Slender body subcritical thrust test device, critical thrust extrapolation method and system

Abstract

The present invention provides a slender body subcritical thrust test device and a critical thrust extrapolation method and system, comprising a test bench, a slender aircraft simulator, and a suspension support device. The test bench comprises a test bench base, a front bearing wall, and a load cell. The slender aircraft simulator comprises an aluminum alloy slender body and a solid rocket motor. The suspension support device comprises a rubber rope, a ball bearing, and a support device. This invention offers a low-risk test process and highly reliable test results, effectively guiding engineering design.

Description

Technical Field

[0001] The present invention relates to the field of solid rocket engine test and structural dynamics technology, and in particular to a slender subcritical thrust test device and a critical thrust extrapolation method and system. Background Art

[0002] Due to the continuous improvement of aircraft's technical and combat indicators such as high acceleration and long range, modern aircraft widely use lightweight materials and adopt large thin-walled structural designs. The aerodynamic layout is generally designed as a slender lifting body layout, which makes the lateral bending characteristics of the slender aircraft structure more and more significant. At this time, the thrust direction of the aircraft's engine will continuously change with the bending vibration of the slender body, forming a follower force. In addition, the widespread application of new energy management technologies such as variable thrust and dual pulse in solid rocket engines causes the output thrust amplitude of the engine to vary significantly during flight. The above two points make the vibration of slender aircraft and their aeroelastic behavior under the action of follower thrust more prominent and complex, posing a safety hazard to flight missions.

[0003] The mechanism of follower forces in engineering is currently controversial. Whether thrust is considered a follower load can lead to different conclusions in the analysis of structural vibration characteristics and dynamic stability, making simulation analysis results difficult to effectively guide engineering design. Therefore, it is necessary to verify and study the impact of follower forces on slender aircraft in engineering applications through ground test runs.

[0004] Patent document CN113447224A (application number: CN202110795429.4) discloses a vibration test and stability boundary prediction method and system under the action of follower thrust, which relates to the field of structural dynamics and structural vibration test technology. The method includes: designing a scaled model test piece of the front body slender body structure, installing an engine on one side of the front body slender body structure, and analyzing the dynamic characteristics and stability of the system; under the action of follower thrust, one end of the engine is fixed to the scaled model test piece of the front body slender body structure, and the other end is in an open state, and a point contact support device is placed under the engine; a scaled model test of the front body slender body structure is carried out; the obtained test data is analyzed and the critical pressure of the front body slender body structure test system is predicted.

[0005] In current conventional ground test runs of solid rocket engines, the solid rocket engine is fixedly mounted on a test bench with all six degrees of freedom fully constrained. The thrust can only act on the engine tail along a fixed axis, and cannot simulate the working condition of thrust as a follower load. At the same time, due to the high risk inherent in solid rocket engine tests, there are relatively few research institutions and scholars qualified to conduct solid rocket engine tests, and there is also relatively little related research. Currently, no patents have been found for technologies similar to the present invention. In the currently available publicly reported literature, foreign scholars conducted test runs on the solid rocket engine thrust acting on the tangent of the free end of the cantilever beam. By adjusting the engine thrust amplitude and the length of the cantilever beam, they conducted multiple tests, and determined whether the critical thrust value had been reached based on whether the cantilever beam amplitude diverged in each test. However, the economic costs of processing, assembling, and testing solid rocket engines, as well as the safety and risk management costs of test sites, resulting from repeated testing, are extremely high. Furthermore, due to overshoot and oscillation in engine thrust during test runs, ideal critical or near-critical thrust test conditions can easily approach supercritical thrust test conditions. Under supercritical thrust test conditions, the slender test object enters a divergent vibration state. Once the slender body structure breaks, the unconstrained solid rocket engine will fly out at high speed, which is extremely dangerous and will cause serious damage to the entire test system. In summary, how to design a safer and more controllable subcritical test, obtain effective system subcritical vibration response data through a smaller number of tests, and extrapolate and predict the system's instability critical thrust from this data, is an urgent problem that needs to be solved in the current ground test of slender aircraft under the action of follower thrust. Summary of the Invention

[0006] In view of the defects in the prior art, the purpose of the present invention is to provide a slender body subcritical thrust test device and a critical thrust extrapolation method and system.

