Turbopump Vibration in Liquid Rocket Engines A Comprehensive Review and Future Perspectives

1. Introduction

Liquid rocket engines are the backbone of modern space exploration, powering spacecraft into orbit and enabling interplanetary missions. At the heart of these engines lies the turbopump, a crucial component that plays a vital role in the engine’s performance and reliability. Turbopumps are responsible for pressurizing the propellants, ensuring a stable and efficient flow into the combustion chamber. However, the harsh operating conditions of liquid rocket engines, including high temperatures, pressures, and rotational speeds, subject turbopumps to severe mechanical and thermal stresses. These conditions often lead to vibration issues, which can have a significant impact on the turbopump’s performance, durability, and ultimately, the success of the space mission.

Vibration in turbopumps can cause a range of problems, such as increased wear and tear on components, reduced efficiency, and even catastrophic failures. It can also lead to premature fatigue of critical parts, shortening the lifespan of the turbopump and increasing maintenance costs. Given the importance of turbopumps in liquid rocket engines, understanding and addressing vibration issues is of utmost importance. This article aims to provide a comprehensive review of the research progress on turbopump vibration problems, exploring the underlying mechanisms, current research methods, and future directions.

2. Structure and Working Principle of Turbopumps

2.1 Structure of Turbopumps

Turbopumps in liquid rocket engines are complex mechanical systems composed of multiple components, each with its own specific function. A typical turbopump consists of a pump, a turbine, a spindle, seals, bearings, and a housing, as shown.

The pump is responsible for pressurizing the propellants, while the turbine provides the mechanical energy to drive the pump. The spindle connects the pump and the turbine, enabling the transfer of rotational motion. Seals are used to prevent leakage of the propellants, and bearings support the rotating components, ensuring smooth operation. The housing encloses all the components, providing structural support and protection.

2.2 Working Principle of Turbopumps

The working principle of a turbopump is based on the conversion of energy. High – pressure, high – temperature gas is expanded through a turbine nozzle, converting the thermal energy of the gas into kinetic energy. The high – speed gas then impinges on the turbine blades, causing the turbine rotor to rotate. The rotational energy of the turbine is transferred to the pump via the spindle, driving the pump impeller to rotate at high speeds. As the pump impeller rotates, it pressurizes the propellants, increasing their kinetic and potential energy. The pressurized propellants are then delivered to the combustion chamber for combustion.

The process can be summarized as follows:

Gas Expansion in Turbine Nozzle: High – pressure, high – temperature gas expands through the turbine nozzle, increasing its velocity and converting thermal energy into kinetic energy.
Turbine Rotation: The high – speed gas impinges on the turbine blades, causing the turbine rotor to rotate. The kinetic energy of the gas is converted into mechanical energy, driving the rotation of the turbine.
Energy Transfer via Spindle: The rotational energy of the turbine is transferred to the pump through the spindle. The spindle ensures the synchronous rotation of the pump and the turbine.
Propellant Pressurization in Pump: As the pump impeller rotates, it accelerates the propellants, increasing their kinetic energy. The propellants are then guided through the pump casing and diffuser, where their kinetic energy is converted into pressure energy, resulting in pressurized propellants.

3. Classification of Turbopump Vibration Problems

Turbopump vibration problems can be classified into three main categories: structure – excited vibration, fluid – induced vibration, and coupled vibration. Each category has its own unique causes, characteristics, and effects on the turbopump’s performance. Table 1 provides a summary of the classification of turbopump vibration problems.

Vibration Classification	Vibration Causes	Vibration Phenomena
Structure – excited Vibration	Rotor unbalance, Rotor – stator rubbing, Shaft looseness and internal friction, Support excitation	Large vibration amplitude, rubbing; High – frequency and fractional – frequency harmonics, circumferential rubbing marks; Sub – synchronous vibration, amplitude fluctuation, friction marks on mating surfaces; Amplitude fluctuation, various harmonic frequencies
Fluid – induced Vibration	Rotor – stator interference excitation, Surge, Cavitation, High – order fluid excitation	Vibration with blade – passing frequency and its harmonics as dominant frequencies; Low – frequency vibration, pressure and flow pulsation; Frequencies near power frequency, fatigue failure of inducer, blade pitting; High – multiple rotation frequencies
Coupled Vibration	Seal – rotor coupling, Turbine flutter, Multi – factor coupling	Sub – synchronous vibration, instability; Flutter of turbine blades, leading to potential structural damage; Complex vibration patterns resulting from the interaction of multiple factors

