Numerical analysis of nozzle erosion in hybrid rockets and comparison with experiments

Nozzle throat erosion in hybrid rocket engines is generally more emphasized than in solid rockets due to a greater concentration of oxidizing species in the combustion products. Therefore, its accurate understanding and modeling is of fundamental importance in order to correctly predict motor performance and possibly to help reducing the associated losses. Moreover, throttling and/or mixture ratio shift can significantly alter the conditions under which the thermal protection material is operating, further complicating the problem. The aim of this work is to numerically investigate the graphite nozzle erosion in hybrid rockets burning high-density polyethylene and oxygen. Nozzle throat erosion rate and wall temperature are evaluated as a function of the operating conditions by performing numerical simulations with a proven in-house computational fluid dynamics solver. The results obtained from a detailed parametric analysis are discussed and then used to derive a general closed-form regression law for rapid estimation of nozzle throat erosion rate for the analyzed propellant combination that is dependent on the operating conditions, chamber pressure and mixture ratio, and on the nozzle geometry. The possible role of combustion efficiency is also discussed. The regression law is then validated through comparison with the experimental results obtained from several lab-scale 2-kN-thrust-class firing tests, underlining a good ability to correctly capture the average nozzle erosion mass flux behavior with the operating conditions and highlighting the different role of the driving parameters.