Performance of Mixture-Ratio-Controlled Hybrid Rockets for Nominal Fuel Regression

This paper discusses the impacts of oxidizer-to-fuel mass ratio (O/F) shifts on the flight performance of single-stage sounding rockets using the flight simulations of three scales of O/F-controlled and O/F-uncontrolled hybrid rockets under a nominal fuel regression behavior without uncertainty. The flight simulation code includes three factors dependent on the O/F: thermodynamic states of the burned gas (theoretical Isp), shifts in c∗ efficiency, and nozzle throat erosion. In the flight simulations, a thrust control law was applied to increase the apogee and evaluate the effects of O/F shifts in the thrust curve including throttling. For the best cases in each scale, O/F-controlled hybrid rockets slightly improved the performance by 2.03–2.42% in the averaged specific impulse. However, the performance of the O/F-controlled sounding rockets is essentially the same as the O/F-uncontrolled type under the median regression behavior: especially when considering the slight increases in the mass and complexity of the oxidizer feed system needed for O/F control. Considerable scale effects on the throat erosion and theoretical Isp were observed, but that of the c∗ efficiency was negligible. The improvement of the theoretical Isp was the primary contributor to flight performance, which was responsible for a larger than 70% share in the total Isp increase. The second largest contribution was the improvement of the c∗ efficiency with a share of 21.8–24.3%. The O/F control gave an improvement of throat erosion corresponding to 5.75% in the total Isp increase for the smallest scale; but, with increasing of the scale, the throat area increase ratio became small so that the throat erosion improvement contribution was reduced to 1.21%