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Combustion wave of metalized extruded double-base propellants
Fuel 237 (2019) 1274–1280
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Combustion wave of metalized extruded double-base propellants Sherif Elbasuney, Ahmed Fahd
T
School of Chemical Engineering, Military Technical College, Kobry El-Kobba, Cairo, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Double-base propellant Combustion Burning rate Aluminum Ballistic performance
Aluminum metal fuel, with combustion heat of 32,000 J/g, is of interest as high energy density material. Aluminum is able to react not only with free oxygen but also with inert combustion gaseous products. This study reports on the impact of ultrafine aluminum particles on combustion characteristics of double-base propellant. Metalized double-base propellant formulations were developed by solventless extrusion process. Combustion characteristics were evaluated using small-scale ballistic evaluation test motor. There was an increase in average operating pressure, burning rate, and characteristic exhaust velocity C∗ with aluminum content. Even though there was no oxidizer available for aluminum oxidation; aluminum alone offered 7.6% increase in average operating pressure, 24.6% increase in burning rate, 7.8% increase in C∗. The main outcome of this study is that this superior performance was achieved at 4 wt% solid loading level. Aluminum particles could alter the combustion wave by increasing the combustion flame temperature, average operating pressure inside the combustion chamber, thermal conductivity of DB propellant. All these combustion criteria could decrease the thickness of the dark zone, allowing the luminous flame to be more adjacent to the burning surface; therefore the reaction could proceed faster. Even though the developed metalized DB formulations were found to be more energetic with an increase in calorific value by 4.6%; they demonstrated good thermal behavior to reference formulation using DSC. These findings confirmed that aluminum is an outstanding energetic ingredient for solid rocket propulsion. It has unique ability to react with inert gases generating substantial heat output.
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1. Introduction
2Al(s) + 3CO(g)
The maximum heat of combustion of double-base propellant is generally limited by the enthalpy of formation of reaction products (CO2, H2O) [1–3]. Higher combustion energies and energy density can be obtained by combusting metal fuels, such as Mg, Al, and B [4]. The considerable strength and energetic formation of the metal-oxygen, account for the excellent fuel properties [5]. Metallic and non-metallic fuels can offer massive heat output up to 70 KJ/g (Fig. 1) [4]. Recent developments in the area of metallic fuels have largely focused on ultrafine powder [6–7]. Exhaust toxicity limits the use of beryllium [8–9]. High cost of boron limits its extended applications [5,10]. Aluminum offers many advantages in terms of performance, cost, availability, safety, as well as chemical stability [11–12]. Aluminum, with combustion heat of 32,000 J/g, is the most common fuel in use in the field of rocket propulsion technologies [13–14]. Aluminum is able to react not only with free oxygen; but also it has unique ability to react with combustion inert gaseous products (i. e. CO2, CO, and N2) (Eqs. (1)-(5)) [12,15].
2Al(s) + 3H2 O(g)
Al2 O3 (s) + 3H2 (g) + 866kJ/mol
(3)
2Al(s) + 3CO2 (g)
Al2 O3 + 3CO + 741kJ/mol
(4)
2Al(s) + 3/2O2
Al2 O3 (s) + 1700 KJ/mol
(1)
2Al(s) + N2 (g)
Al2 O3 + 3 C+ 1251 kJ/mol
2AlN + 346 kJ/mol
(5)
Ultrafine-aluminum can imitate this series of exothermic reactions [5,16–17]. Therefore, a large amount of heat would be liberated with minimum consumption of oxygen [18–21]. One of the main features of aluminum is high chemical stability. The surface of aluminum particles is coated with a protective layer of oxide of 5 nm (Fig. 2) [5,22–24]. This tight surface oxide layer (Al2O3) can prevent the inner metal from further oxidation [5]. Consequently aluminum particles can be stored for extended periods without loss of reactivity [25]. All these features can offer aluminum particles an effective role in the development of DB propellants with wide range of burning rate and specific impulse [26–32]. DB propellants have wide applications in tactical missiles due to their superior mechanical properties, aging capabilities, and good operational characteristics [26–27,33–34]. They have recently been used in booster, sustainer, and dual thrust rocket motors [35].
