Hybrid rocket engine and injection systems. 2011
Simona Silvestri 109
Chapter 6
Conclusions and Future Developments
6.1 Conclusions
The injection unit is the key element of the hybrid rocket engine. The performance (fuel efficiency) and operation stability (absence of instable regimes) are defined by the quality of injector design. One of the main distinctive features of hybrid engines is the possibility of thrust variation. This feature is gained also through the injection system. In fact the control of the hybrid engine is provided mainly and virtually only by the injection system.
The different types of injection, which are used in rocketry, have been considered in this study. In case of hybrid engines, where only one propellant is liquid, 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. When the parameters of hybrid engines are defined, the kinetics of the liquid phase in it can be described using the expressions from the study.
A rocket engine is an integrated unit, and injector systems cannot be considered separately from feed system and solid grain. Under these circumstances the choice of the injection system should not be undertaken before the characterization of the feed system, and grain composition and configuration.
Hybrid rocket engine and injection systems. 2011
Simona Silvestri 110
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.
6.2 Future Developments
In this work a literature research has been done to understand the mechanism of droplet atomization in three different types of injectors. A future development of this work is to do experimental tests to find the best combination of injector type and solid grain configuration to avoid the problem of low regression rate and the loss of propellants during the combustion process.
In the last couple of years the space market is expanding and more money has been allocated for the research. Nowadays four main applications for hybrid engines are: the use of a hybrid upper stage on top of a small launcher (as VEGA type) thanks to a payload mass gain due to a higher hybrid propellants performance (specific impulse is improved); a lunar lander because of the interest of cryogenic oxidizer coupled with a thrust modulation to soft-landing and powerful descent; a martian lander because of the interest of storable oxidizer coupled with a thrust modulation to perform pinpoint landing; and the use of a hybrid 1st stage for a low-cost small launcher (as Falcon1 type) because of the ability of the hybrid system to manage the dynamic pressure and gravity load in atmospheric flight and the flexibility of such an engine to achieve several missions with the same stage but with a different adjustment.
