Fiber-Optic Raman Analysis for Health Monitoring of Solid Rocket Motors Used in the GMD Program

We propose developing a compact, optical fiber-based Raman scattering instrument for rapid, in-situ health monitoring and analysis of solid rocket motors and propellants relied on for the Ground-based Midcourse Defense (GMD) program. The instrument willincorporate diode laser-based, full-spectrum Raman scattering for continuous in-situ remote monitoring of the composite and propellant chemistry. Narrow well defined Raman peaks and lack of sample preparation requirement make Raman scattering well suitedfor in-situ analysis of composites and solid rocket propellants. Raman spectroscopic analysis can be made via small (200 mm core), inert, silica/silica clad optical fibers. One fiber is used for sample excitation via laser diode, and another fibercollects the Raman scattered light and carries the signal to a remote, portable spectrograph. A semiconductor laser diode offers long life, can be very compact, and can operate at near-IR wavelengths where sample fluorescence is minimized while takingadvantage of the very sensitive and full-spectrum capabilities of silicon CCD array Raman spectrum detection. Many optical fibers could be multiplexed to allow depth profiling of selected solid propellant regions such as bond line and areas adjacent tocasing liner. Similar optical fibers could be cast into witness panels of propellant mixtures without damaging the surrounding propellant, liners, insulator, or case structure. The Raman spectra will be used to monitor curing, migration of plasticisersand other contaminants, and other aging effects that could alter propellant chemistry. If this techniques proves valuable, we will address positioning similar optical sensor fibers directly into a rocket motor while the propellant is being cast. Thiswould allow monitoring of propellant chemistry ensuring proper curing conditions and reduce excess cure time and rework. Phase I results demonstrated the ability of Raman scattering to quickly identify the basic components used in polymer composites andsolid rocket propellants. Phase II will demonstrate the ability of casting optical fibers into typical rocket motor propellant systems and using remote Raman scattering analysis to monitor real-time propellant chemistry and basic health characteristics.