A reliability assessment method for the thrust chamber of a pump-type liquid rocket engine

Abstract

The present invention provides a reliability assessment method for the thrust chamber of a pump-type liquid rocket engine. The method identifies the main failure modes of the thrust chamber through a fault tree deduction method. For each main failure mode of the thrust chamber, a unit reliability assessment mathematical model is determined respectively, and the probability of occurrence of each failure mode is quantitatively assessed. The reliability of the thrust chamber is quantitatively assessed by making full use of the test data of the thrust chamber and its components. The method identifies the weak links of the thrust chamber through an importance ranking method, and provides support for the development and reliability improvement of the thrust chamber.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerospace quality and reliability, and in particular relates to a reliability assessment method for a thrust chamber of a pump-type liquid rocket engine. Background Art

[0002] Liquid rocket engines are the heart of launch vehicles. The thrust chamber, the engine's primary combustion device, determines the success or failure of a mission. The thrust chamber generally consists of an injector, a combustion chamber, and a nozzle. Liquid propellant enters the combustion chamber through the injector, undergoing atomization, mixing, and combustion. This produces high-temperature, high-pressure combustion gases, which convert the propellant's chemical energy into thermal energy. In the nozzle, this combustion gas converts this thermal energy into kinetic energy and is discharged at a high velocity from the nozzle's exit section, generating the required reaction force, or engine thrust.

[0003] The thrust chamber is a monolithic welded structure, subject to high operating pressures, temperatures, and heat fluxes. Furthermore, due to weight constraints, it has numerous welds and thin-walled structures. In recent years, thrust chamber failures have frequently led to engine test failures or launch failures. Therefore, the thrust chamber has been a key component in improving engine reliability and a core component in the quantitative reliability control of liquid rocket engines.

[0004] Reliability assessment is a crucial means of quantitatively verifying engine reliability. It's essential for achieving quantitative reliability indicators and completing closed-loop control during the engine development phase. Without a clear understanding of the actual level achieved, it's impossible to determine whether a product is qualified, usable, or in need of improvement. For aerospace products with high reliability requirements, "unmeasurability" leads to "uncontrollability."

[0005] Currently, for liquid rocket engines, there are clear reliability index requirements for the thrust chamber in the early stages of development. However, in the subsequent development process, the reliability of the engine as a whole is only evaluated through the test time of the entire engine, and no quantitative evaluation of the thrust chamber reliability is carried out. This has brought about a series of problems:

[0006] 1) The thrust chamber reliability requirements were not closed-loop controlled during the development process, and the thrust chamber reliability level was unknown;

[0007] 2) During the development process, the thrust chamber reliability level could not be monitored and could only be reflected during the test run in the later stages of development, which increased the development risk;

[0008] 3) After the thrust chamber failure, it was necessary to carry out pump design process improvements. However, after the reliability improvement, the reliability improvement effect could only be verified through the whole machine test run, which could not directly reflect the contribution of the improvement to the thrust chamber reliability.

[0009] 4) Currently, engine reliability assessment relies solely on whole-machine testing, which is extremely expensive and has a long planning cycle. Furthermore, a large amount of thrust chamber test data is not reflected in the engine reliability assessment, resulting in high test costs for verifying engine reliability.

[0010] During the actual development process, extensive verification tests were carried out on the thrust chamber and its components. From the perspective of the sufficiency and effectiveness of the test data, the thrust chamber has the conditions for reliability evaluation, but there is no reliability evaluation method for the thrust chamber yet. Summary of the Invention

[0011] In view of this, the purpose of the present invention is to provide a reliability assessment method for the thrust chamber of a pump-type liquid rocket engine, improve the reliability closed-loop control capability of the thrust chamber, and thus ensure the reliability level of the liquid rocket engine.

