Modeling and rheology of HTPB based composite solid propellants

The propellant with the minimum viscosity required for a defect-free casting can be obtained by proper selection of the size and fractions of solid components leading to maximum packing density. Furnas' model was used to predict the particulate composition for the maximum packing density. Components with certain size dispersions were combined to yield a size distribution that is closest to the optimum one given by Furnas for maximum packing. The closeness of the calculated size distribution to the optimum one was tested by using the least square technique. The results obtained were experimentally confirmed by viscosity measurement of uncured propellants having HTPB binder and trimodal solid part accordingly prepared by using aluminum (volumetric mean particle diameter of 10.4 μm) and ammonium perchlorate with four different sizes (volumetric mean particle diameters: 9.22, 31.4, 171, and 323 μm). The experimental measurements showed that the compositions for the minimum viscosity are in good agreement with those predicted by using the model for maximum packing. The propellant consisting of particles with mean diameters of 10.4, 31.4, and 323 μm was found to yield the minimum viscosity. This minimum viscosity was observed when the fraction of the sizes with respect to total solids was 0.141, 0.300, and 0.559, respectively.     

Burning Behavior of Composite Solid Propellant containing porous ammonium perchlorate

A pilot scale fluidized bed process was developed for preparing porous ammonium perchlorate (PAP) in various particle sizes. The oxidizer, ammonium perchlorate (AP), of composite solid propellant was partially replaced by PAP which was obtained by the fluidization process. The burning rate of propellants containing PAP was found to increase as compared with that of propellants without PAP. In the present study, the effects of percentage content and particle size of PAP incorporated in propellant compositions, on the burning rate were investigated. The results showed that the burning rate increases with increasing of PAP content and with decreasing of PAP particle size for trimodal oxidizer propellants.     

Reducing Agglomeration of Ammonium Perchlorate Using Activated Charcoal

Ammonium perchlorate is the most widely employed oxidizer for composite solid propellants. When exposed to atmosphere, it absorbs moisture and agglomerates. It is usually vacuum dried in order to avoid this agglomeration. When ammonium perchlorate that has been exposed to atmosphere for a certain period of time, is used in making a composite solid propellant, the burning rate is different because of the change in particle size distribution due to its agglomeration. This change in burning rate will change the thrust‐time profile from that of what it is designed for. As one goes to a finer ammonium perchlorate particle size this problem becomes more evident. Experimental studies aimed at reducing the agglomeration of ammonium perchlorate by coating it with activated charcoal. Ammonium perchlorate coated with 1 % activated charcoal showed almost no agglomeration, even when the particle size of ammonium perchlorate is approx. 1 μm. The burning rates also remained unchanged when ammonium perchlorate coated with 1 % activated charcoal was employed in propellant composition, after it has been exposed to the atmosphere for a period of 1 h.     

Combustion of ammonium dinitramide based solid propellant with PBT as energetic binders

Focusing on a new kind of solid propellants, which takes the ammonium dinitramide (ADN) as the high-energetic oxidant, and 3,3-bis(azidomethyl)oxetane and tetrahydrofuran copolymers (PBT) as the high-energetic fuel binder, the burning rates and the theoretical performance of the propellant were measured under different ADN contents, ADN sizes and pressures. The burning rates increased from 7.9 mm/s to 117.4 mm/s and 49.1 mm/s to 74.2 mm/s when the pressure increased from 0.1 MPa to 10 MPa and from 12 MPa to 20 MPa separately, with a singularity in the pressure dependence of the burning rate at around 10 MPa. In terms of the effect of the ADN content, under the experiment pressures from 0.5 MPa to 5.0 MPa, the burning rates of the propellant were promoted by the increase of the oxidizer loading in a range of 50 wt% to 75 wt%, with a transition from a kinetically-controlled reaction to a diffusion-controlled one. Within the condition scope of this study, no obvious effect of the ADN sizes on the propellant combustion properties was observed. This could be attributed to the solid-liquid mixed multiphase layer and the binder which served as a heat sink for the smaller ADN particles.     

