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.

Periyapatna A. Ramakrishna

Department of Aerospace Engineering

M. Gaurav

Agglomeration

Aluminum

Chemical activation

combustion

Composite propellants

particle size

Particle size analysis

Polytetrafluoroethylenes

Propellants

Slags

SOOT BLOWERS

Specific surface area

Thermal conductivity

ALUMINIUM COMBUSTIONS

ALUMINIUM PARTICLES

Composite solid propellant

FLUORINATION REACTIONS

High specific surface area

Large specific surface areas

MECHANICAL ACTIVATION

SOLID PROPELLANT COMBUSTION

Solid propellants

nitrogen

politef

PROPELLANT

article

Bomb

density

Differential scanning calorimetry

Fluorination

heat

powder

priority journal

Scanning electron microscopy

surface area

temperature sensitivity

thermogravimetry

Viscosity

X ray diffraction

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Combustion and Flame

In this work, iron containing hydroxyl terminated polybutadiene (Fe-HTPB) based binder cum burn rate catalyst has been developed without altering the crucial physical properties of HTPB. Ferrocene, the source of Fe in the Fe-HTPB, has been grafted at the terminal carbons of HTPB to ensure no alternation in microstructure of HTPB which in turn helped in retaining physical properties of pristine HTPB. The structure and the presence of ferrocene as the end cap groups of the Fe-HTPB were confirmed by solid-state NMR and MALDI-TOF-MS analysis. Control over the viscosity and Fe content of the Fe-HTPB was achieved by varying the grafting reaction recipes and conditions. The Fe content, as measured by inductively coupled plasma - atomic emission spectroscopy (ICP-AES) in the Fe-HTPB varied from 0.06% to 0.165% (by weight) and found to be responsible for increasing viscosity of Fe-HTPB from 5857 mPa S to 11,890 mPa S. Non aluminized composite solid propellants (CSPs) with 86% (wt%) ammonium perchlorate loading were prepared using Fe-HTPB as a binder for studying the burn rate efficiency. Burn rates of CSPs made from Fe-HTPB binders were found to be enhanced by ∼125% compared to CSPs of pristine HTPB. At 40 bar pressure, the burn rate of CSPs made from Fe-HTPB and pristine HTPB binders are 20.56 and 9.07 mm/s burn rate, respectively. In addition, all the CSPs made from Fe-HTPB were found to be very stable as their pressure index is less than 0.5. © 2019 Elsevier Ltd

Polymer

Ferrocene grafted hydroxyl terminated polybutadiene: A binder for propellant with improved burn rate

This paper deals with the low pressure deflagration limit (LPDL), which is the minimum pressure below which burning of a propellant ceases without the assistance of an external energy source. The LPDL of AP monopropellant reported in literature is around 20 bar and it is reported to shift to different higher pressures with the addition of small fraction of impurities or additives. Most of these additives (such as iron oxide and copper chromite) are known to catalyze AP combustion, but yet their reported LPDL is significantly higher than the LPDL of AP itself. In present study, reasons for this behavior have been explored experimentally. Three additives (each of 1% by mass) namely iron oxide (IO), copper chromite (CC) and activated charcoal (AC) were used. The LPDL of AP with these additives was determined using three methods. Two of these methods (method I and method II) include the effect of ignition dynamics on LPDL while in method III (slow depressurization) ignition dynamics does not play a role. In method I, hot nichrome wire was used for ignition, while in method II, nichrome wire ignition in conjunction with an in-situ propellant was used to ignite the pellets. The effect of convective heat loss on LPDL was studied with the use of silica grease whose role through carefully thought out experimentation has been identified as an insulation on sides of pellet. The LPDL of AP with additives was found to be higher for method I compared to method II. Lastly, method III (slow depressurization) was found to result in lowest LPDL for these additives. The LPDL of pure AP, which has been reported as around 20 bar, was revisited and method III along with the use of silica grease has lowered the LPDL of pure AP to 14 bar. © 2019 The Combustion Institute

Experimental studies on low pressure deflagration limit of ammonium perchlorate with additives

