Shai Rahimi

Development of Laboratory-Scale Gel-Propulsion Technology

Selected gel propellants and simulants were formulated, prepared, rheologically characterized, and tested in the first phase of a program to develop gel-propulsion technology infrastructure. Hydrazine-based fuels, gelled with polysaccharides, were characterized as shear-thinning pseudoplastic fluids with low-shear yield stress (τ y i e l d ), whereas inhibited red-fuming nitric acid (IRFNA) and hydrogen peroxide oxidizers, gelled with silica, were characterized as yield thixotropic fluids with significant τ y i e l d . Safe storage and handling procedures were established. A laboratory-scale experimental setup was used to hot fire successfully a small 100-N nominal thrust rocket engine with selected hypergolic neat-liquid and gelled bipropellant combinations. One-element pentad-type injectors were utilized in the tests to inject the propellants into the combustion chamber. Continuous tests of up to 25-s firing duration and multipulse operations of up to 20 cycles of 0.1-s on/0.5-s off were successfully conducted with gelled-hydrazine/IRFNA bipropellants. Neat-liquid and gelled mono-methyl hydrazine/IRFNA bipropellants were also tested. The combustion pressure ranged between 20 and 35 bars. Experimental characteristic velocity, c* e x p , was determined as a function of the oxidizer-to-fuel (O/F) mass flow rate ratio. Maximum c* efficiency of more than 95 and about 90% was obtained in continuous firings for the neat-liquid and gelled hydrazine/IRFNA, respectively. In both cases, the maximum c* e x p values were obtained at higher O/F ratios than those that yield maximum theoretical c*.

Flow of Gel Fuels in Tapered Injectors

The governing equations of the steady e ow of gel propellants and fuels in a tapered tube injector have been formulated assuming a power-law rheological model. A parametric investigation was conducted to evaluate the effect of the injector geometry and pressure gradient on the e ow rate and the mean apparent viscosity of the gel for various fuels and metal loadings. The theoretical results indicate that the e ow rate increases signie cantly with decreasing thepower-law index and with increasing the pressuredrop in the injector. Theapparent viscosity is not uniformineachcrosssection;itismaximalattheaxisandreachesminimumatthewall.Themeanapparentviscosity of the gel exhibits a signie cant decrease with increasing the convergence angle of the injector. This implies that to obtain betteratomizationofthegel,high convergenceanglesarerequired.AcomparisonbetweenRP-1/Algelswith various aluminum mass fractions was conducted. The results indicate the existence of an optimal metal loading.

Atomization characteristics of gel fuels

In the present research an effort is made to relate the rheological properties of gels propellants with their atomization behavior. The experimental investigation demonstrates that pseudoplastic, viscoinelastic water gels exhibit similar spray pattern with Newtonian liquids, however, they are more difficult to atomize. The experimental results indicate that atomization is significantly affected by the gellant content in the fuel and the injector geometry. The characteristic Sauter Mean Diameter of the spray increases with increasing the gellant content due to the respective increase of the shear viscosity. Wide angles of convergent flow injectors require less upstream pressure to achieve the same atomization performance with regular triplet atomizers. It is demonstrated that gelling agents may be combined to achieve desired rheological characteristics. Nomenclature d Exit diameter D Droplet diameter f Mass fraction of gellant A F fuel FN Flow Number K Power-law consistency index rh Mass flow rate n Power-law rate index Ni Droplet number O Oxidizer P Pressure SMD Sauter Mean Diameter CC Convergence angle [3 Impingement angle * Graduate Student t Senior Lecturer, Senior Member AIAA. Copyright

Preparation and Characterization of Gel Propellants and Simulants

Energetic gels were selected, prepared, characterized and tested for their feasibility as candidates for rocket propulsion and other gas generation systems. For fuel gellation both organic and inorganic gellants were considered. Oxidizers were gelled using inorganic gellants. In addition, gelled propellant simulants were selected and prepared to represent the relevant rheological properties of the actual fuels and oxidizers, and to aid the development of the necessary handling infrastructure for gels. The program for propellant gels and gel simulants characterization, classification, and matching in support of the prospective applications of energetic gels is also described. A capability was demonstrated to prepare gel fuels and oxidizers Theologically matched to each other as well as inert simulant gels matched to propellant gels.

Rheological Matching of Gel Propellants

A SIGNIFICANT effort has been undertaken by several research groups worldwide over the past decades to study various aspects of gel propellants. The principal objective of these basic studies is to realize a flexible energy-management propulsion system,which assures insensitivemunitions compliance. In addition, their increased energy density, whenmetal particles are introduced in the gel matrix, in comparison to neat-liquid propellants presents a significant potential advantage. The non-Newtonian complex rheological character of these propellants and the resulting systemlevel implications make their use in operational rocket engines very challenging. Nevertheless, their potentially full pulse-widthmodulation capabilities, combined with the ability for divert and attitude control system application, make them attractive for future rocket propulsion systems in both tactical and space applications [1– 4]. Key aspects of gel-propulsion technology, propellant preparation, rheology, atomization, and combustion are presented in an extensive review byNatan and Rahimi [5]. A general rheological classification of gel propellants and simulants has been proposed in a recent study on the shear rheology of gel propellants [6]. Rheologicalmatching of gel propellants is an intentional change in one or more relevant properties of the fluids, within specified ranges of temperatures and shear rates, to obtain a desired rheological behavior. Matching is achieved by using suitable techniques, depending on gel composition and preparation procedure. The reasons for rheological matching can vary as follows. 1) Gel-propellant simulant matching. The use of water-based simulants in lieu of toxic and corrosive materials enables the conduction of system development tests without the difficulties inherent in the handling of such materials, allows budget savings, and enhances safety. For example, hydrazine-based gel fuels are attractive in propulsion systems because of their performance characteristics, although safety precautions make the handling of such materials a difficult task. The use of a hydrazine-gel simulant allows adequate handling and testing of the feeding system without being exposed to safety hazards.Water-based gel simulants have been used for sprayflowvisualization [7], thixotropymodeling [8], atomization studies [9,10], elongational behavior [11], and propellant tank expulsion performance [4]. 2) Gel-propellant matching. Fuel and oxidizer can be matched for gel-feeding system design purposes. This matching provides the designer with an additional degree of freedom. In addition, certain fuel and oxidizer gels can bematched to complywith special requirements, such as a reduced temperature sensitivity of their rheological properties within a wide range of temperatures. Shear viscosity of gel-propellant simulants has been found to be significantly affected by shear rate and gel composition (gellant type and mass fraction) [6]. On the other hand, the effects of thixotropy and temperature field were found to be insignificant in comparison to the shear-thinning and composition effects. Rheological matching does not imply that gels of two different materials and gelling agents would have absolutely identical rheological parameters under all circumstances. However, these properties can be brought close enough, in relevant ranges of ambient conditions, such as flow rates, temperature, and external mechanical loads, to satisfy certain requirements. The gellant type and content provide a degree of freedom for the determination of the rheological properties of a gel propellant. The scope of the present study is to demonstrate the rheological matching of fuel, oxidizer, and simulant gels using various gellants, separately or combined. An investigation of the effect of temperature on the rheological parameters of water-based, gel-propellant simulants, formulated by various gellant combinations at different ratios among them, is also presented.