High Shear Rheometry of Unsymmetrical Dimethylhydrazine Gel

Abstract

Gel propulsion systems promise several advantages over conventional solid, liquid, and hybrid propulsion systems and are being explored and developed for about three decades. The major focus, however, has been on monomethylhydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH), and some hydrocarbon-based rocket fuels that are extensively used to prepare shear-thinning (thixotropic) gels for rocket and ramjet propulsion applications. The conversion of liquid fuel to a gelled state enables homogeneous suspension of metal powders into gel matrix. The approach improves propellant energetics on combustion and overall density specific impulse. Another important advantage is that such systems burn at a much reduced oxidizer-fuel ratio leading to reduction in net vehicle hardware weight. Gelled systems also promise considerable storage, handling, logistics, and transportation ease and do not slosh and pose engine restart problem under zero gravity conditions. The gel propellants preferred for rocket engine applications should exhibit shear-thinning behavior when injected into the combustion chamber and to assist in maintaining predictable mass flow rate. As such, these systems are delicately balanced and generally show a complex rheological and high shear flow behavior that is largely dependent on temperature, aging characteristics, etc. This paper aims to discuss key approaches for formulating UDMH-based gelled fuel systems and their mechanism of formation, their metallization using nanometer/micron-size metal powders besides explaining optimization processes. The performance characteristics, ignition, and combustion behavior have also been briefly described before giving an emphasis on low and high shear rate flow behavior of such gels.