Analysis of injector spray dynamics for a stable hybrid rocket motor

Hybrid rocket motors (HRM) are of current interest to space programmes in view of their throttleability, restartability, environmental friendliness and safety. But HRMs like the ones using liquid oxygen-hydroxyl terminated polybutadiene hybrid are prone to combustion instability. In order to develop a stable hybrid rocket motor, detailed characterisation of impinging doublet injector was carried out. The injector head is comprised of 60 identical doublets arranged in three axisymmetric rows that ensure uniform spray of liquid oxygen in the chamber. Jet impingement and fine atomization were demonstrated in the cold flow tests of the injector head. This injector performance was assessed during cold flow tests, both with and without backpressure. The measurements of droplet sizes were corroborated by a linear stability model. The hybrid motor with this injector head was successfully hot tested and stable combustion was demonstrated for a duration of 15 s. The stiffness of the injector was evaluated and found to be adequate to suppress feed coupled oscillation. The details of the configuration, design aspects and measured cold flow spray characteristics of the injector as well as hot test performance of the hybrid motor are presented. The injector configuration plays a crucial role in achieving stable combustion, making it a promising candidate for enhancing the performance of high-thrust hybrid motors.