Characterization of Injector Response in a Hypergolic Pulse Detonation Rocket Engine

Abstract

The strive for developing higher performing rocket engines promotes exploration of technologies departing from conventional rocket combustor design. Closed cycle detonation engines represent a subset of liquid rocket propulsion systems that maintain the potential to provide increases to efficiency and capability by providing thermodynamic advantages over current “constant-pressure” combustion systems and by reducing component complexity and weight. By exploiting constant-volume combustion processes, detonation engines promise increased theoretical performance in ISP and thrust. A valveless pulse detonation rocket engine (PDRE), as discussed hereafter, aims to provide combustion performance gains with the added tangible benefits of reduced inert weight by foregoing the need for high pressure feed systems, strengthened propellant tanks, and active propellant modulation and control systems. The pulse detonation engine (PDE) remains an attractive approach to achieving constant-volume combustion in a rocket engine. Even with the growth in popularity of rotating detonation engines, the prospects of a high-impulse self-regulating rocket engine using storable propellants show promise for particular applications. Developments over the past 60 years in the area of PDEs spreads over a variety of applications from aircraft propulsion systems to rocket cycle devices. The global community continues to contribute to PDE development with aspects ranging from detonation initiation, fuel and oxidizer atomization requirements, the use of gaseous propellants, the use of liquid propellants, thermal acoustic behaviors, performance metrics, and basic deflagration-to-detonation studies. Numerous experimental works have shown that pulse detonation combustion engines are feasible but maintain challenges in practical implementation. To address the need to initiate a new detonation for each cycle of a PDE, researchers sought clever ways to initiate detonations in an alternative to deflagration to detonation transition (DDT) methods. To address the cyclical filling of the detonation tube, some researchers apply fast fluidic valves. Despite these advances, challenges still remain with improving operating frequency, the time between successive detonation events, to levels such that performance surpasses traditional steady state rocket engines. Experimental works with traditional tube PDEs characteristically exhibit operating frequencies below 100 Hz. Improvements to performance in PDE development manifests in thrust augmentation devices such as ejectors or by investigating partial tube fill advantages. The PDE research landscape grows steadily with a practical application in mind. Unfortunately, even with the abundant PDE research available, the application to rocket engines has been limited in scope. The proposed study seeks to take a novel approach in developing a valveless PDRE through the use of tuned hydraulic circuits and hypergolic propellants. The design considerations and fundamental operating concepts draw heavily from the work completed by the PDE and PDRE community. Challenges in achieving high operating frequency, propellant mixing and atomization, and detonation initiation all play a vital role in successful engine cycle operation and have parallels to traditional PDEs. With the introduction of hypergolic propellants to a PDE, additional related research fields appear in the form of hypergolic popping and rocket instabilities associated with the injector, feed lines, and combustor. Research from the Apollo Lunar Module program regarding a phenomena termed “popping,” where