FORMULATION OF GEL PROPELLANTS

Transcription

1 FORMULATION OF GEL PROPELLANTS Boopathi S Assistant Professor, Department of Mechanical Engineering, Jeppiaar Institute of Technology,Chennai ABSTRACT The present study is a review of the main aspects in gel propellants past and current research anddevelopment. The work deals with the following subjects: Advantages of gel propellants in comparison to liquid and solid propellants, Theoretical performance evaluation of different gel propellant formulations for various missions, Rheological characterization, definition of the gel type, rheometry, identification of theparameters that affect the rheological properties, Preparation of gel propellants, gellant,compatibility of the ingredients, Flow behaviour, power law, shear thinning fluids in pipes andinjectors, thixotropic effect of gel fuels in smallmotors. Key word: gel, gel propellants, gel fuels, rheology. INTRODUCTION The demand for high-performance and improved safety propellants for various rocket motor applications hasbeen constantly increasing during the last decades and gels seem to be a promising answer to theserequirements. In this particular solid-liquid state, these propellants combine the advantages of the solids withthose of liquids. Gel Gels are liquids whose rheological properties have been altered by the addition of certain gelling agents(gellants) and as a result their behaviour resembles that of solids. The definition of a gel is not straightforward. D. Jordan Lloyd is quoted by Hermans as having said in her survey on the problem of gelstructure in 1926: "The colloidal condition, the gel, is one which is easier to recognize than to define. Bungenberg de Jong in 1949 defined a gel as "a system of solid character, in which the colloidal particlessomehow constitute a coherent structure." Also in 1949, Hermans used three propositions to define gels:(a) they are coherent colloid systems of at least two components; (b) they exhibit mechanical propertiescharacteristic of a solid; (c) both the dispersed component and the dispersion medium extend themselvescontinuously throughout the whole system." In 1990, Brinker and Scherer provided the definition of a gel as"a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. The continuity ofthe solid structure gives elasticity to the gel." Advantages of Gel Propellants in Comparison to Liquid and Solid Propellants Leaks and spill: The gel surface hardens in contact with a gaseous environment. Hence, in cases offailure in the feeding system or during storage, the leakage rate is reduced, compared to liquids. In casesof accidental spillage due to damage in the fuel and oxidizer tanks, burning will occur only at the fuel oxidizer interface, if they are hypergolic. When contact ceases, the 17

2 rheological nature of the gels willprevent further flow and chemical reaction. The volatility of gels is significantly lower than the volatilityof liquids and in case of leak or spill, much less vapours will be released, thus reducing toxicity hazards. Sensitivity to impact, friction and electrostatic discharge: In comparison to solids where detonation,explosion or deflagration can take place, gels are insensitive, similar to liquids. Accidental ignition: In comparison to solids where motor ignition can lead to a catastrophe, in gels, as inliquids, combustion is controllable and can be prevented. Cracks: Unlike solids, in which cracks in the propellant grain increase burning area and can lead to uncontrolled combustion and explosion, gels are fed into the combustion chamber similar to liquids, thuscracks in the gel structure have no effect. Problems in Gel Propellant Development and Current Research The rheological properties of gel propellants affect several processes in rocketmotoroperation. Thisparticular character of gels causes various problems that should be addressed. Feeding process: In comparison to liquids, increased feed pressures are required for the same propellant mass flow rate. In addition, since the fuel and the oxidizer are non-newtonian fluids, a complex mass flow rate control system is required, especially when the fuel and the oxidizer have different rheological properties. The dependence of these properties on temperature should be considered. Special attention should also be given to the pipe geometry; for example, stagnation regions should be avoided to prevent accumulation of gel. Contact with hot combustion gases at the injector face may result in fast evaporation of part of the liquid content of the gel that leaves a non-volatile residue in the manifold after a firing pulse. This can cause crusting of the gel surface or even pipe plugging and in these cases, significantly higher pressures are required to produce flow. TRW developed a face shut-off injector that prevents plugging even with heavily solid-loaded gels. Particle sedimentation, phase separation and physical instabilities: These may occur during storage orunder in-flight acceleration. Although in comparison to slurries particle sedimentation is significantly lower,at high acceleration levels the solid particles and even the solid phase gellant may separate from the liquid. Cost: Gels are more expensive than both conventional liquids and solid propellants. The price of a gelpropellant can be 30% higher or more than the price of a solid propellant.considering these problems, research has been conducted in several areas and is summarized in Table 1 TABLE 1 Research on Gel Propellants Research Subject Researchers References 18

