint conditions, the Lagrange multiplier method is used to establish the following extreme value function:

[0086]

[0087] In the formula: λ, λ x , λ y , λ z are multipliers.

[0088] According to the principle of minimum strain energy, the actual load loading condition should minimize the deformation energy of the entire system. In order to minimize the deformation energy of the entire system, that is, to make F(λ, λ x , λ y , λz ) Obtain the minimum value. This function takes the partial derivatives of P j equal to zero.

[0089] Let:

[0090]

[0091] We get:

[0092]

[0093] In this way

[0094]

[0095] Substitute Equation (3) into Equation (4) to get:

[0096]

[0097] Use the column pivoting Gauss elimination method to solve Equation (8) to obtain λ, λ x , λ y , λ z Then substitute into Equation (7) to obtain the nodal loads assigned to each finite element node within the influence domain. Execute the above calculation process for all finite element nodes, calculate the loads assigned to each finite element node within the influence domain of each aerodynamic node, and finally accumulate the loads assigned to the same node to obtain the load of that node.

[0098] The strain energy is transformed from the fatigue loads of the aerodynamic nodes to the loads of the finite element nodes for both force and moment through the minimum strain energy algorithm. Develop relevant numerical conversion software. Based on the principles of static equivalence and unchanged force transmission route, the total load, total center of pressure, and force transmission route remain unchanged, ensuring the true transmission of the loads, as Figure 5 shown. Requirements for the deviation before and after the conversion of the aerodynamic fatigue loads: resultant force < 0.5%, resultant moment < 0.5%. The accuracy of the fatigue load assignment is higher, providing a favorable basis for the weight reduction design of the launch vehicle structure.

[0099] 3) Fatigue load post-processing module

[0100] According to the traditional load output format, the loads for the design of large structural sections are usually provided in the form of sectional concentrated forces, while the load output after calculation using a three-dimensional finite element model is the finite element element forces, which need to be synthesized for structural design. After establishing a complete three-dimensional finite element model for analysis, it is planned to realize the post-processing of the load calculation results. Compile the corresponding result post-processing program to automatically extract the internal loads of the required components, synthesize the axial forces, shear forces, and bending moments at the required stations, and perform envelope analysis on the required working conditions; visualize and display the load calculation results in the form of graphs, charts, and contour maps and output them in text.

[0101] Adopt post-processing and visualization technologies to achieve rapid batch processing of the load calculation result data, and realize visual display and text output in the forms of graphs, charts, and contour maps. The main contents include the synthesis of station axis, bending, shear, and torsion loads, the extraction of internal loads, and the interface with structural strength.

[0102] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0103] The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A fatigue load assignment system for a launch vehicle based on a 3D model, characterized in that Including: A three-dimensional modeling module for launch vehicle, a pneumatic fatigue load conversion module, and a fatigue load post-processing module; The three-dimensional modeling module for launch vehicle: Establish a refined three-dimensional finite element model of the launch vehicle; The pneumatic fatigue load conversion module: Obtain the pneumatic fatigue load input from the superior, adopt the minimum strain energy conversion algorithm for pneumatic fatigue load applicable to the launch vehicle to obtain the minimum strain energy; According to the minimum strain energy, obtain the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model and complete the assignment process; The fatigue load post-processing module: Determine whether the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model.

2. The fatigue load assignment system for a launch vehicle based on a 3D model according to claim 1, characterized in that, The three-dimensional finite element model is used to simulate the structural form, layout, centroid, moment of inertia, and connection stiffness of the launch vehicle.

3. The fatigue load assignment system for a launch vehicle based on a three-dimensional model according to claim 2, characterized in that The method for the pneumatic fatigue load conversion module to obtain the minimum strain energy U is specifically as follows: Among them: U j is the strain energy of finite element node j; EJ is the flexural rigidity of the assumed beam; P j is the force of finite element node j; L j is the distance from finite element node j to the nearest aerodynamic fatigue load node; P A is the resultant force of the aerodynamic fatigue model; x j , y j , z j are the three-direction coordinates of finite element node j; x A , y A , z A are the three-direction coordinates of the centroid of the aerodynamic fatigue load model; λ, λ x , λ y , λ z are the multipliers in the extreme value function established by the Lagrange multiplier method, and n is the number of nodes in the finite element model.

