Model Propellant Research Articles

Integrating Moisture Isotherms, Compatibility Assessment, and Model Propellant with ADN as an Eco-Friendly Oxidizer

Integrating Moisture Isotherms, Compatibility Assessment, and Model Propellant with ADN as an Eco-Friendly Oxidizer

Influence of Solid Filler on the Rheological Properties of Propellants Based on Energetic Thermoplastic Elastomer.

Glycidyl azide polymer-energetic thermoplastic elastomer propellant (GAP-ETPE) has high development prospects as a green solid propellant, although the preparation of GAP-ETPE with excellent performance is still a challenge. Focusing on the demand of high-strength solid propellants for free-loading rocket motors, a GAP-ETPE model propellant with excellent overall performance was prepared in this work, and the influence of adhesive structure characteristics on its fluidity was studied. Furthermore, the influence of filler on the rheological properties of the model propellant was investigated by introducing hexogen (RDX) and Al, and a corresponding two-phase model was established. The results may provide a reference for the structural design, molding process, and parameter selection of high-performance GAP-based green solid propellants.

Professor Dr. Thomas M. Klapötke

Professor Dr. Thomas M. Klapötke

Enhanced the thermal conductivity of hydroxyl-terminated polybutadiene (HTPB) composites by graphene-silver hybrid

The heat conductivity affects the burning rate of propellant much, which would influence the properties of power propellant. In this work, hydroxyl-terminated polybutadiene (HTPB) was employed as a model propellant matrix, and series of composites were fabricated by introducing Graphene-Silver (GNs-Ag) hybrid as fillers to improve the heat conductivity of HTPB. GNs-Ag hybrids were prepared via chemical reduction of silver onto the graphene nanosheets, and the effect of preparation conditions on the fabrication of GNs-Ag were studied systematically. With the addition 0.5 wt% of GNs-Ag hybrid, the GNs-Ag/HTPB composites had a 52.6% increase in thermal conductivity compared with that of neat HTPB. The mechanical properties and thermal stability of the GNs-Ag/HTPB nanocomposites were also sightly improved.

Experimental and Numerical Investigation of Stratification and Self pressurization in a High Pressure Liquid Nitrogen Storage Tank

ABSTRACT This paper discusses the evolution of stratification and self-pressurization in a cryogenic storage tank. The heat ingress due to the large temperature difference between ambient and cryogen leads to thermal stratification and self-pressurization. The prediction of the thermodynamic state of cryogen is required for the successful execution of any space mission. An experimental cryogenic test tank which is a combination of an evacuated vacuum jacket and multilayered insulation has been designed, fabricated and is used for stratification studies using liquid nitrogen as the model propellant. Stratification at two conditions were tested; ‘venting’ and ‘non-venting’. During venting condition, only saturated boiling occurs, and the degree of thermal stratification is very less. Whereas during non-venting condition, there is a significant amount of thermal stratification and the degree of thermal stratification increases with working pressure. For a deeper understanding of the phenomenon and better prediction of the state of cryogen, a numerical model had been developed and validated with the experimental result. A stratification parameter has been used to quantify the degree of stratification for both “venting” and “non-venting” conditions. The model developed can be used for accurate prediction of the state of cryogen.

Simulation of solid propellant microstructures by combining the collective rearrangement method with the discrete element method

The polydisperse particulate components in solid propellant are incompact and randomly packed, which determines the microstructural features of the propellants. A packing method, combining the discrete element method (DEM) and collective rearrangement method, was applied to model propellant microstructures. The validity of this method was investigated by comparing the calculated and experimental properties of the monodisperse, bidisperse, and polydisperse random close packed sphere systems. The propellant models were generated using a stepwise approach, and their homogeneity, local randomness, and long-range pattern were analyzed. A statistical study of aluminum (Al) particle distribution was also conducted. The results indicated that this packing method can effectively determine the microscopic characteristics of random close packed monodisperse spheres. The maximum packing fraction of bidisperse and polydisperse spheres had similar trends to those reported in experimental studies and using other packing algorithms. In addition, this method was capable of generating non-compacted propellant structures with uniformly distributed polydisperse particles. The radial distribution functions (RDFs) for Al-Al particles provided information about the Al distribution, but this was mainly related to the size and content of the large particle components.