[0007] The slender body subcritical thrust test device provided by the present invention comprises a test bench, a slender aircraft simulation part and a suspension support device;

[0008] The test bench includes a test bench base, a front bearing wall and a force sensor;

[0009] The slender aircraft simulation part includes an aluminum alloy slender body and a solid rocket motor;

[0010] The suspension support device includes a rubber rope, a ball and a support device;

[0011] One end of the aluminum alloy slender body is connected to a force sensor and fixed to the front bearing wall of the test bench, and the other end of the aluminum alloy slender body is connected to the solid rocket engine through a threaded adapter to form a cantilever structure;

[0012] The supporting device is fixed on the test bench base;

[0013] The balls are used to clamp onto the outer surface of the solid rocket motor to achieve point contact between the solid rocket motor and the top surface of the support device;

[0014] The rubber rope is used to stretch and lift the solid rocket engine upward until the ball is about to leave the top surface of the support device, so as to minimize the friction between the ball and the top surface of the support device.

[0015] The critical thrust extrapolation method for a slender body subcritical thrust test provided by the present invention comprises the following steps:

[0016] Step S1: In a horizontal plane, the free end of the slender aircraft simulation component is moved away from the equilibrium position to form an initial bending deformation, and is then fastened to the test bench with a polyethylene wire;

[0017] Step S2: bundling a black powder bag on a polyethylene wire to burn the thin wire before igniting the solid rocket motor, so that the slender aircraft simulation part forms a free decay vibration under the initial displacement disturbance in the horizontal plane;

[0018] Step S3: igniting the solid rocket motor during the free decay vibration of the slender aircraft simulation component;

[0019] Step S4: measuring the horizontal lateral vibration velocity response of the aluminum alloy slender body using a laser Doppler vibrometer, and measuring the thrust in the combustion chamber of the solid rocket motor using a force sensor;

[0020] Step S5: Changing the thrust by modifying the nozzle throat diameter of the solid rocket motor to obtain solid rocket motors with different thrusts, and repeating steps S1 to S4 to conduct multiple subcritical thrust tests;

[0021] Step S6: Analyze the variation of the modal frequency of the slender aircraft simulation component with thrust from the vibration velocity response of multiple subcritical thrust tests, and further establish a polynomial extrapolation of the critical thrust by combining the relationship between the first two modal frequency differences and thrust.

[0022] Preferably, in step S1, the angle between the initial bending deformation of the slender aircraft simulation piece and the angle before the deformation is less than 10°.

[0023] Preferably, in step S3, the time interval between the ignition time of the black powder bag on the polyethylene line and the ignition time of the solid rocket engine is controlled so that the solid rocket engine is ignited just when the slender aircraft simulation moves to the equilibrium position.

[0024] Preferably, in step S4, the laser Doppler vibrometers are evenly distributed along the axial direction of the aluminum alloy slender body, ensuring that the measurement points of the plurality of laser Doppler vibrometers are not simultaneously located at the vibration mode nodal lines of the slender aircraft simulation component.

[0025] Preferably, in step S6, the polynomial form of the relationship between the first two modal frequency differences and thrust is as follows:

[0026] (Δω 2 ) 2 =A0T 2 +A1T+A2

[0027] Where Δω is the frequency difference between the first two modes; T is the thrust; A0, A1, and A2 are the coefficients to be determined. When the solid rocket motor is not ignited, a no-thrust test with T = 0 is carried out to obtain A2. At least two additional subcritical thrust tests are carried out to obtain A0 and A1 through fitting.

[0028] The critical thrust extrapolation system for the subcritical thrust test of a slender body provided by the present invention includes the following modules:

[0029] Module M1: Move the free end of the slender aircraft simulation away from the equilibrium position in the horizontal plane to form an initial bending deformation, and then tie it to the test bench with polyethylene wire to fix it;

[0030] Module M2: Black powder packages are bundled on polyethylene wires to burn the thin wires before the solid rocket motor is ignited, so that the slender aircraft simulation forms a free decay vibration under the initial displacement disturbance in the horizontal plane;

[0031] Module M3: Ignition of a solid rocket motor during the free-degradation vibration of a slender aircraft simulator;

[0032] Module M4: Measure the horizontal transverse vibration velocity response of an aluminum alloy slender body using a laser Doppler vibrometer and measure the thrust in the combustion chamber of a solid rocket motor using a force sensor;

[0033] Module M5: Change the thrust by modifying the nozzle throat diameter of the solid rocket motor to obtain solid rocket motors with different thrusts. Repeatedly call modules M1 to M4 to conduct multiple subcritical thrust tests.

