• atomization
• hybrid rockets
• injectors

The concept of the hybrid rocket propulsion has been known for a long time. In the case of HRE one propellant is liquid while another is solid. Therefore HRE can potentially have the advantages of solid and liquid rocket engines simultaneously. The attribution for reaching these advantages lies in the injection unit. The injection system is the main and practically the only system of control of a HRE. Therefore the injection unit defines the thrust and performance of HRE.
The current status of HRE technology in Europe is sounding rockets and sub-scale ground tests. In order to pass on technology with real application, the European Commission supports the Operational Research Project on Hybrid Engines in Europe ORPHEE within the 7th Framework Program (FP7). The project aims to develop HRE technology, where the first applications of the technology could be booster engine and upper stage engine.
The aim of this work is to contribute to the ORPHEE project by collecting and presenting the current state of the art. It is clear that the HRE specifies has specific requirements with regard to the injection system different from those for current liquid rocket engines and demand a unique injection system. Thus the dawn of this work is to review the existing LRPE injection systems and determination of kinetics of the liquid phase in the combustion chamber of HRE.
The kinetics of the liquid phase in the hybrid engine, the different types of jet and liquid sheet break-up are examined. Furthermore a correlation between the different atomization processes over a wide range of dimensionless numbers is made.
One of the main distinctive features of hybrid engines is the possibility of thrust variation. The different types of injection, which are used in rocketry, have been considered in this study. In case of hybrid engines shower head, self impinging, swirl injectors, and their combinations can be utilized. The atomization characteristics are defined by Reynolds and Weber numbers, which are dependent on pressure drop, injector design and properties of liquid propellant.
The range of Reynolds and Weber numbers, in which the injector operates, defines the break-up regime of the jet (in showerhead injectors) or of the sheet (in impinging jet and swirl injectors) and defines the maximum size of the droplets injected into combustion chamber with the solid propellant. However, in order to choose the injector that gives the best droplets distribution, it is necessary to know the grain configuration.
In a hybrid engine it is very important to achieve uniform distribution of liquid propellant over the whole surface of solid propellant. From this study, the swirl injector offers more than others because its characteristics are stable and can be varied over a broad range of parameters. Varying the parameters of swirl injection (tangential and radial velocity, nozzle angle) it is possible to get the desired distribution of liquid phase in a short combustion chamber. In the case of a long solid grain a uniform distribution can be achieved using a combination of swirl and impinging jet injectors. However, the choice of one type of injection gives a clear connection between the parameters of injectors and the desired combustion characteristics.