Characterization of the Propulsion System for Submersible Multimedium Robotic Vehicles

Multimedium robots are a new class of bioinspired vehicles with the ability of cross-domain transition and maneuverability across multiple domains. Such hybrid robots can transverse both in air and water, overcoming the stark differences in the motion dynamics within these mediums. Differences in the density, viscosity, and additional fluid-induced forces of the medium make the propulsion system design for multimedium robots a crucial challenge. Both unmanned aerial and remotely operated underwater vehicles rely on electrical propulsion coupled with propellers to navigate in their own respective medium of operation. However, due to the density differences in water and air, which is approximately 103 times, an aerial propeller demands higher RPM to generate significant thrust, unlike an underwater propeller. Higher water density and fluid inertia, on the other hand, demand a greater in-drive torque to spin the propeller in underwater conditions. Other critical parameters to be considered while choosing a suitable propulsion system are propeller diameter, propeller pitch, shaft torque requirements, operational RPM of motor–propeller pair, cavitation, and propeller tip deflection. This article presents an in-depth investigation and evaluation methodology to identify the feasible operating range for propellers and motors that meet the aerial and underwater thrust requirements while minimizing cavitation and propeller deflection. The thrusters (motor-propeller pair) are investigated individually, where propeller performance and physical constraints on the propeller blade are analyzed using blade elemental momentum theory (BEMT), while the motor is studied using the three-constant motor model. These subsystems are then mapped using torque equilibrium to determine their specific operational regime and overall efficiency. Numerical simulation in QPROP demonstrates that aerial thrusters can be utilized for both the mediums but with a sacrifice of overall efficiency. The optimal motor efficiency was 73.06% in aerial operation and 8.1% in underwater operation, but with increased thrust underwater due to higher fluid density, which is essentially a design tradeoff. The results of the analysis are experimentally validated and further presented as a framework for selecting an appropriate propulsion subsystem for multimedium vehicles based on the specific user-defined design requirements.