Influence of the Oxidizer Injector Design on the Performance of a 1-kN Paraffin-Fueled Hybrid Rocket Motor

Hybrid rockets are an attractive alternative to solid and liquid rockets for their unique features, in a world becoming more careful about safety, costs and environmental impact. In particular, hybrid rocket motors based on N2O/Wax or GOX/Wax are good candidates for future transportation systems, permit to hybrid propulsion systems to become a technology asset for launchers and new generation space transportation systems. Hybrid motors combine advantages of both solid and liquid systems. Indeed, these systems use half the piping of liquid propellant rockets but maintain the flexibility of functioning and a low risk of explosion typical of solid rockets. Unfortunately, hybrid motors also have certain disadvantages, the main one is the low rate of fuel regression. International researches are mostly focused on finding a solution to this issue, to develop a competitive hybrid rocket. Several techniques to enhance the regression rate have been tested in recent decades, for instance, the addition of metal powders to increase the radiative heat flux to the surface of the solid fuel and increase of fuel density. This results in higher viscosity and therefore makes the manufacturing process more difficult. One of the earlier tested techniques in large motor thrust is the use of multiport grain, with a wagon wheel geometry, to increase the burning surface instead to use a long motor. However, this technique has some issues, such as the excessive unburned mass fraction, complex design and manufacturing, irregular burning of the individual port, leads to a compromised grain structural integrity, and requirement for a substantial pre-chamber or individual injector for each port. Basically, the limit of the regression rate of conventional hybrid fuels is limited by the physical phenomena of heat and mass transfer from the relatively remote flame zone to the fuel surface. An attractive and promising solution is the use of liquefied fuels, such as paraffin. Laboratory studies of this fuel have shown a regression rate 3 to 4 times higher than that of conventional hybrid fuels. The reason for this improvement in performance is that the combustion mechanisms of paraffin are different from those of traditional fuels. The present research focuses on the use of different type of injectors producing different types of flow, in a hybrid motor with a classical cylindrical solid fuel grain with a circular port. The conducted study is mainly an experimental analysis. First, four different type of injectors are designed and characterized hors motor (cold test). Then, a test bench is developed in order to carry out the firing tests. The results obtained are optimized by improvement of some injectors. Different and significant impacts are observed and measured on the motor performances. In fact, injector and head-end design can significantly affect the overall behavior of the motor because the whole combustion process will be severely affected by the incoming oxidizer flow pattern. The obtained results demonstrate that the overall performance of the motor is substantially affected by changing the injectors, such as: an increase in the regression rate, stability and instability of combustion, flame quenching, homogeneity of fuel consumption, etc.… Therefore, there is a real need to study the injector influence to develop reliable tools to predict the fuel regression rate under different oxidizer flow conditions.Doctorat en Sciences de l'ingénieur et technologie