Design and Test of a Swirling Nitrous Oxide Injection System for a Hybrid Rocket Engine In this work we design an innovative swirling injection system for the N2O hybrid rocket motor of the Department of Mechanical Engineering at the California Polytechnic State University, San Luis Obispo (CalPoly). The past firing tests of the motor were conducted with both cylindrical and annular grain geometries and with axial injectors in a shower-head configuration. Various orifice diameters and number of injectors were employed in different tests, but in all cases the post-burn analysis of the fuel grains showed a non-uniform regression rate along the grain length, with an increased consumption of the solid fuel at the upstream end of the motor.