# A Study on Combustion Characteristics of Insensitive Triple-Base Propellant
## 3 Results and Discussion  
### Self-Illuminating Properties of the Combustion Flame  
With the high-speed camera, the whole ignition and combustion processes were recorded. Figure 2 showed the flame self-illumination image during the ignition delay period, with 2# propellant as an example. The ignition delay time was one of the important parameters to describe the ignition process. There were various definitions for the ignition delay time. Considering the ignition method employed in the present study, the ignition delay time was defined as the period from the moment when the electronic pulse was started to the moment when the initial flame appeared obvious, and the flame area was about 10 mm\({}^{2}\). The initiation time of the pulse igniter was denoted as 0 s. The self-luminous image of the combustion reaction of 200 mg propellant can be obtained by continuous shooting with the high-speed camera. By analyzing the images taken by the high-speed camera, the ignition delay time and combustion time of the same propellant were derived as the average value of repeated experiments. To further understand the ignition and combustion process of the insensitive propellant, the present study took the 2# insensitive propellant as an example and defined the ignition time of the pulse igniter as 0 s. The combustion process of the 2# insensitive propellant was shown in Figure 3. The whole combustion process could be divided into a flame development period (0-0.9 s), a stable burning period (1.0-2.2 s), and a flame decline period (2.3-2.9 s). After the propellant was ignited, it rapidly developed upward in a short time. The burning intensity gradually increased, the flame brightness enhanced, and, then, most of the particles started to burn, entering the stable burning period. As time went by, the flame gradually entered the decline period. The flame height and area decreased. The flame brightness also gradually declined, and it finally completely disappeared. It is obviously seen that the flame development time was longer than the decline period. This might be because the overall energy density was reduced after the addition of the desensitizer, and the burning rate hence decreased. Additionally, the composition of desensitizer had an obstructive effect on flame combustion in the development period. Figure 4 shows the ignition delay time and combustion time of different propellants. It can be seen that, compared to the blank propellant, the ignition delay times of four insensitive propellants were obviously greater. This indicated the application of desensitizer had a partly hindering effect on the early ignition stage, and the insensitive propellants needed higher ignition energy and longer ignition time. The ignition delay period reflects the desensitizing effect of energetic materials to a certain extent. With longer ignition delay period, the thermal response of energetic materials is slower, and the thermal stability is better, and the effect of the desensitizer is more obvious. Specifically, the ignition delay time of 2# was the longest, and that of 4# was the shortest among the four insensitive propellants. Among these four groups of samples, there was no clear correlation between the content of desensitizer and ignition delay time. It should be noted that 2# had the lowest desensitizer content of 1%, while 4# had the highest desensitizer content of 1.5%. This indicated that the ignition process could be affected by both desensitizer content, as well as the desensitization method, and the effect of the desensitization method could be more significant. Regarding the combustion process, the combustion time of insensitive propellants was mostly similar to that of blank propellant, which means the desensitizer had little influence on the intensity of actual combustion. Thus, it was concluded that the performance of the  
Figure 3: The combustion process of the 2# insensitive propellant. Figure 2: The ignition process of the 2# insensitive propellant. propellant was limitedly affected by adding an amount of desensitizer.