Rheological Matching of Gel Propellants

Abstract

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.