Evolution of stratification in a cryogenic propellant tank at reduced gravity environment

The prediction of thermal stratification in a cryogenic storage tank is necessary for the successful execution of space mission. The working fluid may be stored in the sub-cooled conditions and possibility of heat infiltration may lead to the increase of temperature as well as the pressure of the cryogenic fluid. The rise in fluid pressure may also lead to cavitation in the turbo pump which has to be avoided. Commonly used stratification models are based on temperature and velocity correlation developed for flow over a flat plate. An experimental cryogenic test tank is designed, fabricated and stratification is studied using nitrogen as the model propellant. The effect of gravity on the evolution of stratification is studied by using a CFD model. The results show that the fluid velocity will be lesser at microgravity condition which causes the boundary layer fluid to absorb a large amount of heat and the nature of the heat transfer changes from convection to evaporation.

Screen Compliance Experiments for Application of Liquid Acquisition Device in Space

The purpose of a liquid acquisition device (LAD) is to separate liquid and vapor phases inside a spacecraft propellant storage tank in the reduced gravity and microgravity conditions of space so that vapor-free liquid can be extracted to the transfer line. A popular type of LAD called a screen channel LAD or gallery arm, uses a fine porous screen and surface tension forces of the liquid to allow pure liquid to flow through the screen while blocking vapor penetration. To analyze, size, and optimize the design of LADs for future in-space propellant transfer systems, models and data are required for the four fundamental influential factors for LAD systems, including bubble point, flow-through-screen pressure drop, wicking rate, and screen compliance for a wide variety of screen meshes. While there is sporadic data available for three of these parameters, there is no published quantitative data for screen compliance. During the transient startup of propellant transfer, the liquid must be accelerated from rest to the steady flow demand velocity, which causes the screen to deform or comply, so compliance data is required for accurate transient LAD analyses; most design codes only consider steady state analysis. This paper presents screen compliance experiments on 14 different screens, examining the effects of fineness of mesh, open area, and screen metal type on compliance. A basic equation of state is also developed and validated against the data which can be easily integrated into any transient LAD flow code to model propellant transfer.

A Simple and Rapid Method for Deposition and Measurement of Drug Transport Across Air Interface Respiratory Epithelia.

The purpose of this study was to present a novel and simple drug deposition method to evaluate drug transport of aerosol microparticles across airway epithelial cells. Microparticles containing ciprofloxacin HCl (Cip) and doxycycline (Dox), alone or in a 50:50% w/w ratio, were spray dried and suspended using 2H, 3H-perfluoropentane, model propellant. The suspension was then used to assess deposition, and transport of these drug microparticles across sub-bronchial epithelial Calu-3 cells was also studied. In comparison with other methods of depositing microparticles, this proposed method, using drug suspended in HPFP, provides control over the amount of drugs applied on the surface of the cells. Therefore, cell permeability studies could be conducted with considerably smaller and more reproducible doses, without the physicochemical characteristics of the drugs being compromised or the use of modified pharmacopeia impactors. The suspension of microparticles in HPFP as presented in this study has provided a non-toxic, simple, and reproducible novel method to deliver and study the permeability of specific quantity of drugs across respiratory epithelial cells in vitro.

Ignition of a polymer propellant of hybrid rocket motor by a hot particle

The ignition of polymethylmethacrylate (typical model propellant of the hybrid rocket motor) by a hot particle in a shape of parallelepiped, polyhedron, disk is investigated numerically. The initial temperature of a heat source varied within the range 950–1150K, size of particle – within the range 2–6mm. It is established that varying these parameters influenced significantly the main characteristic of the process – ignition delay time under ignition conditions close to critical. For considered shape of particles, ignition delay time is in ascending sequence: parallelepiped, polyhedron, disk. Three polymer ignition regimes, which characterized by the initial temperature of a heat source, ignition delay time and a location of an ignition zone in a vicinity of a hot particle, are emphasized. It is illustrated that taking into account the dependence of thermal and physical characteristics of polymethylmethacrylate on temperature, the ignition delay time increased due to augmentation of energy accumulated by a subsurface layer.

