Method for reducing propellant sloshing during shutdown of a rocket engine and control system therefor

Abstract

The application relates to the field of aerospace technology, in particular to a method for reducing the swing range of propellant when a rocket engine is shut down and a control system thereof. The method comprises the following steps: establishing a propellant swing dynamics model in a tank under axial overload sudden drop; based on the dynamics model, a three-dimensional fluid simulation model in the tank is established, the gas phase region and the liquid phase region are divided, and the gas-liquid interface is tracked to describe the swing state of the propellant; in the three-dimensional fluid simulation model, the propellant swing is simulated under the condition of constant axial overload, and multiple propellant swing phases are selected from the swing evolution process as shutdown moments; the axial overload sudden drop is applied at the shutdown moments respectively to simulate the shutdown of the rocket engine, and the propellant swing evolution data corresponding to each shutdown moment is obtained; based on the propellant swing evolution data, the shutdown moment interval with the minimum propellant swing range variation is determined as the optimal propellant swing phase interval for shutting down the rocket engine.

Description

Technical Field

[0001] This application relates to the field of aerospace technology, and in particular to a method and control system for reducing propellant sloshing when a rocket engine is shut down. Background Technology

[0002] With the rapid development of aerospace technology, spacecraft are constantly evolving towards higher reliability, lower cost, and reusability. Liquid-fueled launch vehicles, with their advantages of adjustable thrust and high specific impulse, are widely used in space launch missions. During the first-stage recovery and reuse, and the upper-stage coasting phase, the main engine needs to be shut down and restarted multiple times to complete attitude adjustments, orbital corrections, and reentry. To ensure normal engine restart, the propellant in the tanks must be at the bottom before ignition. However, during flight, the rocket body is affected by factors such as lateral disturbances, structural vibrations, and engine sway angles, causing significant sloshing of the propellant in the tanks. When the engine thrust decreases rapidly, the axial overload in the tanks drops abruptly, entering a microgravity environment. Under inertia, the liquid propellant is prone to amplified sloshing, floating, or even overshooting, which can affect the reliable restart of the engine.

[0003] Current research primarily focuses on propellant dynamics under complex overload conditions, while numerical studies on propellant sloshing under sudden axial overload drops are insufficient, particularly regarding the mechanism by which the sloshing phase of the propellant amplifies the sloshing amplitude during engine shutdown. In practical missions, the sloshing phase of the propellant (i.e., the fluid state of the propellant) during engine shutdown significantly impacts the change in sloshing amplitude; improper management can affect engine restart. Currently, rocket propellant management schemes mainly fall into several categories, including active, bottom-mounted, and in-tank anti-sloshing structures. Active systems use rubber bladders or metal diaphragms to separate the propellant from the gas cushion, using high-pressure gas to compress the diaphragm and deliver the propellant to the engine. However, these systems are large in size and have a high dry weight, and also present compatibility and fatigue issues with cryogenic propellants. Bottom-mounted systems utilize a bottom-mounted engine to provide acceleration, making the inertial force much greater than the surface tension, thereby suppressing propellant drift and ensuring the propellant is repositioned to the bottom of the tank. However, this requires continuous consumption of propellant or high-pressure gas. In addition, arranging baffles, anti-sloshing plates, perforated plates, and other structures inside the tank can suppress propellant sloshing to a certain extent, but it increases the structural mass.

[0004] Therefore, studying the propellant sloshing behavior under sudden axial overload drop during engine shutdown, especially the influence of different sloshing phases on the sloshing amplitude, is of significant engineering importance. Revealing the hydrodynamic characteristics of this process can provide theoretical basis and technical support for tank structure design, propellant management device optimization, and engine restart strategies. Summary of the Invention

[0005] The purpose of this application is to overcome the aforementioned deficiencies of the prior art, thereby providing a method and control system for reducing propellant sloshing during rocket engine shutdown. This method and system are applicable to situations where the rocket's main engine requires multiple shutdowns and restarts, and sudden changes in axial overload within the propellant tanks easily amplify the sloshing amplitude. This invention significantly reduces the sloshing amplitude by optimizing the engine shutdown timing, ensuring the propellant is in the phase with the weakest sloshing response at the moment of shutdown.