Storage stability of solid rocket fuels. 2. Effect of oxidizer loadings

Ageing behaviour, leading to ballistic changes, has been studied as a function of oxidizer loading in polystyrene/ammonium perchlorate solid-propellants. The ageing studies were carried out at 100 °C in air. Change in burning rate decreased as the oxidizer loading increased from 75 to 80%. The change in thermal decomposition rates both at 230 and 260 °C also decreased as the oxidizer loading in the propellants increased. The shapes of the plots of the changes in burning rate and thermal decomposition rate (230 and 260 °C) at different storage times for different oxidizer-loaded propellants seem to be exactly similar. These results lead to the conclusion that the thermal decomposition of the propellant may be responsible for bringing about the ballistic changes during the ageing process. Infrared studies of the binder portion of the aged propellant indicate that peroxide formation takes place during the course of ageing and that peroxide formation for a particular storage time and temperature increases as the loading decreases.     

Role of Boron in Burning Rate Augmentation of AP composite propellants

Effect of the addition of boron particles on the burning rate of solid propellants was examined. The propellants tested in this study consisted of ammonium perchlorate (AP) as an oxidizer and carboxyl terminated polybutadiene as a fuel binder. The propellant burning rate is increased significantly by the addition of a small amount of boron particles. The burning rate augmentation is dependent largely on the size and concentration of the boron particles mixed. Thermochemical experiments revealed that the boron particles react with the decomposed gases of AP on and just above the propellant burning surface. The heat flux transferred back from the gas phase to the burning surface of the propellant increases with increasing the total surface of the boron particles mixed within the unit mass of propellant. The burning rate augmentation is correlated to the heat of reaction generated by the oxidation reaction of boron particles.     

Is ammonium perchlorate in composite solid propellant under strain?

Thermal decomposition of ammonium perchlorate-polystyrene propellant as a function of oxidizer loading has been found to behave in a fashion analogous to the thermal decomposition of ammonium perchlorate as a function of precompression pressure. It has been argued that the above behaviour of the propellant is due to the strain caused by the binder film on ammonium perchlorate contained in the propellant matrix. The presence of strain has been demonstrated independently by x-ray diffraction peak and infrared peak broadening and strain energy measurements.     

Ammonium perchlorate-based composite solid propellant formulations with plateau burning rate trends

This paper presents burning rates as a function of pressure of several propellant formulations based on ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene cured by isophorone diisocyanate, many of which exhibit significantly low (nearly zero or negative) values of the pressure exponent of the burning rate in distinct pressure ranges, termed as plateau burning rate trends. The propellants contain a bimodal distribution of AP particles with the size of the coarse and fine particles within narrow ranges whose mean values are widely separated. Two mean sizes of fine particles were considered for the propellant formulations in the present work, namely, 5 and 20 μm. These choices are based on the mid-pressure extinction behavior exhibited by the matrix of fine AP and binder contained in the propellants but when tested alone over a wide range of fine AP size and pressure. The propellants that include the fine AP/binder matrixes exhibiting a mid-pressure extinction, in turn, exhibit the plateau burning rate trends within the corresponding pressure ranges. A plateau is also observed at elevated pressures in the burning rates of some formulations, which is related to the diminishing relative importance of the near-surface leading-edge region of the oxidizer/fuel diffusion flame in the gas-phase combustion zone. The choice of the coarse AP size influences the exact pressure range within the mid-pressure extinction domain of the matrix where the propellant exhibits the plateau burning rate trends. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 73–81, July–August, 2007.     

Ammonium Perchlorate,Friend or Foe? Part 1: The Influence of this Oxidizer on the Aging Behavior of Propellant Compositions

Propellants containing nitroglycerine and ammonium perchlorate have been reported to have comparatively shorter shelf lives than analogous energetic materials without this oxidizer. However, investigation into the aging behavior of three compositions containing polyethylene glycol and nitroglycerine revealed that the propellant which included ammonium perchlorate degraded at a slower rate compared with the other materials. It was suggested that ammonium perchlorate might act as an oxygen inhibitor reducing the oxidation rate of the polyethylene glycol binder so decreasing the rate of the propellant decomposition. In addition, at temperatures of 80 °C or lower, ammonium perchlorate initially appears to hinder acid hydrolysis of nitroglycerine which also slows down the degradation of polyethylene glycol based propellant.     