Near-surface flame structure characterization of simplified ammonium perchlorate/hydroxyl-terminated polybutadiene compositions

Simplified model propellant configurations, such as monomodal propellants, can be valuable in the development and validation of predictive numerical tools. These idealized experiments also yield insight into the effect of diffusion length scales on combustion, but comprehensive data covering a large range of diffusional length scales do not currently exist. Here, monomodal propellants with ammonium perchlorate (AP) particle sizes under 800 µm and AP pellets ported and filled with hydroxyl-terminated polybutadiene (HTPB) were used to systematically study the effect of diffusion length scales, or AP equivalent particle sizes) of up to 4.1 mm on flame structure. In general, burning rates increased with pressure and decreasing particle size, as expected. Burning rates for samples with particle sizes greater than 400 µm converged with AP monopropellant burning rate data above approximately 2 MPa, the AP low-pressure deflagration limit (LPDL). For a given pressure above the LPDL, burning rates eventually became constant for both increasing and decreasing particle sizes. Conversely, for a given pressure below the LPDL burning rate was shown to be a function of particle diameter. Flame structures above the composite propellants were observed using 5 kHz OH planar laser-induced fluorescence (PLIF). The transient flames were underventilated (jet-like) over the AP particles at 1 atm while lifted, inverted, and overventilated at 5 atm. Distinct diffusion flame structures were observed visually above the ported samples at 1 atm. Very luminous flames were observed at the interface between the AP and binder. The effect of strain rate on sample combustion was examined using an opposed flow burner; at 1 atm, sample burning rate was not affected by strain rate. At the largest strain rate, the sample self-extinguished after igniter shutoff, indicating that secondary diffusion flames are important in the opposed flow configuration.

Investigations on thermodynamic phenomena of the active-pressurization process of a cryogenic propellant tank

In optimizing the design and operation of a launcher’s cryogenic upper stage, the required pressurant gas mass must be accurately predicted. In order to do so, the appearing thermodynamic phenomena of the initial active-pressurization process of a cryogenic propellant tank need to be understood and assessed. On that account, ground experiments with liquid nitrogen as model propellant, pressurized with either gaseous nitrogen or gaseous helium, as well as numerical simulations were performed, and analytical approaches were applied. It was found that the thermal stratification in the liquid propellant is mostly driven by the increase in saturation temperature at the free surface during pressurization. The mode of phase change during pressurization was found to be mainly dependent on the pressurant gas type. The main mode of heat transfer appears from pressurant gas to the tank wall, through which the heat is then partly conducted into the uppermost liquid layers. After tank pressurization end, a pressure drop is observed and the decrease in vapor temperature is identified as its main driver. Moreover, a correlation is derived for the prediction of the required pressurant gas mass, based on the Jakob number and the thermal expansion Froude number, identifying pressurant gas temperature, phase change, and tank aspect ratio as the most important parameters, determining the required pressurant gas mass for cryogenic propellant tanks.

Solid-phase ignition of a composite propellant by a hot particle under free-convection heat sink into the environment

A mathematical model of the solid-phase ignition of a structurally inhomogeneous metallized composite propellant by an incandescent small particle in the form of a cylindrical disk with allowance for free-convection heat sink into the environment is developed. A numerical study of the ignition delay time, the main integral characteristics of the process, is performed. The calculation results are compared to experimental data on the ignition of model propellant compositions based on ammonium perchlorate, butyl rubber, and ASD-4 aluminum powder.

Isothermal calorimetry: A predictive tool to model drug-propellant interactions in pressurized metered dose systems

The purpose of this work was to evaluate gas perfusion isothermal calorimetry (ITC) as a method to characterize the physicochemical changes of active pharmaceutical ingredients (APIs) intended to be formulated in pressurized metered dose inhalers (pMDIs) after exposure to a model propellant. Spray dried samples of beclomethasone dipropionate (BDP) and salbutamol sulphate (SS) were exposed to controlled quantities of 2H,3H-decafluoropentane (HPFP) to determine whether ITC could be used as a suitable analytical method for gathering data on the behavioural properties of the powders in real time. The crystallization kinetics of BDP and the physiochemical properties of SS were successfully characterized using ITC and supported by a variety of other analytical techniques. Correlations between real and model propellant systems were also established using hydrofluoroalkane (HFA-227) propellant. In summary, ITC was found to be suitable for gathering data on the crystallization kinetics of BDP and SS. In a wider context, this work will have implications on the use of ITC for stability testing of APIs in HFA-based pMDIs.