[0034] Module M6: Analyze the variation of the modal frequency of the slender aircraft simulation parts with thrust from the vibration velocity response of multiple subcritical thrust tests, and further establish a polynomial extrapolation of the critical thrust by combining the relationship between the first two modal frequency differences and thrust.

[0035] Preferably, in the module M1, the angle between the initial bending deformation of the slender aircraft simulation piece and the angle before deformation is less than 10°.

[0036] Preferably, in the module M3, the time interval between the ignition time of the black powder bag on the polyethylene wire and the ignition time of the solid rocket engine is controlled so that the solid rocket engine is ignited just when the slender aircraft simulation moves to the equilibrium position;

[0037] In the module M4, the laser Doppler vibrometers are evenly distributed along the axial direction of the aluminum alloy slender body, ensuring that the measurement points of the multiple laser Doppler vibrometers are not simultaneously located at the vibration mode nodal lines of the slender aircraft simulation component.

[0038] Preferably, in the module M6, the polynomial form of the relationship between the first two modal frequency differences and thrust is as follows:

[0039] (Δω 2 ) 2 =A0T 2 +A1T+A2

[0040] Where Δω is the frequency difference between the first two modes; T is the thrust; A0, A1, and A2 are the coefficients to be determined. When the solid rocket motor is not ignited, a no-thrust test with T = 0 is carried out to obtain A2. At least two additional subcritical thrust tests are carried out to obtain A0 and A1 through fitting.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] (1) In addition to the no-thrust test, the present invention only requires at least two additional engine test runs to analyze the variation of the slender body modal frequency with thrust. The critical thrust can be extrapolated using the thrust polynomial, which is the difference between the first two modal frequencies. This can significantly reduce the costs of solid rocket engine processing, assembly, test runs, and test site safety protection and risk management caused by multiple tests.

[0043] (2) The present invention provides a quantitative theoretical formula for the relationship between the frequency difference of the first two modal frequencies of a slender body and thrust, which provides a theoretical basis and reference for the selection of the subcritical thrust size under test conditions, thereby improving the accuracy of the extrapolation method. The test conclusions are highly reliable and can effectively guide engineering design. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0045] Figure 1 This is a structural diagram of a slender body subcritical thrust test system under follow-up thrust according to an embodiment of the present application;

[0046] Figure 2 This is a top view of the installation of a laser Doppler vibrometer for a subcritical thrust test system of a slender body under follow-up thrust according to an embodiment of the present application;

[0047] Figure 3 This is a flow chart of a critical thrust extrapolation method for a subcritical thrust test of a slender body under follower thrust according to an embodiment of the present application;

[0048] Figure 4 This is a schematic diagram of a fitting curve of the thrust polynomial of the first two modal frequency differences of a slender aircraft simulation part in accordance with a critical thrust extrapolation method for a subcritical thrust test of a slender body under follower thrust in an embodiment of the present application;

[0049] in:

[0050] 1-Test bench base;

[0051] 2-front bearing wall;

[0052] 3-Force sensor;

[0053] 4-aluminum alloy slender body;

[0054] 5-Solid rocket motor;

[0055] 6-Rubber cord;

[0056] 7-ball;

[0057] 8-support device;

[0058] 9-Laser Doppler vibrometer. DETAILED DESCRIPTION

[0059] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0060] Example:

[0061] Reference Figure 1 This embodiment discloses a slender body subcritical thrust test system under follow-up thrust, which uses the following equipment:

[0062] Test benches, slender aircraft simulators and suspension supports.

[0063] The test bench includes a test bench base 1, a front bearing wall 2 and a force sensor 3;

[0064] The slender aircraft simulator consists of a 1m long aluminum alloy slender body 4 fixedly connected to the head of a solid rocket engine 5; one end of the aluminum alloy slender body 4 is connected to the force sensor 3 and fixed to the front bearing wall 2 of the test bench, and the other end is connected to the solid rocket engine 5 through a threaded adapter to form a cantilever structure;

[0065] The suspension support device includes a rubber rope 6, a ball 7, and a support device 8; the support device 8 is fixed to the test bench base 1; the ball 7 is used to clamp the outer surface of the solid rocket engine to achieve point contact between the solid rocket engine 5 and the top surface of the support device 8; the rubber rope 6 is used to suspend the solid rocket engine 5 upward until the ball 7 is about to separate from the top surface of the support device 8, so as to minimize the friction between the ball 7 and the top surface of the support device 8;

[0066] Thus, a test system is built with axial freedom constraint and partial release of horizontal and vertical lateral freedom.

