# A Study on Combustion Characteristics of Insensitive Triple-Base Propellant
## 3 Results and Discussion
### NO and NH Chemiluminescence Characteristics  
The gas-phase products near the combustion surface of propellants can be divided into five main categories: NO\({}_{2}\), CH\({}_{2}\)O, CHO, CH, and CO. They represent oxidant, reducing agent, crackable radical, and the two neutral radicals in sequence [23]. During the reaction process, complex macromolecular propellants decomposed and oxidized continuously, producing gas-phase intermediates, such as NO\({}_{2}\), which could be consumed through the following reactions, and free radicals such as OH and NO would be produced in the whole process. Chemiluminescence refers to the phenomenon that free radicals excited by chemical reactions produce light radiation during their return from the excited state to the ground state. Consequently, information about the flame structure and flame process can be identified through chemiluminescence. Since most energetic materials are nitrogenous substances, the chemiluminescence of NO and NH radicals in energetic materials can provide reliable information and important reference for understanding the combustion reaction process. The moment when the NO signal appeared was defined as the beginning. Figure 8 showed that the distribution of NO chemiluminescence signals in the flame varied with time for different propellants. It can be seen that the overall signal intensity showed a trend of first increasing and then decreasing. The NO signal appeared inside the flame, but the main signal was concentrated in the upstream and midstream wings of the flame. It was speculated that the appearance of the NO signal might originate from two different transformation processes. Figure 8: NO chemiluminescence signal distribution. (**a**) 1\(\#\); (**b**) 2\(\#\); (**c**) 3\(\#\); (**d**) 4\(\#\); (**e**) 5\(\#\). The appearance of NO at the bottom may be derived from the conversion of the intermediate NO\({}_{2}\). NO\({}_{2}\) was an important intermediate product in the pyrolysis of triple-base propellant. The dissociation energy of N-NO\({}_{2}\) and O-NO\({}_{2}\) bonds in the propellant was very low, so NO\({}_{2}\) can appear at early low temperatures [24]. After NO\({}_{2}\) was produced, it trapped protons to form HONO, in which the -OHO bond breaks to form NO [25]. Additionally, the appearance of NO involved the decomposition of solid-phase products, including the NO\({}_{2}\)-catalyzed decomposition of nitrate ester [26; 27; 28], which accelerated the breaking of N-NO\({}_{2}\) and O-NO\({}_{2}\) bonds. As seen in Figure 8, the appearance of NO in the two wings might be caused by the reaction with the intermediate gas-phase products, including CH\({}_{2}\)O and CO. With the increase in flame height, the NO signal was further weakened, and the unstable NO was converted into stable reaction products, including N\({}_{2}\), H\({}_{2}\)O, etc. The NO signal map captured by LaVision was extracted to obtain the change in the maximum signal intensity of NO in the flame over time, as shown in Figure 9. It can be seen that the maximum signal intensity of NO increased with the development of flame and decreased with the decline of flame, and the maximum NO signal appeared in the stable burning period. By comparing the maximum NO signals of five propellants, it was found that the maximum signal intensity of the four insensitive propellants was higher than that of the blank one. This indicated that the concentration of NO generated by the insensitive propellants were higher in the actual combustion process. Regarding NH signal, the moment of NH signal appearance was defined as the beginning. Figure 10 showed the distribution of NH chemiluminescence signals with time for different propellant flames. It could be seen that, similar to the NO signal, the overall NH signal presented a trend of first strengthening and then weakening. The stronger NH signal, i.e., the higher NH radical concentration, was mainly found in the upper part of the flame.