An assessment of beclomethasone dipropionate clathrate formation in a model suspension metered dose inhaler

The aims of this study were to investigate and characterize the physico-chemical properties of beclomethasone dipropionate (BDP) crystallized from tricholoromonofluoromethane (CFC-11). Physical interactions in a model pressurised metered dose inhaler (pMDI) system and changes in surface energy after size reduction (micronization) were determined. Although CFC-11 has largely been phased out of use in pMDIs due to its ozone depletion potential, the BDP CFC-11 clathrate is a stable entity and thus suitable as a model for our initial investigations. In addition, although propellant clathrates have been known for sometime, as far as the authors are aware, their surface energies and adhesive interactions have not been reported. The structure of the clathrate was investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (X-RPD). In addition, atomic force microscopy (AFM) was employed to determine the dispersive surface free energy (SE) and force of adhesion ( F adh) of the BDP CFC-11 clathrate with different pMDI components in a model propellant (decafluoropentane). The dispersive surface free energies for anhydrous BDP (micronized), the CFC-11 clathrate and ball-milled BDP CFC-11 clathrate are (47.5 ± 4.9) mJ m −2, (11.3 ± 4.1) mJ m −2 and (15.2 ± 1.3) mJ m −2 respectively. Force of adhesion results shows that BDP CFC-11 clathrates, even after being ball-milled for 2.5 h, have a lower F adh compared to micronized anhydrous BDP with different pMDI components. This shows that the formation of the crystalline CFC-11 clathrate is advantageous when compared to the micronized anhydrous form, in terms of its surface energy and potential interactions within a suspension MDI formulation. In the wider context, this work has implications for the future development of HFA formulations with APIs which are prone to the formation of propellant clathrates.

Investigation of combustion of HEM with aluminum nanopowders

A number of problems related to the utilization of nanosize aluminum powders as a combustible component of perspective compositions of high-energy materials is considered. The technology of producing nanopowders using the technique of conductor electrical explosion and the method of dispersed powder composition analysis are shown. The results of experimental studies of conductive and radiant ignition of model propellant compositions containing aluminum nanopowder, as well as the results of investigating the rate of stationary and non-stationary combustion of the given systems, are given. The results of an investigation of the ignition and combustion of ecologically safe propellant systems based on a dual oxidant (ammonium perchlorate and ammonium nitrate, which partially replaces it) containing aluminum nanopowder are demonstrated.

Effect of molecular weight and end-group nature on the solubility of ethylene oxide oligomers in 2H, 3H-decafluoropentane and its fully fluorinated analogue, perfluoropentane

Fluorinated liquids possess high chemical and physical stability, are tolerated by the human body and, therefore, show great promise in biomedical fields; however, they require extensive formulation. Phase diagrams are reported here for a series of ethylene oxide oligomeric additives in 2H,3H-perfluoropentane (HPFP), a non-chlorofluorocarbon fluorinated liquid regarded as a model propellant for pressurized metered-dose inhalers. Over a wide range of temperatures and concentrations, dihydroxyl end-capped poly(ethylene glycols) (PEGs) exhibited a lower critical solution temperature (LCST) that was strongly molecular weight dependent. In contrast, monomethyl (and thus monohydroxy) and dimethyl end-capped poly(ethylene oxides) were fully miscible with HPFP over the same temperature and concentration ranges, suggesting that the phase behaviour was dominated by end-group/solvent interactions. By systematically substituting HPFP for the fully fluorinated analogue perfluoropentane, the ability of these end-groups to interact with the solvent was perturbed and LCST-type behaviour was induced in the previously fully miscible monomethyl and dimethyl end-capped PEGs. Concomitantly, with increasing perfluoropentane content, the LCST of the dihydroxyl end-capped PEGs was driven to lower temperatures. Therefore, the phase behaviour of these systems may be controlled by 'tuning' the end-group structure of the ethylene oxide oligomers, and varying the hydrogen bonding capabilities of the fluorinated solvents.