[0012] 1. A reliability assessment method for a pump-type liquid rocket engine thrust chamber, comprising the following steps:

[0013] Step 1: Analyze the thrust chamber reliability requirements, including the thrust chamber's operating conditions, operating hours, and functional performance requirements, thrust chamber failure criteria, and clarify the thrust chamber's reliability parameters and index requirements;

[0014] Step 2: Decompose the thrust chamber structure layer by layer until it is decomposed into thrust chamber components, determine the main failure modes of the components, and use the main failure modes as the bottom events of the fault tree to obtain a fault tree that represents the thrust chamber reliability assessment model, and obtain the logical relationship between each failure mode and the thrust chamber failure;

[0015] Step 3: Select appropriate unit reliability assessment models for each major failure mode based on the major failure mode types and their corresponding reliability characteristic quantities;

[0016] Step 4: Comprehensive assessment of thrust chamber reliability, specifically:

[0017] Step 41, determine the sampling number M;

[0018] Step 42: Generate N random numbers in the range of (0,1) [α 1j ,α 2j ,...,α ij ,...,α Nj ,], where α ij is the random number of the i-th major fault mode sampled at the j-th time, i=1,2,…,N; j=1,2,…,M;

[0019] Step 43: Calculate the unit reliability model of the i-th major failure mode at the quantile α ijReliability R i ;

[0020] Step 44: Calculate the failure probability F of each major failure mode ij :F ij =1-R ij ;

[0021] Step 45: Based on the logical relationship between each failure mode and thrust chamber failure obtained in step 2, the failure probability F of each main failure mode is used. ij Calculate the probability of thrust chamber failure in the jth sampling FS j ;

[0022] Step 46: Calculate the system reliability of the jth sampling: RS j =1-FS j ;

[0023] Step 47: Repeat steps 42-46 for M times to generate M system reliability simulation results [RS1, RS2, ..., RS M ];

[0024] Step 48: [RS1, RS2, ..., RS k ,...,RS M ] are sorted from small to large to obtain the reliability R under the set confidence level. L ;

[0025] Step 48: According to the reliability R L The reliability of the thrust chamber of a pump-type liquid rocket engine was evaluated.

[0026] Furthermore, the method further includes: calculating the relative probability importance of each failure mode based on the logical relationship expressed by the thrust chamber reliability model obtained in step 2 and combining the occurrence probability of each failure mode, and sorting the relative probability importance of each failure mode from large to small according to the importance, thereby identifying the weak links of the thrust chamber.

[0027] Preferably, in step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes:

[0028] For the failure mode of strength failure type, the selection principle is:

[0029] 1) When both the generalized strength and generalized stress of the reliability characteristic quantities have multiple sample measurement values, the stress intensity model is used as the unit reliability assessment model;

[0030] 2) When only one of the reliability characteristic quantities, generalized strength and generalized stress, has multiple sample measurement values, the normal distribution model is used as the unit reliability assessment model;

[0031] 3) When the test corresponding to the strength failure mode is a non-destructive test and the test values of all samples are the same, the reliability assessment method based on the reinforcement coefficient is used to determine the unit reliability assessment model.

[0032] Preferably, when using the enhancement coefficient method, the coefficient of variation is generally taken as 0.1-0.15.

[0033] Preferably, in step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes:

[0034] For the failure mode of fatigue failure type, the selection principle is:

[0035] 1) When the reliability characteristic quantity is the test time or the number of pulse operations, the Weibull distribution model is used as the unit reliability evaluation model;

[0036] 2) When the reliability characteristic quantity is the residual strength or cumulative damage degree corresponding to the mission time, the normal distribution model is used as the unit reliability assessment model;

[0037] 3) When the reliability characteristic quantity is the residual strength or cumulative damage degree after the test and the test time is not exactly the same, the reliability assessment method based on performance degradation data is used to determine the unit reliability assessment model.

[0038] Preferably, in step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes:

[0039] For the failure mode of ablation degradation type, the selection principle is:

[0040] 1) When the reliability characteristic quantity is the degradation quantity corresponding to the task time, the normal distribution model is used as the unit reliability evaluation model;

[0041] 2) When the reliability characteristic quantity is the degradation quantity corresponding to the test and the test time is not exactly the same, the normal distribution model is used as the unit reliability evaluation model.

[0042] Preferably, the confidence level is set to 0.7.

[0043] Preferably, the sampling number M requires:

[0044]

[0045] Where R is the reliability requirement value.

[0046] Preferably, the sampling number M is at least 1000 times.

