Fiber-optic evanescent wave spectroscopy of subsurface layers of solid propellant combustion

We report the development of a novel method of measuring the concentration of chemical species in the subsurface layers of solid propellant burning, using probes made of flattened silver halide optical fibers, which had been preembedded in a burning solid propellant. These probes made it possible to perform high-resolution infrared absorption measurements in the narrow region of the evanescent field of the infrared near the probes. The resolution is in the order of the wavelength λ. Measurements at a wavelength of λ = 4.5 μm revealed the profile of N2O gas emitted in the preheated zone, before the flame front. Other gases can be detected by measurements at different wavelengths. The demonstrated technique can aid the modeling and simulation of combustion thermochemistry.