## 5 Conclusions  
In this study, the effect of a high mass fraction of aluminum particles on the performance and nozzle ablation of hybrid rocket motors is investigated experimentally. The main conclusions are as follows:  
1. The specific impulse of a hybrid rocket motor with 95% hydrogen peroxide and a combination of 38%, 48%, 58%, and 65% mass fractions of aluminum particles and HTPB is 185.79 s, 186.82 s, 188.91 s, and 192.46 s, respectively. The addition of aluminum particles results in an increase in the specific impulse. The addition of aluminum particles decreases the optimal oxygen-to-fuel ratio of the propellant combination, and significantly increases the density-specific impulse in the interval of the oxygen-to-fuel ratio from 1 to 4. The  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Fuel** & **Nozzle Material** & \(d_{1}\) (mm) & \(d_{2}\) (mm) & **Work Time (s)** & **Ablation Rate (mm/s)** \\ \hline
38\%Al + 62\%HTPB & Carbon Ceramic & 15.293 & 15.923 & 4.18 & 0.075 \\
48\%Al + 52\%HTPB & Tungsten-copper Infiltration & 14.882 & 14.562 & 4.82 & \(-\)0.033 \\
58\%Al + 42\%HTPB & Carbon Ceramic & 14.655 & 16.152 & 4.55 & 0.19 \\
65\%Al + 35\%HTPB & Tungsten-copper Infiltration & 15.002 & 14.270 & 3.87 & \(-\)0.099 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Nozzle test results.  
Figure 8: Results of electron microscopy scans of the grain surface. (**a**) 65%Al (front side of grain after test). (**b**) 65%Al (post side of grain after test). (**c**) 58%Al (front side of grain after test). (**d**) 58%Al (post side of grain after test). (**e**) 48%Al (front side of grain after test). (**f**) 38%Al (front side of grain after test). (**g**) 48%Al (front side of grain before test). (**h**) 38%Al (front side of grain before test).  
grain with a 58% mass fraction of aluminum particles has the highest combustion efficiency, which is 88.88%.  
2. Due to the lower pyrolysis temperature of HTPB compared with that of the aluminum particles, the aluminum mass fraction on the surface of the grains at the front of the axial position after the test is much higher than that added to the grains before the test. In contrast, the aluminum content on the surface of the axially positioned grains is slightly higher than that added to the grains before the test, indicating that the reaction and heat transfer on the front surface of the grains is more intense compared with the back surface of the grains.  
3. The ablation rates of carbon ceramic nozzles with grains containing 38% and 58% aluminum mass fractions are 0.075 mm/s and 0.19 mm/s, respectively, and -0.033 mm/s and -0.099 mm/s for tungsten-copper infiltration nozzles with grains containing 48% and 65% aluminum mass fractions, respectively. The tungsten-impregnated copper nozzle is more resistant to ablation than the carbon ceramic nozzle. After four to five seconds of testing, the tungsten-impregnated copper nozzle shows less variation in throat diameter, which can guide the selection of materials for nozzles for motors containing aluminum.  
Methodology, Y.L.; Formal analysis, H.Z.; Investigation, H.T.; Resources, H.T. and G.C.; Data curation, Z.W. and J.G.; Writing--original draft, Z.W.; Writing--review & editing, H.T. and H.Z.; Project administration, G.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding.  
Not applicable.  
Data Availability StatementData is contained within the article.  
The authors declare no conflict of interest.