Characterization of rheological and ignition properties of hypergolic propellants

Hypergolic propellants are widely used by the propulsion community in various applications. A monomethylhydrazine fuel and nitrogen tetroxide-based oxidizer rep- resent the state of the art for hypergolic propellants. The health hazards associated with these propellants has prompted research into gelled formulations due to the inherent safety advantages of gels. Gelling propellants alters many of the chemical and physical properties relevant to the processes encountered in rocket engines. The effect of these altered properties must be understood and quantified before a system utilizing gelled propellants can be fielded. This study explores the rheological properties of gelled monomethylhydrazine and red fuming nitric acid as well as the ignition of liquid and gelled monomethylhydrazine and red fuming nitric acid. Monomethylhydrazine has been gelled with various concentrations of hydroxypropylcellulose. A simple procedure for mixing the gel was developed and the viscosity-shear rate relationship measured with a rotational rheometer. Increasing the hydroxypropylcellulose concentration increased the viscosity of the monomethylhydrazine gels and increased the shear-thinning behavior of the gels. The viscosity- shear rate relationship was modeled using the power-law and Carreau-Yasuda models to quantify the repeatability of the rheological properties. The viscosity measurements for the monomethylhydrazine gels generally varied less than 25% from the average for a single gel formulation. The monomethylhydrazine gels had negligible yield stresses and were not thixotropic. Red fuming nitric acid was gelled with 3, 4, and 5% by mass fumed silica using a mixing procedure developed for silica-hydrocarbon gels. The red fuming nitric acid gels exhibited shear-thinning, thixotropic behavior and had measurable yield stress values. Increasing the fumed silica concentration increased the viscosity, the shear-thinning behavior, and the yield stress of the gels. A power-law curve fit was applied to the viscosity-shear rate results and demonstrated large variations in the rheological measurements. The large deviations resulted from the thixotropic behavior the gels. A method that presheared the gel samples prior to performing the rheological measurements led to improved measurement repeatabililty. The ignition of liquid monomethylhydrazine and red fuming nitric acid was examined through a series of drop tests and impinging jet tests with a unlike-doublet injection system. Drop tests resulted in minimum ignition delays of 1.5 milliseconds characterized by a flame and a loud report. In the impinging jet tests an ignition event defined by a visible flame was observed only at injection velocities from 8 to 20 ft/s. The ignition events were characterized by poor mixing and the burning of large monomethylhydrazine drops with no flame spreading. A series of impinging jet tests with a hydrocarbon-based fuel mixture and 91.2% H2O2 resulted in repeatable ignition delays from 6 to 8 ms at injection velocities from 48 to 138 ft/s, verifying the system functionality. Additional jet tests with a fuel mixture of 2-dimethylaminoethylazide and tetramethylethylenediamine with red fuming nitric acid exhibited similar ignition behavior to the monomethylhydrazine and red fuming nitric acid tests. An investigation into the red fuming nitric acid composition points to the presence of some impurity that may be altering the ignition processes. Impinging jet tests were also performed with monomethylhydrazine and red fuming nitric acid gels. Although an ignition event was generally not observed, the gels exhibited distinct flow patterns related to the rheological properties of the propellants