Preliminary Design of a Throttleable Hybrid Rocket Engine using Multiple Oxidizer Injections

Abstract

Throttling hybrid rocket engines by changing the oxidizer massflow is a key advantage of hybrid propulsion compared to solid propulsion. However, changing the oxidizer massflow usually affects the mixture ratio, which can reduce the engine overall performance. Therefore, engines with two separate oxidizer massflow feed systems are used to control the thrust and mixture ratio independently. One approach is to inject an axial and a tangential massflow. This is also referred to as an alteringintensity swirling-flow-type (A-SOFT) design. The tangential massflow mainly affects the regression rate and the fuel massflow due to the introduced swirl, while the axial massflow mainly adjusts the mixture ratio. It is clear that both massflows simultaneously influence the thrust and the mixture ratio. To investigate the interactions of the massflows further studies and experiments are needed. In this study, an engine suitable for the hybrid rocket test bench at the DLR site Trauen, Germany is designed. Its transient throttling capabilities based on the A-SOFT principle are investigated theoretically. The engine uses hydrogen peroxide as oxidizer and hydroxyl-terminated polybutadiene as fuel. A catalyst chamber is used to decompose the hydrogen peroxide. The chamber is described in detail in a separate study. The design of the engine and the fluid system is shown. The throttling capabilities are investigated by preliminary design methods. Based on these calculations, throttling experiments are proposed for the upcoming test campaigns.