## 1 Introduction  
A hybrid rocket motor is a form of propulsion system that typically uses solid fuel and a liquid oxidizer [1, 2]. Due to the benefits of hybrid rocket motors, such as multiple starts and stops, adaptable thrust adjustments, and superior safety, research on hybrid rocket motors has become a hot topic in recent years [3, 4, 5]. The regression rate is a crucial parameter in the operation of a hybrid rocket motor. The regression process directly contributes to the fuel mass flow rate and the oxygen-fuel ratio of the motor, which determines the specific impulse and thrust of hybrid rocket motors. The combustion of the hybrid rocket motor, which differs from other engines, is diffusion controlled [6, 7, 8]. During the operation of hybrid rocket motors, the gaseous oxidant flows over the channel of the solid fuel, the fuel is heated to produce pyrolysis products, and the pyrolysis products diffuse to react with gaseous oxidants. The heat transfer from the combustion zone maintains the pyrolysis of the fuel and determines the pyrolysis rate of the fuel. Due to diffusion combustion, the application of hybrid rocket motors is generally limited by the low regression rate.  
Many researchers have conducted numerous studies to improve the regression rate of hybrid rocket motors. Injection schemes [9; 10; 11; 12], fuel types [13; 14; 15; 16], and the addition of fuel additives have been demonstrated to be constructive methods for improving the regression rate of the hybrid rocket motor. The injection type variation mainly improves the efficiency of mixing, which in turn enhances the heat transfer between the reaction zone and the fuel wall and consequently the regression rate. The formation of a low-viscosity liquid layer on the grain surface is the intrinsic difference between classical and non-classical combustion mechanisms, and this liquid layer contributes to a higher regression rate during the combustion process [17]. The addition of solid particles to the grain mainly enhances the energy release and results in the improvement of the flame temperature and heat feedback from the combustion products [18; 19]. Meanwhile, the addition of solid particles effectively improves the density-specific impulse of the propellant combinations and reduces the optimal oxygen-to-fuel ratio. [PERSON] et al. [20] reported that adding NaBH\({}_{4}\) and AlH\({}_{3}\) generates up to a 47% and 85% growth in the regression rate, respectively, compared to neat dicyclopentadiene. The characteristic velocity efficiency is not efficient and varies from 80% to 90%. [PERSON] et al. [21] demonstrated that adding metal nanoparticle fuel additives to HTPB-based fuels can significantly benefit the regression rate. Aluminum particles are a common metal addition to grains. The inclusion of aluminum particles within fuels can lead to an increase in the specific impulse, volumetric heat of oxidation, adiabatic flame temperature, heat of combustion, and radiative heat transfer [22]. Our previous findings [23] have shown that the addition of aluminum is more efficient than the addition of aluminum hydride for the enhancement of the regression rate. There exists a limit in how much aluminum can be added to a propellant. The inclusion of aluminum particles increases the regression rate of the propellant combination. However, the combustion becomes less efficient and the nozzle ablation can be aggravated when an excessive amount of aluminum particles are added.  
The purpose of this study is to explore the influence of a high mass fraction of aluminum on the performance of motor and nozzle ablation. 95 HP and HTPB with an aluminum additive are utilized as propellants. The experiment results indicate that adding aluminum particles into solid fuel boosts the average regression rate and reduces the optimal oxygen-to-fuel ratio. The addition of a high mass fraction of aluminum particles has a severe ablative effect on carbon ceramic nozzles, while the effect on tungsten-bronze nozzles is minimal for a hot test lasting four to five seconds.