Advances in science and technology of modern energetic materials: an overview.

An experimental investigation of the regression-rate characteristics of hydroxyl-terminated polybutadiene (HTPB) solid fuel burning with oxygen was conducted using a windowed, slab-geometry hybrid rocket motor. A real-time, x-ray radiography system was used to obtain instantaneous solid-fuel regression rate data at many axial locations. Fuel temperature measurements were made using an array of 25- πm e ne-wire embedded thermocouples. The regression rates displayed a strong dependence on axial location near the motor head-end. At lower mass e ux levels, thermal radiation was found to signie cantly ine uence the regression rates. The regression rates werealso affected by theadditionofactivated aluminum powder.A 20%by weightaddition of activated aluminum to HTPB increased the fuel mass e ux by 70% over that of pure HTPB. Correlations were developed to relate the regression rate to operating conditions and port geometry for both pure HTPB and for HTPB loaded with certain fractions of activated aluminum. Thermocouple measurements indicated that the fuel surface temperatures for pureHTPBwerebetween930 and1190 K.TheHTPBactivationenergywasestimatedat11.5 kcal/mole,suggesting that the overall regression process is governed by physical desorption of high-molecular weight fragments from the fuel surface.

Regression Rate Behavior of Hybrid Rocket Solid Fuels

This volume brings together international scientists in the field of solid rocket propulsion. Thirty-nine papers present in-depth coverage on a wide range of topics including: advanced materials and non-traditional formulations; the chemical aspects of organic and inorganic components in relation to decomposition mechanisms, kinetics, combustion and modelling; safety issues, hazards and explosive characteristics; and experimental and computational interior ballistics research, including chemical information and the physics of the complex flow field.

Solid propellant chemistry, combustion, and motor interior ballistics

Alex powder (an ultra-fine aluminum powder produced by the plasma-explosion process) has been shown to be a very effective burning rate enhancer for solid propellants and fuels. The objective of this research is to investigate the mechanisms responsible for the beneficial effect of Alex. A TGA and DTA were employed to examine the thermal bahavior of Alex and regular aluminum particles in different gases. An environmental scanning electron microscope (ESEM) was utilized to study the particle surface bahavior when samples were heated in air and nitrogen. A transmission electron microscope (TEM) was employed for examining the microstructure of Alex particles. Alex powder was demonstrated in TGA and DTA experiments to behave very differently from the regular aluminum in air, oxygen and nitrogen. Rapid weight gain of Alex particles in air and nitrogen environment occurred around 548°C and 700°C, respectively. The low-temperature reaction in air corresponds to an oxidation reaction. Comparing DTA traces of Alex and regular aluminum in air, it was found that oxidation of Alex particles occurs at a lower temperature with a higher degree of reaction. This is believed to be mainly due to the higher reactivity associated with greater surface area of the Alex particles. In the comparison of DTA traces of Alex and regular aluminum in nitrogen, the rapid nitridation reaction of Alex particles occurs at temperature around 680°C and proceeds to near completion.

Comparison of Thermal Behavior of Regular and Ultra-fine Aluminum Powders (Alex) Made from Plasma Explosion Process

Aseriesofstaticenginee ringswereconductedtoinvestigatethesolid-fuelregressionratebehaviorandoperating characteristicsofvortexhybridrocketengines.Thevortexhybridenginecone gurationischaracterizedbyacoaxial, coswirling, countere owing vortex combustion e eld in a cylindrical fuel port. To generate this e owe eld, oxidizer is injected through a swirl injector located between the aft end of the fuel grain and the inlet to the converging portion of the exit nozzle. Test e rings with thrusts up to 960 N were conducted with gaseous oxygen and hydroxylterminated polybutadiene solid fuel. Average fuel regression rates up to seven times larger than those in similar classical hybrids were measured. Empirical correlations were developed to describe accurately the experimental regression rates over more than an order of magnitude variation in mass e ux. In addition to local mass e ux and oxygeninjectionvelocity,geometricenginevariables,suchasenginecontractionratioand length-to-diameterratio, had a signie cant ine uence on the measured regression rates. Nondimensional regression rate and heat transfer correlations were also developed. Throttling and restart capability were demonstrated. Nomenclature A = cross-sectional area, cm 2 B = blowing parameter cp = isobaric specie c heat, J/kg ¢K D = port diameter, cm Ea = activation energy, kcal/mole G = local mass e ux, kg/m 2 ¢s Ginj = injection mass velocity, kg/m 2 ¢s GO = gaseous oxygen mass e ux, kg/m 2 ¢s

