Heterogeneous Propellants Research Articles

A model for the combustion behavior of AP/HTPB propellant containing ultrafine aluminum

AbstractNano‐aluminum, renowned for its remarkable efficacy in augmenting propulsion efficiency and regressive velocity, stands out as a compelling choice for incorporation into heterogeneous propellant compositions. Nevertheless, the considerable expense of this ultrafine aluminum variant, coupled with the inherent hazards associated with conducting propellant trials, may deter extensive allocation of resources and efforts towards the intricate processes of amalgamation, molding, and comprehensive analysis of the combustion characteristics of freshly devised propellant blends. To provide theoretical support for predicting the chemical properties of nano‐Al composite propellant, we establish a numerical framework to study the combustion characteristics under the working environment of a solid rocket motor. A new five‐step kinetic mechanism is developed in this model to describe the reaction process in the gas phase while accounting for heat conduction, radiative effects, and non‐planar moving surfaces. The comparison between our theoretical work and experimental results confirms that making no distinction among Al particle sizes below 3 μm is reasonable. Finally, the effects of nano‐Al on combustion, burning rate variation, and temperature sensitivity are analyzed in detail.

Effects of Thermal Expansion and Pressure on Far-Field Fluctuations of Heterogeneous Propellants

In this work, we carry out three-dimensional mesoscale simulations of heterogeneous solid propellant combustion. We solve the reactive low-Mach-number equations in the gas phase with complete coupling to the solid phase. The model takes into account thermal expansion and deformation in the solid phase by using a hypoelastic law in the quasi-static limit. To account for morphology, we select two different propellant formulations with different particle size distributions and present the results as a function of pressure. The thermomechanical behavior is accessed by examining quantities such as strain and stress in the propellant as a function of pressure and propellant morphology. We also show that temperature and velocity fluctuations exist in the far field above the propellant surface and that these fluctuations can be significant. To better understand the nature of these fluctuations, we vary the pressure and make relevant plots of normal velocity and temperature probability density functions, as well as time autocorrelations. Such descriptions are necessary to account for the coupling between the mesoscale and the macroscale, where the fluctuations at the mesoscale can affect quantities at the macroscale, such as head-end pressure, trigger parietal vortex shedding, and aeroacoustics.

Modified multiflame model for AP-HTPB composite propellant combustion

Among composite solid propellants, AP/HTPB is the most widely used. In spite of being studied for the last few decades, modeling for predicting the burn rate of these heterogeneous propellants still remains an active research problem, and new models for the same have been proposed in the recent literature. Amongst them, BDP and Cohen and Strand formulation are mostly used, which uses phenological models to predict the burn rate and parameter sensitivity in a quick time. A modified model is proposed in the present work. The modifications include (1) Three flame structure along with separate surface temperature for oxidizer and binder, (2) Detailed energy balance equations at surfaces, (3) Modified surface heat release term, (4) Flame temperature as a function of initial temperature, pressure, and fuel/oxidizer ratio, and (5) Modification of diffusion flame stand-off distance formulation. The predicted burn rates from the modified model are compared against experimental and other theoretical results for AP monopropellants, AP-HTPB monomodal, and multi-modal configurations, including packs. The new modified model gives better predictions than the existing ones in all the cases considered in this study. The temperature sensitivity of the AP monopropellant model comes well within the range of experimental values available in the literature, and the value drops slightly as chamber pressure increases. For composite AP/HTPB, the temperature sensitivity lies within the experimental range. However, it does not give any particular trend and is highly dependent on the type of flame dominance for that particular composite configuration and pressure. The current modified model is highly robust as there is excellent agreement with experimental data from different research groups incorporating a wide range of formulations covering monopropellant to composite multi-modal packs accounting for a range of particle size and solid loading variation. The model gives the results quickly, making the model very efficacious in terms of time, effort, and computational resources.

Experimental and Numerical Investigation into the Mechanical Behavior of Composite Solid Propellants Subject to Uniaxial Tension.

