Propellant and System Performance Analysis of a Water-Breathing Ramjet for Underwater Propulsion

High-speed torpedoes exceeding 100 m/s generate substantial ram pressure that can be leveraged to enhance propulsion through water ingestion into the combustion chamber. The water-breathing ramjet mixes the ingested water with hot combustion products, vaporizes it into steam, and increases the nozzle mass flow rate. When hydro-reactive metals are present, the ingested water acts as an oxidizer as well, releasing additional energy. This work presents a systematic thermochemical analysis of propellant formulations for water-breathing ramjets, evaluating hybrid and solid systems with different fuels, oxidizers, and hydro-reactive metal additives. Key performance metrics, including specific impulse and density-specific impulse, are examined as functions of vehicle velocity, operating depth, and water-to-propellant mass flow ratio. Hybrid propellants based on polyester and liquid oxygen achieve specific impulses up to 470 s, representing 55% improvement over conventional hybrid rockets, whereas hydrogen peroxide provides a storable alternative with favorable density-specific impulse. Solid propellants exhibit a lower baseline specific impulse but attain a competitive density-specific impulse due to their higher oxidizer density. Metal additives further enhance performance, with boron at 30 wt% offering the most significant improvement and producing theoretical specific impulses of about 545 s in hybrid systems and 558 s in solid propellants, roughly a 230% increase over conventional rockets. Sample calculations using VA-111 Shkval geometry indicate that water-breathing ramjets can accomplish representative supercavitating missions with roughly half the propellant mass and volume of conventional rockets. These results demonstrate the potential of water-breathing ramjets and provide guidance for selecting propellant compositions based on mission requirements and constraints.