Combustion Processes in Hybrid Rockets

This paper presents the latest results achieved at the Space Propulsion Laboratory of Politecnico di Milano in the area of hybrid propulsion. Focus is put on four specific research topics currently under investigation and strongly linked: (1) solid fuel formulations development; (2) investigation of the burning behavior of liquefying fuel formulations and of the entrainment phenomenon; (3) development of a vortex flow pancake (VFP) designed for in-space missions; and (4) numerical simulation approaches. A wide chemical, thermal, rheological, mechanical, and ballistic investigation of traditional polymeric formulations and paraffin-based solid fuels has been performed in the last years and is shortly summarized here. Firing tests are performed in a radial lab-scale burner enabling time-resolved regression rate measurements. The results of this activity pave the way to the challenging horizon of liquefying fuel formulations. The entrainment of melted fuels is investigated by a dedicated setup designed for the study of the oxidizer stream/melt surface interaction under cold-flow conditions, to understand the droplet formation mechanism and to measure their size distribution. The effects of liquid layer entrainment on the combustion processes seem attractive for the development of unusual geometries, such as the VFP. The VFP hybrid rocket configuration offers a compact implementation with motor length-to-diameter ratio lower than 1, giving a breakthrough opportunity for in-space missions that could strongly benefit from the system affordability, with low recurring costs joined to high operating flexibility. The VFP development requires a strong support of numerical simulation activities, developed through OpenFOAM, and described in the last part of the paper.