3.1 Structure – excited Vibration

Rotor Unbalance: Rotor unbalance is a common cause of structure – excited vibration in turbopumps. It occurs when the mass center of the rotor does not coincide with the axis of rotation, resulting in an unbalanced centrifugal force during rotation. This unbalanced force can cause large – amplitude vibrations, which may lead to increased stress on the rotor, bearing wear, and even rotor – stator rubbing.
Rotor – stator Rubbing: In turbopumps, to achieve high efficiency, small clearances are often maintained between the rotor and the stator. However, under certain conditions, such as thermal expansion, mechanical misalignment, or excessive vibration, the rotor and the stator may come into contact, resulting in rubbing. Rotor – stator rubbing can generate high – frequency and fractional – frequency harmonics in the vibration spectrum and cause circumferential rubbing marks on the components.
Shaft Looseness and Internal Friction: The high – speed rotation and thermal cycling in turbopumps can cause shaft looseness, especially in components with interference fits or spline connections. Shaft looseness can lead to changes in the rotor’s balance state and cause internal friction between components. This internal friction can excite sub – synchronous vibrations, which may be detrimental to the turbopump’s stability.
Support Excitation: The supports of the turbopump, such as bearings, play a crucial role in determining the vibration characteristics of the rotor system. Uneven support stiffness, bearing wear, or misalignment can introduce support excitation, causing amplitude fluctuations and the appearance of various harmonic frequencies in the vibration spectrum.

3.2 Fluid – induced Vibration

Rotor – stator Interference Excitation: The interaction between the rotating and stationary components in the turbopump, such as the impeller and the diffuser, can cause fluid – induced vibration. The pressure pulsations generated by the relative motion of the rotating and stationary parts can excite vibrations at the blade – passing frequency and its harmonics, which may lead to structural fatigue and failure.
Surge: Surge is a self – excited instability phenomenon that occurs in pumps when the flow rate is below a certain critical value. It is characterized by low – frequency pressure and flow pulsations, which can cause significant vibrations in the pump and the entire system. Surge can lead to performance degradation, mechanical damage, and even system failure.
Cavitation: Cavitation is a phenomenon that occurs when the local pressure in the fluid drops below the vapor pressure, resulting in the formation and collapse of vapor bubbles. In turbopumps, cavitation can occur in the inducer or the impeller, causing vibrations, noise, and damage to the components. Cavitation – related vibrations can be classified into different types, such as rotating cavitation, cavitation surge, and rotating flutter.
High – order Fluid Excitation: In recent years, high – order fluid excitation has become an increasingly important issue in turbopump design. It refers to the fluid – induced vibrations with frequencies higher than the rotating frequency of the inducer. These high – order vibrations can be potentially destructive as they may approach the natural frequencies of the turbopump components, leading to resonance and structural failure.

3.3 Coupled Vibration

Seal – rotor Coupling: Turbopumps often use small – clearance annular seals, such as floating ring seals and labyrinth seals, to control the internal flow. The flow of fluid through these seals can interact with the rotor’s rotation and whirling motion, generating forces that can cause the rotor to vibrate. This phenomenon is known as seal – rotor coupling vibration, which can lead to sub – synchronous vibrations and instability of the rotor system.
Turbine Flutter: With the increasing use of integrated bladed disks in rocket engine turbopumps, the problem of turbine flutter has become more prominent. Turbine flutter is a fluid – structure interaction phenomenon in which the aerodynamic forces acting on the turbine blades cause the blades to vibrate self – destructively. This can lead to blade fatigue, cracking, and even disk rupture, posing a serious threat to the engine’s safety.
Multi – factor Coupling: In real – world turbopump operation, multiple factors, such as fluid forces, structural stiffness, thermal effects, and internal friction, often interact with each other, resulting in complex vibration patterns. This multi – factor coupling vibration, such as the sub – synchronous vibration of the rotor, is difficult to analyze and control, and requires a comprehensive understanding of the underlying physical mechanisms.