[0067] The present invention also provides a critical thrust extrapolation method for a subcritical thrust test run of a slender body under follow-up thrust, wherein the method uses one of the above-mentioned subcritical thrust test run test systems for a slender body under follow-up thrust, such as Figure 3 , the method comprises the following steps:

[0068] Step S1: In a horizontal plane, the free end of the slender aircraft simulation component is moved away from the equilibrium position to form an initial bending deformation, and is then fastened to the test bench with a polyethylene wire;

[0069] Step S2: bundling a black powder bag on a polyethylene wire to burn the thin wire before igniting the solid rocket motor 5, so that the slender aircraft simulation part forms a free decay vibration under the initial displacement disturbance in the horizontal plane;

[0070] Step S3: During the free decay vibration of the slender aircraft simulation component, the solid rocket motor 5 is ignited;

[0071] Step S4: Reference Figure 2 , using a laser Doppler vibrometer 9 to measure the horizontal lateral vibration velocity response on the aluminum alloy slender body 4, and using a force sensor 3 to measure the thrust in the combustion chamber of the solid rocket engine 5;

[0072] Step S5: Changing the thrust by modifying the nozzle throat diameter of the solid rocket motor 5 to obtain solid rocket motors 5 with different thrusts, and repeating the above steps S1 to S4 to carry out several groups of subcritical thrust tests;

[0073] Step S6: Analyze the variation of the modal frequency of the slender aircraft simulation component with thrust from the vibration velocity response of several subcritical tests, and further establish a polynomial extrapolation of the critical thrust by combining the relationship between the first two modal frequency differences and thrust;

[0074] In step S1, the angle between the initial bending deformation of the slender aircraft simulation piece and the angle before the deformation is about 5°;

[0075] In step S3, the time interval between the ignition time of the black powder bag on the polyethylene wire and the ignition time of the solid rocket motor is set to 250ms, so that the solid rocket motor is ignited just when the slender aircraft simulation moves to the equilibrium position.

[0076] In step S4, four laser Doppler vibrometers are distributed inwardly along the free end of the aluminum alloy slender body, with an interval of 0.2 m between adjacent laser Doppler vibrometers, and no measuring point is set at the root where the amplitude is smaller.

[0077] In step S6, the vibration equation of the slender aircraft simulation member with the follower thrust acting on the free end is:

[0078]

[0079] Where m, c, and k are the structural mass, damping, and stiffness matrices of the slender aircraft simulation component, respectively; w = {w1, w2, …, w e} T is the lateral displacement of the bending deformation; T is the thrust; for the case of small elastic deformation, the thrust component along the axial direction of the slender structure k nc is the thrust component in the lateral direction The non-conservative forces induce additional stiffness matrix.

[0080] Take the normalized vibration mode Φ of the mass of the slender aircraft simulation component in the no-thrust state and separate the variables of the bending deformation lateral displacement w:

[0081] w(x,t)=Φ(x)q(t)…………(2)

[0082] Substitute equation (2) into equation (1) and multiply it by Φ on the left. T have to:

[0083]

[0084] Where q is the generalized coordinate; C is the generalized damping matrix; is a diagonal matrix composed of the squares of the first n-order natural circular frequencies of the slender aircraft simulation under no-thrust state; K c =Φ T k c Φ is the conservative force additional generalized stiffness matrix caused by the thrust along the axial component, K c Is a diagonal matrix, whose diagonal elements K c (i,i) represents the degree of weakening of the i-th order lateral bending modal stiffness by the thrust; K nc =Φ T k nc Φ is the additional generalized stiffness matrix of non-conservative force caused by the thrust component along the lateral direction of the slender structure, K nc is a non-diagonal matrix, and its non-diagonal elements Knc (i,j), i≠j represents the stiffness coupling between the i-th and j-th order modes caused by thrust.

[0085] Ignoring the system damping, the vibration equation of the undamped system corresponding to formula (3) is:

[0086]

[0087] The generalized eigenvalue of this equation is the first n-order natural circular frequencies ω1, ω2,…, ω of the slender aircraft simulation under the action of follower thrust. n .

