## 4 Results and Discussion
The addition of aluminum particles leads to an improvement in the specific impulse and regression rate, and the grain with a 58% mass fraction of aluminum particles has the highest combustion efficiency. Figure 6 shows the CT scan images of the grains after the test, from which it can be seen that there is a distinct area of end-burning at the front and post-side of all the grains. The end-burning area on the front face is caused by the direct exposure to high-temperature peroxide decomposition products, while the end-burning area on the post-face is attributed to the swirling effect of the post-combustion chamber. The effect of the direct exposure to high-temperature peroxide decomposition products on end-burning at the front face is much more dramatic than the swirling effect of the post-combustion chamber. All four sets of grain CT images in Figure 6 show this trend. Figure 7 shows the axial distribution of the regression rate. The curves in Figure 7 show the effect of removing the front- and rear-end combustion regions. At the front of the grain, the maximum speed of the regression rate occurs. As the axis distance increases, the regression rate decreases rapidly until it reaches  
Figure 5: Instantaneous regression rate versus time and oxidizer mass flux. (**a**) Instantaneous regression rate versus time. (**b**) Instantaneous regression rate versus oxidizer mass flux. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Fuel**} & **Oxidizer Mass Flow** & \multirow{2}{*}{\(p_{\text{c}}\)**(MPa)**} & \multirow{2}{*}{**O/F**} & \multirow{2}{*}{\(I_{\text{sp}}\) (s)**} & **Average Regression** & **Combustion** \\  & & & & & **Rate (mm/s)** & **Efficiency (\%)** \\ \hline
38\%Al + 62\%HTPB & 165.2 & 1.53 & 3.13 & 185.79 & 0.78 & 87.68 \\
48\%Al + 52\%HTPB & 157.3 & 1.74 & 2.71 & 186.82 & 0.96 & 85.35 \\
58\%Al + 42\%HTPB & 155.3 & 1.70 & 2.29 & 188.91 & 1.07 & 88.88 \\
65\%Al + 35\%HTPB & 163.3 & 1.81 & 1.94 & 192.46 & 1.12 & 85.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Performance results. a minimum value, then it increases slowly and gradually decreases. The reasons for this trend are as follows: at the front part of the grain channel, the flame layer is relatively thin and closest to the grain surface, resulting in a high temperature gradient between the flame layer and the grain surface. A higher heat flow density leads to a higher regression rate. However, at the same time, due to the continuous addition of fuel mass flow from the upstream grain, the total mass flow in the grain channel is increasing, which causes an increase in the heat flow. The combined effect of these two factors leads to the above trend in the axial distribution of the regression rate. Figure 6: CT scan images of the grains after the test. Figure 8 shows the results of the electron microscopy scan of the grain surface. The spot scans of the electron microscope are repeated three times and the normalized results are shown in the graph. The scanned areas in the graph show that the mass fractions of aluminum on the front surface of the grain after the test are 83.69%, 86.76%, 58.76%, and 51.34% for grains with 65%, 58%, 48%, and 38% mass fractions of aluminum, respectively. It is true that an elemental analysis by electron microscopy cannot detect hydrogen, and the elemental content selected is mainly based on the composition of the main elements blended in the grain before the test; therefore, it will result in a slight overexvaluation of the aluminum content on the surface of the grain after the test. The presence of HTPB can be clearly seen on the surface of the grain before the test. After the test, it can be seen that the mass fraction of aluminum on the front surface of the grains is higher than that of the initial mass fraction of the grains, and as the mass fraction of aluminum of the grains increases, the front surface of the post-test grains gradually becomes encapsulated by spherical crystals, making the presence of the HTPB matrix more and more difficult to identify.