## 2 System Composition and Working Sequence  
### Composition of Electric Pump Feed System  
The propellant of the engine is hydrogen peroxide (90 wt.%) and kerosene. This study focused on a hydrogen peroxide feed system including a gas tank, an oxidant tank, two electric pumps, motors, motor controllers, oxidant valves, pipelines, and other components, as shown in Figure 1.  
The pressurized gas system contains a gas tank, a pneumatic valve, a pressure regulator, etc. Since the system uses an electric pump to pressurize the propellant, the pressure of the regulator outlet does not need to be too high. This pressure is only needed to ensure that no cavitation occurs at the pump inlet. The gas tank contains high-pressure nitrogen. The nitrogen passes through the pressure regulator and reaches the required pressure. The oxidant tank is mainly used to store high-concentration hydrogen peroxide. Downstream of the tank is a pneumatic valve, which is mainly used to isolate the propellant in the tank and the downstream pipe. The high reliability of the pneumatic valve can ensure the effectiveness of the isolation. After the propellant passes through the pneumatic valve, it is divided into two paths and enters the centrifugal pump separately. The centrifugal pump is a device that pressurizes the propellant from a low pressure to a high pressure. There are solenoid valves at the inlet and outlet of the centrifugal pump to control the propellant. The power source to drive the centrifugal pump is a high-speed permanent magnet synchronous motor (PMSM) [27]. The propellant passing through the two electric pumps converges into the flow regulating valve. The flow-regulating valve follows the  
Figure 1: Schematic diagram of oxidant feed system of the engine. principle of cavitation to control the flow into the thrust chamber. The flow of propellant in the line is controlled by changing the throat area of the flow-regulating valve. The thrust chamber mainly consists of the injector, the catalytic bed, the combustion chamber, and the nozzle. Injectors enable the impact and atomization of the propellant, which plays a very important role in combustion. The catalytic bed is the device that catalyzes the decomposition of high-concentration hydrogen peroxide into high-temperature oxygen and water vapor. High-concentration hydrogen peroxide is storable at room temperature, green, and nontoxic [28; 29], which has led to its widespread use. The combustion chamber is the main place where the hydrogen peroxide decomposition products are burned with kerosene [30; 31]. The main purpose of the nozzle is to enable the expansion of the high-pressure gas expand, thus increasing the gas velocity, which in turn generates thrust. The temperature of the high-concentration hydrogen peroxide decomposition products is much higher than the ignition point of kerosene; thus, no ignition device is needed [32]. The decomposition products of high-concentration hydrogen peroxide and kerosene are capable of self-ignition and continuous combustion. The above-described rocket engine was used in a combined power vehicle in operating conditions at high altitude with a high Mach number. It had a maximum thrust of 33.6 kN and an operating condition adjustment ratio of 4:1. The engine consisted of three different operating stages. To ensure the performance of combustion, the mixing ratio was set to 7.5, which is basically near the optimal mixing ratio. With such a large thrust, it is difficult for one electric pump to achieve such a large power. Therefore, two electric pumps were used in parallel in this study, concentrating on the starting and regulating scheme between the two electric pumps. The technical parameters of the system are shown in Table 1.  
The system underwent the operating process shown in Table 2. The first and third stages of the operation were relatively long. The first stage denotes the minimum operating condition, and the third stage denotes the maximum operating condition.