4. Research Status of Turbopump Vibration Problems

4.1 Structure – excited Vibration

4.1.1 Rotor Unbalance

Rotor unbalance is a well – studied issue in turbopump vibration. Researchers have proposed various methods to address this problem, including rotor dynamic balance techniques and improved manufacturing and assembly processes.

Rotor Dynamic Balance Techniques: Rotor dynamic balance is a common method to reduce unbalance – induced vibrations. Different balance methods have been developed, such as the modal balance method and the three – circle balance method. For example, Chen Xi et al. proposed a rotor modal balance method under elastic support conditions and verified it through experiments. Liu Gangqi et al. developed a no – trial – weight modal balance method for rotors passing through the second – order critical speed.
Improved Manufacturing and Assembly Processes: In addition to dynamic balance techniques, improving the manufacturing and assembly processes can also reduce the occurrence of rotor unbalance. Huang Jinping et al. analyzed the influence of factors such as coaxiality deviation of the shaft center hole and spline positioning surface on the unbalance of the shaft system by performing unbalance response sensitivity analysis for the turbopump rotor system.

To further reduce the vibration during the rotor’s passage through the critical speed, advanced damping devices have been developed. For instance, Guo Baoting et al. developed a metal – rubber damper for the ultra – low – temperature working environment of hydrogen – oxygen rocket engine turbopumps. This damper, when combined with an elastic support, can effectively reduce the vibration of the rotor passing through the critical speed.

4.1.2 Rotor – stator Rubbing

Research on rotor – stator rubbing in turbopumps mainly focuses on monitoring, prevention, and understanding the vibration characteristics during rubbing.

Monitoring and Prevention: Given the potential hazards of rotor – stator rubbing, effective monitoring and prevention methods are crucial. Monitoring the vibration signals of the turbopump can help detect the occurrence of rubbing at an early stage. In addition, setting appropriate clearances between the rotor and the stator, considering factors such as thermal expansion, pressure deformation, and centrifugal deformation, can prevent rubbing.
Vibration Characteristics: Many studies have investigated the vibration characteristics of rotor – stator rubbing. Wang Hui et al. established a motion differential equation for the full – circle rubbing state between the rotor and the casing and analyzed its vibration characteristics. They found that rubbing can generate high – order and sub – harmonic vibrations, and as the friction intensifies, the rotor may change from forward whirl to backward whirl, eventually leading to system instability.

4.1.3 Shaft Looseness and Internal Friction

Shaft looseness and internal friction in turbopumps can cause sub – synchronous vibrations, which can be a threat to the stability of the rotor system.

Causes and Effects: Research has shown that factors such as the centrifugal deformation of the disk and the thermal – deformation differences of the shaft – system components can lead to shaft looseness. For example, the LE – 7 hydrogen turbopump experienced sub – synchronous vibration during the ground test phase due to relative sliding between the impellers and insufficient shaft – system damping.
Solutions: To address this problem, researchers have proposed several solutions. Tan Dali et al. analyzed the internal friction between cylindrical – surface clearance fits and suggested that it can be equivalent to an additional internal damping to the rotor. Luo Qiaojun et al. found that increasing the shaft – system pre – load can significantly suppress abnormal vibrations caused by shaft looseness. Jin Lu et al. established a rotor – shaft – sleeve internal friction model with clearance and provided design suggestions for the stability of the rotor system with a shaft – sleeve structure.

4.1.4 Support Excitation

The support system of a turbopump has a significant impact on the vibration characteristics of the rotor. Research in this area mainly focuses on the influence of support stiffness and bearing parameters on rotor vibration.

Influence of Support Stiffness: The stiffness of the support system can affect the vibration response of the rotor. In the case of the ATD HPOTP for the Space Shuttle Main Engine, the deformation of the casing under pressure led to a decrease in the support stiffness of the pump – end bearing, resulting in an increase in synchronous vibration.
Bearing Parameters: The parameters of the bearings, such as clearance and pre – load, also play an important role in rotor vibration. Deng Si’er et al. and He Zhixian et al. established dynamic models of the rotor – bearing system to analyze the influence of bearing parameters on rotor vibration characteristics. In China, Jin Lu et al. studied the influence of bearing radial clearance on the unbalance response of a rigid rotor and found that applying axial pre – load can reduce the bearing radial clearance and the vibration amplitude of the rotor at low speeds.

