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This work analyzes and presents theoretical performance of a marine water-breathing ramjet propulsor. A conceptual scheme of the motor is shown, the equation of thrust is presented, and the dependence on cruise velocity and depth are discussed. Different propellant compositions, representing a wide variety of formulations suitable for propelling a water-breathing ramjet, are investigated. The theoretical results reveal that the specific impulse of a water-breathing ramjet can increase by as much as 30% compared to a standard rocket, when using a conventional hydroxyl terminated polybutadiene (HTPB)-ammonium perchlorate (AP) propellant, which does not react chemically with the water. When employing a water-reactive propellant containing metal particles such as magnesium or aluminum, the specific impulse may be more than doubled. The thrust coefficient of the propulsor was computed at different cruise velocities and depths and was found to be greater than the predictable drag even at significant depth.

This research focuses on theoretically and experimentally evaluating the performance of a metallized hybrid-propellant, water-breathing ramjet. The aluminum and/or magnesium particles added to a polymeric (polyester) fuel grain are hydro-reactive, using the surrounding water as an oxidizer, in addition to a source of gas. Theoretically, the metal additives significantly increase the specific impulse of the motor, and as the percentage of the hydro-reactive ingredient increases, the theoretical performance increases as well. Additionally, aluminum is more energetic than magnesium. However, it was experimentally discovered that the addition of aluminum beyond 20% resulted in a slag formation and did not increase the specific impulse. Adding 30% of magnesium was relatively favorable to aluminum due to its better reactivity, enabling the achievement of an actual specific impulse of up to 485 s at standard conditions, approximately double the performance of common solid rockets.

This paper presents, for the first time, firing tests and results measured during a hybrid-propellant marine water-breathing ramjet operation at underwater cruise conditions. The firing test demonstrated ignition under water, stable combustion, and a significant increase in chamber pressure due to the introduction of water by the ram effect. The exclusive test facility used to accommodate this unique experiment is described along with a detailed explanation of the motor design. In addition, a theoretical model to predict the performance of the motor is shown along with the numeric process used to solve the model. Measurements taken during the dynamic firing test reveal good compatibility with the theoretical model. The theoretical model was also used to evaluate values that could not be measured during the experiment. The results found during this work fit previous results attained by static firing tests, demonstrating significant improvement in specific impulse due to addition of water. An improvement of 50% in specific impulse was demonstrated when comparing the results of an underwater dynamic firing test of a marine ramjet to those of a parallel rocket test conducted under identical conditions in the water basin.

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