Evaluation of paraffin-based fuels for hybrid rocket engines

This paper summarizes the investigations on the combustion behavior of paraffin-based hybrid rocket fuels with gaseous oxygen (GOX) as oxidizer. Combined experimental activities have been done at the DLR Institute of Space Propulsion in Lampolds hausen and at the Space Propulsion Laboratory (SPLab) of Politecnico di Milano. Regression rate tests have been done in a 2D radial micro burner at the DLR and at the SPLab. Fuel samples have been characterized by viscosity measurements, tensile tests and differential scanning calorimeter (DSC). Tensile tests shows significant improvement in maximum stress and elongation when polymers in low concentration are added to the paraffin samples. The values of the liquid fuel viscosities differ significantly between the fuels. This affects the droplet entrainment process during combustion and also the regression rates of the fuels. Entrainment and regression rate increase for decreasing fuel liquid layer viscosity. An exponential relation has been found between the liquid fuel layer viscosity and the regression rate, which can be used to predict the regression rate of new liquefying fuels by measuring their viscosity.