4.2 Fluid – induced Vibration

4.2.1 Rotor – stator Interference Excitation

Rotor – stator interference excitation is a major source of fluid – induced vibration in turbopumps. Computational fluid dynamics (CFD) and experimental methods are widely used to study this problem.

CFD Analysis: CFD has become an important tool for studying the flow field in turbopumps and predicting rotor – stator interference excitation. For example, during the development of the LE – 7A rocket engine in Japan, CFD methods such as large – eddy simulation and one – way coupling were used to analyze the flow field of the hydrogen fuel pump, accurately capturing the rotor – stator interference phenomenon and predicting the vibration response of the engine.
Experimental Research and Solutions: In China, during the development of the new – generation liquid oxygen – kerosene engine, the oxygen pump experienced excessive vibration caused by rotor – stator interference. By replacing the vane – type diffuser with a new – type diffuser, the vibration level of the turbopump was significantly reduced, and the vibration surge at high – load conditions was eliminated.

4.2.2 Surge

Surge is a complex instability phenomenon in pumps, and research on surge in turbopumps mainly focuses on understanding its mechanism and developing prevention and control methods.

Mechanism and Characteristics: Surge occurs when the pump operates in the region of positive slope of the flow – head curve. It is characterized by low – frequency pressure and flow pulsations, which can cause significant vibrations in the pump and the system. For example, the hydrogen pump inducer of the LE – 7 engine in Japan experienced severe surge, and the surge frequency was as low as 2 Hz.
Prevention and Control: To prevent and control surge, various methods have been proposed, such as adjusting the operating conditions of the pump, optimizing the pump design, and using surge – control valves. However, due to the complexity of the surge mechanism, further research is still needed to develop more effective prevention and control strategies.

4.2.3 Cavitation

Cavitation is a common problem in turbopumps, and it can cause various types of vibrations, such as rotating cavitation, cavitation surge, and rotating flutter.

Rotating Cavitation: Rotating cavitation occurs at low cavitation numbers and can cause significant vibrations and damage to the components. For example, the high – pressure oxygen turbopump of the Space Shuttle Main Engine and the LE – 7 engine’s liquid hydrogen turbopump inducer both experienced rotating cavitation, resulting in component damage and mission failures. In China, by adopting a new – type inducer design, the rotating cavitation problem in the oxygen pump of the large – thrust liquid – coal engine was effectively suppressed.
Cavitation Surge: Cavitation surge occurs when the cavitation coefficient drops below a critical value, leading to strong pressure and flow pulsations in the pump and the system. Research has shown that the cavitation surge frequency depends on the cavitation coefficient, and it is also related to the POGO (Propellant Oscillation in Gas – filled Oscillator) phenomenon in liquid rocket propulsion systems.
Rotating Flutter: Rotating flutter is a relatively new type of cavitation – related instability. It occurs when the performance of the inducer drops sharply due to cavitation, and the rotating shaft of the inducer experiences strong vibrations. This phenomenon has been observed in the oxygen pump of the Vinci engine and the hydrogen turbopump of the H – 3 rocket in Japan.

4.2.4 High – order Fluid Excitation

High – order fluid excitation in turbopumps is a relatively new research area. Fujii et al. discovered high – order surge and high – order rotating cavitation phenomena in the inducer of the LE – 7 rocket engine oxygen pump through experimental research. The self – excited frequencies of these high – order vibrations are approximately three times the rotating frequency of the inducer shaft. The existence of high – order fluid excitation in the hydrogen pump inducer of the LE – 7 rocket engine also indicates its potential threat to the stability of the turbopump. Further research is needed to understand the mechanism and develop effective control methods for high – order fluid excitation.

4.3 Coupled Vibration

4.3.1 Seal – rotor Coupling Vibration

Seal – rotor coupling vibration is a complex phenomenon that has been studied extensively. The main focus of research is on understanding the mechanism of fluid – induced excitation in seals and developing effective control strategies.