4. The fatigue load assignment system for a launch vehicle based on a 3D model according to claim 2 or 3, characterized in that, The method for the fatigue load post-processing module to determine whether the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements is specifically as follows: 21) Calculate the resultant force F and resultant moment M of the pneumatic fatigue load relative to the centroid of the launch vehicle; 22) Calculate the resultant force and resultant moment of the finite element load based on the centroid according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model; 23) Obtain the difference x1 between the resultant force of the pneumatic fatigue load and the resultant force of the finite element load, and obtain the difference x2 between the resultant moment of the pneumatic fatigue load and the resultant moment of the finite element load; 24) If x1 < 0.5%F and x2 < 0.5%M; then it is determined that the pneumatic fatigue load conversion algorithm meets the usage requirements; otherwise, it is determined that the pneumatic fatigue load conversion algorithm does not meet the usage requirements.

5. A method for assigning fatigue loads of a launch vehicle based on a three-dimensional model, characterized in that Including the following steps: 1) Establish a three-dimensional finite element model of the launch vehicle and perform mesh division; 2) Obtain the pneumatic fatigue load input from the superior, adopt the minimum strain energy conversion algorithm for pneumatic fatigue load applicable to the launch vehicle to obtain the minimum strain energy; 3) According to the minimum strain energy, obtain the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model and complete the assignment process; 4) Determine whether the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model; If not, re-divide the mesh and return to step 1) until the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements (different repeated cycles change the mesh division, that is, the three-direction coordinates of the three-dimensional finite element nodes, and repeat steps 1) to 3)).

6. The method for assigning fatigue loads of a launch vehicle based on a 3D model according to claim 5, characterized in that In step 1), the three-dimensional finite element model simulates the real structural form, layout, centroid, moment of inertia, and connection stiffness of the launch vehicle.

7. The method for assigning fatigue loads of a launch vehicle based on a three-dimensional model according to claim 6, wherein The method for obtaining the minimum strain energy in step 2) is specifically as follows: Where: U j is the strain energy of finite element node j; EJ is the flexural rigidity of the assumed beam; P j is the force of finite element node j; L j is the distance from finite element node j to the nearest aerodynamic fatigue load node; P A is the resultant force of the aerodynamic fatigue model; x j , y j , z j are the three-direction coordinates of finite element node j; x A , y A , z A are the three-direction coordinates of the centroid of the aerodynamic fatigue load model; λ, λ x , λ y , λ z are the multipliers in the extreme value function established by the Lagrange multiplier method, and n is the number of nodes in the finite element model.

8. The method for assigning fatigue loads of a launch vehicle based on a 3D model according to claim 6, wherein The method for determining whether the pneumatic fatigue load conversion algorithm of the launch vehicle meets the usage requirements in step 4) is specifically as follows: 21) Calculate the resultant force F and resultant moment M of the pneumatic fatigue load relative to the centroid of the launch vehicle; 22) Calculate the resultant force and resultant moment of the finite element load based on the centroid according to the force and moment of the fatigue load relative to the centroid in the three-dimensional finite element model; 23) Obtain the difference x1 between the resultant force of the aerodynamic fatigue load and the resultant force of the finite element load, and obtain the difference x2 between the resultant moment of the aerodynamic fatigue load and the resultant moment of the finite element load; 24) If x1 < 0.5%F and x2 < 0.5%M, it is determined that the aerodynamic fatigue load conversion algorithm meets the usage requirements; otherwise, it is determined that the aerodynamic fatigue load conversion algorithm does not meet the usage requirements.

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