Gel Propellants

The transformation of a liquid propellant into a semi-solid gel state paves way for energetically improved, reliable, safer, and possibly green futuristic propellant for rocket and ramjet propulsion. A review of the progress achieved on gel propellants so far is essential to expand the capabilities of gel propellants by adopting new avenues for research and development towards actual space flights. In keeping with this objective, we comprehensively present various aspects of gel propellants in this review. Physically, gel propellants are the fluids whose rheological properties are altered by gelling agents so that they behave as solids at rest and can be atomized and combusted like conventional liquid propellants. The current definition of the gel propellants emphasizes their ability to flow under shear and gel propellants are almost exclusively intended to function like liquid propellants. Gel propellants have a considerable history; and research and development is continuously expanding across the world. Different research groups are working on the relevant areas of this field and research has gathered momentum in the first two decades of the present century. Hundreds of studies on all the aspects of gel propulsion including formulation, rheological behavior, atomization, combustion, and applications, have appeared. Gel propellants have been applied in rocket motors, ramjets, furnace combustion, and afterburners of jet engines. On this background, compilation and organization of the accumulated knowledge in this field, identification of relevant issues, unraveling of the links among the different areas of study, and identification of the open gaps for future study have become essential and these are the chief objectives in organizing the present review. This review encompasses the formulation, flow characterization, atomization, and combustion of gel propellants. We begin with a perspective on gel propellants and discuss the relevant aspects of conventional liquid propellants and previous efforts to improve their energetic performance. Inherent advantages of gel propellants and the challenges faced in their realization are considered. Formulation of gel propellants and their simulants is covered with emphasis on the description of different types of gelling agents and methods of their incorporation into the liquid propellants. Our emphasis is on the connection between formulation parameters and flow properties of gels. Of special significance are the recent attempts at formulating hypergolic gel propellants other than the hydrazine class. Flow properties of gels are of critical importance in tailoring their behaviors. Therefore, non-Newtonian flow properties of the gel propellants are discussed. Gels are 'complex fluids' and a number of rheological functions and measurement methods are necessary for comprehensively characterizing the complexity. Hence, we describe the rheological properties of many gel propellants and focus on their interpretation. Rheological matching of gels to develop gel propellant simulants is considered. Simulants provide a useful strategy to conduct flow and atomization experiments for hazardous or expensive propellants. Atomization of gel propellants is discussed with the aid of results in which various aspects of atomization of gel propellants using conventional and specially designed atomizers have been addressed. We also discuss the role of rheological properties in determining the behavior of gel fragments, ligaments, and droplets.A large amount of quantitative data on the spray properties are compiled and discussed in this review. Theoretical analyses of gel propellant breakup are reviewed. Discussion on combustion is divided into droplet combustion and spray combustion. In droplet combustion, we include the studies on hypergolic and non-hypergolic gel propellants and consider the processes during heating, vaporization, and combustion of tethered and freely falling droplets of gel propellants. Attempts at theoretical analysis of these processes are considered. In case of hypergolic gels, current understanding of their ignition is reviewed using modeling studies. Spray combustion of gel propellants is considered with reference to the modeling studies on canonical spray diffusion and premixed flames and experimental observations corroborating their findings are discussed. A majority of spray combustion studies reported to date have been carried out on small-scale rocket motors. These studies are reviewed here and the reported performance parameters are compiled. In each of the sections, we also review the status of metalized gel propellants with consideration to choice of metals and experience with metal slurries. A roadmap for future progress is proposed and recommendations are made based on the discussion in the preceding sections. We have given particular attention to the linkages among the four areas of gel propellant combustion. Selected data have been extensively tabulated for many of the specific topics considered in this review and it is supplemented by a separate comprehensive table. (C) 2020 Elsevier Ltd. All rights reserved.