[0006] To solve the above-mentioned technical problems, the technical solution of this application provides a method for reducing propellant sloshing during rocket engine shutdown, comprising the following steps: Step 1: Establish a dynamic model of propellant sloshing within the tank under axial overload sudden drop; Step 2: Based on the aforementioned dynamic model, establish a three-dimensional fluid simulation model inside the tank, divide the gas phase region and liquid phase region, and track the gas-liquid interface to describe the sloshing state of the propellant through the gas-liquid interface; Step 3: In the three-dimensional fluid simulation model, simulate propellant sloshing under constant axial overload conditions, and select multiple propellant sloshing phases from the sloshing evolution process as shutdown moments; apply axial overload sudden drops at each shutdown moment to simulate rocket engine shutdown, and obtain propellant sloshing evolution data corresponding to each shutdown moment; Step 4: Based on the propellant sloshing evolution data, determine the shutdown time interval with the smallest change in propellant sloshing amplitude, which is taken as the optimal propellant sloshing phase interval for shutting down the rocket engine.

[0007] As an improvement to the above method, step 2 specifically includes: establishing a three-dimensional geometric model based on the tank structure; dividing the fluid region inside the tank into structured meshes and locally refining the mesh at the gas-liquid interface; using no-slip boundary conditions on the solid wall of the tank; dividing the fluid region into two regions, gas phase and liquid phase; using the fluid volume method to track the evolution process of the gas-liquid interface; and setting the corresponding physical property parameters of the gas phase and liquid phase.

[0008] As an improvement to the above method, an axial overload drop is applied at the shutdown moment to simulate engine shutdown, and propellant sloshing evolution data corresponding to each shutdown moment is obtained. Specifically, this includes: in a three-dimensional fluid simulation model, by setting an axial acceleration that varies with time, simulating the axial overload drop process at the moment of engine shutdown; during the axial overload drop process, simulating the sloshing response of the propellant, thereby obtaining the propellant sloshing evolution simulation data corresponding to the propellant sloshing phase at each shutdown moment.

[0009] As an improvement to the above method, the three-dimensional fluid simulation model uses a double-precision solver to numerically calculate the sloshing response of the propellant in the tank under the condition of sudden axial overload drop, and obtains simulation data of the sloshing evolution of the propellant; wherein, the discrete control equation adopts the second-order upwind scheme, the pressure is discretized by the pressure interpolation method, the volume fraction is discretized by the compression scheme, and the pressure-velocity coupling is corrected by the pressure implicit splitting operator method.

[0010] As an improvement to the above method, the three-dimensional fluid simulation model calculates the sloshing period of the propellant based on the tank geometry, propellant liquid level, and the equivalent gravitational acceleration of the propellant.

[0011] As an improvement to the above method, the different initial sloshing phases are selected from different moments within the sloshing period of the propellant.

[0012] As an improvement to the above method, the simulation data of propellant sloshing evolution includes: propellant distribution pattern diagram, propellant sloshing animation, propellant center of mass change curve and / or propellant kinetic energy change curve.

[0013] As an improvement to the above method, the three-dimensional fluid simulation model is used to simulate the sloshing evolution data after the engine is shut down under different propellant margins, so as to obtain the optimal sloshing phase range at the shutdown time under different propellant margins.

[0014] To achieve another objective of the present invention, the present invention also provides a shutdown control system for a rocket engine, comprising: Sensor components for monitoring the sloshing of propellant within the tank; and, The shutdown control module is used to receive engine shutdown commands; to estimate the current propellant sloshing phase based on the propellant sloshing state; and to trigger the engine shutdown command when the engine shutdown command is received and the propellant sloshing phase is within the optimal propellant sloshing phase range obtained in claim 1, thereby shutting down the engine, reducing the propellant sloshing amplitude after engine shutdown, and ensuring that the propellant sinks to the bottom.

[0015] As an improvement to the above system, the sloshing state includes: propellant liquid level height, propellant liquid level tilt angle, and propellant sloshing amplitude.

[0016] Compared to existing technologies, it has at least the following advantages: 1. Effectively reduce propellant sloshing amplitude: By analyzing the evolution of propellant sloshing under different shutdown phases, the optimal engine shutdown time is selected to avoid amplification of sloshing amplitude and propellant floating caused by sudden changes in axial overload, ensuring that the propellant can reliably sink to the bottom before engine restart.

[0017] 2. Low implementation cost: The core of this invention is to effectively reduce propellant sloshing by optimizing the control algorithm and adding a small number of sensors to obtain the propellant sloshing state based on the existing flight control system. This solution does not require anti-sloshing plates, baffles, or other structures inside the propellant tank, nor does it involve any major hardware modifications, thus resulting in low implementation cost.

