Hybrid Rocket Engine Injector Design & Test

This was a first-year engineering project at Ecole Centrale Paris, done with the CNRS EM2C combustion lab. The goal: improve the feed system of a hybrid rocket engine that uses N2O (nitrous oxide) as oxidizer and ABS plastic as fuel.

Hybrid rockets are interesting because they're simpler and safer than liquid engines, but they come with their own headaches — oxidizer sloshing in the tank during flight, and getting the injection right so combustion stays stable.

During flight, the rocket vibrates and the liquid oxidizer sloshes around in the tank. This messes with the feed rate and can interrupt combustion entirely. We designed a device that combines rings and plates inside the tank to damp all three sloshing modes (anti-symmetric, rotating, and chaotic).

The whole thing weighed 350g — about 7% of the filled tank mass. We tested it on a dedicated bench with a transparent Plexiglas tube so we could actually watch the liquid behavior. The device cut wave energy by a factor of 3 across the full frequency range.

Standard axial injectors give you stable combustion but poor atomization. Swirl injectors atomize well but create pressure oscillations. We designed something in between — an axi-swirl injector with a central axial orifice surrounded by a swirl vortex chamber.

We machined the prototype from brass, ran CFD simulations to validate the flow split (about 40% axial, 60% swirl), then tested it with cold water flow and live fire. The reacting tests used a modified Sky Ripper hybrid motor. The reference axial injector produced 310N of thrust with an ISP of 186.4 seconds. Our axi-swirl design showed an 11-second ISP improvement for the same combustion chamber geometry.

The work was published at the 5th European Conference for Aeronautics and Space Sciences (EUCASS) in Munich, 2013. It was a proper research contribution — not a student showcase, an actual conference paper with peer review.