Hydroxyl-terminated Polybutadiene Propellants Research Articles

Analysis of thermomechanical coupled accelerated aging of HTPB propellants

AbstractThis study employed macroscopic uniaxial compression tests at low and medium strain rates, coupled with microscopic electron microscopy, to extensively analyse the impact of thermomechanical coupled aging on the accelerated aging of Hydroxyl‐terminated Polybutadiene (HTPB) propellants, contrasting it with the effects of isolated factors such as heat and dynamic reciprocating force. Results indicate that at various environmental temperatures (323 K, 343 K, and 363 K), thermomechanical coupled aging more significantly affects HTPB propellants than isolated factors. This effect is macroscopically evident in increased ease of deformation, permanent deformation during aging, continual increase in dissipated energy, and a decrease in average stress and ultimate strain post‐aging. Microscopically, the effect predominantly arises from the interplay between matrix thermal degradation and particle fragmentation, which rapidly accumulate and substantially impact the material's macroscopic mechanical properties. Furthermore, as the aging temperature rises, the alterations in both macroscopic mechanical properties and microscopic morphology of HTPB propellants become more pronounced. However, overly high temperatures may swiftly result in substantial material performance deterioration. Consequently, while elevating temperature effectively accelerates thermomechanical aging, the potential adverse effects on material performance must be judiciously considered. This underscores the necessity of balancing temperature regulation with aging efficiency enhancement in HTPB propellants to ensure effective control and quantitative assessment of the aging process, while minimizing material degradation.

High-Pressure Combustion Characteristics of Hydroxyl-Terminated Polybutadiene Propellants

Combustion mechanisms of propellants under high pressures are necessary to solve the problems of a high burning rate pressure exponent in high-pressure solid rocket motors. In this study, the combustion characteristics of hydroxyl-terminated polybutadiene (HTPB) propellants were investigated by analyzing the burning rate, the quenched surface, and the combustion wave up to pressures of 28 MPa. The results showed that the pressure exponent presented an obvious break above 17 MPa, which increased from below 0.4 to approximately one. The characteristic pressure where the exponent break occurred decreased from 19.00 to 17.90 MPa with the increase of coarse ammonium perchlorate (AP) particles and decreased from 18.73 to 16.96 MPa with the addition of octogen. Coarse-AP particles exposed on the burning surface recessed relative to the binder below the characteristic pressure and protruded from the binder above the characteristic pressure. This finding was attributed to the difference in the local mass transfer between the gas phase and the burning surface under high pressures. An important finding of this study was the decline at the AP/HTPB interface above the characteristic pressure, indicating that the local heat transfer might play a crucial role in the pressure exponent break of HTPB propellants under high pressures.

Strain-rate correlation of biaxial tension and compression mechanical properties of HTPB and NEPE propellants

An effective biaxial tension and compression test method is proposed based on the shortcomings of current research for the mechanical properties of solid propellants under complex stress states. The equal proportion biaxial tension and compression test of HTPB (Hydroxyl-terminated polybutadiene) and NEPE (NitrateEster Plasticized Polyether) solid propellants is performed at different rates while at room temperature, and the damage morphology of the tension–compression zone is analyzed using micro-CT. The results show that the failure mode of the solid propellant under biaxial tension and compression loading is similar to that under uniaxial tension. Meanwhile, the compressive strength is much greater than the tensile strength, which will eventually cause tensile failure. With an increased loading rate, the growth trend of the initial modulus, ultimate strength, and maximum elongation of the propellant is gradually flattened, and the damage degree is gradually reduced. Additionally, damage that forms in the HTPB propellant is from dewetting and particle fracture while that for the NEPE propellant is from matrix tearing. The porosity can be used as the meso-damage parameter of the propellant.

The ablation and ionization characteristics of a rectangular pulsed plasma thruster with hydroxy-terminated polybutadiene propellant

Pulsed plasma thrusters (PPTs) are attractive micro-thrusters for their advantages such as compactness and lightweight compared to other electric propulsion systems. Accordingly, PPTs have been used in station-keeping, drag makeup, and attitude control for small satellites. Instead of the most common PPT propellant PTFE (polytetrafluoroethylene), hydroxyl-terminated polybutadiene (HTPB) propellant was used and some interesting experimental results were obtained; however, the ablation and ionization characteristics of such propellant have never been investigated. In this research, performance experiments using rectangular breech-fed PPT were conducted with HTPB propellant. Mass shot, electrical parameters, and plasma properties were obtained. The ablation process was further analyzed from three perspectives: electrical thermal energy, chemical energy, and ablation energy. The experimental result shows that all mass shots of different HTPB propellants are less than that of PTFE propellant, and the mass shot of HTPB propellant increases with increasing oxidant content. The difference in ablation between HTPB propellant and PTFE propellant could be explained by ablation energy while the difference between HTPB propellants with different proportional compositions could be elucidated by chemical energy. It was observed that the electron densities of these two kinds of propellants were not significantly different. This could be explained by the fact that HTPB propellant produced about three times more electrons than PTFE did even though less mass shot was produced by HTPB propellant.

