A Laser Absorption Spectroscopy Sensor Suite for High-Speed Measurements in a Model Solid Fuel Ramjet: Thermometry With Inlet H 2 O and Exhaust CO Concentrations

Two laser absorption spectroscopy diagnostics were developed and deployed to measure temperatures and species concentrations at the inlet and exhaust of a solid fuel ramjet (SFRJ) test facility. Custom optically accessible hardware enabled non-intrusive, in-situ measurements under representative facility conditions and was designed for seamless integration into the settling chamber and exhaust throat for the inlet and exhaust diagnostics, respectively. The inlet diagnostic used two distributed-feedback lasers near 1.4 μm, targeting H2O transitions, using a time-division multiplexed, scanned direct absorption scheme at 5 kS/s. The exhaust sensor used a quantum cascade laser near 4.9 μm to probe two CO transitions with a scanned wavelength-modulated spectroscopy technique at 4 kS/s. Time-series measurements at a baseline condition (505 K, 2.72 atm, and 0.34 kg/s at the inlet) demonstrate the sensors’ ability to resolve thermodynamic behavior, as well as initial low-frequency facility-driven fluctuations consistent across both measurement locations. Furthermore, estimates of soot volume fraction in the exhaust are provided. These initial results provide the foundation for a complete sensor suite to support future full-facility characterization and numerical model validation.