Experimental regression rate profiles of stepped fuel grains in Hybrid Rocket Engines

Abstract

In this work, we assess the impact of stepped hybrid rocket solid fuel grains on the performance of the engine. The influence of Forward Facing Steps (FFS) and Backward Facing Steps (BFS) on the regression rate is analysed on two different cylindrical motors and one slab burner with optical access. Accessible post-processing allows to quantify the local regression rate profiles induced by the steps for the cylindrical motors. For the slab burner, novel image processing approaches enable the authors to correlate the findings of the cylindrical motors precisely to the visual inspection of the slab burner. Using two different cylindrical motors with different fuel grain lengths stressed the importance of the total motor length on the effect of the steps. Shorter grains (118 mm) exhibit higher regression rates for FFS than for BFS (+41.3% versus +15.7%), whereas longer motors (500 mm) profit more from BFS than FFS (+26.3% versus ±0%). Moreover, using three different oxidizers (nitrous oxide, hydrogen peroxide and gaseous oxygen) and two different fuels (high density polyethylene and paraffin) at different O/F ratios, we showed that the beneficial properties of steps are valid for numerous configurations. This would allow the employment of stepped geometries to a variety of existing motors. Finally, the authors propose the possibility to approximate fuel grain designs by a set of multiple steps.