Testing of a Throttleable Hybrid Rocket Engine using multiple Oxidizer Injections

Abstract

Throttling capabilities through oxidizer massflow modulation represent a key advantage of hybrid rocket engines over solid-propellant systems. However, conventional single oxidizer massflow injection systems inherently couple thrust control with mixture ratio variation, often leading to suboptimal combustion conditions and performance losses. To overcome this limitation, multiple oxidizer injections systems can enable independent control of thrust and mixture ratio, offering a significant performance and operational advantage. One promising configuration, the AlteringIntensity Swirling-Flow-Type (A-SOFT) design, employs separate axial and radial oxidizer injection. The radial flow induces swirl, enhancing fuel regression rate and fuel mass consumption, while the axial flow primarily governs the mixture ratio. Together, both flows jointly influence thrust and combustion stability, creating complex interaction dynamics that require thorough investigation. This study presents the design and experimental validation of a hybrid rocket engine tailored to the test facility at DLR Trauen, Germany, using the A-SOFT principle for transient throttling. The engine utilizes hydrogen peroxide (H2O2) as oxidizer and hydroxyl-terminated polybutadiene (HTPB) as fuel, with a dual massflow catalyst chamber enabling independent decomposition of the oxidizer massflows. A novel nozzle design featuring a carbon/carbon–silicon carbide (C/C-SiC) throat ensures high thermal resistance and structural integrity under high heat loads. Regression rate behaviour is estimated from measured thrust and chamber pressure data, enabling insight into the coupling between swirl-induced flow dynamics and fuel consumption.