3 Rheological characterization. Definition of the geltype, measurement of the rheological properties,identification of the parameters that affect theirbehaviour. Processing. Preparation of gel propellants, gellant, compatibility of the ingredients. Flow behaviour. Flow of shear-thinning fluids in pipes and injectors, thixotropic effect. Li-jun Yang et al. Kubal et al. Gupta et.al Rahimi and Natan Rapp and Zurawski Wickman and James Dove et al. Varghese et al. Changjin Yoon et al. Rahimi and Natan Kraynik et al ,11, ,11 16 GEL PROPELLANT RHEOLOGY The key to gel propellant science and technology is rheology. The rheological properties of a gel control itscombustion, atomization and flow characteristics. These properties depend on the chemical structure of thegel. Gel propellants are mainly of type 3, however, it has to be noted that though some gels (for example silicategels) appear outwardly like organic gels, their structure is not the same. Polymerization occurs in threestages: (1) polymerization of monomer to form particles, (2) growth of particles, (3) linking of particles intochains, then networks that extend throughout the liquid medium, thickening it to a gel. Figure 1Shear stress shear rate behaviour of Newtonian and non-newtonian fluids. Non-Newtonian fluids, in general, and gels, in particular, can be classified according to their rheological properties. A schematic classification is presented in Fig

4 A constitutive equation is a fundamental relation between force and deformation in materials, primarily liquids. For a general viscous fluid the stress tensor, τ, depends only on the rate of deformation tensor,2d, and the relation is described by: τ =η 2D (1) The most widely used form of the general viscous constitutive equation is the power law(p-l) model formulated by Ostwald and de Waele in 1923 (also known as the Ostwald-de Waelemodel). For a steady shear the power law becomes: (2) or (3) Where, is the shear rate, η is non-newtonian viscosity, K is the consistency index and η is the power law or rate index. The power law has been used extensively in polymer process models. Nearly all non-newtonian materials show shear thinning behaviour (0<n<1, viscosity decreases with increasing shear rate) but some, dilatant liquids, particularly concentrated suspensions, show regions of shear thickening (n>1, viscosity increases with increasing shear rate). In Newtonian fluids, viscosity is independent of the shear rate (n=1). The behaviour of these fluids is shown schematically in Fig. 1. Certain non-newtonian fluids exhibit time-dependent behaviour and are distinguished to thixotropic (viscosity decreases under constant shear rate) and rheopectic (viscosity increases under constant shear rate). Gel propellants are power law, shear thinning, thixotropic fluids. To give Newtonian regions at both low and high shear rates other models have been proposed and are described in Tables 2, 3 and 4.η 0 is the viscosity at rest while η is the viscosity at infinite shear rate. TABLE 2 Constitutive Equations of Time-independent Fluids. Model Equation Ostwald-de Waele, power law model (2) Cross Carreau-Yasuda (4) (5) 20

5 In several cases, the yield stress has to be taken into account (particle sedimentation, initial pressurecalculation etc.). The most common model is the Herschel-Bulkley model. Timeindependent constitutive equations for fluids with yield stress are presented in Table 3. TABLE 3 Constitutive Equations of Time-independent Fluids with Yield Stress. The adoption of a constitutive model in gel propellants is rather common. In most cases the P-L model isused because of its simplicity and its ability to describe quite well the behaviour of gel propellants in a wideshear rate range. The rheological parameters are measured by rheometers that operate at different conditionsand principles and provide data at different shear rate ranges. A list of rheometers is given by Macosko. The rheological properties of gel propellants were measured in various studies and the P-L model wasadopted. The P-L indici in those studies are presented in Table4. TABLE 4 Rheological Properties of Various Gel Propellants Propellant K, pa-s n n Shear rates s -1 Authors Ref. Water gels ,000 Chojnaki and Feikema[15] Water gels ,000 Rahimi and Natan [10-12] Water gels Green et al. [16] RP-1/Al Green et al. [16] Ethane, Propane 0.22, Starkovich and palaszewski [7] UDMH Gupta et al. [9] DMH (30%) Varghese et.al [2] 21

