Polybutadiene Composite Propellants Research Articles

Aluminum agglomeration of AP/HTPB composite propellant

Aluminum (Al) powder agglomeration is one of the main reasons for the degradation in the performance of aluminized solid propellant rockets and so, understanding the combustion behavior of aluminum in solid propellants is of great importance. In this work, a laser ignition test bench was used to study the behavior of Al on the burning surface of an aluminum/ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellant, under atmospheric pressure. Based on the images captured by a high-speed camera, the agglomeration process and behavior of the agglomerates were analyzed in detail. The size distribution and speed of motion of the agglomerates away from the burning surface were also considered. Results show that the formation of an agglomerate on the burning surface from multiple aluminum particles include three stages: accumulation, aggregation, and agglomeration. Local ignition promotes the collapse of the aggregate into a spherical agglomerate. Before detachment, the agglomerates often roll around on the burning surface and pick up more aluminum, promoting self-growth. The interesting phenomenon of an agglomerate droplet rupturing and ejecting liquid alumina on the burning surface was clearly captured for the first time. This is believed to indicate the heterogeneous composition characteristics of the agglomerate. The transformation of the polar oxide cap on one part of the agglomerate surface into an alumina shell that fully covers the droplet surface was also captured for the first time. The agglomerates have different shapes, diameters, and velocities when they leave the burning surface. The formation of non-spherical agglomerates consisting of more than one aluminum droplet is ascribed to the propellant microstructure. The 400 agglomerates that were counted had diameters that ranged from 51 μm to 815 μm and the majority of them (nearly 98%) were below 400 μm. The velocities of motion of 176 agglomerates exhibited great dispersion, with the maximum and minimum velocity being 196 cm/s and 13 cm/s, respectively. In general, the moving velocities of the agglomerates decreased with increasing diameter.

Ignition Delay Times of Composite Solid Propellants Using Novel Nano-Additive Catalysts

The ignition delay times of propellants provide insight for the fundamental understanding of the ignition process. This study demonstrated a refined method to measure the of ammonium-perchlorate-based propellants at pressures between 3.5 and 15.5 MPa. Both aluminized and nonaluminized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants were studied with and without metal oxide nanoparticle catalysts. A laser was used to obtain quantifiable and reliable ignition times over a 30–100 W power range. The results were compared to literature values for similar power fluxes at comparable pressures and to a simple ignition model based on heat conduction. Catalytic additives were seen to modify the , depending primarily on whether they impact the lower- or higher-temperature decomposition of ammonium perchlorate. Examining the effects of in situ titania nanoparticles on the ignition delay times led to the conclusion that the nano-additives only altered the ignition behavior of the aluminized samples, but had no effect on the nonaluminized ones. Conversely, when nano-sized was used, the was halved at lower pressures. As a result, additives that assist in the low-temperature decomposition of ammonium perchlorate, such as , are believed to have the greatest impact on ammonium perchlorate/hydroxyl-terminated polybutadiene ignition delay times.

Rheological and wall-slip behaviour of composite propellant suspension containing Al-nanopowder

ABSTRACTHydroxyl terminated polybutadiene composite propellant suspension mainly containing ammonium perchlorate (70%) and aluminium nanoparticles (ANP) (0–6%) was evaluated rheologically to determine the effect of wall slip occurring due to the formation of an apparent slip layer. True rheological properties have been obtained from gap-dependent steady-shear data using Tikhonov regularization method for the composite suspension. The advantage of this method is that it converts the gap-dependent steady-shear data into true rheological properties. The two-stage method can successfully establish both the true shear stress vs. shear rate behaviour and wall-slip parameters. The errors during rheological experimentation are analysed by determining the slip velocities and slip layer thickness. Slip velocity is observed to increase linearly with shear stress. Also, the slip layer thickness decreases with the increase in ANP content in the composite suspension. The maximum slip layer thickness of 2.13 µm is obtained for composition in which ANP is absent, and the same decreased to 0.24 µm for the composition containing 6% ANP. The rheological measurements show least deviation from gap-independent values as the amount of ANP in the propellant increases. Finally, a correlation of apparent slip layer thickness with normalized filler fraction is investigated to check the effect on wall-slip behaviour.

Ignition and combustion behavior of mechanically activated Al–Mg particles in composite solid propellants

Metallized propellants typically produce large agglomerates that result in two-phase flow losses. Tailoring composite metal fuel particles can improve ignition and combustion characteristics while reducing product droplet sizes. In this work, mechanically activated (MA) aluminum (Al) and magnesium (Mg) powders are synthesized, characterized and compared to magnalium (Mag) alloy, neat Al, neat Mg and their physical mixtures (PM) at the same 1:1 mass ratio as MA and Mag powders. CO2 laser ignition tests showed that the MA powders are more reactive than those of Mag and exhibit particle fragmentation upon ignition. Mag powders only show fragmentation and microexplosions at high heating rates. The burning rates of the ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants containing MA powders were the highest, compared to Mag, neat Al, PM and neat Mg in decreasing order. High-speed imaging of the propellants and product collection showed that MA, Mg and Mag powders produce much smaller agglomerates than neat Al or PM at lower pressures due to fragmentation and Mg vaporization. However, the microexplosion tendency of the MA and Mag particles in the propellants was reduced at higher pressures due to reduced vapor bubble growth. Out of all the materials investigated, MA particles provide the best combination of burning rate and product sizes.

