What is a hybrid rocket engine?
Like all chemical rocket engines, hybrid propulsion uses an oxidizer and fuel. The term hybrid is derived from the respective aggregate state of the two components. While oxidizer and fuel are used in liquid form in liquid propulsion systems, they are present in solid form in solid propellants. Hybrid systems combine the states of aggregation, whereby the oxidizer is usually found in liquid form and the fuel in solid form.
Why a hybrid rocket engine?
By combining the drive concepts, the following advantages of liquid and solid drives are combined:
- Safety: Spontaneous ignition is not possible due to the local and physical separation of the chemical components
- Control: Unlike a solid motor, the oxidizer supply can be controlled and the journey stopped at any time
- Costs: Due to its low complexity and the omission of a pump for the oxidizer, the hybrid drive is comparatively inexpensive
- Research and development: Compared to other propulsion systems, very little research has been done on a hybrid drive. Especially the current developments in additive manufacturing open up new possibilities
Areas of responsibility
The tank must not only withstand the oxidizer's working pressure of approx. 70 bar, but must also be made as light as possible by skillful selection of materials and manufacturing technologies. Thermodynamic simulations are used to determine the load situation, which serves as an input parameter for a necessary FEM simulation of the tank structure.
Security plays a major role here. The oxidizer must be fed into the combustion chamber in a controlled manner and encounter as little resistance as possible. The main task is the thermodynamic calculation and selection of fluid line components.
Important for efficient combustion is the dispersion of the oxidizer, which is achieved with the help of the injector. With different geometries and drillings, the flow can be influenced in various ways and the oxidizer can be atomized.
A calculation model provides the thermal and mechanical loads that must later be absorbed by the housing, insulation, seals, and other machine elements. At a pressure of up to 35 bar and a gas temperature of more than 3000 K, this application requires special materials to meet the lightweight construction requirements.
There are different fuels, mostly kerosene wax, HTPB, ABS, or other plastics are used. Especially with ABS, the shear profile of the drive can be significantly influenced by additive manufacturing processes. The exact chemical composition finally influences the thrust and the burn-up rate of the fuel and is optimized with additives to our requirements.
An optimally designed Laval nozzle provides the necessary thrust with maximum efficiency of the drive. Here too, high-temperature-resistant materials such as graphite are used to withstand the thermal loads over the entire combustion period.
We follow the principle: "Test as you fly", which means that we use the same components for our test bench as for our rocket. This means that the proven system can be integrated into the rocket without having to do major modifications. Another important point besides the components and system tests are the "cold-flow tests" and "hot-fire tests". In a old-flow test, the engine is not ignited, which allows us to test the injector and fluid flow system without stressing the combustion chamber and seals. In a hot-fire test, the engine is fired and develops its full thrust. The most important measured data for verifying our calculations are temperatures, pressures, oxidizer mass flow, and thrust.
Since many components are modified by us and we work with high-energy materials, it is essential that we can guarantee maximum safety for everyone involved. For this reason, the test facility is fully automated. Besides, several redundant safety devices ensure the overall safety of the system.