Rheology and Thermal Characterization of Lecithin-Based Kerosene Gels for In-Space Propulsion

Gel fuels bridge the gap between liquid and solid propellants by improving handling and safety, while making them energy dense, an attractive fuel class for rocket and satellite propulsion. In this study, kerosene is gelled with lecithin, a naturally derived organic gellant and a low concentration of CaCl2, producing a fully combustible, optically transparent gel. CaCl2 promotes lecithin self-assembly into long worm-like micelles that form a mesh-like network and immobilize kerosene, yielding a viscoelastic material whose state shifts from near viscous-liquid to solid-like as gellant concentration increases. Rheology reveals two desirable properties for gel propellants: strong shear thinning and thermoresponsiveness, where the viscosity drops with increasing shear and temperature. In the case of gel's response to deformation, the viscosity decreases by 2 orders of magnitude when the shear rate increases from 1 to 100 s-1, enabling atomization before combustion. While heating the gels, the viscous effect takes dominance over the elastic effect at temperature ranging from 59 °C to 68 °C depending on the gellant concentration, melting the gel to a liquid state and providing an additional handle for flow control during operation. In addition to being responsive to shear and temperature, a unique aspect of the gel is that it liquefies almost instantly upon the addition of ethanol, which disassembles the micellar network structure. Flow curves show yielding at 14-40 Pa with yield stress increasing with the density of the mesh network. Oscillatory tests identify a predominantly elastic linear viscoelastic regime below 1% strain that supports long storage stability, and mitigates leakage and sloshing while the fuel is at rest. Finally, Large Amplitude Oscillatory Shear (LAOS) resolves the nonlinear response by decoupling elastic and viscous contributions at large deformations with increasingly distorted intracycle stress-strain responses indicating progressive network breakdown as deformation proceeds.