On Soot and Its Environmental Impact in Hybrid Rocket Engines

Hybrid rocket engines (HREs) are increasingly considered for space transportation due to their inherent safety, cost-effectiveness, and operational flexibility. However, their diffusion flame structure promotes soot formation, leading to potentially significant emissions of black carbon (BC) directly into the stratosphere. This review provides a comprehensive analysis of soot formation and evolution in hybrid rocket combustion, covering fuel pyrolysis, polycyclic aromatic hydrocarbon (PAH) growth, particle nucleation, surface growth, and oxidation processes. The physico-chemical properties and nanostructural evolution of soot are discussed in relation to combustion conditions typical of HREs. Particular emphasis is placed on the environmental and climatic implications of BC emissions, including radiative forcing, atmospheric lifetime, and heterogeneous chemical interactions. Existing diagnostic techniques for soot measurement are critically assessed, highlighting their limitations under the extreme conditions of rocket exhaust plumes. A key outcome of this review is the identification of a major knowledge gap: although preliminary experimental emission-index measurements for HRE soot have recently become available, the database remains extremely limited, fuel- and configuration-specific, and not yet supported by standardised diagnostic protocols. Addressing this gap is essential for accurate environmental impact assessments and for the development of sustainable propulsion technologies.