Observation on ignition process in small rocket thruster fueled by gaseous oxygen and gaseous methane

Ignition process in the reaction control thruster fueled by gaseous oxygen and gaseous methane is experimentally observed with high-speed photography. The effects of the spark plug type, recess of the high-voltage plug, and mixture ratio on the ignition process are studied. The results show that the start-up process of the engine includes two sections, ignition process and transition process. The ignition process involves six phases in turn: spark formation, fire core formation, fire core grow-up, filling, fluctuation, and stable flame. However, during the transition process, with the increase of the fuel flow rate, the mixture ratio decreases to the stable condition. Experimental studies show that with enough spark energy, the flame is primarily controlled by the mixture ratio, and the range of the mixed ratio 2.01-18.3 has been validated. On the other hand, the duration from spark to stable flame is determined by the instantaneous mixture ratio at the spark time. In conclusion, the effect of the spark plug type does not play a significant role, while long recess of the spark plug always produces long duration of fire core propagation and short grow-up duration.