6 Compatibility studies: The compatibility of aluminium with gelled IRFNA was investigated by Dove et al.and Hallit et al. Gellation of IRFNA is obtained using silicon dioxide, which reacts with the HF inhibitorin IRFNA to form silicon-fluorine or silicon-fluorine-oxygen compounds that can increase corrosion in thealuminum tanks. In general, the corrosion rates measured in gelled and non-gelled IRFNA werecomparable. TABLE 5 Gel Propellants and Gellants Propellant Gellant Additives Reference Fuels Hydrazine UDMH RP-1 JP10 Kerosene Carbopol, Sulphated gatactose polymers Methyl Cellulose Sio 3 Cellulose compounds Organophilic clay complex Al, Al (0%, 40%) Al (30%, 5%-40%) Al (0%, 55%) Boron Carbide Al (30%, 5%-40%) 4,5,17 2,9 1,7,8 3,4 2 Oxidizer IRFNA RFNA Sio 3 Sodium Silicate LiNo 3, HF AP (0%-3%) 6 5 The Flow of Gel Propellants In the feeding process of the propellant, the gel fluid passes through a pipe and finally is injected into the combustion chamber. The injectors are small in both length and crosssection area size and the fluid stay there for a very limited time (fraction of 1 ms). The shear rates developed in the injectors due to the sudden decrease in the cross-section area are very large ( 10 4 s -1 ) and the shear thinning effect is dominating. The thixotropic effect should be considered in pipes where the fluid flows through for a long time and the shear rates are considerably lower ( s -1 ). Shear thinning flows: Since the gel viscosity is shear-rate dependent, by applying high shear rates during injection it is possible to reach low viscosities and even liquefaction near the injector exit. Injection of the gel propellant through a converging injector causes the shear rate to increase due to the fluid acceleration, consequently resulting in a decrease in viscosity along the axis of the injector. Thixotropic flows: The thixotropic effect in inorganic gel fuels was investigated by Rahimi and Natan and the results indicate that it is insignificant in this kind of propellants. In any case, the diameter/length ratio of the pipe might be an important design parameter. CONCLUDING REMARKS In the present study many aspects of the gel propellants past and current research and development wereaddressed. It seems that gel propulsion has not yet reached maturity, however the day that such a system willbe fully operational is not far away. 22

7 Rheology is the key element to realize the theoretical benefits of metallized propellants. Suspension of metalparticles, stability, smooth flow, fine atomization and efficient combustion depend on the rheological characteristics. Gellant chemistry and its effects on the gel structure and rheological properties are mostimportant. The author strongly believe that gels will find the right place among the solid and liquid propellants infuture propulsion systems. REFERENCES 1. Rapp, D.C. and Zurawski, R.L., "Characterization of Aluminum / RP-1 Gel Propellant Properties," AIAApaper , (also) NASA TM , July Varghese, T.L., Gaindhar, S.C., John D., Josekutty J., Muthiah, Rm., Rao, S.S., Ninan, K.N. andkrishnamurthy, V.N., "Developmental Studies on metallized UDMH and Kerosene Gels," DefenseScience Journal, Vol. 45, No. 1, January 1995, pp Mueller, D.C. and Turns, S.R., "Ignition and Combustion Characteristics of Metallized Propellants -Phase II," Annual Report , NASA Lewis Research Center, NASA-CR Robbins, J.M. and Feist, R.W., "The China Lake Propulsion Laboratories," AIAA paper , July Tarpley, W.B., "Thixotropic Liquid Propellant Compositions with Solid Storage Characteristics," U.S.Patent No. 3,470,040, patented Sept. 30, Schindler, R.C., Olson, A.M. and Arnold, C.J., "A Gelled Propellant Sustainer Stage," AIAA paper , Feb Palaszewski, B., "Upper Stages Using Liquid Propulsion and Metallized Propellants," NASA TP-3191,Feb Palaszewski, B. and Powell, R., "Launch Vehicle Performance Using Metallized Propellants," J. ofpropulsion and Power, Vol. 10, No. 6, 1994, pp Gupta, B.L., Varma, M. and Munjal, N. L., "Rheological Studies on Virgin and MetallizedUnsymmetrical Dimethyl Hydrazine," Propellants, Explosives, Pyrotechnics, Vol. 11, No. 2, 1986, pp Rahimi, S. and Natan, B., "The Flow of Gel Fuels in Tapered Injectors," J. of Propulsion and Power,Vol. 16, No. 3, 2000, pp Rahimi, S. and Natan, B., "On the Thixotropic Effect of Inorganic Gel Fuels," accepted for publication inj. of Propulsion and Power, Feb Rahimi, S. and Natan, B., "Atomization Characteristics of Gel Fuels," AIAA paper , July Wickman, S. and James, E., "Gelled Liquid Oxygen / Metal Powder Monopropellants," AIAA paper , July Dove, M.F.A., Norman, L., Mauger, J.P., Allan, B.D., Arndt, R.E. and Hawk, C.W., "Aluminum AlloyCompatibility with Gelled Inhibited Red Fuming Nitric Acid," J. of Propulsion and Power, Vol. 12, No.3, 1996, pp Chojnacki, K.T. and Feikema, D.A., "Study of Non-Newtonian Liquid Sheets Formed by ImpingingJets," AIAA paper , July