Optimal design of hybrid rockets with self-pressurising oxidizer

A nested direct/indirect method is used to find the optimal design of a microgravity platform which consists of a hybrid sounding rocket that uses a self-pressurizing oxidizer, namely, nitrous oxide. The direct optimization of the parameters that affect the engine design is coupled with the trajectory indirect optimization to maximize a given mission performance index. Different models can be used to describe the self-pressurizing behavior of the oxidizer in the tank. The simplest model assumes liquid/vapor equilibrium. A two-phase model is also proposed: saturated vapor and superheated liquid are considered and the liquid/vapor mass transfer is evaluated making reference to the liquid spinodal line. Results show that the different models have a limited impact in the optimal engine characteristics. The performance are slightly modified due to the different mass of the residual oxidizer. A performance comparison with different propellant combinations is also shown.