This paper examines the effect of mixer size on the density of the propellant and it also explores the role of the propellant density with regards to burning rate and burn rate pressure index. Propellant samples were prepared in the four different size (15, 70, 200 and 1000 g) mixers with identical input of ingredients to examine the effect of mixer size on density and burning rate of the composite solid propellant. It was noticed that density and burning rate of the propellant changes significantly with change in the mixer size from 15 g to 1000 g. This is because for the smaller mixers the surface area to volume ratio is large and the actual percentage of aluminum and ammonium perchlorate (especially coarse) that goes into the propellant reduces. High burning rate with decrease in mixer size is also accompanied with increase in burn rate pressure index. The composition analysis was also carried out and it is noticed that the actual percentage of the ingredients is different from the intended percentage of the ingredients and percentage change is more when the mixer size decreases. © 2013 IAA.

Periyapatna A. Ramakrishna

Department of Aerospace Engineering

Sumit Verma

AMMONIUM PERCHLORATES

Burn rates

Burning rate

Composite solid propellant

COMPOSITION ANALYSIS

Different sizes

surface area

VOLUME RATIO

Composite propellants

Density (specific gravity)

Propellants

Mixers (machinery)

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IIT Madras

Acta Astronautica

This paper presents a comparison of actual solid propellants with a computationally generated propellant pack, which is intended for use in combustion modelling studies. X-ray computed tomography (XCT) was used to obtain a 3D reconstruction of an actual propellant sample. Random distribution of spherical particles of ammonium perchlorate (AP) has been used to model a composite solid propellant (random pack). The simulated random pack and 3D reconstructed propellant are compared based on three parameters capturing volume, area and length metrics, viz., density, exposed AP area and AP/binder intercept lengths. The density match is within 1.5 % between the actual propellant and the simulated propellant pack. The exposed AP surface area lies in the range 53–63 % for the 80–86 % solid loaded non-aluminized propellants considered in the study. The histogram of the intercept lengths matched reasonably as well capturing the variation in particle size and solid loading of the propellant. However, the comparison between simulated and XCT reconstructed packs was found to be limited by the resolution of the XCT reconstruction of actual propellant samples. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Propellants, Explosives, Pyrotechnics

Use of X-Ray Computed Tomography for Validation of Random Packs of Composite Solid Propellants

An experimental investigation was conducted to study the combustion characteristics of aluminum and water, gelled using polyacrylamide as gelling agent. These propellants were tested for burn rates and residue in a Crawford bomb. The present study used flakelike aluminum particles with large specific surface area (∼22.5 m 2 ∕g) called pyral. Burn rates and residue were measured for oxidizer-to-fuel ratios (O/F) of 0.8, 1, and 1.2. Gelled propellants prepared with pyral had a maximum burn rate of 7.5 mm∕s at 60 bar pressure at an O/F of 0.8. However, these propellants had a very large residue of 70–80% of the initial mass. To overcome this, propellants were prepared with pyral, which was mechanically activated with polytetrafluoroethylene. The reduction in residue was mainly attributed to smaller thermal profile depth associated with mechanically activated aluminum and to the higher heat of combustion observed with such propellants. Through careful experimentation, the reason for the residue has been established to be due to heat loss from the sides of the propellant strand. The thermal conductivity and propellant diameter play a vital role in determining the heat loss from the condensed phase and in turn determining burn rates and residue. Copyright © 2018 by M. G. Gautham and P. A. Ramakrishna.

Journal of Propulsion and Power

Combustion characteristics of aluminum–water gelled composite propellant

Aluminum is used in solid propellants to increase the specific impulse (Isp). It is desirable to have high propellant loading in any stage as it reduces the structural coefficient and an end burning grain is known to be the one with the highest propellant loading. As aluminum combustion is a slow process, the time available for aluminum combustion in an end burning configuration will be very small at the start of the combustion process. This demands an increase in the reactivity of the aluminum. This study is built on the fact that mechanical activation of aluminum powder with PTFE (poly-tetra-flouro-ethylene) enhances the reactivity of aluminum powder. This study also deals with the use of this activated aluminum powder in conjunction with various other methods to enhance the burn rates of the solid propellant. The temperature sensitivity was also measured. Based on these results, new designs with end burning grains for the third stage of Polar Satellite Launch Vehicle (PSLV) and for the second and third stage of Pegasus launch vehicle have been proposed to increase the payload capacity. With this new design, it is seen that the payload can be increased by 12.7 % and 17.6 % for PSLV and Pegasus, respectively. The novelty of this design is that with no changes to any other hardware of the above two systems the increase in payload can be achieved. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Enhancement of Aluminum Reactivity to Achieve High Burn Rate for an End Burning Rocket Motor

