Arie Peretz

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*.

HEAT FLUX AND INTERNAL BALLISTIC CHARACTERIZATION OF A HYBRID ROCKET MOTOR ANALOG

The results of lab-scale hybrid rocket motor test firings and solid fuel pyrolysis experiments were combined with several analytical and numerical techniques to develop a semi-empirical method to analyze and correlate the experimental data with descriptive non-dimensional parameters. The experimental regression rate database was combined with a solid-fuel pyrolysis model and the energy flux balance equation to determine the total heat flux reaching the fuel surface. The component heat fluxes due to convection, radiation from the gas-phase combustion products, and radiation from soot were determined using various methods. The results of the analysis showed that under some conditions the magnitude of the overall radiant heat flux to the fuel surface was quite significant compared to the convective heat. Though CO2 represented the most important radiating gas-phase combustion product, radiation from soot was found to account for 80 to 90% of the total radiant heat flux. Dimensionless regression rate correlations indicated that the classical hybrid boundary layer correlation must be modified to account for the effects of radiation and variable fluid properties across the boundary layer. The Boltzmann number and velocity ratio between the flame and bulk flow were employed to make these corrections. In addition, it was found that the deduced Stanton number can sometimes be larger than the reference Stanton for turbulent flow over a flat plate due to various effects in the hybrid shear flow. The effects of variable transport properties were not found to be important in correlating the data, and no evidence for the existence of rate-limiting chemical kinetics was observed even for relatively large mass fluxes and low pressures. Finally, for the same percent weight mass addition, Alex powder (an ultra-fine aluminum powder, 0.05 to 0.1 |J.m) caused a substantially greater increase in regression rate than did conventional aluminum powder (5 to 15 urn).

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.

Pyrolysis and combustion of solid fuels in various oxidizing environments

An experimental study was conducted to determine the dependence of the regression rate of two solid-fuel formulations (cured HTPB and JIRAD fuel) on operating conditions near the head-end of a hybrid motor. Cylindrical fuel samples were burned in a windowed combustor at pressures ranging from 0.79 to 3.55 MPa. The burning was sustained by a diffusion flame created over the fuel surface by an impinging oxidizer jet. The gaseous oxidizer was a mixture of oxygen and nitrogen with the oxygen mass fraction ranging from 0.21 to 1.00. For both fuels, the regression rate increased with oxidizer mass flow rate, oxygen mass fraction, and oxidizer temperature, but decreased at higher pressures in the range tested. Measured regression rates, surface temperatures, and operating parameters were used to validate a simple power-law regression rate correlation for each fuel. A modified form of Marxmans analysis was also developed to consider the effects of species diffusion, heterogeneous surface reactions, and fluid-dynamic/heat transfer processes. The effect of heterogeneous reactions was found to be important in the range of parameters tested. For both fuels, the activation energies of the heterogeneous reactions were lower than the pyrolysis activation energies, with HTPB being more reactive than the JIRAD fuel. Nomenclature A pre-exponential coefficient [mm/s] B Spalding transfer number cf skin friction coefficient CP specific heat [kJ/kg-K ] Ea activation energy [kcal/mole] Ah enthalpy difference between the flame and the surface [kJ/kg] AHf° heat of formation [kJ/mole] hpy effective heat of pyrolysis [kJ/kg] hpych chemical enthalpy of fuel pyrolysis [kJ/kg] If heat flux feedback due to radiation [W/m] Krf, concentration of reactants below the flame m mass flow rate [g/s or kg/s] O/F oxidizer-to-fuel ratio P pressure in chamber [MPa or Pa] Q heat release [kJ/kg] rb solid-fuel regression rate [mm/s] Re Reynolds Number Ru universal gas constant [kcal/K-mole] T temperature [K] u velocity of gas [m/s] Y species mass fraction (j) velocity ratio A, thermal conductivity of the species [W/m-K] p density [g/cm] Subscripts b flame boundary e edge of the boundary layer EX oxidizer below the flame f solid-fuel F fuel F,O species and oxidizer g gas-phase h heterogeneous 1 initial condition/ i* species o initial condition O,T oxidizer and energy 02 oxygen OX oxidizer P products of combustion py pyrolysis r reactants of combustion ref reference condition s solid-fuel surface st stoichiometric condition «> free-stream quantity

Rheological Matching of Gel Propellants

Distinguished Professor and Director of the High Pressure Combustion Laboratory. AIAA Fellow. § Ph.D. Student. Student Member AIAA. 1 Ph.D. Candidate, AFRL Palace Knight. Member AIAA * Visiting Professor, on sabbatical leave from Rafael, Israel. Associate Fellow AIAA. ** Research Assistant. * Hybrid Propulsion Engineer, Advanced Propulsion Systems. Member AIAA. t Hybrid Propulsion Engineer, Advanced Propulsion Systems. Copyright

REGRESSION RATE AND HEAT TRANSFER CORRELATIONS FOR HTPB/GOX COMBUSTION IN A HYBRID ROCKET MOTOR

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.

Development of a Laboratory-Scale System for Hybrid Rocket Motor Testing

A data analysis program was developed to correlate the experimental solid-fuel regression-rate and thermal-pyrolysis data obtained in a lab-scale hybrid motor, a thermal-pyroly sis test apparatus, and a GC/MS system. The code employs an interfacial energy flux balance, various methods to determine the component heat fluxes due to radiation and convection, transient mass and energy conservation equations for the local gas properties, and chemical equilibrium computations for the flame properties. Separate dimensionless correlations were developed to accurately predict the regression rates in the upstream developing-flow region of the motor and the downstream developed-flow region. Radiation, variable fluid properties across the boundary layer, and axial variations in the gas temperature and velocity gradients had significant effects on the regression rate behavior in both regions. The radiant heat flux from soot contributed significantly to the overall surface heat flux under relatively low mass flux and low O/F ratio conditions. Correlations were also developed for the deduced Stanton and Nusselt numbers. The regression rate correlations predicted independent data from a tube burner to within ±5% error.

Regression-Rate and Heat-Transfer Correlations for Hybrid Rocket Combustion

DOI: 10.2514/1.47521 Theresultsofeffortstoestablishalaboratory-scalesetupforhot-firetestingofamodularhybridrocketmotorand experimental evaluation of the effect of several additives on the regression rates of hydroxyl-terminated– polybutadiene-based fuels are presented. Supercharged nitrous oxide was used as the oxidizer in all tests. The main objectivesofthereportedtest-programphasewerethedesignandbuildupoftheexperimentalfacilityandtosortout, by hot-fire tests, various fuel compositions with respect to the fuel regression rate and combustion–extinction ability at oxidizer shutoff. The cartridge-loaded fuel grains had an initial circular port diameter of 42 mm, a length of 400 mm, and a web thickness of 15 mm. Five fuel composition families with various additive contents of fine ammoniumperchlorate,polystyrene,andaburning-ratecatalystweretested.Thefuelregressionrateincreasedwith the increase in the ammonium perchlorate portion, but combustion extinguishment was not possible with high ammonium perchlorate content. A significant enhancement of the regression rate of up to 3:5 mm=s at an average oxidizer mass flux of about 140 kg=m 2 s (0:2 lbm=in 2 s) was obtained with the addition of both fine ammonium perchlorate and large particles of polystyrene. Full combustion extinguishment and multipulse operation with an insignificant effect on the regression rate was demonstrated by the use of large particles of polystyrene additive and no addition of ammonium perchlorate.