Modernization of CFD Tools for Advanced Propulsion Applications

Predictive, analytical modeling of subcritical combustion physics is a challenge that bridges several state-of-the-art and state-of-the-practice limitations of currently available models. Practically all spacecraft propulsion systems operate in a subcritical propellant injection regime during nominal operations. The maturation of advanced chemical propulsion systems with high efficiency and reliability is critical to NASA's missions for human exploration on the Moon and Mars. Development of these systems requires robust and accurate computational fluid dynamics (CFD) models to help guide design decisions and understand combustion dynamics and performance observed in testing or operation. Rotating Detonation Rocket Engines (RDREs) operate by injecting liquid fuel and/or oxidizer into an annular combustion chamber. These propellants are initially in a liquid state and undergo liquid break-up and atomization once introduced into the combustion chamber as dictated by their fluid properties and injection dynamics. In an RDRE application, a supersonic combustion wave impacts the injected propellent stream, further atomizing the injected fuel and oxidizer streams to enhance combustion in an unsteady combustion environment. The potential advantages of RDREs have led to great interest in development of flight-scale RDREs, but with limited success to date due in part to a lack of understanding of the complex multiphase fluid injection and combustion physics. In traditional, subsonic combustion rocket engines, the initial start-up operation of the engine requires that the fuel and oxidizer are injected in a subcritical fluid regime. The success of the rocket engine performance relies on characterization of ignition and combustion dynamics. In the case of liquid hypergolic rocket engines, liquid fuel and oxidizer are injected, break up and atomize, undergo extremely energetic liquid-liquid chemical reactions and simultaneous vaporization followed by gas-phase chemical reactions, with a range of timescales from microseconds to tens of milliseconds. Hypergolic engines have been plagued with damaging transient combustion events throughout the history of their development, with mitigations typically resulting from extensive test campaigns to develop operational keep out zones, with significant residual risk due to a lack of physical understanding of these transient combustion events. Predictive analytical models could potentially avoid engine development issues and ensure reliable, predictable combustion dynamics across the range of liquid rocket engines from traditional chemical, hypergolic, to RDRE. There are currently no validated or even pragmatic analytical tools that can predictively model the physics of subcritical propellant injection, break-up, phase change and combustion. Model development in these areas of subcritical liquid injection and break-up, interface tracking, phase change, and multiphase combustion kinetics mechanisms are needed in order to develop predictive analytical tools that can be used in the design of advanced chemical propulsion systems. Particular focus should be placed on coupling subcritical approaches to existing methodologies used for trans- and supercritical regime flows.