Mechanism and Theoretical Models: Since the discovery of seal – fluid excitation in 1940, numerous theoretical models have been proposed. Alford’s top – clearance excitation theory in 1965 laid the foundation for subsequent research. Muszynska and Bently proposed the Muszynska model after extensive experiments, identifying the rotational effect of fluid – exciting forces as a major factor causing rotor instability.
Control Strategies: To suppress seal – fluid excitation, various control strategies have been developed. These include using damping seals, anti – swirl flows, and optimizing seal structures. For example, in the high – pressure fuel turbopump of the US Space Shuttle Main Engine, changing the inter – stage labyrinth seal from a grooved three – stage structure to a smooth three – stage structure reduced the cross – stiffness of the seal fluid and eliminated rotor sub – synchronous vibration. In China, researchers have also conducted in – depth studies on the dynamic characteristics of different seal structures for turbopumps, providing a basis for seal design and optimization.

4.3.2 Turbine Flutter

Turbine flutter is a critical issue in the design of turbopumps with integral bladed disks. Research in this area focuses on understanding the aero – elastic stability of turbine blades and developing methods to improve flutter resistance.

Aero – elastic Stability Analysis: The aero – elastic stability of turbine blades is affected by factors such as blade geometry, flow conditions, and structural damping. Nowinski et al. found that the position of the disk torsion axis has a significant impact on flutter stability. Schuff et al. analyzed the variation of aerodynamic damping under different modes and inter – blade phase angles to determine the aero – elastic stability range.
Flutter Suppression Methods: To improve the flutter resistance of turbine blades, methods such as structural damping improvement, aero – dynamic structure optimization, and active detuning design have been proposed. Holmedahl introduced a roof – shaped damper between the oxygen turbine blades of Vulcain 2, and the test results showed that it effectively improved the flutter performance. Peeren et al. proposed a blade – shape improvement scheme based on the influence mechanism of shock waves on flutter and successfully applied it to the quasi – three – dimensional cascade oscillation problem.

4.3.3 Multi – factor Coupling Vibration

Multi – factor coupling vibration, such as rotor sub – synchronous vibration, is a complex phenomenon that involves the interaction of multiple factors. Research in this area is still in its infancy, and more in – depth studies are needed to understand the underlying mechanisms and develop effective control methods.

Causes and Occurrences: Rotor sub – synchronous vibration is caused by factors such as small – clearance seal – fluid excitation, non – continuous rotor stiffness characteristics, rotor – structure internal friction, and fluid – exciting forces from the pump and turbine. This type of vibration has occurred in the development of engines such as the SSME and the Japanese LE – 7 engine, as well as in a certain type of hydrogen turbopump in China.
Research Challenges: Due to the complexity of multi – factor coupling, accurately predicting and controlling this type of vibration is a significant challenge. It requires a comprehensive understanding of the physical mechanisms of each factor and their interactions, as well as the development of advanced multi – disciplinary modeling and analysis methods.

5. Summary of Research Achievements and Existing Problems

5.1 Research Achievements

Over the years, significant progress has been made in the research of turbopump vibration problems. Through the efforts of researchers around the world, a relatively complete theoretical system has been established, covering aspects such as structure – excited vibration, fluid – induced vibration, and coupled vibration.

Theoretical System: In the field of structure – excited vibration, methods for analyzing rotor unbalance, rotor – stator rubbing, shaft looseness, and support excitation have been well – developed. For fluid – induced vibration, the mechanisms of rotor – stator interference excitation, surge, cavitation, and high – order fluid excitation have been deeply explored. In the area of coupled vibration, research on seal – rotor coupling and turbine flutter has also achieved certain results.
Engineering Applications: Many of these research results have been applied in engineering practice, effectively solving a series of vibration problems in turbopumps. For example, advanced dynamic balance techniques have reduced the impact of rotor unbalance; optimized seal structures have suppressed seal – rotor coupling vibration; and improved inducer designs have mitigated cavitation – related vibrations. These applications have improved the performance, reliability, and safety of liquid rocket engines.