[0047] The present invention has the following beneficial effects:

[0048] The present invention provides a reliability assessment method for the thrust chamber of a pump-type liquid rocket engine. The method identifies the main failure modes of the thrust chamber through a fault tree deduction method. For each main failure mode of the thrust chamber, a unit reliability assessment mathematical model is determined respectively, and the probability of occurrence of each failure mode is quantitatively assessed. The reliability of the thrust chamber is quantitatively assessed by making full use of the test data of the thrust chamber and its components. The method identifies the weak links of the thrust chamber through an importance ranking method, and provides support for the development and reliability improvement of the thrust chamber. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a flow chart for implementing reliability assessment of a liquid rocket engine thrust chamber according to the present invention;

[0050] Figure 2 This is a flowchart for implementing the comprehensive reliability evaluation of the thrust chamber of the present invention;

[0051] Figure 3 This is the fault tree model of the thrust chamber. DETAILED DESCRIPTION

[0052] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0053] In view of the structural characteristics of the thrust chamber of a liquid rocket engine and in combination with the test verification of the thrust chamber, the present invention proposes a reasonable and feasible thrust chamber reliability assessment method to support the reliability assessment of the thrust chamber and to be used for the closed-loop control of the thrust chamber reliability and the verification of the reliability growth effect. Figure 1 The specific implementation process is as follows:

[0054] Step 1: Thrust chamber reliability requirements analysis

[0055] The thrust chamber reliability requirement analysis mainly sorts out the thrust chamber's working conditions, working hours and functional performance requirements, lists the thrust chamber's failure criteria, and clarifies the thrust chamber's reliability parameters and index requirements.

[0056] Working conditions generally include the working temperature of the thrust chamber, working medium, etc.

[0057] The working time of the thrust chamber is generally consistent with the working time of the engine, that is, after the engine is started, the thrust chamber starts working, and when the engine is shut down, the thrust chamber stops working.

[0058] Functional performance requirements generally include thrust chamber pressure, injection pressure drop, cooling jacket pressure drop, combustion efficiency, etc.

[0059] The fault judgment criteria of the thrust chamber should be quantifiable parameters that can determine whether the thrust chamber is faulty. The fault judgment criteria should be able to fully reflect the functions and performance requirements of the thrust chamber and be detectable.

[0060] The reliability index requirements of the thrust chamber are generally obtained through reliability distribution of the engine, which is the mission reliability of the thrust chamber.

[0061] Step 2: Thrust chamber fault tree modeling and failure mode analysis

[0062] Because the thrust chamber's structure and functions are relatively complex and have multiple failure modes, a system reliability assessment model is needed to characterize the relationships between its various components and failure modes. This paper employs a fault tree modeling approach to construct a thrust chamber reliability model. This model aims to identify the thrust chamber's primary failure modes and to express the logical relationships between component failure modes and thrust chamber failures, providing model support for subsequent thrust chamber reliability assessments. Fault tree construction is implemented in accordance with GJB / Z768, the "Guidelines for Fault Tree Analysis."

[0063] Based on the thrust chamber failure criteria and the thrust chamber failure mode and effects analysis (FMEA) process, the authors used a fault tree model and deductive methods to decompose the thrust chamber structure layer by layer, down to its components. The primary failure modes of these components, representing relatively high-risk failure modes, were identified and used as the bottom events of the fault tree. This fault tree was then used as a reliability assessment model for the thrust chamber, and the logical relationships between each failure mode and the thrust chamber failure were expressed through logical relationship gates within the fault tree, such as OR gates, AND gates, and voting gates.

[0064] As an integral welded structural component, the thrust chamber has the following failure modes: strength failure, fatigue failure, ablation and degradation failure, etc. The thrust chambers of different engine models have different main failure modes due to the different media, structural design characteristics and loads they bear during operation. The main failure modes of the thrust chamber should be identified based on the characteristics of this model.

[0065] Step 3: Calculate the probability of occurrence of the main failure modes of the thrust chamber

[0066] (1) Determination of unit reliability assessment model for failure mode:

[0067] For each major failure mode, the unit reliability assessment model corresponding to the failure mode is determined to calculate the occurrence probability value of each major failure mode of the thrust chamber.

[0068] First, based on the main failure modes of the thrust chamber and combined with the test and measurement results of the engine, thrust chamber, and its components, the thrust chamber reliability characteristics are analyzed and determined. These are variables that can be measured during testing or flight and comprehensively reflect the thrust chamber reliability level. Subsequently, an appropriate unit reliability assessment model is selected based on the main failure modes and the determined reliability characteristics. The main unit reliability assessment models involved in the thrust chamber reliability assessment are listed in Appendix 1. Other unit reliability assessment models are referred to in Q / QJA 307, "Requirements for Reliability Assessment of Standalone Aerospace Products." The appropriate unit reliability assessment mathematical model can be selected based on the main failure modes of the thrust chamber and the data characteristics.