Effect of Organic Fluoride on Combustion Performance of HTPB Propellants with Different Aluminum Content

ABSTRACT Aluminum is an important contributor to energy of solid propellants. Completeness of aluminum combustion has key effect on the release of energy. In this paper, hydroxyl-terminated polybutadiene (HTPB) based propellants with aluminum content from 16% to 24% were prepared with a kind of organic fluoride (OF). Meanwhile, propellants without OF were also prepared as a comparison. The theoretical specific impulse and the heat of explosion of these propellants were calculated and measured, respectively. The particle size, morphology and elemental composition of the condensed combustion products for these propellants were characterized by laser measurement of particle size, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The burning rates of propellants were tested by CCD line-scanning camera and a high-pressure chamber measurement system. The high-speed photographic system was used to study the agglomeration process of the melting aluminum particles on the propellant burning surface. The results indicate that with increasing aluminum content, both the theoretical specific impulse and the measured heat of explosion show a trend of increasing until an aluminum content of 20%. The large particle size and residual active aluminum in the condensed combustion products increase with increasing aluminum content in the propellants, but decrease significantly after addition of OF. Under the same pressure, the burning rates of the blank and OF propellants decrease with increasing aluminum content, but the burning rates of OF propellants are slightly higher than those of the blank propellants. For the propellants with OF, the melting aluminum particle on the burning surface is less agglomerated and brighter, meaning more complete aluminum combustion.

Aging Effects on the Burning Rates of Composite Solid Propellants with Nano-Additives

The aging behavior of composite propellant is a key parameter in determining the usable service life of an energetic system. The effect of aging on composite propellant burning rates is an understudied topic in the open literature. Recent research in the authors’ laboratory has led to the development of high-burning rate propellants containing titania nanoparticles and fluorescing propellants containing quantum dots. The current study assessed the aging behavior of composite ammonium perchlorate (AP)/hydroxyl terminated polybutadiene propellants, with specific emphasis on fluorescent properties and burning rates. Propellant formulations were based on either 80% monomodal or 85% bimodal AP distributions with and without titania nanoparticles and quantum dots. Propellant samples were thermally aged at temperatures of 63 and 85°C and humidity levels of 25 and 75% for up to 195 days to yield approximate service lives in the range of 0–25 years. Fluorescent imaging experiments were conducted with an ultraviolet light source to measure the uniformity of propellant samples. Ballistic testing was conducted in a constant-volume strand burner over the pressure range of 500–2250 psia (3.5–15.5 MPa). Spatial uniformity of the propellant fluorescence was not significantly altered with propellant age but did decrease in intensity to some degree. In general, the burning rate and pressure index of propellants decreased with increasing service life. Propellant formulations containing 85% bimodal AP were found to be more stable than formulations containing 80% monomodal AP. Inclusion of quantum dots or titania nanoparticles in the propellant formulation increased aging stability.

Modeling and simulation of the debonding process of composite solid propellants

In order to study the damage evolution law of composite solid propellants, the molecular dynamics particle filled algorithm was used to establish the mesoscopic structure model of HTPB(Hydroxyl-terminated polybutadiene) propellants. The cohesive element method was employed for the adhesion interface between AP(Ammonium perchlorate) particle and HTPB matrix and the bilinear cohesive zone model was used to describe the mechanical response of the interface elements. The inversion analysis method based on Hooke-Jeeves optimization algorithm was employed to identify the parameters of cohesive zone model(CZM) of the particle/binder interface. Then, the optimized parameters were applied to the commercial finite element software ABAQUS to simulate the damage evolution process for AP particle and HTPB matrix, including the initiation, development, gathering and macroscopic crack. Finally, the stress-strain simulation curve was compared with the experiment curves. The result shows that the bilinear cohesive zone model can accurately describe the debonding and fracture process between the AP particles and HTPB matrix under the uniaxial tension loading.

Preparation, characterization and catalytic behavior of CdFe 2O 4 and Cd nanocrystals on AP, HTPB and composite solid propellants, Part: 79

CdFe 2O 4 and Cd nanocrystals were synthesized by wet chemical and hydrazine reduction methods, respectively. These nanocrystals were characterized by XRD and TEM. Their catalytic activity was investigated on the thermal decomposition of ammonium perchlorate (AP), hydroxyl terminated polybutadiene (HTPB) and composite solid propellants (CSPs) using TG and TG-DSC. Burning rate of CSPs was considerably enhanced by these nanocrystals. Rapid thermolysis of the CSPs and AP has also been studied using ignition delay measurements.

