# A Review of Progress and Hydrodynamic Design of Integrated Motor Pump-Jet Propulsion
## 1 Introduction  
An integrated motor pump-jet (IMP) is a propeller in which the motor stator is embedded in the pump-jet duct and the motor rotor is integrated with the tips of the pump-jet impeller blades rotating together at the same speed and in the same direction, with an air gap between the motor stator and rotor. It is a modern kind of underwater vehicle propulsion machine, corresponding to the modular design model, and consists of a propulsion system and an electrical system. The working principle of the IMP is that after energization, the motor air gap is opened into an air-gap magnetic field, and the current-carrying armature winding and the air-gap magnetic field interact to produce raw electromagnetic torque. The electromagnetic torque drives the motor rotor and, at the same time, drives the pump spray rotor circumferential rotation of the water work backward spray and the water flow to the pump spray axial reaction force to drive the propulsion carrier forward, which is used to ensure the rapidity of the propulsion carrier indicators. The most important characteristics of this system are the compactness, high reliability, and low noise. This system could be directly driven by a high-power-density engine, specifically adapted to low noise, low vibration, and a finite volume as the main design goals of the underwater vehicle. The core technical advantage of the pump-jet is the complete integration of the shaftless propulsion technology motor with the outstanding advantages of the pump-jet's low noise and high critical speed, which allows the advantages of the pump-jet's acoustic performance to be exploited while eliminating the shaft system's acoustic excitation. At the same time,the transplantation of the propulsion motor from inside the boat to the interior of the thruster allows for a significant improvement in the effective use of space within the cabin. As with conventional pump-jets, the pump-jets can also be divided into two types: front-mounted stator shaftless pump-jets and rear-mounted stator shaftless pump-jets, depending on the axial position of the pump-jet stator blades in relation to the rotor blades. In recent years, pump-jet thrusters (also known as pump-jets) have been widely used in underwater vehicles for their advantages such as their high propulsion efficiency, low radiation noise, strong anticavitation ability, and high critical speed [1], especially in submarines and torpedo propulsion. For example, the American \"MK48\" torpedo, the Seawolf-class submarine, the Virginia-class submarine, and the British Smart-class submarine all make use of this technology. The propeller is attached to the shaft to drive the propulsion, while the opposite side of the shaft is put on the engine in conventional propulsion (Figure 1a). The main engine is located in the hull, so that the entire structure extends through the nacelle space. The bearings are installed on the stern with the middle circumferential and radial fixed shafts. In addition to this, it is necessary to decelerate the speed using a gearbox for the main propulsion device of a high-speed ship. When the underwater vehicle is operating, considerable frictional power loss, high levels of noise, and high levels of vibration may occur in the shafts, bearings, and gearboxes, etc. Since the 1990s, researchers have been interested in podded propulsion (Figure 1b), which is now a common design in the marine industry [1]. The pod propeller has a uniformly operated wake, low vibration, and low noise, and it can also provide thrust at multiple angles. Therefore, it plays the role of a rudder compared to the traditional propulsion method. Furthermore, it is beneficial to the interior of the engine room's space layout, due to its flexible arrangements. A propeller, a bracket, and a pod make up the pod propeller, whereas the general propeller is a four-blade fixed-pitch propeller.