[0069] For the failure mode of strength failure type, the normal distribution reliability assessment method, stress intensity model reliability assessment method, and reliability assessment method based on strengthening coefficient can be selected. The selection principle is:

[0070] 1) When there are multiple sample measurements of the generalized strength and generalized stress of the reliability characteristic quantities corresponding to the strength-type failure mode, the stress-intensity model reliability assessment method is adopted;

[0071] 2) When only one of the generalized strength and generalized stress of the reliability characteristic quantities corresponding to the strength-type failure mode has multiple sample measurement values, the normal distribution reliability assessment method is adopted;

[0072] 3) When the test corresponding to the strength failure mode is a non-destructive test and the test values of all samples are the same, a reliability assessment method based on the enhancement coefficient is adopted. When using the enhancement coefficient method, if the coefficient of variation cannot be obtained by calculation, it can be determined through engineering experience. The coefficient of variation is generally 0.1-0.15. It is recommended that the enhancement coefficient of the thrust chamber of a mature engine be 0.1 and that of the thrust chamber of a newly developed engine be 0.15.

[0073] For fatigue failure type failure modes, the Weibull distribution reliability assessment method, the normal distribution reliability assessment method, and the reliability assessment method based on performance degradation data can be selected. The selection principles are as follows:

[0074] 1) When the reliability characteristic quantity corresponding to the fatigue failure mode is the test time or the number of pulse operations, the Weibull distribution reliability assessment method is used. When the number of failures is greater than or equal to 3, the shape parameter is calculated according to the method in Appendix A. When the number of failures is less than 3, the shape parameter is determined based on engineering experience. The general value range is [1,3] and it should not be less than the shape parameter determined by the reliability assessment of the entire engine.

[0075] 2) When the reliability characteristic quantity corresponding to the fatigue failure mode is the residual strength or cumulative damage degree corresponding to the mission time, the normal distribution reliability assessment method is adopted;

[0076] 3) When the reliability characteristic quantity corresponding to the fatigue failure mode is the residual strength or cumulative damage degree after the test and the test time is not exactly the same, the reliability assessment method based on performance degradation data is adopted.

[0077] For the failure mode of ablation degradation, reliability assessment methods based on performance degradation data and normal distribution reliability assessment methods can be selected. The selection principles are as follows:

[0078] 1) When the reliability characteristic quantity corresponding to the ablation degradation failure mode is the degradation quantity corresponding to the mission time, the normal distribution reliability evaluation method is adopted;

[0079] 2) When the reliability characteristic quantity corresponding to the ablation degradation failure mode is the corresponding degradation quantity after the test and the test time is not exactly the same, the normal distribution reliability evaluation method is used.

[0080] The typical failure modes, reliability characteristics and unit reliability evaluation model of the thrust chamber are shown in Table 1.

[0081] Table 1 Typical failure modes and reliability characteristics of thrust chamber

[0082]

[0083]

[0084] In addition to the above methods, the thrust chamber can also select other unit reliability assessment methods according to the specific model characteristics and actual failure mode types to quantify the probability of occurrence of failure modes.

[0085] For each bottom-level event in the fault tree model, if the probability of a failure cannot be quantified due to limitations in test data and testing methods, a qualitative analysis should be conducted on the bottom-level event to demonstrate that the uncertainty of the failure is within an acceptable range. Failure modes that are still not well understood should be specially marked and given special attention as undetectable and unmeasurable.

[0086] (2) Data collection, including:

[0087] a) Basic information collection items mainly include the name and code of the assessment object, reliability index requirements, composition and working principle, function and performance description, mission profile and fault judgment criteria;

[0088] b) Collect the failure mode of the thrust chamber;

[0089] c) Thrust chamber test data collection items include reliability characteristic quantities corresponding to failure modes;

[0090] d) A data collection card should be prepared and reviewed and approved by the relevant person in charge of the data providing unit.

[0091] The scope of data collection includes:

[0092] a) The information collected for thrust chamber reliability assessment includes test data from the development phase and data from the operational phase, and should be of the same technical status as the assessment object;

[0093] b) Data from thrust chamber component tests, thrust chamber tests, and engine run tests may be used. Generally, the number of test samples for reliability testing shall be no less than 5.