To further explore the quasi-static mechanical characteristics of composite solid propellants at low strain rates, an investigation was conducted on the mechanical behavior and damage mechanisms of a four-component hydroxy-terminated polybutadiene (HTPB) propellant by means of experiments and numerical simulation. A uniaxial tensile test and scanning electron microscope (SEM) characterization experiment were carried out. A microstructural model, which accurately represents the mesoscopic structure, was developed via the integration of micro-CT scanning and image-processing techniques. The constructed microstructural model was utilized to conduct a numerical simulation of the mechanical behavior. The experimental results demonstrated that the maximum tensile strength increases with increasing strain rate, and the primary cause of propellant failure at low strain rates is the dewetting phenomenon occurring at the interface between the larger particles and the matrix. The maximum tensile strength is 0.48 MPa when the strain rate is 0.00119 s-1, and the maximum tensile strength is 0.37 MPa when the strain rate is 0.000119 s-1. The simulation results indicated a consistent trend in variation when comparing the simulation and experimental curves. This suggested that the established model exhibits a high level of reliability, and provides a promising approach for carrying out microstructural simulations of heterogeneous propellants in future. The mechanical behavior of the propellant can be effectively described by utilizing a mesoscopic finite element model that incorporates the superelastic constitutive model of the matrix and the bilinear cohesive model. This framework facilitates the representation of mesoscopic damage evolution, which consequently provides insights into the damage mechanism. Additionally, the utilization of such models assists in compensating for the limitations of damage evolution characterization experiments.

Unsteady heat and mass transfer mechanism in AP/HTPB propellant combustion under rapid depressurization

This work is to investigate the dynamic instability and unsteady heat and mass transfer mechanism of Ammonium perchlorate/Hydroxyl-terminated polybutadiene (AP/HTPB) propellant in the chamber of solid rocket motor (SRM). The environment of exponential depressurization is imposed on the heterogeneous propellant combustion to simulate the actual working condition of SRM. The microstructure of AP/HTPB propellant is generated by a code based on the sequential algorithm. The combustion model consists of gas reaction, heat conduction in the solid phase, and the non-planar moving surface. A quenched model under the rapid pressure decay is established. It is found that the rapid depressurization and the change of surface morphology are responsible for the combustion fluctuation. Finally, quenched times of different propellant packs are calculated. It is found that different AP particle positions in the propellant pack will also cause a change in the quenched time.

Isocyanate-free Carboxyl-terminated Polybutadiene and Epoxidised Hydroxyl-terminated Polybutadiene Cross-linking System as a Binder in Solid Heterogeneous Propellants

Isocyanate-free Carboxyl-terminated Polybutadiene and Epoxidised Hydroxyl-terminated Polybutadiene Cross-linking System as a Binder in Solid Heterogeneous Propellants

Modern Competing Flames Model for Composite Ammonium Perchlorate/Hydroxyl-Terminated Polybutadiene Propellant Combustion

The competing flames model, also termed the Beckstead–Derr–Price model, for steady-state heterogeneous propellant combustion has been widely used but has not been sufficiently updated in decades or compared to modern propellant combustion databases. In the current study, historical competing flames modeling approaches were thoroughly documented; and an improved framework was outlined and updated to include several improvements, such as variable flame temperatures, specific heat capacities, and latent heat terms. Model parameters were initially taken from previous literature, but the fuel and diffusion flame parameters were optimized based on a compiled database of unimodal propellant burning rates from the literature spanning a wide range of ammonium perchlorate (AP) particle sizes (), AP mass concentrations (70–87.5%), and combustion pressures (0.7–20.7 MPa). The improved model was compared to AP monopropellant, unimodal, and multimodal propellant burning rate databases from the literature. General dependencies of the burning rate-to-oxidizer concentration and size were accurately captured. The predictive capability of the improved model for AP monopropellant burning rates and unimodal propellant formulations was excellent, where the only significant discrepancies were noted for very fine AP particles (). Model predictions for multimodal formulations were moderate and could be improved by alternative pseudopropellant apportionment and statistical accounting schemes.

