Overview
Flexible inhibitors have been used in the solid rocket motors (SRMs) to control the burning of propellant. Vortices generated by the flow of propellant around the flexible inhibitors have been identified as a driving source of instabilities that can lead to thrust oscillation in launch vehicles. Potential coupling between SRM thrust oscillations and structural vibration modes is an important risk factor in launch vehicle design. The capability to accurately predict these oscillation features is crucial to the development of NASA’s new Space Launch System (SLS).
Engineers at NASA Marshall Space Flight Center have applied a newly developed, multidisciplinary simulation tool to investigate the interactions between pressure waves and the three flexible inhibitors inside the SLS’s reusable SRM. This tool solves the complex, fully coupled fluid-structure interactions involved in rocket motor combustion and vibration, and provides a valuable asset for designing safe propulsion systems.
Project Details
Solving fluid-structure interaction problems requires coupling two totally different solvers: one for computational fluid dynamics (CFD) and one for computational structural dynamics (CSD). The new simulation tool couples the Loci/CHEM CFD solver with the CoBi CSD solver through the interchange of boundary variables across the solid-fluid interface. This modular approach enables the application of well-established and optimized methods for the flow and the structure, respectively. The tool also employs a special time iteration technique to ensure strong coupling between the two solvers. The SLS SRM simulations use a production-level CFD large-eddy simulation turbulence model with a grid resolution of 80 million cells. Three flexible inhibitors are modeled with around 2,500 3D shell elements.
Results and Impact
Verification of the tool’s structural solver shows excellent agreement with analytical results for displacement distributions and modal frequencies. The preliminary coupled results indicate that, due to acoustic coupling, the dynamics of one of the more flexible inhibitors shift from free vibration to forced vibration at the first acoustic frequency of the solid rocket motor.
The multidisciplinary tool will also be used to study other fluid-structural phenomena in the SLS propulsion system, such as:
- Liquid propellant tank breathing due to propellant interaction with the flexible tank shell.
- Interactions between the water suppression system on the launch pad and ignition overpressure waves during liftoff.
- Fluid-induced vibration in delivery pipes with bellows.
- Fluid-thermal-structural coupling in rocket engine nozzles.
- Cavitation-induced vibration in turbopump inducer blades.
Role of High-End Computing Resources
Capturing the detailed vortex structures and interactions inside SRMs requires very fine grid resolutions, which make these simulations both computationally intensive and memory intensive. All of our simulations were performed on NASA’s Pleiades supercomputer, using 1,000 processors and 1 to 3 weeks of runtime for each case.H. Q. Yang, NASA Marshall Space Flight Center
hong.q.yang@nasa.gov Jeff West, NASA Marshall Space Flight Center
jeffrey.s.west@nasa.gov