5.2 Existing Problems

Despite the achievements, there are still many problems that need to be addressed in the research of turbopump vibration problems.

Multi – factor Coupling: The understanding of multi – factor coupling vibration, such as rotor sub – synchronous vibration and turbine flutter, is still insufficient. The complex interactions between different physical fields make it difficult to accurately predict and control these vibrations, and more in – depth multi – disciplinary research is required.
Variable – condition Operation: With the development of large – thrust and reusable engines, turbopumps need to operate under a wider range of working conditions. However, current research on the vibration characteristics of turbopumps under variable – condition operation is relatively limited, and it is necessary to strengthen research in this area to ensure the stable operation of turbopumps under different conditions.
Lack of Experimental Data: In some cases, the lack of sufficient experimental data limits the accuracy of theoretical models and the verification of research results. More experimental studies are needed to obtain comprehensive and accurate data, which will help to improve the understanding of turbopump vibration mechanisms and the development of more effective control methods.

6. Future Research Directions

6.1 Relationship between Vibration – induced Dynamic Stress and Fatigue Life of Critical Components

For large – thrust and reusable engines, the dynamic stress caused by vibration in critical components of turbopumps can lead to structural failure and fatigue life reduction. Future research should focus on establishing a quantitative relationship between vibration – induced dynamic stress and fatigue life.

Experimental and Numerical Studies: Conduct in – depth experimental and numerical studies on the fatigue characteristics of turbopump components under vibration conditions. Use advanced testing techniques to measure the dynamic stress distribution of components during operation and combine it with fatigue life prediction models to accurately evaluate the fatigue life of components.
Life Prediction Models: Develop more accurate fatigue life prediction models that consider factors such as vibration frequency, amplitude, and the material properties of components. These models can provide a basis for the design, maintenance, and replacement of turbopump components, ensuring the long – term reliable operation of engines.

6.2 Fluid Excitation Mechanisms under Off – design Operation of Turbopumps

During the large – range variable – condition operation of engines, turbopumps often deviate significantly from the design conditions, resulting in complex fluid – excitation problems. Future research should focus on understanding the fluid – excitation mechanisms under off – design conditions.

CFD and Experimental Research: Use advanced CFD methods and experimental techniques to study the flow field characteristics of turbopumps under off – design conditions. Analyze the generation and evolution of fluid – excitation forces and their effects on the vibration of turbopumps.
Design Optimization: Based on the research results, optimize the design of turbopumps to improve their adaptability to off – design conditions. This can include adjusting the blade shape, improving the flow passage design, and adding flow – control devices to reduce the impact of fluid excitation and ensure the stable operation of turbopumps.

6.3 Multi – factor Coupled Modeling Analysis and Active Vibration Control

To meet the requirements of high thrust – to – mass ratio and long life of engines, future research should focus on multi – factor coupled modeling analysis and active vibration control of turbopumps.

Multi – disciplinary Modeling: Establish multi – disciplinary coupled models that consider the interaction between fluid and rotor, as well as the influence of temperature and pressure on the structure deformation and flow state. Use these models to analyze the complex vibration mechanisms of turbopumps and provide a theoretical basis for vibration control.
Active Vibration Control: Develop active vibration control strategies for turbopumps. This can include using smart materials, such as piezoelectric materials, to actively adjust the vibration characteristics of components, and designing advanced control algorithms to achieve real – time control of turbopump vibrations, thereby improving the structural safety and working life of turbopumps.

7. Conclusion

Turbopump vibration is a complex problem that involves multiple disciplines and has a significant impact on the performance, reliability, and safety of liquid rocket engines. Through years of research, great progress has been made in understanding the mechanisms, classification, and control methods of turbopump vibration. However, with the development of modern space technology, new challenges have emerged, such as the requirements for large – thrust, reusable engines and wide – range variable – condition operation.

Future research on turbopump vibration should focus on solving the problems of multi – factor coupling vibration, understanding the vibration characteristics under variable – condition operation, and establishing a relationship between vibration – induced stress and fatigue life. By strengthening research in these areas, we can develop more advanced vibration control technologies, improve the design level of turbopumps, and ensure the successful implementation of space missions. This will also contribute to the continuous development and innovation of the aerospace industry.