As efforts to access and explore space increase, emerging rocket technologies will undeniably continue to rely on chemical propulsion, as it remains the only feasible way of providing Earth-to-orbit access and has proven invaluable in extending in-space capabilities. Accordingly, advancements are needed to improve combustion performance and the understanding of underlying chemical and physical phenomena governing chemical propulsion systems, as they have a substantial impact on the mission capabilities of flight vehicles and spacecrafts. While modeling efforts have made significant progress in recent years, empirical studies remain a necessity in the development of advanced propulsion systems; however, limits of traditional instrumentation often preclude definitive interpretation of flow-field phenomena in experimental tests and are not suitable for in-situ measurements at the extreme temperatures and pressures present in rocket propulsion systems. As such, optical diagnostics have become an attractive tool in combustion science due to the non-intrusive nature of the measurement and flexibility in the measured properties (temperature, species concentration, pressure, velocity, number density, etc.) inferred via spectroscopic interaction. Therefore, to advance the next generation of chemical propulsion systems, advanced optical diagnostic tools need be developed for characterizing propulsion test facilities and developing/validating computational models for complex chemically reacting flow-fields.
The work herein describes novel advancements in laser absorption spectroscopy for characterizing liquid- and hybrid-propellant rocket combustion systems with support of a new High-Enthalpy Shock Tube (HEST) facility at UCLA. Using the shock tube facility to emulate the high temperatures (T > 3000 K) and high pressures (P > 100 bar) present in liquid-propellant rocket combustors, a novel laser absorption spectroscopy sensor that exploits line-mixing effects in the infrared spectra was developed for temperature, carbon monoxide (CO), and carbon dioxide (CO2) measurements. This sensor was then demonstrated on a liquid-propellant rocket combustor at the Air Force Research Laboratory (AFRL) in Edwards Air Force Base with kerosene (RP-2)/oxygen and methane/oxygen propellant combinations. Successful thermochemistry measurements were obtained at pressures up to 105 bar–marking a significant improvement in the pressure capability of optical diagnostic tools. In addition to these liquid-propellant rocket combustor measurements, a unique approach was developed for investigating hybrid rocket propulsion flows. Using laser absorption tomography, spatially-resolved measurements of temperature, carbon monoxide (CO), carbon dioxide (CO2), and water (H2O) were obtained in the reaction layer of a hybrid-propellant rocket combustor with poly(methyl methacrylate) (PMMA)/oxygen and high-density polyethylene (HDPE)/oxygen propellant combinations and two injector geometries (single port and axial showerhead). These measurements highlight combustion physics and thermochemical energy conversion in the spatial domain and help identify mechanistic losses in combustion efficiency for different engine configurations. Lastly, in efforts to develop and refine combustion models for real fuels used in chemical propulsion systems, a novel time-resolved, laser absorption spectroscopy technique was developed for measuring the formation of isotopically-labeled carbon monoxide (12CO and 13CO) in shock tube oxidation experiments of isotopically- labeled fuel blends. The technique was demonstrated by examining competitive oxidation of methane (CH4) with differing C2 hydrocarbon functional groups (alkane, alkene, alkyne), namely acetylene (C2H2), ethylene (C2H4), and ethane (C2H6). By isotopically-labeling specific fuel components of the overall fuel mixture and simultaneously measuring both 12CO and 13CO, individual reaction pathways and rates are distinguishable, providing kinetic targets for reaction mechanisms used to model fuel blends present in chemical propulsion systems.