Enhancement of fuel regression rate for hybrid rockets by introducing novel coaxial tube injector

Low fuel regression rate is one of major drawback of hybrid rocket motors. A new scalable injector concept is proposed in this study to provide an enhancement for fuel regression rate. As different than the classical head-end axial injectors, offered injector called Coaxial Tube Injector-CTI is lying through the fuel grain centre and could capable to inject oxidizer both axial and radial directions. Fuel used in this study is paraffin with some polymeric additives and the oxidizer is N2O (L) under the gaseous oxygen supercharge. 30 hot firings were conducted, and results revealed that the CTI configuration provides deterministic increase in fuel regression rate which is 5.3 times higher than paraffin motor with axial injector. This great enhancement gives ability to the designer to change the motor sizing sharply according to the requirements. Verifying regression rate enhancement with CTI configuration under the insight of hot firings provides to develop a non-dimensional, scalable regression rate relation. Conducted tests revealed that the radial fluxes have great contribution to the regression rate rather than the axial fluxes and non-dimensional equation was developed under this information. Influence of each parameters in non-dimensional equation were determined by curve-fit effort. Developed scalable regression rate equation was used to design the internal ballistic of motors for last two tests to verify the equation and results showed the developed rule works well. Consequently, very high regression rates could be achieved with proposed CTI configuration and non-dimensional regression rate relation was developed for CTI configuration as a design tool for scale-up systems.