[0094] Step 4: Comprehensive evaluation of thrust chamber reliability

[0095] The unit reliability assessment mathematical models corresponding to each failure mode in the reliability assessment model are different. It is necessary to adopt a comprehensive system reliability assessment method. According to the logical relationship expressed by the reliability model, the probability information of each failure mode is integrated to calculate the reliability level of the thrust chamber.

[0096] According to the type of reliability assessment model and each unit reliability assessment mathematical model, the Monte Carlo simulation algorithm is used. According to the probability distribution of the failure mode of each component, a random sampling method is used to propagate the uncertainty of the thrust chamber in the reliability assessment model, and finally the probability distribution information of the thrust chamber reliability is obtained, and then the reliability level of the thrust chamber is determined. The input information of the model is the probability distribution of each bottom event (failure mode of each component) of the fault tree model. The Monte Carlo simulation method is used to obtain the probability of occurrence of the top event of the fault tree. The comprehensive assessment process of the thrust chamber reliability is as follows: Figure 2 As shown, the specific implementation process is:

[0097] Step 41: Determine the number of simulations M

[0098] Determine the number of simulations based on the system reliability index requirements. The number of simulations generally requires:

[0099]

[0100] Where M is the number of simulations and R is the reliability requirement.

[0101] For the convenience of subsequent statistics, the number of simulations is generally set to an integer multiple of 10. For example, if the system reliability index is required to be 0.99, the number of simulations should be at least 1000.

[0102] Step 42: Generate N random numbers in the range of (0,1)

[0103] Count the number of components N in the system and generate N random numbers (0,1) [α 1j ,α 2j ,...,α ij ,...,α Nj ,], where αij is the random number of the jth sampling of the i-th unit;

[0104] Step 43: Calculate the unit reliability model of each failure mode according to the unit reliability model of each failure mode. ij Percentile R i

[0105] For the unit reliability model corresponding to each failure mode of the thrust chamber, according to the meaning of the reliability confidence lower limit, N random numbers [α 1j ,α 2j ,...,α ij ,...,α Nj ,] as the quantile (confidence γ=1-α), and bring it into the mathematical model of unit reliability assessment to calculate the reliability of each unit (γ confidence) [R 1j ,R 2j ,...,R ij ,...,R Nj ], where R ij is the reliability of the jth sampling of the i-th unit.

[0106] Step 44: Calculate the failure probability, F ij =1-R ij

[0107] Step 45: Calculate the probability of thrust chamber failure in the jth sampling

[0108] For the thrust chamber fault tree model, [F 1j ,F 2j ,...,F ij ,...,F Nj ] is input, and the probability of occurrence of the top event of the fault tree is calculated according to the corresponding mathematical model of the system fault tree model, that is, the failure probability of the thrust chamber FS j ;

[0109] Step 46: Calculate the system reliability of the jth sampling: RS j =1-FS j ;

[0110] Step 47: Loop calculation

[0111] Repeat steps 42-46 for M times to generate M system reliability simulation results [RS1, RS2, ..., RS M ]. In order to improve computational efficiency, it is recommended to adopt stratified sampling method.

[0112] Step 48: Calculate system reliability

[0113] [RS1,RS2,...,RS k,...,RS M ] are sorted from small to large, and the obtained elements are [RS [1] ,RS [2] ,...,RS [k] ,...,RS [M] ], where RS [k] is the reliability of the system with sequence number k after sorting. According to the meaning of the lower confidence limit, the reliability under the confidence level of 0.7 is:

[0114] Pr(Rs>R L )=0.7

[0115] Then Pr(Rs≤R L )=0.3

[0116] Then the reliability of the system (0.7 confidence level) R L RS [0.3×M] .

[0117] Step 5: Identify weak links

[0118] Based on the logical relationship expressed by the thrust chamber reliability model and the probability of occurrence of each failure mode, the relative probability importance method is used to calculate the relative probability importance of each failure mode, and then sort them from high to low according to importance to identify the weak links of the thrust chamber. The calculation method of relative probability importance is:

[0119]

[0120] Where, Q(q1,q2,...,q n ) is the probability function of the top event of the fault tree in the thrust chamber reliability model, q i is the probability of occurrence of the i-th bottom event, and n is the total number of bottom events in the fault tree. The importance is sorted from large to small, and this is used as the risk ranking table for the main engine failure modes. The higher the ranking, the greater the risk.

