Hybrid rocket motors are generally rated with lower average impulses than solid motors. Why is this?
Part 1: Well, there are a couple of things that make this a reality. In solid motors, when designers want to increase the average impulse, they can add oxidizer, modify grain geometry and modify the nozzle throat diameter to influence higher combustion rates. To add oxidizer in hybrids, the injector can be opened to a larger diameter thereby causing the N20 to exit the tank at a higher rate allowing a more rapid oxidization of the fuel source, resulting in higher average thrust levels. In theory this sounds great, but in reality there are problems. When dumping a large amount of oxidizer through a single injector positioned over the center of a cylindrical grain, a sizeable percentage of the oxidizer will get spit thru the nozzle without ever oxidizing anything. The fuel can be placed into the flow of the oxidizer, but that involves expensive fuel grain manufacturing techniques that, to this point, have not been explored by manufacturers in the sport rocketry hybrid market. It is also
