Particle induced laser ignition and transient flame behavior in hybrid rockets
Keywords: Engineering.
Aeronautics and Astronautics.

In comparison to liquid and solid propulsion alternatives, hybrid propulsion systems have historically been precluded from in-space applications due to lack of development and heritage. However, the increasing research interest in small-scale, experimental hybrid design and the rapidly emerging small-satellite market motivate the future use of hybrid rockets. Small-satellite propulsion systems require high-performance, low cost, and reliable motors. Crucially, hybrid motors are easier to downscale and present fuel solutions that are stable and non-toxic. Currently, hybrid rockets are classified as low technological readiness systems. As such, substantial research efforts are required in order for motor design and operation to be effectively improved. This thesis focuses on the refinement and utilization of the novel, lightweight and low-cost hybrid laser ignition scheme in supplementary testing conditions. This research focuses on two primary goals. The first goal is the refinement of the original laser ignition theory. The theory suggests that particle size significantly affects the success of ignition and requires correlations to the effect of dynamically changing local flows on the non-ideal shapes of encountered particles. The second goal is to provide unique insight into the combustion behavior of standard hybrid motors by utilizing the laser ignitor. The laser delivers precise ignition and development of a small initial flame kernel which is then observed in high fidelity as it propagates across a fuel surface. Experimental investigations in an optically accessible combustion chamber on fuel samples with size controlled additives resulted in an analysis of ignition behavior and verification of a refined ignition theory. These results depict an ignition process that involves the heating of a fuel surface via the laser and the emergence and heating of a charred particle upon fuel decomposition. The particles are carbon based, residual material of fuel pyrolysis and can be capable of carrying energy sufficient for the ignition of an oxidizer-fuel vapor propellant mixture. These studies demonstrate the ignition reliability of fuels which is dependent on the sizes of the particles, the concentration of the particles, and the rate of the oxidizer flow. Fundamentally, the data and theory supports the use of a range of intermediary ($\approx$ 40$\mu ) sized particles at a 2\% concentration by mass for reliable ignition. This infrastructure also permitted studies to be performed on the combustion process over flat fuel sample surfaces in two axial directions. Experimental investigations performed in a small-scale hybrid motor with a transparent single-port combustion chamber resulted in an analysis of the combustion transient of an ignition performed down the length of the port. These studies were informed by flat surface fuel sample flame propagation trends and demonstrate inversely proportional flame behavior in varying opposing oxidizer flow conditions. At the low initial flow rates encountered in the former infrastructure, flame propagation rates increase in response to increasing oxidizer flow rate and they proceed in an non-acceleratory, circular pattern. In a channel port geometry, the flame propagation rates increase as the fuel port regresses, correlating to a decrease in the local oxidizer flow rate. Additionally, the flame initially proceeds in a stable manner similar to what is observed over a flat sample but eventually begins to accelerate closer to the fore-end of the chamber and displays unstable flame characteristics. Overall, the results of these tests help provide a more robust physical basis for the use of a laser igniter and provide unique data pertaining to the complex gas-dynamics-thermal behavior inherent to hybrid rocket motor operation. Finally, these results provide insight into ways to improve laser ignition performance in the future.