[0121] Example:

[0122] 1. Analysis of thrust chamber reliability requirements

[0123] 1) Structure and main functions

[0124] The thrust chamber primarily consists of a head, a body, and external structural components (including the turbine exhaust inlet). The head and body are connected using argon arc welding. The thrust chamber's primary function is to atomize, mix, and combust the two propellants, generating high-temperature, high-pressure combustion gas that is accelerated and discharged through the nozzle (including the nozzle and nozzle extension) to generate thrust.

[0125] 2) Working conditions

[0126] Oxidant: Nitrogen tetroxide, Fuel: Unsymmetrical dimethylhydrazine. Temperature Range: Oxidant: ××°C; Fuel: ××°C; the absolute temperature difference between the two components shall not exceed ×°C. Operating Environment: Ignition Altitude: Approximately ×× km for the first ignition and approximately ×× km for the second ignition.

[0127] 3) Reliability index requirements

[0128] The cumulative working time of the thrust chamber is 500s, and the reliability requirement at the end of the mission is 0.97, evaluated at a confidence level of 0.7.

[0129] 4) Fault judgment criteria

[0130] If, under the specified conditions and within the specified operating time (or procedure) of the engine, the thrust chamber parts and components fail to perform the specified functions or their performance exceeds the allowable limit of any one of the design requirements, it can be determined that a thrust chamber failure has occurred.

[0131] 2. Reliability modeling and failure mode analysis

[0132] On the basis of the FMEA work of the thrust chamber, taking the thrust chamber failure as the top event, the fault tree modeling method is used to construct the thrust chamber fault tree model, identify and screen the main failure modes of the thrust chamber, and the fault tree model is as follows: Figure 3 shown.

[0133] 3. Calculation of the probability of occurrence of major failure modes

[0134] According to the main failure mode types of the thrust chamber, combined with the experiments and tests of the engine, thrust chamber and its components, the reliability characteristic quantities of the thrust chamber are analyzed and determined, and the unit reliability evaluation model is selected, as shown in Table 2.

[0135] Table 2 Main failure modes of thrust chamber and unit reliability evaluation model

[0136] Serial number Failure Mode Reliability characteristic Unit reliability assessment model 1 Damage to the head structure Starting pressure peak Reliability Assessment Method of Stress Intensity Model 2 Damage to body structure New product bursting pressure Reliability Assessment Method Based on Strengthening Factor 3 Laryngeal ablation Ablation amount Reliability Assessment Method Based on Performance Degradation Data 4 Plating peeling Coating bonding strength Normal distribution reliability assessment method 5 Nozzle ablation Antioxidant time Weibull distribution reliability assessment method

[0137] For the above main failure modes of thrust chamber, relevant test data are collected according to the reliability characteristic quantities, and the unit reliability evaluation model is used to calculate the occurrence probability value of each failure mode.

[0138] (1) Reliability assessment of head structure damage

[0139] 1) Data Source

[0140] The head structure strength data obtained through mechanical testing are shown in Table 3. The head starting pressure peak data measured through test runs are shown in Table 4.

[0141] Table 3 Head structure strength test results

[0142] Product Code 0001 0002 0003 0004 0005 Structural strength (MPa) 16 18 14 15 17

[0143] Table 4 Starting pressure peak measurement values

[0144] Product Code X001 X002 X003 X004 X005 Starting pressure peak (MPa) 10 11.5 11 10.5 10

[0145] 2) Unit reliability assessment

[0146] Based on the head structure strength data and starting pressure peak data, the stress-strength model reliability assessment method was used to carry out calculations. The reliability obtained was 0.99532 (0.7 confidence level), and the probability of head structure damage was calculated to be 0.00468 (0.7 confidence level).

[0147] (2) Reliability assessment of body structure damage

[0148] 1) Data Source

[0149] Three units were tested for 12MPa pressure on the thrust chamber body, and all suffered structural failure. The actual working pressure of the thrust chamber is 7MPa.

[0150] 2) Unit reliability assessment

[0151] Based on the thrust chamber pressure test data, the reliability assessment method based on the strengthening coefficient was used to carry out the calculation. According to engineering experience, the coefficient of variation was conservatively estimated to be 0.15, and the reliability was assessed to be 0.99879 (0.7 confidence level). The probability of body structure damage was then calculated to be 0.00121 (0.7 confidence level).

