Stabilizing additives are added to solid rocket propellant systems to slow the break-down of energetic nitrogen-based compounds utilized in solid rocket propellants. Over time this results in a reduction of stabilizers and an increase of inert compounds, which decrease propellant performance. Raman spectroscopic techniques can detect changes in chemical concentrations due to the strong spectrum that these compounds demonstrate. In this study, two wavelengths, 532 nm, and 785nm are used to analyze the Raman spectra of samples to characterize the changes to compounds over time. Computational techniques are demonstrated to mitigate fluorescence and improve the signal-to-noise ratio of chemical peaks specific to stabilizer compounds. Fluorescence in the 532 nm Raman spectrum is examined as a method for characterizing propellant compounds, as 2-Nitrodiphenylamine (2-NDPA) traditionally has more fluorescence than Nmethyl- 4-nitroaniline (MNA), and the 532 nm Raman system traditionally detects more fluorescence than the 785 nm Raman system. Detection of the stabilizer, MNA, in concentrations ranging from 0.38% to 0.75% is demonstrated. Raman spectroscopy is shown to provide a rapid method for analyzing high and low concentrations of stabilizer compounds to determine the remaining viability of the propellant.
The primary objective of this effort is to demonstrate the efficacy of the Raman spectroscopy technique for detecting and evaluating the health of propellant stabilizers commonly used in missiles stored under a range of ambient conditions. Tincured silicone rubber doped with a commonly used propellant stabilizer N-methyl-4-nitroaniline (MNA) and ammonium nitrates used in explosives has been investigated using 532 nm and 785 nm wavelength laser Raman systems. The detected propellants’ Raman peak intensity ratios are used to analyze the results. Calibration curves with error bars are created using more than 30 data runs. The results indicate both systems are suitable to detect fractions of these chemicals as low as 0.2 percent within a few seconds of integration time. The calibration curves created for all the samples measured show a consistent linear increase to the ratio indicating the reliability of the measurements.
Energetic nitrogen-based compounds utilized in solid rocket propellants break down under typical environmental conditions. The breakdown of these energetic propellant compounds requires stabilizing additives to absorb excess acids that form. These chemical changes result in a reduction of stabilizers and an increase of inert compounds over time which decrease propellant performance. Vibrational spectroscopic techniques such as Raman can detect changes in chemical concentrations due to the strong spectrum that these compounds demonstrate. In this study two wavelengths, 532 nm and 783 nm, are used to analyze the Raman spectra of propellant samples to characterize the changes to compounds over time. Computational techniques are demonstrated to mitigate fluorescence and single out the ratio of chemical peaks specific to stabilizer compounds. In addition, fluorescence in the 532 nm spectrum is examined as a method for characterizing propellant compounds, as 2NDPA traditionally has more fluorescence than MNA, and the 532 nm Raman system traditionally detects more fluorescence than the 785 nm Raman system. Detection of the stabilizer MNA in concentrations of greater than .70% and lower than .40% are demonstrated. Raman spectroscopy is shown to provide a rapid method for analyzing high and low concentrations of stabilizer compounds to determine the remaining viability of propellant.
A sensitive Raman spectroscopy technique is used for detection and possible quantification of the propellant stabilizer, nmethyl nitroaniline (MNA), in solid rocket propellants used in multiple domestic missile systems. Over time, the energetic ingredients of the propellant will degrade and react with the stabilizer, causing issues with the propellant useful safe life. Currently, there are no non-destructive analytical techniques for which MNA can be detected in solid rocket fuel inside a missile. Therefore, after a certain amount of time, missiles in inventory must be disassembled and tested for reliability and safety. This methodology is labor intensive, costly, and time consuming so a less intrusive approach is warranted to determine a missile useful safe life. Raman spectroscopy provides a possible solution to this problem, where a small fiber optic probe line may be inserted into the rocket motor of the missiles, which can be tested within seconds without the need for dismantling the missiles. A 785 nm portable Raman analyzer is used for all measurements reported in this paper with integration times ranging from 10 to 60 s. It is found that Raman sensing is a viable option for detection of MNA in solid rocket fuels.
Stabilizers are added to nitrate ester-based rocket motor propellants to form a stable product. The products added to stabilize the propellants react with NOx and are gradually exhausted over a period of time. In this paper, we demonstrate the efficacy of Raman spectroscopy technique for nondestructive, inexpensive, and rapid evaluation and monitoring of the depletion of rocket motor propellant stabilizers. Preliminary results show that concentrations as low as 0.1% of both MNA and 2-NDPA dissolved in DMSO (Dimethyl sulfoxide) can easily be detected at 1 second integration time using a 785 nm wavelength Raman system. In addition, MNA concentrations between 0.37% and 0.54% are detected in propellant samples containing energetic constituents using a 60 second integration time.
The objective of this study is to demonstrate a sensitive Raman technique for sensing degradation of propellant
stabilizers like MNA and 2-NDPA that are commonly used in some missiles. The functionality of missiles and rockets
are often evaluated by being fired or decomposed at routine time-intervals after prolonged storage. However, these
destructive testing techniques for determining long-term rocket motor aging and shelf-life are extremely costly. If
successful, the Raman technique could be utilized to determine the health of propellant stabilizers without dismantling
the missiles as is commonly done at present. Raman technique is to measure concentrations of propellant stabilizers
between 0.1-2% in glycerin. Two different lasers at 785 nm and 532 nm are used for developing this technique. A
secondary objective is to develop a theoretical model that predicts temperature as a function of time and position inside
the cylindrical storage container of MNA or 2-NDPA stabilizer. This model can help in understanding the thermal
degradation of propellant stabilizers.
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