Integrated Design of Hybrid Rocket Upper Stage and Launcher Trajectory

A three-stage launcher, with solid-propellant first and second stage and a hybrid-propellant third stage is considered. The design of the hybrid-propellant upper stage and the whole ascent trajectory are simultaneously optimized by means of a nested direct/indirect procedure, while the characteristics of the first and second stages are assigned. Direct optimization of the parameters that affect the motor design is coupled with indirect trajectory optimization to maximize the launcher payload. A mission profile based on the Vega launcher is considered. The feed system exploits a pressurizing gas, namely helium, with hydrogen peroxide as the oxidizer, and polyethylene as the fuel. The simplest blowdown design is compared with a more complex pressurizing system, which has an additional gas tank that allows for a phase with constant pressure in the oxidizer tank. The optimization provides the optimal values of the main engine design parameters (pressurizing gas mass, nozzle expansion ratio, and initial values of tank pressure, mixture ratio and thrust), the corresponding grain and engine geometry, and the control law (thrust direction during the ascent trajectory and engine ignition and shutoff times). Results show that a hybrid-propellant third stage may be a viable option for small launchers, with improved performance and similar cost compared to an all-solid rocket. The results of the optimization also offer interesting theoretical insight into the problem. © 2009 by the American Institute of Aeronautics and Astronautics, Inc.