[0018] 3. Good versatility and scalability: It is suitable for various tank configurations and propellant types, can adapt to fluid management requirements under different overload conditions, has good engineering applicability, and can be expanded to propellant management and control in other variable overload scenarios. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the initial state of the propellant in the tank, where, Figure 1 The blue part is liquid propellant, with an initial liquid surface tilt angle of 10°; Figure 2 This is a flowchart illustrating the method for reducing propellant sloshing during rocket engine shutdown as described in this application. Figure 3 Flowchart for selecting control system for engine shutdown time; Figure 4 This is a graph showing the overload variation. Figure 5 The diagram shows the phase change of propellant sloshing, where t1-t6 correspond to the sloshing phases at the shutdown time of operating conditions 1-6, respectively. Detailed Implementation

[0020] The technical solutions provided in this application are further illustrated below with reference to the embodiments.

[0021] This application provides a method and control system for reducing propellant sloshing during rocket engine shutdown. This method is applicable to scenarios involving the recovery and reuse of the first stage and the upper stage coasting phase of liquid-fueled launch vehicles, where the main engine requires multiple shutdowns and restarts, addressing the problem of amplified propellant sloshing caused by sudden axial overload changes. First, a propellant sloshing dynamics model and a three-dimensional fluid simulation model are established to analyze the influence of the sloshing phase of the liquid at different shutdown times on the fluid response characteristics. Based on numerical simulation results, the optimal phase range for minimizing propellant sloshing after shutdown is determined. This shutdown time window is then integrated into the rocket flight control system. By monitoring the propellant sloshing state in real time using sensors, the control system executes the shutdown command within the optimal window based on phase judgment, thereby avoiding amplified sloshing caused by sudden overload changes and ensuring propellant settling and reliable engine restart.

[0022] Example 1 This embodiment provides a method for reducing propellant sloshing during rocket engine shutdown. This method analyzes the evolution of propellant sloshing under different sloshing phases of the liquid at different shutdown times, determines the optimal shutdown phase, and integrates it into the flight control system to achieve effective management of propellant sloshing. Figure 2 As shown, the specific implementation steps are as follows: Step (1) Establish the swaying dynamics model A dynamic model of propellant sloshing under sudden axial overload is constructed to describe the energy changes and sloshing response characteristics corresponding to different sloshing phases when the engine is shut down.

[0023] Step (2) Establish a three-dimensional fluid simulation model of the storage tank. A three-dimensional fluid model was established based on the Cassini tank. Structured meshes were used for both the liquid and gas phases, with localized mesh refinement at the gas-liquid interface. The VOF (Volume of Fluid) method was employed to track the gas-liquid interface, and variable overload was achieved through time-varying axial acceleration.

[0024] Step (3) Conduct three-dimensional fluid simulation of the propellant in the tank. Multiple engine shutdown times were set so that the propellant was in different sloshing phases at the moment of shutdown, and three-dimensional numerical simulations were performed on each operating condition to obtain the dynamic response corresponding to different phases.

[0025] Step (4) Determine the wobbling phase at the optimal shutdown time. Based on the numerical simulation results, key data such as the propellant distribution pattern, the liquid centroid variation curve over time, the kinetic energy variation curve over time, and the sloshing process animation were extracted. The variation law of propellant sloshing amplitude under different shutdown phases was analyzed, and the phase interval with the minimum sloshing amplitude was identified, thereby determining the optimal shutdown time window of the engine. Step (5) Select shutdown time using the application control system The optimal shutdown phase interval determined in step (4) is integrated into the rocket flight control system, such as... Figure 3 As shown, the system monitors the propellant sloshing state in real time using sensors, and selects the shutdown time based on the sloshing phase, thereby reducing the increase in sloshing amplitude caused by sudden changes in axial overload.

[0026] Calculation and interpretation of main physical quantities: (1) Propellant sloshing period For the liquid sloshing inside a cylindrical tank, the first modal period is:

[0027] in: The propellant level (m); Equivalent gravitational acceleration (m / s²) 2 ); For wave number, approximately: ; Where the radius of the storage tank is (m); (2) Location of the center of mass The coordinates of the centroid are:

[0028] in: , , These are the coordinates of the centroid in the X, Y, and Z directions, respectively. , , These are the coordinates of the fluid element in the X, Y, and Z directions, respectively. The volume occupied by the liquid; This is the density of the liquid.

[0029] (3) Liquid kinetic energy

[0030] in: Indicates the density of the fluid; The volume occupied by the liquid; It is the velocity potential function; Let be the gradient of the velocity potential function, whose squared magnitude is equal to the squared magnitude of the velocity.

[0031] (4) Froude number (Fr)

[0032] in To characterize the velocity of liquids, The characteristic length represents the ratio of inertial force to gravity.