Study of the Mechanical Properties of a CMDB Propellant Over a Wide Range of Strain Rates Using a Group Interaction Model

Composite modified double base (CMDB) propellants are heterogeneous propellants in which properties are significantly improved by adding solid particles into the polymer matrix. A molecular group interaction model that can predict the mechanical properties of polymers through a molecular structure is used to predict the viscoelastic behavior of the CMDB propellant. Considering that the addition of solid particles will improve the crosslinking degree between polymer molecules and reduce its secondary loss peak, the input parameters of the model are modified through dynamic mechanical analysis (DMA) experimental data. By introducing the strain rate into the expression of model glass transition temperature, the mechanical properties of propellant over a wide strain range ( 1.7 × 10 − 4 s-1 ~ 3000 s-1) are obtained. The reliability of the model is verified by comparison with uniaxial compression test data. By modifying the input parameters of the model, the effects of different mass ratios of nitrocellulose (NC)/nitroglycerin (NG) on the mechanical properties of the CMDB propellant were analyzed. The results show that the glass transition loss increases with increasing mass ratio of NC/NG, while Young’s modulus and yield stress decrease.

Heterogeneous propellant combustion with conjugated interface conditions

A numerical framework for three-dimensional simulation of heterogeneous propellant combustion with conjugated interface conditions is proposed. Evolutions of the solid and gas phase are resolved in their respective but overlapping grids. The two-way coupling between the solid and gas phase is realized by performing the interface conditions on the regression surface using the level-set method. The morphology of the propellant is characterized by ingredients generated using a random packing algorithm. The conservation of mass as well as energy in the gasification process can be ensured by imposing appropriate source terms in the immersed boundary cells in the gas phase. The obtained test case results match the experimental trend satisfactorily, which justifies the validity of the present algorithm designed for solid fuel combustion problems.

Prediction Method of Heterogeneous Propellant Relaxation Moduli Based on Viscoelastic VCFEM

Prediction Method of Heterogeneous Propellant Relaxation Moduli Based on Viscoelastic VCFEM

Theoretical Analysis of Ammonium-perchlorate Based Composite Propellants with RDX Containing Small Size Particles of Beryllium

Rocket engines have been developed for at least the last six decades. There is a need to improve the actual solid propellant grain for rocket engines through the addiction of metallic fuels in the mixture as well as the addiction of energetic binders to stabilize the combustion. The rocket industry expects the launchers to be reliable, to be faster, stable and to have longer times of operation for the most possible payload weight (operational envelope). New propellants should have optimized ignition and combustion time rates reducing the possibility of negative oxygen balance thus reducing detonation process. Deflagration process should be optimized for best performance of the rocket. In this evolution, small quantities of explosives have been used in the propellant in order to increase the operational burning time, hence, the specific impulse. Adding metallic fuels such as aluminum, boron or beryllium on double based composite propellants and ammonium perchlorate are expected to increase the propellant density over chemical stability and aging resistance. The study of heterogeneous propellants containing large amounts of fine beryllium and ammonium perchlorate, it is necessary to understand the combustion products so to a proper evaluation of specific impulse, Mach number and mass flow of the mixture. In this study a mixture with nitramides (RDX – Cyclotrimethylene trinitramide) and ammonium perchlorate was analyzed with and without the addiction of small size particles of beryllium using a numerical algorithm. Therefore, this study relates the influence of beryllium in the performance parameters of ammonium perchlorate based composite propellants.
 Keywords: Propellant, Rocket engine, RDX, Ammonium perclorate

Investigating Dimensional Effects on Predicting Burning Rates of Heterogeneous Solid Propellants

