# A Study on Combustion Characteristics of Insensitive Triple-Base Propellant
## 4 Conclusions  
In this paper, spontaneous luminescence, NO, NH chemiluminescence, and OH-PLIF methods were used to investigate the combustion phenomena of five triple-base propellants. The combustion performance of different propellants was comparatively evaluated. The main conclusions are as follows.  
(1) The insensitive propellant needs higher ignition energy and longer ignition time, and the duration of flame development is longer than that of decline, which means the desensitizer has an obstructive effect on flame propagation in the early ignition stage and the development period. However, the combustion time of insensitive propellants are mostly similar, showing the desensitizer has little influence on the intensity of whole combustion.  
(2) The intensity of early combustion of some insensitive propellants are lower, but higher in late stage of development, which means the combustion progressivity are more prominent. In the actual process of desensitizer, the desensitizer concentration and desensitizing methods all affect the performance of the insensitive propellant.  
(3) The NO signal concentrates are in the interior of the flame. The OH signal locates in outer part of the flame, indicating that there might be transformation between NO/NH and OH. The NH radical mainly appears upstream of the flame. Additionally, there might be a competitive relationship between the formation of NO and NH radicals during the reaction.  
(4) The maximum signal intensity of NO and NH are different for the five propellants, demonstrating that the desensitizer effect is affected not only by the concentration of desensitizers, but also by the desensitizing methods. It is interesting that the signal of NH in insensitive propellants is significantly lower than blank one. Future mechanism research should focus these initial findings of the formation and change in NH radical in the reaction process of insensitive propellant.  
The investigation of the complete combustion process, as well as the distribution and concentrations of NO, NH, and OH radicals, can characterize the microscale flame structure to a certain extent, help to identify the dominant factors in combustion, and also help modelers to create dynamic and flame structure models. However, the composition of the tri-base propellant is complex, so further kinetic work is necessary to learn the detailed chemical reaction process and the influence factors.  
Conceptualization, Q.L. and J.W.; methodology, Y.Y. and Z.Y.; validation, Y.Y., T.Z. and Z.Y.; resources, B.L.; funding acquisition, B.L., Q.L. and Z.H.; writing--original draft preparation, Y.Y.; writing--review and editing, Q.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Natural Science Basic Research Plan in Shaanxi Province of China (2020 JM-053). [PERSON] from Liao Ning Qingyang Chemical Industry Corporation was acknowledged for providing the experimental materials.  
The data presented in this study are available upon request from the corresponding author (Q.L.).  
The authors declare no conflict of interest.