E-mail address: [email protected] (S. Elbasuney). https://doi.org/10.1016/j.fuel.2018.10.018 Received 5 February 2018; Received in revised form 25 September 2018; Accepted 2 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
Fuel 237 (2019) 1274–1280
S. Elbasuney, A. Fahd
Fig. 1. Gravimetric and volumetric heats of oxidation for metallic and non-metallic additives.
decomposition reactions occurs very rapidly. IV. Dark zone (induction zone) In this zone, very slowly oxidation reactions occur. This zone length lies in the range 2–20 mm depending on the operating pressure [44–45]. V. Flame zone In this zone, the final combustion products are formed, where they reach the thermal equilibrium state. Aluminum metal fuel can positively impact the combustion flame temperature and increasing the operating pressure. Aluminum particles have the potential to induce highly exothermic reactions in the dark zone (induction zone), decreasing its thickness, and hence could increase the burning rate [46–48]. Furthermore aluminum can enhance the thermal conductivity of DB propellant allowing the combustion process to proceed faster. In this study ultra-fine aluminum particles of 5 µm average particle size were developed by wet milling. The starting aluminum particles (particle size ≤ 32 μm) were dispersed in 250 ml ethanol. The colloidal particles were ground using ball mill Retch 100 for 12 h. Ethanol was employed as the grinding medium in order to dissipate any generated heat (during grinding process) and to protect ultra fine aluminium from further oxidation. The size of developed aluminum particles was visualized using scanning electron microscope SEM, Zeiss EVO-10 by Carl Zeiss Corporation. Colloidal aluminum particles were integrated into DB propellant formulation using screw extrusion process. The impact of Al metal fuel on combustion characteristics as well as ballistic performance, particularly on the operating pressure, burning rate, and C* were evaluated using small-scale ballistic evaluation test motor; which is established as the most representative mean for combustion characteristic evaluation [8]. Aluminum particles demonstrated a dual effect by increasing the average operating pressure inside the rocket motor and increasing the burning rate. These combustion characteristics resulted an enhanced characteristics exhaust velocity C* by 24.6%.
Fig. 2. Aluminium nanoparticles with passivated layer of Al2O3 [12]
The burning rate and specific impulse (thrust per unit weight of propellant) are the main parameters which need to be precisely optimized [8,36–38]. High reaction rates and specific impulse can be achieved by increasing the combustion temperature, an/or operating pressure inside the rocket motor [39–41]. It is widely accepted that the combustion wave of DB propellant consists of five distinctive zones (Fig. 3) [42–43]. I. Heat conduction zone No chemical changes occur at this zone; thermal effect by heat conduction from the burning surface causes temperature increase to the onset decomposition temperature. II. Solid phase reaction zone It is a very thin zone with slightly exothermic reactions. Temperature in this zone is approximately equal to the burning surface temperature (Ts). This zone length is 2–3 mm.
2. Theoretical evaluation of combustion characteristics
III. Fizz zone
Thermochemical methods have been widely accepted as a valuable fast tool to predict the combustion characteristics of different solid
This zone is adjacent to the burning surface; where a series of 1275
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Fig. 3. Distinctive combustions zones of DB propellant with associated chemical reaction [11]. Table 1 The chemical composition of the investigated DB propellant formulations. Formulation
NC (wt %)
NG (wt %)
Stabilizers (wt %)
Other additives (wt %)
Al (wt %)
F0 F1 F2 F3
56.20 55.07 54.51 53.95
28.55 27.98 27.69 27.41
2.1 2.06 2.04 2.02
13.15 12.89 12.76 12.62
0 2 3 4
(Reference) (2 wt% Al) (3 wt% Al) (4 wt% Al)
propellant compositions [12,49–50]. The impact of Al metal fuel on the combustion characteristics of DB propellants particularly the combustion temperature (Tc) and specific impulse, were evaluated using thermodynamic code named ICT (Institute of Chemical Technology in Germany, version 2008). This code is based on the chemical equilibrium and steady-state burning model; which is based on two methods of calculation: frozen equilibrium and shifting equilibrium. The employed frozen equilibrium model is based on the assumption that the composition is invariant (the product composition at the nozzle exit is identical to that in combustion chamber). The chemical compositions of the investigated DB propellant formulations are listed in Table 1. Thermochemical calculations demonstrated an increase in the combustion temperature and specific impulse with the increase of aluminum metal fuel content. Fig. 4 demonstrates the theoretical impact of aluminum particles on specific impulse and combustion temperature of DB propellant. The great impact of aluminum particles on specific impulse and combustion temperature could be assigned to the exothermic oxidation of aluminum with combustion heat of 32,000 J/g. This criteria could enhance the combustion flame temperature and increase the operating pressure inside the rocket motor [35,51]. Aluminum can also enhance the heat transfer from the burning zone to the adjacent layers of unreacted composition due to its high thermal conductivity [52]. Thus Al has the potential to speed up the burning rate of DB propellant formulations.
