Rheological Characterization of Monomethylhydrazine Gels

In this paper, viscometry, yield stress, and small shear strain oscillation experiments on monomethylhydrazine gels are presented. The goal is to define a gel formulation with a measurable yield stress under low-shear conditions and shear-thinning behavior at shear rates representative of typical rocket injectors. Hydroxypropylcellulose, fumed silica, and a hybrid of the two materials are investigated as gelling agents for monomethylhydrazine. The two gel formulations that included hydroxypropylcellulose exhibited similar shear-thinning behavior with power-law flow indices of similar to 0.1 to 0.2. The pure fumed silica gel samples exhibited highly shear-thinning behavior with negative power-law flow indices indicative of thixotropic behavior. Tests performed after a preshear cycle showed shear-thinning power-law flow indices of similar to 0.3. Unlike the pure hydroxypropylcellulose and hybrid gels, the fumed silica gel exhibited a well-defined, measureable yield stress. Small-strain oscillatory experiments showed that hydroxypropylcellulose and hybrid gels behave like linear viscoelastic liquids below a critical strain value, whereas the fumed silica gel behaves like a viscoelastic solid with a distinguishable yield stress. The phase angle of these gelled propellants is also discussed as a function of frequency, as are the implications of these results to the physical processes of interest.