Solid-Fuel Regression Rate Behavior of Vortex Hybrid Rocket Engines

A series of static engine  rings, thermal pyrolysis experiments, and gas chromatograph/mass spectrometer tests were conducted to investigate the solid-fuel regression rate and heat-transfer behavior in a lab-scale hybrid rocket motor burning hydroxyl terminated polybutadiene/gaseous oxygen. A real-time, X-ray radiography system was used to determine the local, instantaneousregression rates. A dataanalysisprogramwas developedto help correlate the experimental data. The semi-empirical regression-rate correlation showed that, in addition to convection, radiation from soot and variable  uid properties across the boundary layer had signi cant effects on regressionrate behavior. The radiant heat  ux from sootwas relativelymore signi cant under lowmass  ux and low oxidizerto-fuel ratio conditions. Radiation from CO2 , H2O, and CO was quite small compared to convection and soot radiation. The nondimensional regression-rate correlation agreed with the experimental data to within § 3%. Stantonand Nusselt-number correlations were also developed and found to depend on both  ow regime and radiant heat  ux. The regression-rate correlations predicted independent data from both a lab-scale tube burner and the 11-in. (28 cm) JIRAD motor to very reasonable accuracy.

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

An experimental investigation of the thermal pyrolysis behavior of several hybrid-rocket solid fuels under rapid heating conditions was conducted to determine pyrolysis laws and to identify and quantify the products of fuel pyrolysis. The study focused on four fuel formulations: pure hydroxyl-terminated polybutadienes (HTPB), 80% HTPB/20% Alex, 80% HTPB/20% Al, and the Joint Industrial Research and Development fuel formulation. A rapid conductive-heating technique was developed and employed to determine Arrhenius-type pyrolysis laws. All four fuels displayed two sets of Arrhenius parameters, depending on the range of surface temperature. For pure HTPB,Ea = 4:91 kcal/mol andA = 11.04mm/s above722K, whileEa = 13:35 kcal/mol andA = 3965mm/s below 722 K. These results agree well with those obtained previously using a lab-scale hybridmotor operating under realistic conditions. The gas chromatograph/mass spectrometer tests of the nonmetalized fuels, using a  ash-heating oven, indicated that the relative concentrations of the pyrolyzed species depended strongly on temperature. For pure HTPB seven major products were identi ed, with 1,3-butadiene representing the dominantproduct at all temperatures tested, up to 1073 K. The measured mole fractions of the pyrolysis products and deduced pyrolysis laws of the fuels studied can be utilized in a comprehensive model simulation for combustion performance predictions.

Pyrolysis Behavior of Hybrid-Rocket Solid Fuels Under Rapid Heating Conditions

Combustion characteristics of boron/poly(BA MO/NMMO) fuel-rich solid propellants have been studied due to their potential application to solid-fuel ramjets (SFRJ). For the same boron content, BAMO/NMMO copolymer-based fuels are superior to conventional hydroxyl terminated polybutadien (HTPB) fuels due to their vigorous pyrolysis characteristic for dispersing boron particles into the main reaction zone. However, their specific impulses are generally lower than that of HTPB, in spite of high positive heats of formation. Formation of hexagonal crystalline boron nitride (BN) has been found in the combustion of this family of propellants studied. Favorable conditions for the formation of BN have been identified. BN also has a significant effect on the theoretical performance for high equivalence ratio conditions. The burning rate was found to depend strongly upon pressure and nonmonotonically on boron content. An "energy sink" hypothesis is proposed to explain this observation. Fine-wire thermocouple measurements support this hypothesis.

Combustion behavior of boron-based BAMO/NMMO fuel-rich solid propellants

Pyrolysis behavior of hybrid rocket solid fuels under rapid heating conditions

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).

Arie Peretz

Papers

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

Pyrolysis Behavior of Hybrid-Rocket Solid Fuels Under Rapid Heating Conditions

Combustion behavior of boron-based BAMO/NMMO fuel-rich solid propellants

Pyrolysis behavior of hybrid rocket solid fuels under rapid heating conditions

Heat flux and internal ballistic characterization of a hybrid rocket motor analog