Iron oxide and copper chromite have been used as burn-rate modifiers of composite solid propellants. In recent times, studies have been reported on potassium-doped ammonium perchlorate and on dry activated charcoal. These have shown an increase in composite solid propellant burning rates. The studies on composite solid propellants using dry activated charcoal have also shown a reduction in the burn-rate pressure index for a composite solid propellant. With this knowledge, experiments are conducted to develop compositions that result in both high burning rates and low burn-rate pressure index of the propellant.In additiontothese, the study also reports propellants with dual burnrate pressure index. The change in the burn-rate pressure index is from 0.2 to 0.65 in one of the propellants. These could be very useful in that the same grain can act as boost and sustain grain depending on the combustion chamber pressure. With the addition of different burn-rate modifiers, the pressure at which the change in burn-rate pressure index is observed shifts. This could be a very useful tool for propellant designers. It was observed that, with the use of activated charcoal along with potassium-doped ammonium perchlorate, the burn-rate pressure index was brought down from around 0.65 to 0.38, whereas the burn rates were still higher (60%) than the base propellant. Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Burning rate studies: Potassium-doped ammonium perchlorate with other burn-rate modifiers

This study deals with the mechanical activation of pyral with PTFE. Pyral is a flake-like aluminium particle with a large specific surface area (~22.5m2/g). This paper, through a variety of tests like the TGA, DSC, particle size analysis, SEM analysis and other tests tries to establish that this mechanically activated aluminium is a good replacement for conventional aluminium used in a solid propellant. Burn rates were also measured at pressures ranging from 10 to 70bar using a modified Crawford bomb. The burn rates of solid propellants prepared with this mechanically activated pyral powder were found to be higher than that of non-activated pyral powder and much higher than that of those reported in literature with similar mechanically activated aluminium. This has been explained to be due the large specific surface area of aluminium used in these experiments which undergo exothermic fluorination reactions with PTFE. Two phase losses and slag accumulation are the two problems encountered when aluminium is used in composite solid propellants. Slag accumulation has shown a significant reduction with the use of this activated powder over non-activated pyral. © 2016 The Combustion Institute.

Combustion and Flame

Effect of mechanical activation of high specific surface area aluminium with PTFE on composite solid propellant

Through careful experimentation, this paper establishes that moisture trapped inside the pores of activated charcoal was the reason for the burn-rate enhancement of composite solid propellant observed in literature. This paper demonstrates that, even with the presence of moisture in activated charcoal, the propellants can not only be cured to be free from blow holes and voids but also have good mechanical properties. Experiments were also carried out to examine the possible mechanism for high burn rates and burn-rate pressure index observed in composite propellants when moisture was present in activated charcoal. Scanning-electron-microscope images show that moisture in activated charcoal reduces the binder melt flow. It is noticed that formation of the bubbly layer is more when moisture is present in the binder. This bubbly layer might be exploding near the surface of the propellant and taking away the binder melt layer. This could be the cause for the observed increase in the burn rate and burn-rate pressure index of aluminized composite propellants.

Investigations on activated charcoal, a burn-rate enhancer in composite solid propellant