The combustion of a randomly packed, heterogeneous propellant consisting of ammonium perchlorate and hydroxyl-terminated polybutadiene is investigated in a two-dimensional and three-dimensional numerical model to better understand the dimensional effects on the propellant burning rate. A ammonium perchlorate particle is set in a premixed hydroxyl-terminated polybutadiene binder and investigated using a two-dimensional model with finite-rate chemistry and detailed transport properties. The results from the detailed model are compared to two-dimensional and three-dimensional burning-rate codes with reduced chemistry. The gas-phase combustion chemistry is modeled using a reduced four-step mechanism developed from a detailed microscale model of ammonium perchlorate/hydroxyl-terminated polybutadiene combustion. Simple configurations of a single ammonium perchlorate particle and sandwich propellants at varying pressures are compared to determine leading-order effects of two-dimensional and three-dimensional approaches. The reduced chemistry models are then applied to real-world propellants for which a random packing algorithm is used to construct the ammonium perchlorate/hydroxyl-terminated polybutadiene propellant in a periodic cube for a given size distribution of spherical ammonium perchlorate particles and mass ratio of the oxidizer and binder. The decomposition of the ammonium perchlorate and hydroxyl-terminated polybutadiene on the surface is determined at discrete locations and is used to determine the average burning rate over a period of time. This approach combines a more detailed description of the gas-phase combustion processes, the condensed-phase morphology, and physics in a time-dependent simulation in hopes of developing a more predictive model of solid propellant combustion.

ESTIMATION OF THERMODYNAMIC AND THERMOCHEMICAL PROPERTIES OF SOLID HETEROGENEOUS ROCKET PROPELLANTS CONTAINING SCAVENGERS

A new trend in development of solid heterogeneous rocket propellants (SHRP) is a reduction of emission of gases, generated during combustion of propellants, leaving a trace of white smoke behind the nozzle as a result of condensation of combustion products including above all the hydrogen chloride. This smoke is called as secondary smoke in accordance with the AGARD classification. The SHRP based on ammonium perchlorate (commonly used oxidizer) generates significant amounts of hydrogen chloride in combustion products. There are various methods to reduce the emission of secondary smokes – one of them is the ad-dition of scavengers to SHRP. Alkaline earth metal salts are used as scavengers. The estimation of thermodynamic and thermochemical properties of SHRP containing selected scavengers was carried out by using the ICT-Code software (The ICT-Thermodynamic Code). The influence of different scavengers and their amount on selected properties, e.g. oxygen balance, density, caloricity, specific impulse, and amount of hydrogen chloride in combustion products, was determined.

Analysis of Thermal Decomposition of Solid Rocket Propellants

The paper presents results of thermal decomposition analysis of selected solid rocket propellants. Homogeneous propellant PAC and heterogeneous propellant H2 were subjected to simultaneous thermal analysis with the use of NETZSCH STA 2500 Regulus device with five heating rates of 2.5, 5, 7.5, 10 and 15 K/min. The method combines TG, DTG and DTA analytical techniques in a single measurement. The aim of the conducted experiments was to study thermal decomposition of these energetic materials as well as to determine activation energy of the decomposition process and the preconditioning factor from the TG curves. The tested materials properties and chemical composition along with a brief description of the experimental procedure are described. The inverse procedure of calculating the activation energy, based on the Ozawa-Flynn-Wall model is described. Finally, the results of thermal decomposition of two tested solid rocket propellants are presented along with maximum decomposition rates and percentage of mass loss.

THERMO-MECHANICAL ANALYSIS OF HETEROGENEOUS SOLID ROCKET PROPELLANT

The article presents results of thermo-mechanical analysis of heterogeneous solid rocket propellant H2, with special attention devoted to determining the glass transition temperature and softening temperature. Mechanical properties such as storage modulus (E’), loss modulus (E’’) and tan(δ) were measured using NETZSCH DMA 242C analyzer within temperature range from -120oC to +80oC at 2 K/min of heating rate and frequency of applied force f = 1 Hz. Relative thermal expansion as well as the Coefficient of Linear Thermal Expansion (CLTE) of H2 sample were determined using NETZSCH DIL 402C dilatometer within temperature range from 30oC to 80oC at 1 K/min of heating/cooling rate. The thermophysical properties including thermal conductivity, thermal diffusivity, and specific heat were determined within temperature range from -20oC to +80oC using KD2 Pro apparatus.