Surface Energy and Interparticle Force Correlation in Model pMDI Formulations

To compare experimental measurements of particle cohesion and adhesion forces in a model propellant with theoretical measurements of the interfacial free energy of particulate interactions; with the aim of characterizing suspension stability of pressurized metered dose inhalers (pMDIs). Interparticulate forces of salbutamol sulfate, budesonide, and formoterol fumarate dihydrate were investigated by in situ atomic force microscopy (AFM) in a model propellant 2H,3H perfluoropentane. The surface thermodynamic properties were determined by contact angle (CA) and inverse gas chromatography (IGC). Experimental data were compared with theoretical work of adhesion/cohesion using a surface component approach (SCA), taking into account both dispersive and polar contributions of the surface free energy. Results indicated that the measured forces of interaction between particles in model propellant could not be accounted for by theoretical treatment of the dispersive surface free energies via CA and IGC. A correlation between theoretical work of adhesion/cohesion and AFM measurements was observed upon the introduction of the polar interfacial interactions within the SCA model. It is suggested that the polar contributions of the surface free energy measurements of particles may play a crucial role in particle interaction within propellant-based systems. Together with the application of a SCA model, this approach may be capable of predicting suspension stability of pMDI formulations.

Surfactant solubility and aggregate orientation in hydrofluoroalkanes

Purpose: To find surfactants soluble in the two hydrofluoroalkane (HFA) propellants, HFA-134a and HFA-227ea; to compare surfactant solubility in the two propellants with those in 2 H,3 H-decafluoropentane (DFP) in order to assess latter's suitability as a liquid model propellant and to investigate surfactant aggregation and aggregate orientation in HFAs. Methods: To assess surfactant solubility, HFA was added to a known amount of surfactant until dissolution was visibly apparent. An iodine solubilization method was used to determine surfactant aggregation behaviour in DFP. Fluorescence spectroscopic investigations on the surfactant orientation in aggregates were carried out in HFAs using a microviscosity sensitive fluorescent probe (1,3-dipyrenylpropane). The aim was to assess viscosity changes in the microenvironment of this lipophilic probe upon incorporation into surfactant aggregates. Results: Soluble surfactants could be found among the polyoxyethylene-ethers and POE–PPO-block copolymer surfactants. Solubility in DFP appears to correlate with solubility in HFA-134a, but not HFA-227ea. Iodine solubilization indicates micellization of Brij 30 in DFP at a cmc (type II association behaviour). L-44 in DFP, on the other hand, does not exhibit a well defined cmc, but shows continuous surfactant aggregation (type I association behaviour). The fluorescence spectroscopic studies showed evidence for probe incorporation into surfactant aggregates in HFAs. Conclusions: DFP proved to be a good model for HFA-134a only. An L 1-aggregate orientation was shown for surfactants in HFAs and is in marked contrast to the chlorofluorocarbon propellant where a L 2-aggregate orientation exists.

Capillary Rewetting of Vaned Containers: Spacecraft Tank Rewetting Following Thrust Resettling

Recent investigations have successfully demonstrated closed-form analytical solutions of spontaneous capillary flows in idealized cylindrical containers with interior corners. In this work, the theory is extended and applied to complex containers modeling spacecraft fuel tanks employing propellant-management devices (PMDs) consisting of networks of vanes. The specific problem investigated is one of spontaneous rewetting of a typical partially filled liquid-fuel/cryogen tank with PMD after thrust resettling. The transients of this flow impact the logistics of orbital maneuvers and potentially tank thermal control. The general procedure to compute the initial condition for the closed-form transient flows is first outlined and then solved for several complex cylindrical tanks exhibiting symmetry. The utility and limitations of the technique as a design tool are discussed in a summary, which also highlights comparisons with NASA flight data of a model propellant tank with PMD