[0033] (5) Amplification factor Based on the principle of energy conservation before and after shutdown, assume that the amplitudes of liquid sloshing before and after shutdown are respectively , The corresponding accelerations are respectively , Phase angle of the shaking wave when the device is turned off The amplification factor for the propellant sloshing amplitude is:

[0034] If the computer is turned off = 0 or 180°, meaning the amplification is most significant at a flat liquid surface; the amplification factor for the propellant sloshing amplitude is:

[0035] Assuming the main engine accelerates to 4g when shut down, and the acceleration during coasting is 4 × 10⁻⁶. -4 g. Based on this calculation, the amplification factor K = 100. If the sway amplitude before shutdown is 0.3 m, after being amplified 100 times, the maximum sway amplitude reaches 30 m. Since the total height of the tank is 13.6 m, there is a possibility that the propellant may overshoot.

[0036] The following example uses the Cassini tank (propellant is liquid oxygen) as an example (e.g.) Figure 1 As shown in the figure, corresponding operating conditions are set for different engine shutdown times. By analyzing the propellant sloshing characteristics after shutdown, the specific implementation steps of this method are explained.

[0037] (1) Establish a three-dimensional fluid simulation model of the storage tank. The following are the specific steps for establishing a three-dimensional fluid simulation model of the tank using computational fluid dynamics (CFD) software: 1) Geometric Model and Mesh Generation: A three-dimensional geometric model was established based on the Cassini tank structure, with tank parameters of a diameter of 3.8m, column height of 11m, and base height of 1.3m. The physical structure was simplified, ignoring non-critical components such as the liquid oxygen delivery pipe and pressurization inlet, as well as internal baffles. A structured mesh was used for the internal fluid region, and local mesh refinement was applied to the gas-liquid interface to improve computational accuracy.

[0038] 2) Model Selection: The fluid region is divided into two phases: gas and liquid. The VOF (Volume of Fluid) method is used to track the gas-liquid interface, and the density, viscosity, and other physical properties of the gas and liquid phases are set. The initial liquid surface inclination angle is set to 10°, and the liquid region is shown in blue in Figure 1.

[0039] 3) Boundary conditions: Use no-slip boundary conditions for the wall.

[0040] 4) Variable overload setting: By setting the axial acceleration that varies with time, the overload change process at the moment of engine shutdown is simulated. 5) Solver Setup: A double-precision solver was used for numerical calculations. The second-order upwind scheme was used for the discrete control equations. The pressure term was discretized using PRESTO! (pressure interpolation scheme), the volume fraction was discretized using Compressive (interface compression scheme), the pressure-velocity coupling term was corrected using the PISO algorithm (pressure-velocity coupling algorithm), the turbulence model was the RNG k-ε model (turbulence model), and the calculation time step was set to 0.001s.

[0041] (2) Three-dimensional fluid simulation of propellant in storage tanks 1) Operating conditions settings: Set the 12 operating conditions shown in the table below.

[0042]

[0043] The selection of t1~t6 in the table above is based on dividing the first T / 4 time period of the second propellant sloshing cycle into 5 equal parts, resulting in six representative shutdown moments. The corresponding sloshing phase angles are 0.5π, 0.4π, 0.3π, 0.2π, 0.1π, and 0°, respectively. Figure 5 As shown. This effectively avoids transient interference in the initial stage of calculation, and based on the symmetry of the sloshing, the selected liquid surface tilt angle can characterize the main sloshing phase within a cycle.

[0044] 2) Changes in axial overload under various working conditions, as follows: Figure 4 As shown: tn (n=1, 2, ..., 6) is the moment of shutdown, and the shutdown effect lasts for 1.5s.

[0045] 3) Simulation result acquisition: Extract key data from the simulation results, including propellant distribution pattern diagram, center of mass change curve over time, kinetic energy change curve over time, and animation of propellant sloshing process, etc.

[0046] (3) Determine the optimal shutdown time and integrate it into the control system. 1) Determine the optimal shutdown time window: Based on the numerical simulation results, compare the propellant sloshing amplitude, center of mass change and kinetic energy change at different shutdown times, identify the phase interval that minimizes sloshing, and thus determine the optimal shutdown time window.

[0047] 2) Control System Integration: Integrate the safe shutdown time window into the rocket's flight control system. Using sensors (such as 3D image sensors), the propellant status is monitored in real time. As the shutdown time approaches, the system automatically assesses whether the current phase is within the optimal window. If not, the shutdown command is adjusted to prevent amplified propellant sloshing after engine shutdown. Example 2 This embodiment provides a rocket engine shutdown control system, including: (1) Propellant Condition Monitoring Module Sensors are installed inside the propellant tank to monitor parameters such as propellant level, surface tilt angle, and sloshing amplitude in real time. The collected data is then transmitted to the flight control module to determine the current sloshing phase.