STUDIES OF THE INFLUENCE OF ENERGETIC ADDITIVES ON SELECTED PROPERTIES OF HETEROGENEOUS SOLID ROCKET PROPELLANT WITH LOW CONTENT OF HCL IN COMBUSTION PRODUCTS

The paper presents influence of additives like aluminium, magnesium, AMD (aluminium-magnesium dust) and boron on selected properties of heterogeneous solid rocket propellants (HSRP) based on HTPB in which ammonium chlorate was partly replaced by sodium nitrate. The presence of sodium nitrate reduces the content of hydrogen chloride (HCl) in combustion products. Theoretical values of thermochemical and thermodynamical properties like isochoric heat of combustion (Q), specific impulse (Isp) and combustion products in motor chamber and nozzle were identified by ICT-Code program. A laboratory rocket motor (LRM) was used to examine ballistic properties for prepared samples of propellants. Their temperature of decomposition, heat of combustion and hardness were tested both with sensitivity to mechanical stimuli (impact, friction) and rheological properties at curing.

Rheological and thermal properties of mixtures of hydroxyl-terminated polybutadiene and plasticizer (Rapid communication)

Mixtures of hydroxyl-terminated polybutadiene (HTPB) and dioctyladipate (DOA) as a plasticizer, which after crosslinking are applied as binders in heterogeneous propellants, were studied.The influence of mixture composition on rheological and thermal properties, important from technological and utility viewpoint were specified. The temperature dependence of prepared mixtures viscosity were studied. Systems were subjected to crosslinking and values of pot life (tpl) were determined. It was stated that tpl rose with increasing plasticizer content. Differential scanning calorimetry and thermogravimetry techniques were used toanalyze thermal stability of mixtures. It was concluded that applied component scould not decompose in the temperature range of 290–350 K and studied mixtures did not cause the hazard of thermal explosion.

STUDIES OF HIGH EXPLOSIVES IMPACT ON REDUCTION OF HCL IN HETEROGENEOUS SOLID ROCKET PROPELLANTS

The paper describes an influence of high explosives: hexogene (RDX), octogene (HMX), and dinitro-diaminoethene (FOX-7) on the properties of heterogeneous solid rocket propellant (HSRP) prepared on the base of Hydroxy Terminated Polybutadiene (HTPB) in which ammonium perchlorate (AP) was partially replaced by sodium nitrate (SN). It reduced the content of HCl in combustion products. Theoretical values of thermochemical and thermodynamic properties such as isochoric combustion heat (Q), specific impulse (Isp) and contents of combustion products in motor combustion chamber and nozzle have been identified by using the ICT-Code program. The rheological properties (virtual viscosity) of the propellant slurry during curing process, the sensitivity to mechanical stimuli (impact, friction), decomposition temperature, calorific value and hardness of propellants containing explosive materials were tested by instruments and ballistic properties were investigated by laboratory rocket motor (LRM).

Theoretical analysis of ammonium-perchlorate based composite propellants containing small size particles of boron

One of the main goals for the scientific/ military rocket industry is to increase the operational burning time, thus the specific impulse. New homogeneous and heterogeneous propellants were tested and metallic fuels were added in the mixture to obtain best performance. To study heterogeneous propellant, containing large amount of fine boron and ammonium perchlorate, it is appropriate to estimate the combustion products to evaluate/obtain the values of the specific impulse, density, Mach number and mass flow of the mixture. Several composite propellant mixtures, ammonium perchlorate, nitramides (RDX – Cyclotrimethylene trinitramide), were defined with or without addiction of small particles of Boron and modeled. The energetic properties of boron and progress of boron particles on the burning surface of the ammonium perchlorate based composite propellants was modeled used a numerical algorithm. This paper reports the analysis of the influence of boron in the performance parameters for ammonium perchlorate based composite propellants.

Studies of the Influence of Nano Iron(III) Oxide on Selected Properties of Solid Heterogeneous Propellants Based on HTPB

Studies of the Influence of Nano Iron(III) Oxide on Selected Properties of Solid Heterogeneous Propellants Based on HTPB