[0088] As the thrust increases, the first two frequencies of the slender aircraft simulation under the action of follower thrust will gradually approach each other. When the thrust increases to the critical thrust, the difference between the first two frequencies will drop to 0. According to this rule, only the first two modes are retained, and q = {q1, q2} T Perform modal truncation and Substituting into formula (4), rewriting it into matrix form:

[0089]

[0090] Where a1 = K c (1,1), a2=K c (2,2), b1=K nc (1,2), b2=K nc (2,1). If this matrix is established, it must satisfy:

[0091]

[0092] The frequency equation is thus:

[0093]

[0094] Solve equation (7) for ω 2 The quadratic equation of one variable is obtained by the square difference of the first two order frequencies of the system (Δω 2 ) 2 :

[0095]

[0096] Simplifying equation (8), the square difference of the first two order circular frequencies of the system and the thrust can be expressed as a quadratic polynomial relationship:

[0097] (Δω 2 ) 2 =A0T 2 +A1T+A2…………(9)

[0098] Where:

[0099] A0=(a1-a2) 2 +4b1b2

[0100]

[0101]

[0102] When T=0, the first two circular frequencies of the slender aircraft simulation under the action of follower thrust are equal to the first two natural circular frequencies of the system under no thrust state. This indicates that A2 can be obtained from subcritical tests without thrust. Furthermore, only two additional test runs at different subcritical thrusts are needed to obtain two additional sets of data, from which A0 and A1 can be calculated. The critical thrust corresponding to a modal frequency difference of zero can then be extrapolated using the thrust polynomial representing the difference between the first two modal frequencies.

[0103] refer to Figure 4 In this embodiment, in addition to the no-thrust test, three groups of subcritical thrust tests were carried out. The thrust corresponding to the modal frequency difference of 0 in the first two-order modal frequency difference-thrust polynomial fitting curve is the critical thrust.

[0104] In the description of this application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0105] Those skilled in the art will appreciate that, in addition to implementing the system, device, and various modules provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, embedded microcontrollers, and the like by logically programming the method steps. Therefore, the system, device, and various modules provided by the present invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; the modules for implementing various functions can also be considered both software programs for implementing the method and structures within the hardware component.

[0106] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A slender subcritical thrust test device, characterized in that: Includes test bench, slender aircraft simulator and suspension support device; The test bench comprises a test bench base (1), a front bearing wall (2) and a force sensor (3); The slender aircraft simulation part comprises an aluminum alloy slender body (4) and a solid rocket engine (5); The suspension support device comprises a rubber rope (6), a ball (7) and a support device (8); One end of the aluminum alloy slender body (4) is connected to a force sensor (3) and fixed to the front bearing wall (2) of the test bench, and the other end of the aluminum alloy slender body (4) is connected to a solid rocket engine (5) through a threaded connection to form a cantilever structure; The supporting device (8) is fixed on the test bench base (1); The ball (7) is used to clamp onto the outer surface of the solid rocket engine (5) to achieve point contact between the solid rocket engine (5) and the top surface of the support device (8); The rubber rope (6) is used to stretch and lift the solid rocket engine (5) upward until the ball (7) is about to leave the top surface of the support device (8), so as to minimize the friction between the ball (7) and the top surface of the support device (8).

2. A critical thrust extrapolation method for a slender body subcritical thrust test, characterized in that: The slender body subcritical thrust test device according to claim 1 includes the following steps: Step S1: In a horizontal plane, the free end of the slender aircraft simulation component is moved away from the equilibrium position to form an initial bending deformation, and is then fastened to the test bench with a polyethylene wire; Step S2: bundling a black powder bag on a polyethylene wire for burning the thin wire before igniting the solid rocket motor (5), so that the slender aircraft simulation part forms a free attenuation vibration under the initial displacement disturbance in the horizontal plane; Step S3: igniting the solid rocket motor (5) during the free attenuation vibration of the slender aircraft simulation component; Step S4: using a laser Doppler vibrometer to measure the horizontal lateral vibration velocity response on the aluminum alloy slender body (4), and using a force sensor (3) to measure the thrust in the combustion chamber of the solid rocket engine (5); Step S5: changing the thrust magnitude by modifying the nozzle throat diameter of the solid rocket motor (5) to obtain solid rocket motors (5) with different thrusts, and repeating steps S1 to S4 to conduct multiple subcritical thrust tests; Step S6: Analyze the variation of the modal frequency of the slender aircraft simulation component with thrust from the vibration velocity response of multiple subcritical thrust tests, and further establish a polynomial extrapolation of the critical thrust by combining the relationship between the first two modal frequency differences and thrust.

