Overview
There are three common ways of propelling a rocket: Solid, hybrid and bi-liquid motors. All three motor types require both a fuel and an oxidizer, which can be burned when mixed together. This combustion releases the necessary energy to move the rocket upwards.
So far, TUDSaT has only flown rockets using commercial solid rocket motors, in which fuel and oxidizer come in pre-mixed propellant grains. The teams overarching goal is to fly bi-liquid engines, in which both propellants are handled in a liquid state (e.g. liquid oxygen and liquid methane - a common mix for new orbital rockets). Hybrid propulsion is - as the name suggests - a hybrid between these two: while one propellant is handled in a solid grains, the other one is carried as a liquid. The lessons learned from this hybrid engine will get us closer to our bi-liquid goal.
TUDSaT’s Hybrid Engine
As opposed to the commercial solid rocket motors TUDSaT flew before, the hybrid engine is developed completely in-house. The design philosophy is simplicity while building the foundation for the switch to bi-liquid engines. The engine uses nitrous oxide as oxidizer, which is pressurized by nitrogen, not unlike to typical bi-liquid designs. Paraffin is used as the base component of the fuel grain, while additives improve the characteristics of the grain. The engine burns 7 kg of fuel-oxidizer mixture within 5 s to produce a thrust of up to 2.5 kN.
The oxidizer tank and the combustion chamber adhere to the design guideline of simplicity and are constructed from aluminium tubing with machined bulkheads. Combined with the high pressure nitrogen tank and the valvery, the complete stack stands 2m tall and has a mass of around 24 kg. The most interesting features however are found in and on the combustion chamber.
Armored Grain
In simple terms, the fuel grain is simply a cylinder of solid fuel with a single or multiple holes along is axis, called “the port”. Paraffin is a great choice of fuel as it has a high regression rate, i. e. it melts and vaporizes quickly, making large quantities of fuel available for combustion in a small amount of time. This allows for short and high thrust burns without the need of complex port geometries. However, paraffin is prone to fracturing and cracking under the high mechanical loads experienced during the burn. This issue is solved by introducing a combination of additives. TUDSaT's custom fuel formulation therefore uses a macroscopic “armoring” gyroid structure 3D printed from ABS, as well as Carbon Black and EVA polymer additives. This ensures a high regression rate while achieving acceptable mechanical properties.
Injector Plate
The injector plate forms the barrier between the liquid nitrous oxide at 55bar of tank pressure and the hot gases inside the combustion chamber at a pressure of 25bar. It is machined precisely to allow around 1kg/s of nitrous oxide mass flow to enter the combustion chamber at a pre-defined pressure drop of 30bar. Equally important, the injector plate converts the stream of nitrous oxide to a fine spray of particles. This ensures good mixing of oxidizer and fuel in the melt layer near the surface of the fuel grain.
The injector plate is designed as inclined shower head injector to achieve these properties. It has 55 tiny holes that produce a fine spray. The holes are angled from the vertical by 10° to reduce the particle size even further.
Igniter
For the ignition of the hybrid engine, we use a second hybrid engine - just scaled down by two orders of magnitude. Due to the smaller scale, the heat released from a glow plug is sufficient to light this pilot engine. The hot gases then start the vaporization of fuel from the fuel grain in the main combustion chamber and eventually ignite the combustion of fuel and oxidizer there. This innovative approach allows us to check engine parameters during ignition and have the capability for cancelling ignition in case of parameters being out of certain bounds. Compared to torch igniters, our design makes the rocket a fully self-contained system, exploring possibilities for in-flight ignition for projects in the future.
Nozzle
Our Nozzle is subdivided into two parts: a throat insert and an insulator. The throat is the one of the most critical parts in the engine, as it constricts the exhaust flow, experiencing extreme thermal and mechanical stress. To withstand these harsh conditions, we are pioneering the use of fiber-enhanced concrete - a material that has, to our knowledge, never been flown on a student rocket before. The insulator will prevent the immense heat at the throat from damaging structure-critical components.
Test Fires
As of now, we are preparing for a test fire campaign - updates will follow soon!