Solid‐Liquid Adhesion Energies of Composite Propellant Components Determined by Solution Calorimetry

The energetic or adhesive interaction between binder matrix and solid fillers as well as plasticizers and fillers is a property determining mechanical properties as strain capacity and stress at maximum or stress at break. To know such interaction energies is therefore of interest. An isoperibolic solution calorimeter is used to measure such interaction energies or heats of immersion of the solid propellant components ammonium perchlorate (AP) and aluminium (AL) with the liquid plasticizers bis‐(2‐ethylhexyl) adipate (DEHA) and azido‐terminated oligomeric glycidylazide (GAP−A). The determined heats indicate wettability and interaction energy in units of energy per surface area of the solid component. For AP, three different grain sizes with mean diameter about 200, 90, 42 μm were used. Results indicate dependence of AP wetting on mean particle diameter: Successful wetting occurs for AP at diameters from above 90 μm. The polar interactions between the ionic AP and GAP−A greatly enhance wetting as compared to DEHA.     

Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder for composite solid propellants

The synthesis and application of hydrogenated hydroxy-terminated polyisoprene (HHTPI) to a fuel binder of composite solid propellants were attempted. An HHTPI prepolymer was synthesized through the hydrogenation for the hydroxy-terminated polyisoprene (HTPI) in the presence of nickel and zirconium catalysts over kieselguhr in 2.0 MPa hydrogen and at 443 K – 453 K for 24h. A prepolymer of a number-averaged molecular weight 2500–3800, provided a viscosity level required for the use of a fuel binder from which solid propellant can be possibly made by means of direct casting method. Thermal stability and aging characteristics of HHTPI elastomer against environmental attacks are superior to those of HTPB. Some plasticizers and bonding agents can bring about the acceptable mechanical properties to the propellant grains mainly composed of HHTPI, ammonium perchlorate and aluminium powder. The linear burning rates of HHTPI-based propellants are at the same level with that of HTPB-based propellants. However, the composition that gives the maximum performance with HHTPI-based propellants, shifts to 1–2 wt% fuel-rich side from the most adequate fuel content 12 wt% in HTPB/AP/Al. The HHPTI propellants indicated the similar burning rate as HTPB-based propellants in the linear burning rates in spite of the comparatively poor ignitability. Nevertheless, the static tests of 100 mm dia. sounding rocket motors are successfully performed by an ignition operation at the pressurized condition. The ballistic performances are not inferior to those of the HTPB-based propellants.     

Effect of the physical state of the activators on the combustion of composite rocket propellants. I. The applied catalysts

The effects on the burning and thermal decomposition of composite rocket propellants, based on ammonium perchlorate and butyl rubber, of oxide coated catalysts applied to tge surface of the ammonium perchlorate crystals and introduced into the propellant in the form of a colloidal suspension are investigated. It is shown that the possibility of changing the burning rate by means of applying the catalyst on the oxidizer crystal surface is determined by the chemical nature, the content of the compounds deposited on the oxidizer surface, and by the structure of the coating formed on the ammonium perchlorate surface. Excluding the agglomeration of the catalytic additives using the developed methods, the variation in their dispersivity and the nature of localization in the propellant are the indicators of the propellant's performance efficiency within the region of small additive concentrations (up to 0.5%) in the propellant. Scientific Research Institute for Physicochemical Problems, Belorus State Univ., 220050 Minsk. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 6, pp. 82–88, November–December, 1995.     

Agglomeration Characteristics of Aluminum Particles in AP/AN Composite Propellants