Finite element implementation of a thermo-damage-viscoelastic constitutive model for hydroxyl-terminated polybutadiene composite propellant

A thermo-damage-viscoelastic model for hydroxyl-terminated polybutadiene (HTPB) composite propellant with consideration for the effect of temperature was implemented in ABAQUS. The damage evolution law of the model has the same form as the crack growth equation for viscoelastic materials, and only a single damage variable $S$ is considered. The HTPB propellant was considered as an isotropic material, and the deviatoric and volumetric strain-stress relations are decoupled and described by the bulk and shear relaxation moduli, respectively. The stress update equations were expressed by the principal stresses $\sigma_{ii}^{R}$ and the rotation tensor $M$ , the Jacobian matrix in the global coordinate system $J_{ijkl}$ was obtained according to the fourth-order tensor transformation rules. Two models having complex stress states were used to verify the accuracy of the constitutive model. The test results showed good agreement with the strain responses of characteristic points measured by a contactless optical deformation test system, which illustrates that the thermo-damage-viscoelastic model perform well at describing the mechanical properties of an HTPB propellant.

Mechanical Erosion of Graphite Nozzle in Solid-Propellant Rocket Motor

A detailed theoretical/numerical framework is established to study the mechanical erosion of graphite-nozzle materials in solid rocket motors with aluminized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants. The analysis is based on a combined Eulerian–Lagrangian approach for treating multiphase motor flowfields. The multicomponent reacting gas-phase dynamics is formulated using the conservation equations of mass, momentum, and energy in the Eulerian framework. Turbulence closure is achieved using the standard two-equation model. The dispersed phase, consisting of aluminum and alumina droplets, is treated in the Lagrangian framework. Combustion of aluminum droplets to aluminum-oxide smoke is considered. Two empirical correlations are first calibrated and then employed to predict the mechanical-erosion rate of the nozzle surface. The estimated erosion rates fall within the range of the available experimental data. Mechanical erosion is prevalent in the convergent section of the rocket nozzle due to the particle impingement on the nozzle surface. No such erosion, however, is observed at the nozzle throat or downstream because the droplet trajectories move away from the nozzle surface in those regions.

Industrial Adaptation of Ultrasonic Technique of Propellant Burning Rate Measurement Using Specimens

A method of solid-propellant burning rate measurement by ultrasonic technique using small cylindrical specimens was developed for routine measurements required in the solid-propellant manufacturing industry. The measurement procedure is discussed in detail. Emphasis is given to accuracy in measurements, ease of operation, minimization of manpower, and savings of cost and time. With this in view, new adaptations in specimen preparation, test procedure, and data processing are incorporated. Burning rates of more than 300 batches of aluminized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellant of three different compositions were measured. The results showed that the standard deviation in the measurements for a propellant batch was less than 1%. It was also demonstrated that the performance of full-size solid motors can be predicted with good accuracy by these burning rates with a small augmentation factor.

Effect of Surface Roughness and Radiation on Graphite Nozzle Erosion in Solid Rocket Motors

A numerical analysis is performed to study the effect of surface roughness and radiation on graphite nozzle erosion in solid rocket motors loaded with non-metalized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants. An integrated theoretical/numerical formulation established previously to predict the chemical erosion of nozzle material has been extended to investigate the effects of surface roughness and radiation. The framework takes into account propellant chemistry, detailed thermofluid dynamics for a multi-component flow, energy transport in the solid phase, heterogeneous reactions at the nozzle surface, and nozzle material properties. Typical combustion species of non-metalized composite propellant at practical motor operating conditions are considered at the nozzle inlet. The two-layer turbulence model has been modified to account for surface roughness. The contribution from radiative heat transfer is included in the overall energy balance at the gas-solid interface. Results show that the heat- transfer rate to the nozzle material is enhanced for a rough surface, leading to a higher erosion rate than that for a smooth surface. For non-metalized propellant, the erosion rate has been found to decrease slightly due to radiation, and change in the rate strongly depends on the gas-phase emissivity.

Effect of Catalyst Addition on Subatmospheric Burning Surface Temperature of Composite Propellants

The subatmospheric burning with its higher catalytic effectiveness, lower temperature gradient, and slower combustion wave offers a Ž tting environment to study the effect of catalysts on the burning surface temperature of composite propellants. Using platinum and platinum-13% rhodium 7.5m thermocouples in uncatalyzed as well as copper–chromite-catalyzed ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants, the subatmospheric-burning surface temperatures were measured. The results of the present experimental study are in close agreement with the established trend; the surface temperature increases with the increase in pressure. Some experimental studies of others failed to give an observable change in surface temperature with pressure. This is argued to be because of the dimensional inadequacy of detectors in the very high-temperature-gradient environment. The measured surface temperature of the catalyzed propellant is signiŽ cantly higher than that of the uncatalyzed one. The study shows that the increased surface and subsurface heat release caused by catalyst addition causes this temperature enhancement.