Multiphysics Framework for Prediction of Dynamic Instability in Liquid Rocket Engines

Mitigation of dynamic combustion instability is one of the most difficult engineering challenges facing NASA and industry in the development of new continuous-flow combustion systems such as the combustion chambers in liquid-fueled rocket engines (LREs). Combustion instabilities are spontaneous, self-sustaining oscillations that tie the combustor acoustics to the combustion reaction itself. These oscillations can lead to a wide range of problems from off-design performance to catastrophic failure. Efforts to predict instabilities at design-time is hindered by the complex, multi-physics nature of the acoustics and chemistry, typically requiring multiple iterations of time and resource intensive system prototyping. The proposed Phase I STTR project aims to develop a simulation framework that will enable accurate, design-time prediction of instabilities. This framework will leverage the capabilities of Loci/CHEM for massively parallel, multi-physics flow simulations to generate low-order, independent models of combustion and acoustic response to perturbations. By solving for simultaneous solutions of these low-order perturbation models, it will be possible to numerically map the acoustic modes of the system to their stability characteristics, providing a means to predict instability. Phase I will develop critical additions to Loci/CHEM's combustion modeling capabilities, develop the appropriate acoustic models, develop a test plan for experimental validation of the combustion model, and conclude with a proof-of-concept demonstration of the full framework. In Phase II, an experimental campaign will be carried out to validate the combustion modeling tools developed in Phase I and augment the simulation framework with multi-phase modeling appropriate for full-scale LRE combustion chambers.

NASA is spearheading the creation of this next generation of LREs through their Evolvable Mars Campaign and ongoing development of the Space Launch System (SLS) and Commercial Crew/Cargo launch vehicles. A number of engine development programs are currently underway and could benefit from the improved predictive accuracy resulting from this technology; examples include the design of J-2X (or RL-10) as the upper stage engine for SLS, and adaptation of the Space Shuttle main engine (RS-25) for use on the SLS core stage. Similarly, potential future programs for which combustion instability will be a key factor include development of a common upper stage/in-space stage engine and booster options for SLS Block 2, including a Large Oxygen-Rich Staged Combustion (ORSC) cycle engine and a Large Gas Generator (LGC) cycle engine. As prime contractor for many of the liquid propulsion systems used by NASA, the DoD, and international space agencies, Aerojet Rocketdyne is a key potential customer of this technology. In addition, customers of the modeling tools or ATA's accompanying services include engineers at the NASA centers developing the aforementioned missions and at companies developing propulsion systems for other launch vehicles (e.g., Blue Origin's BE-4 staged-combustion rocket engine to power the United Launch Alliance (ULA) next-generation launch system, Vulcan and SpaceX's development of the Raptor engine for their planned Interplanetary Transport System).

Combustion instability poses significant technological and economic challenges in the gas turbine power generation and aviation propulsion fields. GE, Siemens, and others have invested heavily in combustion instability research over the last few decades as they have sought practical low NOx gas turbine combustors for power generation. Numerous mitigation strategies, both passive and active, have been studied and implemented as instabilities arise during testing, however the ability to predict instabilities and make system changes at design time has remained elusive. In the aviation community, the challenges of combustion instability have largely precluded the use of low emissions combustors in favor of safer, but less efficient systems. The challenges that we seek to address for rocket combustion chambers are analogous and in many respects, more extreme than those facing gas turbines. The tools resulting from this effort will be directly applicable to design-time analysis for power generation and aviation combustors.

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