[0152] (3) Reliability evaluation of throat ablation

[0153] 1) Data Source

[0154] After the whole machine test run was completed, it was disassembled for inspection and the throat erosion was measured. The measurement data are shown in Table 5. According to engineering analysis, when the erosion is greater than 1.2 mm, the throat is faulty.

[0155] Table 5 Measurement values of throat ablation

[0156] Product Code C001 C002 C003 C004 C005 C006 C007 Test time (s) 1500 1000 1000 750 1000 1000 2000 Ablation amount (mm) 1 0.2 0.4 0.7 0 0 1.1

[0157] 2) Unit reliability assessment

[0158] According to the engine test time and throat ablation measurement data, the reliability evaluation method based on performance degradation data was used to calculate the reliability of 0.99963 (0.7 confidence level). The probability of body structure damage was calculated to be 3.7×10 -4 (0.7 confidence level).

[0159] (4) Reliability assessment of coating peeling

[0160] 1) Data Source

[0161] The bonding strength of the coating was measured, and the results are shown in Table 6. The required bonding strength of the coating is 80 MPa.

[0162] Table 6 Measurement values of coating bonding strength

[0163] Product Code K001 K002 K003 K004 K005 Coating bonding strength (MPa) 160 150 180 130 170

[0164] 2) Unit reliability assessment

[0165] Based on the engine test time and throat ablation measurement data, the normal distribution reliability assessment method was used to carry out calculations, and the reliability obtained was 0.99854 (0.7 confidence level), and the probability of body structure damage was calculated to be 0.00146 (0.7 confidence level).

[0166] (5) Reliability assessment of nozzle ablation

[0167] 1) Data Source

[0168] For this type of thrust chamber, the main cause of nozzle ablation is coating shedding. During the development process, in order to verify the coating life, 5 products were sampled and subjected to 2-hour antioxidant tests. The test results showed that none of the products failed.

[0169] 2) Unit reliability assessment

[0170] According to the product antioxidant test time, the measurement data was used to carry out calculations using the Weibull distribution reliability assessment method. Since the test was zero failure, the Weibull distribution shape parameter was determined to be 2 based on engineering experience. The reliability was evaluated to be 0.99884 (0.7 confidence level), and the probability of structural damage was calculated to be 0.00116 (0.7 confidence level).

[0171] (6) Summary

[0172] The occurrence probabilities of the main failure modes of the thrust chamber obtained through calculation are shown in Table 7.

[0173] Table 7 Statistics of the probability of occurrence of various failure modes of thrust chamber

[0174] Serial number Failure Mode Failure mode probability (0.7 confidence level) 1 Damage to the head structure 0.00468 2 Damage to body structure 0.00121 3 Laryngeal ablation <![CDATA[3.7×10 -4 ]]> 4 Plating peeling 0.00146 5 Nozzle ablation 0.00116

[0175] 4. Comprehensive reliability assessment

[0176] According to the fault tree model and the probability of occurrence of each bottom event, the Monte Carlo simulation method is used to calculate the thrust chamber reliability to be 0.976 (0.7 confidence level), which meets the reliability index requirement of 0.97.

[0177] 5. Importance Ranking

[0178] Based on the fault tree model and the probability of occurrence of each bottom event, the relative probability importance calculation method is used to calculate the relative probability importance of each bottom event and sort them in descending order. The sorting results are shown in Table 8. According to the sorting results, it is found that the risk importance of the first three failure modes is similar, and the risk of the last two is relatively small.

[0179] Table 8 Thrust chamber importance ranking

[0180] Serial number Failure Mode Relative probability importance 1 Damage to the head structure 0.3826 2 Nozzle ablation 0.3213 3 Damage to body structure 0.2718 4 Plating peeling 0.0120 5 Laryngeal ablation 0.0116

[0181] 6. Conclusion

[0182] The thrust chamber mission reliability was assessed at 0.976 (0.7 confidence level), meeting the required reliability index of 0.97. A ranking of importance revealed that the three failure modes—head structure damage, nozzle ablation, and side structure damage—had relatively high risks and warranted particular attention, while plating loss and throat ablation had relatively low risks.

