Experimental Investigation of Stepped Fuel Grain Geometries in Hybrid Rocket Engines

Hybrid Rocket Engines (HREs) are a promising technology to achieve a cheaper, more sustainable and competitive access to space. So far, liquid rocket engines and solid rocket motors have been preferred over HREs for launcher applications. This is due to the low maturity of HREs, as well as challenges related to low regression rate, combustion inefficiency, oxidizer-to-fuel ratio (O/F) shift during operation and high residual inert mass. Nonetheless, in recent years, the space launcher market calls also for cheap micro- and nano-launchers. HREs could help to fill this niche, given their advantages (e.g. high performance, intrinsic safety, throttle-ability, flexibility and low cost), provided that some of the disadvantages are being solved. In this paper, the effect of Forward Facing Step (FFS) and Backward Facing Step (BFS) on the HRE performance (namely regression rate and combustion efficiency) is investigated through test firings. The application of diaphragms, resembling both a FFS and BFS at the same time, and other turbulence and heat exchange enhancing devices in the solid fuel grain have proven to increase both regression rate and combustion efficiency due to the formation of recirculation zones. These zones increase turbulence, mixing as well as heat transfer and, thus, leading to improved performance of the HREs. In order to gain further insight into this phenomenon, an experimental campaign is carried out on a lab-scale HRE using nitrous oxide (N2O) and High-Density Polyethylene (HDPE) as propellants. The cylindrical fuel grain with an external diameter of 130 mm and a length of 117 mm is separated halfway into two different internal diameters. Varying these diameters in the two sides allows creating different FFS and BFS geometries. The focus of the test campaign can be summarized as follows: First, the influence of the step height on the length of the recirculation zone is investigated. Therefore, the fuel grain is probed after the test for indications of the recirculation zone length. Further, the performance impact of BFS and FFS on the HRE is compared under identical conditions. This enables to distinguish the effects for BFS and FFS. Another measurement is the influence of the steps on regression rate and combustion efficiency by weighing the fuel grains before and after the tests and quantifying the thrust respectively. Lastly, the evolution of the steps over time can be assessed by comparing the final geometry of the stepped grain after different burn time lengths. To recapitulate, assessing the test campaign will allow to judge the practically of steps as performance enhancer in HREs.