Rocket propulsion technology relies on extensive knowledge and understanding of the combustion properties of propellants. Understanding the thermochemical processes in solid propellants' burning layer is crucial for rocket motor design and development. Full understanding of the governing processes requires the rate and type of chemical reactions involved.

Fiber evanescent wave spectroscopy is used for detecting small traces of gases in liquids and the air. Using this method can help investigate specrtoscopically the thin burning layer in solid propellants, as the sensing evanescent field is in the order of the wavelength. Since the chemical mole fraction gradients near the burning layer of a solid propellant are very steep, a mid-IR absorption probe based on the evanescent field can achieve a resolution of a few microns, depending on the optical properties of the media involved and the exact wavelength.

The work that will be presented shows the development of such a probe, based on silver halide IR-transmitting fibers that were flattened to increase sensitivity and greater compatibility with the flat burning surface. Experiments were performed on a nitramine propellant, which has a liquid burning layer, using a special burning chamber. In addition microthermocouple probes were placed alongside the optical probe, to allow temperature measurement as well. Using a wide thermal source and narrow IR filters, the vibrational absorption lines of N₂O gas were distinguished at the wavelength of 4.5 microns, and the spatial profile of its relative molar fraction was revealed. This is the first glimpse of the sub-surface thermo-chemical process underlining the burning of solid propellants.