UC Davis

In this work, a hydrogen-oxygen torch igniter was developed and utilized to study the combustion of gaseous oxygen (GOx) and solid acrylonitrile butadiene styrene thermoplastic (ABS) in conjunction with gaseous hydrogen (GH2) in application to hybrid rocket motors. The effects thereof are not presently found in literature. Initial pyrolysis of the ABS fuel grain was accomplished via products of GH2 and GOx combustion from a spark initiated torch igniter. In order to isolate the effects of hydrogen to the overall combustion, the mass flow rate of hydrogen and its duration was varied in a factorial design of experiment. It was found that an increase of the hydrogen mass flow rate at constant GOx mass flow rate led to an increase in the average linear regression rate for lean to stoichiometric operating conditions of the torch igniter. Additionally, extending the duration of hydrogen flow, from 10% to 50% of the total burn time, led to an increase in the regression rate. The author posits that the increases in average linear regression rate are due to the hydrogen-oxygen flame applying high heat and mass flux to the surface of the fuel grain, helping to overcome the rate-limiting, endothermic pyrolysis step. However, an increase in the hydrogen mass flow rate, from stoichiometric to rich operating conditions, led to decreases in the average linear regression rate. It is posited that at the fuel-rich operating condition, the hydrogen consumes most of the available oxidizer molecules. This resulted in a competition between the hydrogen gas and the ABS fuel grain for available oxidizer molecules. This theory is further supported by the turbulent boundary layer combustion model presented by Marxman and Gilbert [1]. These results suggest that the hydrogen-oxygen torch igniter can be used not only as a reliable starter, but also for regression rate enhancement that targets the rate limiting step, pyrolysis, for hybrid rocket combustion at stoichiometric to fuel-lean operating conditions of the torch igniter.