Multi-Wavelength Laser Opacity Study of a Hybrid Rocket Plume

An instrumentation system was developed to measure the opacity of a hybrid rocket plume as a function of optical wavelength. The source consisted of collineated beams from two lasers, providing seven wavelengths in a single probe beam. Detection was accomplished with a spectrograph equipped with a photodiode array. Previous work with a two-wavelength system demonstrated the ability to follow the changes in opacity level of a hybrid rocket plume during the various stages of a typical firing cycle. The present work was to investigate the feasibility of using a multiple wavelength system to acquire more detailed information about the particulates present in the hybrid rocket plume. Qualitative analysis of the plume particulates was done by comparison of the relative extinction coefficients of the laser wavelengths with published extinction coefficient curves from Mie scattering theory. While it was found that light level fluctuations in the system prevent definitive conclusions, the data suggests that the particulate matter in the plume may consist of some optically transparent material. This is in contrast to the absorbing, soot-like material that might be expected in a hybrid rocket plume.