Most solid rockets are powered by ammonium perchlorate (AP) composite propellant including aluminum particles. As aluminized composite propellant burns, aluminum particles agglomerate as large as above 100 μm diameter on the burning surface, which in turn affects propellant combustion characteristics. The development of composite propellants has a long history. Many studies of aluminum particle combustion have been conducted. Optical observations indicate that aluminum particles form agglomerates on the burning surface of aluminized composite propellant. They ignite on leaving the burning surface. Because the temperature gradient in the reaction zone near a burning surface influences the burning rate of a composite propellant, details of aluminum particle agglomeration, agglomerate ignition, and their effects on the temperature gradient must be investigated. In our previous studies, we measured the aluminum particle agglomerate diameter by optical observation and collecting particles. We observed particles on the burning surface, the reaction zone, and the luminous flame zone of an ammonium perchlorate (AP)/ammonium nitrate (AN) composite propellant. We confirmed that agglomeration occurred in the reaction zone and that the agglomerate diameter decreased with increasing the burning rate. In this study, observing aluminum particles in the reaction zone near the burning surface, we investigated the relation between the agglomerates and the burning rate. A decreased burning rate and increased added amount of aluminum particles caused a larger agglomerate diameter. Defining the extent of the distributed aluminum particles before they agglomerate as an agglomerate range, we found that the agglomerate range was constant irrespective of the added amount of aluminum particles. Furthermore, the agglomerate diameter was ascertained from the density of the added amount of aluminum particles in the agglomerate range. We concluded from the heat balance around the burning surface that the product of the agglomerate range and the burning rate was nearly constant irrespective of the added amount of aluminum particles. Moreover, the reduced burning rate increased the agglomerate range.     

Ammonium Perchlorate,Friend or Foe?

In part 1 of this paper, it was demonstrated that a nitroglycerine and polyethylene glycol based propellant containing ammonium perchlorate degraded at a slower rate at temperatures of 80 °C or less compared with the other two energetic materials studied which did not have this oxidizer present. It was suggested that ammonium perchlorate might act as an oxygen inhibitor reducing the oxidation rate of the polyethylene glycol binder which decreases the rate of propellant decomposition. In part 2, the specific interaction between ammonium perchlorate, nitroglycerine and polyethylene glycol is reported. It has been shown that at temperatures lower than 90 °C, if there is any uncured and unstabilised PEG present, nitroglycerine rapidly degrades in the presence of ammonium perchlorate but this is prevented if stabiliser is added. In addition, ammonium perchlorate initially appears to hinder acid hydrolysis of nitroglycerine which also slows down the degradation of polyethylene glycol based propellants. However, in the long term at low temperatures, or short term at higher temperatures, AP accelerates the decomposition of NG.     

Mechanical Properties of Smokeless Composite Rocket Propellants Based on Prilled Ammonium Dinitramide

Ammonium dinitramide (ADN) is a high performance solid oxidizer of interest for use in high impulse and smokeless composite rocket propellant formulations. While rocket propellants based on ADN may be both efficient, clean burning, and environmentally benign, ADN suffers from several notable disadvantages such as pronounced hygroscopicity, significant impact and friction sensitivity, moderate thermal instability, and numerous compatibility issues. Prilled ADN is now a commercially available and convenient product that addresses some of these disadvantages by lowering the specific surface area and thereby improving handling, processing, and stability. In this work, we report the preparation, friction and impact sensitivity and mechanical properties of several smokeless propellant formulations based on prilled ADN and isocyanate cured and plasticized glycidyl azide polymer (GAP) or polycaprolactone‐polyether. We found such propellants to have very poor mechanical properties in unmodified form and to display somewhat unreliable curing. However, by incorporation of octogen (HMX) and a neutral polymeric bonding agent (NPBA), the mechanical properties of such smokeless formulations were significantly improved. Impact and friction sensitivities of these propellants compare satisfactorily with conventional propellants based on ammonium perchlorate (AP) and inert binder systems.     

Rheology of HTPB propellant. I. Effect of solid loading,oxidizer particle size,and aluminum content

Composite solid propellants based on hydroxyl-terminated polybutadiene (HTPB) have become the workhorse propellants in the present-day solid rocket motors. The other major ingredients of a composite propellant are the crystalline oxidizer and metallic fuel. As the solid loading of such propellants is as high as 86–90%, their rheological behavior is very complex. The propellant slurry needs to have reasonably low viscosity and a long pot life for better casting and, hence, for a defect-free rocket motor. The primary factors affecting the solid propellant viscosity are solid content, particle size, shape, and distribution. The present study concerns the variations of solid loading from 80 to 89% at constant aluminum cotent, variation of aluminium from 0 to 22% at constant solid loading, and the coarse-to-fine ratio of the oxidizer. The plots of yield stress, consistency index, pseudoplasticity index, and thixotropic index at different time intervals are drawn for all these parametric changes. Based on these rheological studies, the optimum ratio of oxidizer coarse-to-fine ratio, aluminum content, and level of solid loading have been determined.     