Fig. 4. Theoretical impact of aluminum particles on combustion characteristics of DB propellant.
aluminum particles are represented in Fig. 5. MDB propellant formulations, based on different percentages of Al metal fuel (Table 1), were manufactured by screw extrusion. Screw extrusion technique emphasizes mixing of different ingredients to molecular level, good homogenization, high density, and dimensional stability [33]. This technique includes many stages in consequence for instance: blending, rolling, grinding, granulation, and finally extrusion to obtain grains of desired shape and dimensions (Fig. 6). The maximum solid loading level was limited to 4 wt% to secure DB propellant formulations with good homogeneity and integrity (free from defects and cracks). For each investigated formulation, a tubular specimen of accurate dimensions 29 mm OD, 10 mm ID, and 200 mm length was manufactured for combustion characteristics evaluation using small-scale ballistic evaluation test motor. 4. Combustion characteristics of metalized DB propellants
3. Manufacture of tested MDB propellant formulations
Small-scale ballistic evaluation motor is widely accepted to measure combustion characteristics of solid propellants during static firing tests [8,33]. The burning rate and the specific impulse are the main parameters which need to be precisely optimized and evaluated during
Colloidal aluminum particles with an average particle size of 5 µm were developed by wet milling. SEM micrographs of developed 1276
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Fig. 5. SEM micrographs of aluminum particles developed by wet milling.
Fig. 6. Schematic diagram for solventless extrusion process.
Fig. 8. Practical static fire test using small scale rocket motor.
combustion process to meet specific propulsion requirements (booster, sustainer, or dual thrust rocket motor). Combustion characteristics of metalized DB propellant formulations were evaluated using one-inch test motor (33 mm ID and 225 mm length) (Fig. 7). The pressure (p) inside the combustion chamber was recorded with time (t) during static firing process. The moment of static fire test is represented in Fig. 8. On the basis of p(t) curve, the average operating pressure, burning time, burning rate, and characteristic exhaust velocity can be evaluated [53]. All developed metalized DB propellant formulations exhibited ideal stable burning during the static firing process. It is obviously clear that there is an increase in the average operating pressure and burning rate with Al content (Fig. 9).
It is apparent that Al metal fuel had a dual effect by increasing the average operating pressure inside the rocket motor and increasing the burning rate. This effect was illustrated by the increase in the combustion temperature as well as the increase in thermal conductivity of DB propellants which accelerate the heating of the next burning surfaces [48]. The main combustion characteristic parameters, including burning rate, average operating pressure, and total impulse (IPT) of the developed formulations, are summarized in Table 2. The total area under the P-t curve (IPT) was calculated to retrieve the characteristic exhaust velocity using Eq. (6).
C =
IPT At Wt
Fig 7. Schematic for employed small-scale ballistic evaluation test motor during static firing test. 1277
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Fig. 9. Pressure-time curves of developed aluminized MDB propellants. Table 2 Results on ballistic (performance) parameters of reference DB propellant formulation and MDB developed propellant formulations using the one-inch small-scale ballistic test motor.
Fig. 10. Impact of Al metal fuel on MDB characteristic exhaust velocity & burning rate.
where: C*, IPT, At, Wt are the characteristic exhaust velocity, the total impulse, throat area, and propellant mass respectively. Al particles demonstrated dramatic increase in burning rate as well as C* (Fig. 10).