The Effect of Aluminum and Ammonium Perchlorate Content on Temperature Sensitivity, Mechanical Properties and Performance of HTPB Propellants

This paper describes the effect of aluminum content variation of HTPB (hydroxyl terminated polybutadiene) propellants on temperature sensitivity, mechanical properties and performance values. Aluminum (Al) content was applied as 16, 18 and 20 percent to the propellant compositions and those compositions were tested at small test motors with three different propellant area ratios (K=200, 250 and 300) by keeping the propellant grain at three different temperatures (231, 258 and 323 K). As a result of this, a relationship such as P=aesT was determined with different coefficients for each of the three compositions and propellant area ratios. The lowest temperature sensitivity value was obtained for the composition containing 20% Al and 65% AP. The second relation as P=bKy was derived for three propellants and temperatures. The highest pressure was found with the K value of 300 for the propellant including 16% Al and 69% AP at 323 K. The third relationship such as r=cPn was also determined for three propellant compositions and temperatures. It was observed that the propellant having 16% Al and 69% AP had the highest burning rate at 323 K. The highest maximum tensile stress and elongation values were obtained for the propellants including 20% Al and 16% Al respectively. Evaluation of performance values for those propellants exhibits that the maximum characteristic velocity value is attained for the propellant containing 16% Al and 69% AP.

Burning of Ammonium-Perchlorate Ellipses and Spheroids in Fuel Binder

Our simulations of heterogeneous propellant combustion have always assumed that the oxidizer particles (ammonium perchlorate) are disks (two dimensional) or spheres (three dimensional). Here the effects on the burning rate are examined when the disks/spheres are replaced by ellipses/ellipsoids. In two dimensions, it is shown that an area-preserving deformation of a pack of disks, generating a pack of ellipses, can lead to significant variations in the burning rate. However, if the ellipses are randomly packed, so that the alignment of their axes is random, the shape effect is small. In three dimensions, volume-preserving deformation, which generates ellipsoids, leads to burning rate changes no greater than those in two dimensions. Absent a random packing algorithm for ellipsoids, it is speculated that here also a random alignment of the axes would eliminate the effect. ECENT years have seen significant advances in the modeling of heterogeneous propellant combustion, specifically in the context of ammonium‐perchlorate (AP)/hydroxyl-terminatedpolybutadiene propellants. In Ref. 1, a two-dimensional treatment, a numerical code is described in which the combustion field is fully coupled to heat conduction in the solid via a nonuniformly regressing nonplanar surface. The corresponding three-dimensional problem is described in Refs. 2 and 3, the first using two-step kinetics and the second three-step kinetics. Comparisons between the numerical results and experimental results of Miller 4 show that the effects of propellant morphology on burning rates can be satisfactorily predicted over a wide pressure range. At fixed pressure, the variations in burning rate due to different morphologies are as much as threefold, so that these are nontrivial predictions. In this connection, note how we chose the values of the numerous parameters for this problem, parameters identified in Refs. 1‐3. Most are typical values defined by the community at large. Some are uncertain, of course, particularly the surface pyrolysis parameters, but once a choice was made no post hoc adjustments were made to better fit the three-dimensional data of Miller. An important parameter subset is those of the gas kinetics, the reaction rates for the two-step and three-step kinetics. There, we used one-dimensional burning rate data for pure AP and for homogeneous blends of AP and binder. The use of a three-step mechanism enabled us to fit blend burning rates over a wide range of mixture ratios, and it is presumably for this reason that the three-step results are more accurate than the two-step results. Most important, our calculations are not a massive exercise in curve fitting. This does not mean that better choices for some of our parameter values are not possible. Although we reasonably capture qualitative aspects of sandwich burning, 3 most notably the surface topography, the surface topography in our three-dimensional simulations does not show concavity in the surfaces of large AP particles at pressures for which it is observed experimentally. It seems probable that this

Three-dimensional heterogeneous propellant combustion

A numerical framework is described which permits the calculation of the three-dimensional combustion field supported by a heterogeneous propellant, allowing for complete coupling between the condensed-phase physics, the gas-phase physics, and the unsteady uneven, regressing surface. A random-packing algorithm is used to construct models of ammonium perchlorate (AP) in hydroxyl-terminated polybutadiene propellants which minic experimental propellants designed by R. Miller, and these are numerically burnt, Mean burning rates are compared with experimental data for four packs, over a pressure range of 7–200 atm. The effect of the local propellant morphology on the local surface regression speed is examined, with particular attention to the behavior in the neighborhood of large AP particles, At first, these burn slowly and protrude significantly above the surrounding surface, but later they are rapidly consumed and the surface flattens. In all cases, mean burning rates can be accurately calculated using an Oseen approximation for the velocity field, rather than solving the Navier-Stokes equations.

Combustion Studies of Boron, Magnesium, and Aluminum Composite Propellants

Abstract The macroscopically steady combustion behavior of AP (ammonium perchlorate)-HTPB (hydroxyl-terminated polybutadiene) propellants containing boron and magnesium was examined using thermocouples and fiber optics and compared with previous results for similar aluminized propellants. Addition of magnesium at constant AP-HTPB ratio was found to increase propellant burning rate moderately while boron increased burning rate substantially. These increases were greater than those previously observed for similar aluminized propellants, A sensitivity analysis was performed to determine the relative importance of various mechanisms such as conductive heat feedback, radiative feedback, and condensed phase heat release. For boron and magnesium the propellant burning rate was found to be most sensitive to changes in condensed phase heat release while the aluminized propellant was found to be most sensitive to radiative feedback. These observations are attributed to the properties of the various metals and their...