The combustion characteristics of Ammonium Perchlorate (AP) monopropellant have been revisited. The use of silica grease to prevent heat loss from the sides of the pellet and the ignition methodology opened up new area; one of them being the low pressure deflagration limit (LPDL) of AP itself and the other being the burn rate of AP at different pressures. The new burn rate law for pure AP was obtained for conditions close to adiabatic. Using slow depressurization method, wherein the LPDL was independent of ignition transients, the LPDL of pure AP was found to be 14 bar The burn rate of pure AP at 70 bar was found to be 10.66 mm/s, which was much higher than the ones reported in literature. The pressure index for this case was found to be 0.71. This new result completely changes the understanding of AP combustion and called for a new set of parameters to be developed to predict the results obtained with silica grease on the sides. Using the same set of parameters, along with a heat loss model the predictions for the case when there was no silica grease applied was made. These results were in reasonable agreement with the experimental data reported in literature without the silica grease on the sides of the AP pellet. This brings out the critical role of heat loss in AP combustion. © 2019 The Combustion Institute

Combustion of Ammonium Perchlorate monopropellant: Role of heat loss

Polytetrafluoroethylene is known to enhance the reactivity of aluminum. The enhancement in the reactivity is produced by mechanically activating the aluminum using polytetrafluoroethylene. In this study, mechanically activated aluminum powder is used along with paraffin wax as the fuel grain for a hybrid propellant. Here, gaseous oxygen is used as oxidizer. To achieve an optimum composition for the best specific impulse, the aluminum fraction in the composition is restricted to 50% of the base fuel composition along with 50% wax/ethylene vinyl acetate (combination of wax and ethylene vinyl acetate, in a ratio of 4∶1). The polytetrafluoroethylene in mechanically activated aluminum powder is added at the expense of wax/ethylene vinyl acetate in the fuel composition. When compared with the composition having pyral aluminum powder (which has flake like particles of very high specific surface area), mechanically activated powder increases the fuel regression rate by ∼40% at an oxidizer mass flux of 35 g∕cm2-s. It is also observed that the combustion efficiency of the hybrid motor is 59% with the use of mechanically activated aluminum powder in comparison to 39% obtained with micron-sized aluminum powder. Copyright © 2018 by Indian Institute of Technology, Madras, India.

Journal of Propulsion and Power

Utilization of mechanically activated aluminum in hybrid rockets

This studymakes an attempt to demonstrate a technique to embed the catalyst on ammoniumperchlorate surface. In the present study, various techniques like differential scanning calorimeter, thermogravimetric analysis, scanning electron microscope, and x-ray computed tomography scan analysis as well as others were used to establish the effectiveness of this technique.Micron-sized iron oxide was used in this study for embedding the catalyst on ammonium perchlorate. Burn rates of pellets containing different fractions of iron oxide from various sources were measured to establish an optimumfraction for highest burn rate of ammoniumperchlorate pellets achievable with iron oxide. These burn ratesweremeasured using aCrawford bomb, and a fraction of1%iron oxide (Sigma-Aldrich)was highestamong them for pellets. The burn rate of the ammoniumperchlorate pellet with 1%iron oxide at 70 bar pressure recordedwas 13.33 mm/s. This fraction of iron oxide was also used to study the effect of embedding the catalyst on ammonium perchlorate surface when used in nonaluminized composite solid propellants. It is observed that catalyst-embedded ammonium perchlorate yields a higher burn rate as compared to the mechanically mixed catalyst. ©2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Effects on burn rates of pellets and propellants with catalyst-embedded AP

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.

