The engine project of our rockets

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Chimæra is the first Hybrid Rocket Engine, developed and built by Skyward, with the specific aim of flying on a student-developed sounding rocket. It will participate to 2023 EuRoC competition and push Skyward’s rocket to 3000m apogee.

Its design exploits the know-how acquired through-out the tests performed with our test motor: HRE-Mini. The propellant used is hybrid: nitrous oxide as bi-phasic oxidant, and 3D printed ABS composing the solid fuel grain (a plastic filament with which Lego is also made).

Chimæra is composed of three smaller subsystems:
– combustion chamber, containing the fuel grain and the injection assembly
– tank and the supply lines, allowing for proper storage and feeding of oxidant to the engine
– ignition system, allowing for the ingnition of the engine

Chimaera Skyward

Combustion Chamber

Chimæra’s combustion chamber contains the injection assembly, which includes a 15-holes injection plate, the graphite convergent-divergent nozzle, and the 1260 g ABS grain. The combustion of the propellant mix produces around 1300 N of average thrust for 9.8 s, with an initial peak of 1800 N. The total impulse is around 13000 Ns while the specific impulse is approximately 200 s. The grain geomtry chosen for the first tests is the single circular perforation and its combustion is able to develop a mean regression rate of 0.8 mm/s.

Tank and Feed Line

The custom tank the Propulsion Team developed, consists of a cylindrical aluminum body and two threaded caps, containing the nitrous oxide at a pressure of 57.7 bar. A safety valve and a venting line are connected on the top cap during the refuelling phase, once the rocket is stationary on the ramp. The test cofiguration features also two pressure transducer and one thermocouple.

The supply line is a steel pipe long around 100 mm and 1/2″ in diameter, on which a custom servo-valve is mounted: this allows the connection between the tank and the combustion chamber.

injection subsystem

The injection subsystem consists of an injection flange, which aim is to keep the combustion chamber airtight and accommodate the injection plate. The latter is used to atomize the oxydizer entering the chamber through an important pressure drop. This way, oxydizer evaporation is favored and the combustion speed enhanced.

Ignition Subsystem

The ignition subsystem consists of a pyrotechnic igniter. The latter is a solid propellant disc made of potassium nitrate and sorbitol. The ignition of the propellant takes place thanks to an incandescent Nickel-Chromium filament placed inside the propellant disc during the casting phase of the same, and remotely activated. The ignition of the solid propellant generates the pyrolysis of the solid grain and the developement of hot gases to start the combustion of the engine.


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