Spatially Offset Raman Spectroscopy for Depth Resolved Propellant Analysis

Physical Sciences Inc. (PSI) will develop a compact, time-gated Raman sensor for non-contact, measurements of solid rocket motor (SRM) propellant. The Raman sensor, currently under development at PSI, uses a high throughput spatial heterodyne interferometric spectrometer (SHS). The SHS is more sensitive to Raman photons than a conventional diffraction grating based slit-spectrometer operating with the same spectral resolution. The Phase I effort focused on measuring energetic SRM propellant samples that were generated in-house, and characterizing a limit of detection. The limit of detection for the Phase I breadboard was determined to be 0.25 % wt. at an SNR of 5 for stabilizer in uncured energetic propellant samples. However, fluorescence was observed in cured samples. The current Raman design will be modified in the Phase II to include time-gating, which will allow for detection of stabilizer within SRM propellant while rejecting fluorescent signal. This system will utilize a commercial 532 nm laser, as well as a borescope to increase the sensor’s standoff distance. This will allow for the borescope to be inserted through the nozzle of the motor and interrogate the propellant, removing the need for sample preparation prior to measurements.