This paper reports the discovery of activated charcoal as an effective burn rate enhancer of aluminized composite solid propellant. Experiments were carried out using a strand burner at pressures ranging from 10 to 70. bar with varying fractions of activated charcoal. The results show that with the addition of activated charcoal in an aluminized composite propellant higher burn rates (in excess of 25. mm/s at 70. bar) were obtained. This paper also explores the possibility of further enhancing the burn rates of aluminized composite propellant by using iron oxide along with activated charcoal. Burn rates recorded with iron oxide and activated charcoal both present in the propellant were in excess of 50. mm/s at 70. bar. It was noticed that these high burn rates were accompanied by a higher (0.65) pressure index of combustion. A series of experiments were designed to understand the burn rate enhancement with activated charcoal in aluminized composite propellant. TGA and DSC tests were carried out on AP pellets with and without activated charcoal to detect any condensed phase activity of activated charcoal. These tests did not show any significant difference between the AP pellets with and without activated charcoal. The AP pellets with and without activated charcoal were also burnt in a strand burner to understand if any gas phase reaction was being catalyzed by activated charcoal. AC does not enhance any gas phase activity while addition of AC in AP increased the LPDL of AP and reduced the burn rate. Lastly, sandwich propellant with activated charcoal at the interface was prepared to determine if interface reactions were being affected with the addition of activated charcoal. It has been observed that with the addition of activated charcoal at the AP-binder interface the burn rates are higher than those recorded without activated charcoal at the AP-binder interface. © 2009 The Combustion Institute.

Activated charcoal - A novel burn rate enhancer of aluminized composite propellants

Nano-aluminium particles of ∼50 nm size, produced at this laboratory, are added to composite solid propellants based on ammonium perchlorate and hydroxyl-terminated poly-butadiene binder that exhibit plateau burning rate trends and those including burning rate catalysts. The nano-aluminized propellant burning rates are compared with corresponding micro-aluminized and non-aluminized ones in the 1-12 MPa pressure range. The mid-pressure extinction of the matrixes containing the fine-sized ammonium perchlorate particles in the propellant along with the binder is investigated in all the cases to understand the mechanism of plateau-burning. Further, the variations in aluminium content, the aluminium size (within nano- and micro-ranges), bimodal combination of nano- and micro-aluminium are considered. Ferric oxide and titanium dioxide are the burning rate catalysts considered. Large scale accumulation of aluminium is observed not only in micro-aluminized matrixes, but also in nano-aluminized ones as clusters of 1-5 μm size. Since aluminium is added at the expense of the coarse ammonium perchlorate particles to preserve the total-solids loading in the present formulations, addition of micro-aluminium decreases the burning rate; whereas, nano-aluminized propellants exhibit ∼80-100% increase in the burning rate under most conditions. The near-complete combustion of nano-aluminium close to the burning surface of the propellant provides heat feedback that controls the burning rate. Mid-pressure extinctions of matrixes and plateau burning rates of propellants are washed out when nano-aluminium is progressively added beyond 50% in bimodal aluminium blends, but low pressure-exponents are observed in the nano-aluminized propellant burning rates at elevated pressures. Adjusting the plasticizer content in the binder alters the pressure range of plateau burning rates in non-aluminized propellants. Catalysts increase the burning rate by ∼50-100% in non-aluminized and micro-aluminized propellants, but in nano-aluminized propellants, the μm-sized catalyst does not affect the burning rate significantly; whereas, the nanometre size catalysts increases the burning rate merely by ∼5-15%. © 2009 The Combustion Institute.

Effect of nano-aluminium in plateau-burning and catalyzed composite solid propellant combustion

Subatmospheric combustion characteristics of ammonium perchlorate (AP)/hydroxyl-terminated-polybutadiene (HTPB) propellants prepared using two different catalyst mixing procedures were studied. In the first one, identified as the wet-mixing process, copper chromite catalyst (CC) was mixed with the binder (HTPB + di-2-ethylhexyl adipate); and to this CC-binder mix, AP and the curing agent [toluene di-isocyanate (TDI)] were then added. In the second, identified as the dry-mixing process, CC was first mixed with AP, and to this dry-mixture, the binder and toluene di-isocyanate were then added. For such propellants of different mixing procedures, with various AP particle sizes, the measurements of burning rates up to 1 bar, low-pressure deflagration limit (LPDL), and the scanning electron microscope (SEM) study of extinguished samples at LPDL were carried out. Results of the study show a higher burning rate for dry-mix propellents, and this higher burning rate is AP particle-size and pressure dependent. LPDL is also AP particle-size dependent, and for catalyzed propellant it is lower than that of an uncatalyzed one. The two procedures of CC mixing do not demonstrate any significant difference in LPDLs. The micrographs of wet-mix and dry-mix propellants look similar; however, the strong effect of low pressure in increasing the surface and subsurface reactions is brought out by the SEM study.

Journal	Acta Astronautica
ISSN	00945765
Open Access	No