Numerical analysis of nozzle transient heating and erosion in hybrid rockets burning HDPE

The erosion of the nozzle throat can represent one of the major limitations against the widespread use of hybrid rocket engines (HRE) in the space industry. In fact, nozzle erosion in HRE can be generally more severe than in solid rockets due to the higher concentration of oxidizing species in the combustion products, and to mixture ratio shifts and/or throttling, which can affect the exhaust gas composition. Therefore, an accurate understanding of the erosion phenomenon is of fundamental importance for the technological advancement of HRE. This work is focused on the investigation of graphite nozzle erosion and transient heating in HRE burning high-density polyethylene (HDPE) and two different oxidizers (oxygen and nitrous oxide). Firstly, results of a computational fluid dynamics (CFD) parametric analysis are used to derive closed-form regression laws for the rapid estimation of nozzle throat erosion and wall temperature depending on chamber pressure, and mixture ratio. Then, a one-dimensional transient heat conduction solver is loosely coupled with the aforementioned regression laws, allowing to get information about the heating process within the solid without a dedicated coupling with the CFD solver. Finally, the obtained results are validated against experimental test campaigns performed at the Hokkaido University. The closed-form regression laws for nozzle throat erosion and wall temperature can aid the preliminary design of HRE ablative thermal protection systems.