Acta Astronautica

Dependence of density and burning rate of composite solid propellant on mixer size

Iron oxide and copper chromite are the known burn rate enhancers used in a composite solid propellant. Lot of research has been carried out to understand the mechanism or location of action of the burn rate modifiers so as to better tailor the burning rate of a composite propellant. The literature is still very confusing in affirming the mechanism. Here, a systematic study has been carried out, by undertaking experiments at varying levels of combinations of the individual components (ammonium perchlorate, which is oxidizer and hydroxyl terminated poly butadiene, which is both fuel and binder) of composite solid propellant. Firstly, thermal gravimetric analysis, differential scanning calorimetry and burning rate measurements on the individual components are carried out to study the effect of iron oxide and copper chromite on the components themselves. It has been noticed that though both iron oxide and copper chromite are effective on ammonium perchlorate, iron oxide is slightly more effective than copper chromite. Also, copper chromite enhanced the binder melt flow, while iron oxide reduced it. These are followed-up by experiments on sandwich propellants, which give greater insight and enables better understanding of the behavior of iron oxide and copper chromite in composite propellants, as these are simple two-dimensional analogue of the composite solid propellants. Finally, experiments are carried out on the composite solid propellants to obtain a holistic understanding of the behavior/location of action of iron oxide and copper chromite in them. These studies are used to explain certain unexplained but observed phenomena, at the same time elucidating the location of action of these burn rate modifiers in composite solid propellant combustion. Based on these observations, it has been proposed that both iron oxide and copper chromite are primarily acting on the condensed phase. These studies are further complimented with experiments to analyze the thermal conductivity measurements of various propellant samples. This is pursued to understand the reason for the differences in burn rate pressure index for the composite propellants with iron oxide and with copper chromite. It has been understood from these studies that the thermal conductivity of a composite propellant is a key parameter, which affects the burn rate pressure index. Literature has never addressed it from this perspective. © 2014 The Combustion Institute.

Studies on the role of iron oxide and copper chromite in solid propellant combustion

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 proposes the use of micron-sized aluminum (pyral, average particle size 3.66 μm) with a higher specific surface area as a good candidate to enhance the burn rate of the composite propellant. Experiments were performed in the pressure range of 10 to 70 bar in a window bomb for measuring the burn rate. Comparison of these burn rate results with those obtained using micron- and nanosized aluminum found that the performance of pyral was in between that of micron- and nanosized aluminum. The reason for the high burn rates observed with pyral is due to the flake like appearance of pyral with a large specific surface area. It is argued that, if the specific surface area is large, then the thickness becomes the characteristic length scale. This ensures the heat release from the aluminum combustion to occur closer to the propellant surface as the thickness of pyral is in nanometers. Both the x-ray diffraction and heat of formation analyses indicated that pyral had higher purity than nanoaluminum, which has an implication to the specific impulse of the propellant. In addition to this, it shows that the micron-sized catalyst is effective with pyral, whereas it is ineffective with nanoaluminum. This study also reports the mechanical properties of the propellant containing pyral.

Effect of specific surface area of aluminum on composite solid propellant burning

An experimental investigation has been carried out to measure the size of nano-aluminium agglomerates emerging from the combustion of nano-aluminized sandwiches and composite solid propellants. Nano-aluminium of median size of 50 nm produced in-house by the electrical wire explosion method is used in these samples. Propellants with different sizes of coarse and fine ammonium perchlorate are considered. Surface features of sandwiches and a propellant whose burning was interrupted by rapid depressurization are examined in a scanning electron microscope. The combustion products of the sandwiches and propellants are quenched close to the burning surface and collected in a quench collection set-up. The surface features of rapid-depressurization quenched sandwiches exhibit relatively large nano-aluminium clusters - of the order a few micrometres - particularly in the binder lamina. Quench-collected nano-aluminium exhibits significant agglomeration, but only a small fraction of the agglomerates are in the 1-3 μm range, except for both the coarse and fine AP particles used in the formulation being large, but even there they do not exceed ∼5 μm in size. This is expected to be benign for reduced smoke propellant applications from exhaust signature point of view, and to decrease the specific impulse losses without sacrificing the energetics of the propellant. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Proceedings of the Combustion Institute

Quench collection of nano-aluminium agglomerates from combustion of sandwiches and propellants

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

Journal	Data powered by TypesetCombustion and Flame
Publisher	Data powered by TypesetElsevier Inc.
ISSN	00102180
Open Access	No