Rocket engines are the perfect creation of the human mind, incorporating our existing knowledge in aerodynamics, thermodynamics, solid and fluid mechanics, and rotor dynamics. Believe it or not, rocket engines designs contain turbopumps that move fuel and the oxidizer into a combustion chamber creating the perfect conditions for their burning and high-efficiency rocket motion. The word “turbopump” means that the pump is driven by the turbine installed on the same shaft or connected to it through a gearbox. This thrilling tandem results in a bunch of rotor dynamics effects inherent in pumps, turbines, high-speed rotors, cryogenic temperature materials, etc. And all these effects must be carefully taken into account during rotor dynamics studies.
A standard schematic of an internally geared turbopump consists of the liquid hydrogen (LH2, fuel) and liquid oxygen (LO2, oxidizer) rotors.
Although the rotor dynamics model is usually simpler than the CAD models, it looks quite complicated in the case of the turbopump. The rotors contain sections that are hollow and sections with some elements inside the hollow space.
Lateral rotor dynamics are considered for separate rotors in the assembly. It incorporates a huge amount of effects arising from the complex structure of the rotor and cryogenic conditions of the pumped liquid.
These effects include:
- The aerodynamic cross-coupling generated in the turbine impeller.
- The static radial forces in pump volutes due to uneven clearances in them.
- The hydrodynamic imbalances in the pump impellers.
- The added masses of the pumped liquid around the turbopump shaft.
- The actual stiffness and damping coefficients of rolling element bearings supporting the rotor.
- The liquid annular seal stiffness and damping affecting the lateral rotor dynamics response.
The maneuver loads generating the rotor prestressed state due to the rocket acceleration
The features of the rocket turbopump rotors mentioned impose strict limitations on the structural finite-element code which an engineer uses to predict the rotor vibration response.
This code should be capable of calculating the dynamic characteristics of the rotor auxiliary components and hydrodynamic issues, such as bearings, supports, seals, and aerodynamic cross-coupling.
In addition, the code should provide the comprehensive rotor modeling capability including the shaft design; lumped mass-inertia elements; bearings and supports; couplings; forces and accelerations affecting the rotor stress state and its prestress conditions, etc.
Finally, comprehensive solution settings and post-processing capabilities are necessary for obtaining accurate results which correspond to the modern requirements and allow engineers to analyze the rotor’s safe operation with regards to its stress state, vibration response, stability, transient issues occurring during its operation, etc.
The torsional vibrations of the turbopump rotor assembly are not a less important part of the rotor dynamics analysis. The whole rotor assembly connected through the gearbox modeled as flexible couplings should be analyzed to check for the possible resonances due to different excitation sources occurring within or near the operational speed range which can be observed on the Campbell diagram.
The differences in the rotor rotation speed due to various gear ratios should be obligatorily taken into account during these calculations. In addition, the transient excitation sources should be analyzed carefully to check for the rotor’s safe operation influenced by unsteady torsional moments due to aerodynamics issues – peak stress amplitudes in the rotor should not exceed their steady limits and fatigue conditions should be satisfied.
As you can tell from the above, analyzing a space vehicle’s engine turbopump rotor dynamics requires engineers and software to work at the cutting edge of technology just like the rockets themselves do.
To analyze the turbopump rotor dynamics, a huge number of different effects and conditions should be taken into account. This requires the comprehensive calculations of the rotor auxiliary components, bearings, and different types of analyses. AxSTREAM Bearing and RotorDynamics provide users with a lot of capabilities to calculate the dynamic properties of liquid annular seals; rolling element bearings; aerodynamic cross-coupling; as well as the whole spectrum of lateral and torsional rotor dynamics analyses with observations of the criteria for rotor non-resonant, safe and stable operation according to the modern standards.