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Typical hybrid rockets, using solid plastic fuel and liquid oxidizer, have the potential to become non-explosive propulsion systems based on boundary layer combustion (Takahashi and Shimada, 2018). This feature enables hybrid rockets to be both safe and low-cost vehicles. However, there are two primary disadvantages in their use as propulsion systems. One is the low fuel regression rate, which results in a low thrust density. The other is the variation in the oxidizerto-fuel mass ratio (O/F shift) during combustion. This is typically observed in a hybrid rocket engine using a single-port fuel grain. To increase the fuel regression rate, low melting temperature fuels, such as paraffin wax, have been applied (Karabeyoglu, 2002). The authors used paraffin wax in a swirling-oxidizer-flow-type hybrid rocket engine (Saito et al., 2012). The fuel regression rate of paraffin wax reached 5 mm/s, which was approximately seven times greater than that of polypropylene under the same oxidizer mass flux. For the O/F shift problem, one of the solutions is the end-burning hybrid rocket engine. In this engine, as the combustion occurs at the end surface of the cylindrical fuel grain, the burned area does not vary with time and, therefore, constant O/F would be realized under constant oxidizer injection conditions. Saito et al. demonstrated constant O/F burning in their axial-injection end-burning hybrid rocket motor under various operation conditions, including thrust throttling (Saito et al., 2018). The authors investigated the swirling-injection end-burning hybrid rocket engine not only to solve the O/F shift problem, but also to obtain a higher regression rate with swirling oxidizer injection. Compared to other studies (Haag et al., 2000; Rice et al., 2003), the novel approach in this study is the combination of swirling-injection end-burning and the use of paraffin wax fuel. Previous studies (Hayashi and Sakurai, 2015; Oishige et al., 2016) reported that the fuel end-surface regressed in the grain axial direction, as anticipated, and the local fuel regression rate had characteristic Abstract Burning experiments were conducted to better understand the fuel regression behavior in a swirling-injection end-burning hybrid rocket engine using paraffin wax/gaseous oxygen propellant. The oxidizer mass flow rate, grain diameter, and the distance between the oxidizer injector and the grain end-surface were the variable parameters taken as influencing the regression rate. The engine attained an overall axial regression rate as high as approximately 5 mm/s, whereas unstable combustion occurred with increasing burning time owing to low melting temperature of paraffin wax. The fuel grain with a diameter of 90 mm also resulted in unstable combustion caused by the initial shallow crack of the cast grain. The radial distribution of the local regression rate exhibited dependency on the radial position and had two peaks: close to the periphery and the middle of the chamber. From the analogy of the heat transfer at the end surface in a vortex flow chamber, the controlling parameter of the overall axial regression rate was derived. Since this parameter depends on the distance between the oxidizer injector outlet and the fuel grain end-surface, to hold the grain end-surface by using some kind of actuator is necessary to attain constant O/F combustion.

Fuel regression behavior of swirling-injection end-burning hybrid rocket engine