3. The critical thrust extrapolation method for the subcritical thrust test of a slender body according to claim 2, characterized in that: In step S1, the angle between the initial bending deformation of the slender aircraft simulation piece and the angle before the deformation is less than 10°.

4. The critical thrust extrapolation method for a slender body subcritical thrust test according to claim 2, characterized in that: In step S3, the time interval between the ignition time of the black powder bag on the polyethylene line and the ignition time of the solid rocket engine (5) is controlled so that the solid rocket engine (5) is ignited just when the slender aircraft simulation part moves to the equilibrium position.

5. The critical thrust extrapolation method for the subcritical thrust test of a slender body according to claim 2, characterized in that: In the step S4, the laser Doppler vibrometers are evenly distributed along the axial direction of the aluminum alloy slender body (4), ensuring that the measurement points of the plurality of laser Doppler vibrometers are not simultaneously located at the vibration mode nodal lines of the slender aircraft simulation part.

6. The critical thrust extrapolation method for a slender body subcritical thrust test according to claim 2, characterized in that: In step S6, the polynomial form of the relationship between the first two modal frequency differences and thrust is as follows: (Δω 2 ) 2 =A0T 2 +A1T+A2 Where Δω is the frequency difference between the first two modal modes; T is the thrust; A0, A1, and A2 are the coefficients to be determined. When the solid rocket engine (5) is not ignited, a no-thrust test with T=0 is carried out to obtain A2. At least two additional subcritical thrust tests are carried out to obtain A0 and A1 through fitting.

7. A critical thrust extrapolation system for subcritical thrust test of a slender body, characterized in that: The slender body subcritical thrust test device according to claim 1 includes the following modules: Module M1: Move the free end of the slender aircraft simulation away from the equilibrium position in the horizontal plane to form an initial bending deformation, and then tie it to the test bench with polyethylene wire to fix it; Module M2: Black powder packages are bundled on polyethylene wires to burn the thin wires before ignition of the solid rocket motor (5), so that the slender aircraft simulation part forms a free decay vibration under the initial displacement disturbance in the horizontal plane; Module M3: igniting the solid rocket motor (5) during the free decay vibration of the slender aircraft simulation; Module M4: Using a laser Doppler vibrometer to measure the horizontal transverse vibration velocity response on the aluminum alloy slender body (4), and using a force sensor (3) to measure the thrust in the combustion chamber of the solid rocket motor (5); Module M5: changing the thrust by modifying the nozzle throat diameter of the solid rocket engine (5) to obtain solid rocket engines (5) with different thrusts, and repeatedly calling modules M1 to M4 to carry out multiple subcritical thrust tests; Module M6: Analyze the variation of the modal frequency of the slender aircraft simulation parts with thrust from the vibration velocity response of multiple subcritical thrust tests, and further establish a polynomial extrapolation of the critical thrust by combining the relationship between the first two modal frequency differences and thrust.

8. The critical thrust extrapolation system for subcritical thrust test of a slender body according to claim 7, characterized in that: In the module M1, the angle between the initial bending deformation and the angle before deformation of the slender aircraft simulation piece is less than 10°.

9. The critical thrust extrapolation system for subcritical thrust test of a slender body according to claim 7, characterized in that: In the module M3, the time interval between the ignition time of the black powder bag on the polyethylene line and the ignition time of the solid rocket engine (5) is controlled so that the solid rocket engine (5) is ignited just when the slender aircraft simulation moves to the equilibrium position; In the module M4, the laser Doppler vibrometers are evenly distributed along the axial direction of the aluminum alloy slender body (4), ensuring that the measurement points of the multiple laser Doppler vibrometers are not simultaneously located at the vibration mode node line of the slender aircraft simulation part.

10. The critical thrust extrapolation system for subcritical thrust test of slender body according to claim 7, characterized in that: In the module M6, the polynomial form of the relationship between the first two modal frequency differences and thrust is as follows: (Δω 2 ) 2 =A0T 2 +A1T+A2 Where Δω is the frequency difference between the first two modal modes; T is the thrust; A0, A1, and A2 are the coefficients to be determined. When the solid rocket engine (5) is not ignited, a no-thrust test with T=0 is carried out to obtain A2. At least two additional subcritical thrust tests are carried out to obtain A0 and A1 through fitting.

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