[0048] (2) Flight control module The built-in shutdown time selection algorithm evaluates the sloshing phase in real time based on data provided by the propellant condition monitoring module. When the propellant sloshing phase approaches the optimal shutdown time window, the engine shutdown command is automatically triggered; if it is not within the safe window, the shutdown command is adjusted.

[0049] (3) Execution module It receives the shutdown command from the flight control module and controls the engine to shut down. Simultaneously, it monitors the propellant settling status to ensure that the propellant remains stable at the bottom of the tank after the engine shuts down.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for reducing propellant sloshing during rocket engine shutdown, characterized in that, Includes the following steps: Step 1: Establish a dynamic model of propellant sloshing within the tank under axial overload sudden drop; Step 2: Based on the aforementioned dynamic model, establish a three-dimensional fluid simulation model inside the tank, divide the gas phase region and liquid phase region, and track the gas-liquid interface to describe the sloshing state of the propellant through the gas-liquid interface; Step 3: In the three-dimensional fluid simulation model, simulate propellant sloshing under constant axial overload conditions, and select multiple propellant sloshing phases from the sloshing evolution process as shutdown times; Axial overload was applied at the shutdown moment to simulate rocket engine shutdown, and propellant sloshing evolution data were obtained at each shutdown moment; Step 4: Based on the propellant sloshing evolution data, determine the shutdown time interval with the smallest change in propellant sloshing amplitude, as the optimal propellant sloshing phase interval for shutting down the rocket engine; Step 2 specifically includes: establishing a three-dimensional geometric model based on the tank structure; dividing the fluid region inside the tank into structured meshes and locally refining the mesh at the gas-liquid interface; using no-slip boundary conditions on the solid wall of the tank; dividing the fluid region into two regions, gas phase and liquid phase; using the fluid volume method to track the evolution process of the gas-liquid interface; and setting the corresponding physical property parameters of the gas phase and liquid phase. Axial overload drop was applied at the shutdown moment to simulate engine shutdown, and propellant sloshing evolution data corresponding to each shutdown moment was obtained. Specifically, in a three-dimensional fluid simulation model, the axial overload drop process at the moment of engine shutdown was simulated by setting an axial acceleration that varies with time; during the axial overload drop process, the sloshing response of the propellant was simulated, thereby obtaining the propellant sloshing evolution simulation data corresponding to the propellant sloshing phase at each shutdown moment. The three-dimensional fluid simulation model uses a double-precision solver to numerically calculate the sloshing response of the propellant in the tank under axial overload and sudden drop conditions, and obtains simulation data of the sloshing evolution of the propellant. Among them, the discrete control equation adopts the second-order upwind scheme, the pressure is discretized by the pressure interpolation method, the volume fraction is discretized by the compression scheme, and the pressure-velocity coupling is corrected by the pressure implicit splitting operator method. The three-dimensional fluid simulation model calculates the sloshing period of the propellant based on the tank geometry, propellant liquid level, and the equivalent gravitational acceleration of the propellant. The simulation data of the propellant sloshing evolution includes: propellant distribution pattern diagram, propellant sloshing animation, propellant center of mass change curve and / or propellant kinetic energy change curve.

2. The method for reducing propellant sloshing during rocket engine shutdown according to claim 1, characterized in that, The different initial sloshing phases are selected from different moments within the propellant's sloshing period.

3. The method for reducing propellant sloshing during rocket engine shutdown according to claim 1, characterized in that, In the three-dimensional fluid simulation model, simulation data of sloshing evolution after engine shutdown under different propellant margins are simulated respectively, so as to obtain the optimal sloshing phase range at the shutdown moment under different propellant margins.

4. A shutdown control system for a rocket engine, used to implement the method for reducing propellant sloshing during shutdown of the rocket engine as described in any one of claims 1-3, characterized in that, include: Sensor assembly for monitoring the sloshing of propellant within the tank; The engine shutdown control module is used to receive engine shutdown commands; It is used to estimate the current propellant sloshing phase based on the propellant sloshing state; when an engine shutdown command is received and the propellant sloshing phase is within the optimal propellant sloshing phase range, the engine shutdown command is triggered to shut down the engine, thereby reducing the propellant sloshing amplitude after the engine is shut down and ensuring that the propellant sinks to the bottom.

5. The rocket engine shutdown control system according to claim 4, characterized in that, The sloshing state includes: propellant liquid level height, propellant liquid level tilt angle, and propellant sloshing amplitude.

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