[0183] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A reliability assessment method for a pump-type liquid rocket engine thrust chamber, characterized in that: The steps include: Step 1: Analyze the thrust chamber reliability requirements, including the thrust chamber's operating conditions, operating hours, and functional performance requirements, thrust chamber failure criteria, and clarify the thrust chamber's reliability parameters and index requirements; Step 2: Decompose the thrust chamber structure layer by layer until it is decomposed into thrust chamber components, determine the main failure modes of the components, and use the main failure modes as the bottom events of the fault tree to obtain a fault tree that represents the thrust chamber reliability assessment model, and obtain the logical relationship between each failure mode and the thrust chamber failure; Step 3: Select appropriate unit reliability assessment models for each major failure mode based on the major failure mode types and their corresponding reliability characteristic quantities; Step 4: Comprehensive assessment of thrust chamber reliability, specifically: Step 41, determine the sampling number M; Step 42: Generate N random numbers in the range of (0,1) [α 1j ,α 2j ,...,α ij ,...,α Nj ,], where α ij is the random number of the i-th major fault mode sampled at the j-th time, i=1,2,…,N; j=1,2,…,M; Step 43: Calculate the unit reliability model of the i-th major failure mode at the quantile α ij Reliability R i ; Step 44: Calculate the failure probability F of each major failure mode ij :F ij =1-R ij ; where R ij is the reliability of the jth sampling of the unit with the i-th failure mode; Step 45: Based on the logical relationship between each failure mode and thrust chamber failure obtained in step 2, the failure probability F of each main failure mode is used. ij Calculate the probability of thrust chamber failure in the jth sampling FS j ; Step 46: Calculate the system reliability of the jth sampling: RS j =1-FS j ; Step 47: Repeat steps 42-46 for M times to generate M system reliability simulation results [RS1, RS2, ..., RS M ]; Step 48: [RS1, RS2, ..., RS k ,...,RS M ] are sorted from small to large to obtain the reliability R under the set confidence level. L ; Step 48: According to the reliability R L The reliability of the thrust chamber of a pump-type liquid rocket engine was evaluated.

2. The reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1, wherein: Also includes: According to the logical relationship expressed by the thrust chamber reliability model obtained in step 2, combined with the occurrence probability of each failure mode, the relative probability importance method is used to calculate the relative probability importance of each failure mode, and then sort them from large to small according to importance to identify the weak links of the thrust chamber.

3. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: In step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes: For the failure mode of strength failure type, the selection principle is: 1) When both the generalized strength and generalized stress of the reliability characteristic quantities have multiple sample measurement values, the stress intensity model is used as the unit reliability assessment model; 2) When only one of the reliability characteristic quantities, generalized strength and generalized stress, has multiple sample measurement values, the normal distribution model is used as the unit reliability assessment model; 3) When the test corresponding to the strength failure mode is a non-destructive test and the test values of all samples are the same, the reliability assessment method based on the reinforcement coefficient is used to determine the unit reliability assessment model.

4. The reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 3, wherein: When using the enhancement coefficient method, the coefficient of variation is taken as 0.1-0.

15.

5. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: In step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes: For the failure mode of fatigue failure type, the selection principle is: 1) When the reliability characteristic quantity is the test time or the number of pulse operations, the Weibull distribution model is used as the unit reliability evaluation model; 2) When the reliability characteristic quantity is the residual strength or cumulative damage degree corresponding to the mission time, the normal distribution model is used as the unit reliability assessment model; 3) When the reliability characteristic quantity is the residual strength or cumulative damage degree after the test and the test time is not exactly the same, the reliability assessment method based on performance degradation data is used to determine the unit reliability assessment model.

6. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: In step 3, the method of selecting a suitable unit reliability assessment model for each major failure mode includes: For the failure mode of ablation degradation type, the selection principle is: 1) When the reliability characteristic quantity is the degradation quantity corresponding to the task time, the normal distribution model is used as the unit reliability evaluation model; 2) When the reliability characteristic quantity is the degradation quantity corresponding to the test and the test time is not exactly the same, the normal distribution model is used as the unit reliability evaluation model.

7. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: The confidence level is set to 0.

7.

8. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: The sampling times M require: Where R is the reliability requirement value.

9. A reliability assessment method for a pump-type liquid rocket engine thrust chamber according to claim 1 or 2, characterized in that: The sampling number M is at least 1000 times.

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