## 4 Results and Discussion  
Figure 4 shows the measured combustion chamber pressure and the thrust of the test. The oxidizer valve opening and closing once in a short time is one pulse. In order to achieve a rapid ignition, five pulses are added before ignition to raise the temperature of the catalytic bed, thus promoting the efficiency of the decomposition of hydrogen peroxide. The duration of each pulse is 100 ms, and the gap for each pulse is 600 ms. After five pulses, the oxidizer valve opens at 4.5 s and closes at 9.5 s. The test lasts for five seconds. It is obvious from the combustion chamber pressure curve and thrust curve that an unstable combustion is generated. Furthermore, this oscillation does not occur at the beginning of the engine build-up, but after a period of operation. A possible explanation is that the addition of the high fractional aluminum particles results in an unstable combustion. The unique combustion mechanism of the hybrid rocket motor is diffusion combustion. A vast number of aluminum particles are blown into the flow field as droplets for combustion [24], and the aluminum particles are highly susceptible to coupled oscillations with the airflow. Possible ways to suppress this instability are to reduce the particle size of the aluminum particles and to increase the space of the pre-combustion chamber [25]. Figure 5 shows the instantaneous regression rate versus time and oxidizer mass flux. The instantaneous regression rate in Figure 5a is calculated according to the method described above. As can be seen from Figure 5a, the regression rate shows a general trend of a rapid decline with time. The instantaneous regression rate can be calculated from Equation (7). Therefore, the instantaneous regression rate generally oscillates significantly after the engine has been operating steadily for 1-2 s synchronized with the combustion chamber pressure. As can be seen in Figure 5b, the grain with a 65% mass fraction of aluminum particles decreases from 1.57 mm/s to 1.08 mm/s in the oxidizer mass flux range of 130-160 kg\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\), which is higher than the grain with 58% and 48% mass fractions of aluminum particles. Furthermore, in the oxidizer mass flux range of 110-130 kg\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\), the grain with a 65% mass fraction of aluminum particles decreases from 1.08 mm/s to 0.75 mm/s, which is between the grain with 58% and 48% mass fractions of aluminum particles. The regression rate of the grain adding a 38% mass fraction of aluminum particles is between that of the grains with 58% and 48% mass fractions of aluminum particles in the oxidizer mass flux region from 105 kg\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\) to 120 kg\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\). Among the four  
Figure 4: Measured combustion chamber pressure and thrust of test. grains with aluminum particle mass fractions of 38%, 48%, 58%, and 65%, the grain with the mass fraction of 58% decreases the most smoothly with the diminishing oxidizer mass flux. Compared with neat HTPB grains [18], it is clear that the increase in the aluminum mass fraction of the grain enhanced the regression rate. The regression rate of the 90 HP and pure HTPB is 0.37 mm/s with an oxidizer mass flux of 93 kg\(\cdot\)s\({}^{-1}\cdot\)m\({}^{-2}\). The regression rate of the grain with a mass fraction of 38% aluminum particles is 0.56 mm/s, which has increased by 51.35%. Table 3 shows the performance results. As can be seen from Table 3, the specific impulses of the hybrid rocket motor with 95% hydrogen peroxide and a combination of 38%, 48%, 58%, and 65% mass fractions of aluminum and HTPB are 185.79 s, 186.82 s, 188.91 s, and 192.46 s, respectively. The average regression rates of the grains containing 38%, 48%, 58%, and 65% mass fractions of aluminum are 0.78 mm/s, 0.96 mm/s, 1.07 mm/s, and 1.12 mm/s, respectively.