Effect of ultrafine aluminum on the combustion of composite solid propellants at subatmospheric pressures

This paper presents the results of an experimental study of the combustion of composite solid propellants with a double oxidizer (ammonium perchlorate/HMX) at pressures of 0.03–0.1 MPa. Systems containing a micron-sized aluminum powder (ASD-4) and the Alex ultrafine aluminum powder were investigated. It was shown that the replacement of ASD-4 by Alex in propellant systems led to an increase in the burning rate. The aluminum particle size and the oxidizer excess coefficient were found to affect the exponent in the power-law burning-rate dependence. The range of the oxidizer excess coefficient was determined that corresponded to the effective replacement of micron-sized aluminum by ultrafine aluminum for which the exponent in the power-law burning rate dependence decreases. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 47–55, January–February, 2009.     

Developing a Combustion Model of a Solid Propellant Containing Capsules of Liquid Oxidizer

A combustion model of a solid propellant containing liquid oxidizer capsules was developed. The model is based on the combustion cycle of a unit cell, an oxidizer droplet surrounded by the adjusting amount of binder. Three combustion stages are considered: (1) binder decomposition parallel to oxidizer heating up to boiling temperature, (2) simultaneous binder and oxidizer gasification, and (3) gasification of the remaining condensed phase species. Simple global kinetics are assumed for the gas phase reactions and binder decomposition abiding Arrhenius law, and boiling process for the oxidizer abiding Clausius‐Clapeyron curves. The obtained results show similar trends as do other relevant models and experimental results, especially for the effect of droplet size and propellant composition on the burn rate. The predicted effect of initial temperature on the burn rate is less significant than for common solid propellants. The results indicate that for small droplets the oxidizer will heat up to boiling temperature even before it is revealed on the burning surface, due to heat conduction through the surrounding binder.     

Burning Behavior of Composite Propellant Containing Fine Porous Ammonium Perchlorate

A fine porous ammonium perchlorate (FPAP) was prepared by the spray-drying method. The burning behavior of a propellant containing FPAP was investigated and compared with that of a propellant containing a fine ammonium perchlorate without porosity (FAP), of which the mean diameter was almost the same as that of FPAP. The results are as follows: (1) The burning rate of the propellant containing FPAP increases with increasing FPAP content. (2) When the propellants containing FPAP or FAP have the same composition, the burning rate of the propellant containing FPAP is larger than that of the propellant containing FAP at various pressures. (3) The temperature sensitivity of a propellant containing FAP is relatively constant at various pressures. However the temperature sensitivity of the propellant containing FPAP decreases with increasing pressure. (4) It was found that the differences in the burning behavior of the propellant containing FPAP and the propellant containing FAP are caused by the porosity of FPAP.     

Effect of tension of a composite propellant on its burning rate

Existing concepts of the effect of tensile strains on the burning rate of propellants are analyzed. It is demonstrated that the basic mechanism of increasing burning rate of composite propellants under tension is spalling of the binder from oxidizer particles, formation of an additional burning surface, and changes in the combustion-zone structure. To describe this effect, a rheological model of a composite solid propellant is developed, which takes into account separation of the binder from disperse particles of the fillers (oxidizers, coolants, metals, etc.). A criterion is found, which describes the difference between the propellant behavior under tension with spalling of the binder from the particles and the tension of the same material without the emergence of internal defects. A method of experimental determination of the number of defects arising in the propellant under tension, based on analyzing the tensile stress-strain diagram of the material, is proposed. A mathematical model of composite propellant combustion is developed, which takes into account separation of the binder from the oxidizer particles and formation of an additional burning surface. A correlation between the change in the burning rate of the propellant under tension and parameters of the propellant tensile diagram is found. A method for predicting the change in the burning rate of the propellant under tension on the basis of the propellant tensile diagram shape is developed.