F3 with 4 wt% Al offered enhanced burning rate and C* by 17.26% and 10.52% respectively. Aluminum particles are able to react not only with free oxygen but also they are also able to react with inert 1278
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Fig. 11. Flame behavior of double base propellant under different pressure values [56–57].
Fig. 12. DSC thermograms for metalized DB propellant to reference formulation.
can decrease the dark zone thickness allowing the combustion flame to become more adjacent to the burning surface (Fig. 11). Fig. 10 demonstrated how aluminum particles can alter the combustion wave of DB propellant by increasing the combustion flame temperature, thermal conductivity of DB propellant, and average operating pressure inside the rocket motor. All these criteria could result in a decrease in the dark zone thickens allowing the luminous flame zone adjacent to the burning surface [58].
Table 3 Auto-ignition temperature, Calorific value, and DSC outcomes of metalized DB propellant to reference formulation. DSC results
F0 (Reference) F3 (4 wt% Al)
Total heat released [J/g]
Max. peak temperature [°C]
1591 1655
194.48 191.85
Calorific value (Qv) kJ/g
Ignition temperature [°C]
Density [g/cm3]
3891 4071
171 172
1.612 1.619
5. Thermal behavior of metalized DB propellant The impact of ultrafine aluminum particles on the thermal stability as well as thermal behavior of DB propellant was evaluated. The autoignition temperature was measured using cook off test; the calorific value was evaluated using bomb calorimeter; thermal behavior was investigated using DSC. While metalized DB propellants exhibited an increase in calorific value; they exhibited onset decomposition temperature similar for reference formulation. This thermal behavior was further verified using DSC (Fig. 12). Aluminum particles did not greatly impact the maximum peak temperature; but greatly impact the total heat released by 65 J/g. The
decomposition gaseous products and add much more heat to the combustion process [54–55]. Aluminum particles, with high heat of combustion and excellent thermal conductivity, have the potential to positively impact the burning rate [35]. The high heat output of ultra-fine aluminum particles could inhanced the increased the average operating pressure inside the rocket motor. The operating pressure and flame temperature are the most effective factors to control the combustion mechanism [48]. It has been reported that the increase in operating pressure inside the rocket motor 1279
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main thermal properties including calorific value, auto ignition temperature, and DSC outcomes of developed metalized DB propellant to reference formulation, are tabulated in Table 3. While, metalized DB propellant formulations were found to be more energetic; they exhibited similar thermal behavior to the reference formulation. 4 wt% Al offered enhanced combustion heat (C*) by 10.5%.
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AIAA 1982;20(1):100–5. [40] Kubota N. Propellants and explosives: thermochemical aspects of combustion. Wiley-VCH; 2002. p. 123–56. [41] Kubota N. In: Propellant and Explosives, Thermo chemical Aspects of combustion, V. VCH, Editor; 2002. [42] BL Crawford HC, Mc Brady JJ. The mechanism of the double base propellants. J Phys Colloid Chem 1950;54:854–62. [43] Rice OK GR. Theory of burning of double-base rocket propellants. J Phys Colloid Chem 1950;54:885–917. [44] Fahd A, Mostafa HE, Elbasuney S. Certain Ballistic Performance and Thermal Properties Evaluation for Extruded Modified Double-Base Propellants. Central Eur J Energ Mater 2017:14(3). [45] Kubota N. ed. Propellants and Explosives Thermochemical Aspects of Combustion. WILEY-VCH: Weinheim; 2002. [46] Yaman H, A.M., Çelik V. Investigation of the effects of metallic additives on the performance of solid propellant. In: 11th International combustion symposium, Sarajevo, Bosnia Herzegovina; 2010. [47] Naya T, K.M.. 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Evaluation of methods for solid propellant burning rate measurement. Defense technical information center (DTIC) Document, USA; 2002. [54] Yen NH WL. Reactive metals in explosives. Propellants Explos Pyrotech 2012;37:143–55. [55] Zarko VE GA. Energetic nanomaterials synthesis, characterization, and application. Amsterdam: Elsevier; 2016. [56] N., K. Propellants and explosives: thermochemical aspects of combustion. WileyVCH; 2002. [57] Yang R, T.P., Liau Y-C, Yang V. Formation of dark zone and its temperature plateau in solid-propellant flames: a review. Combust Flame 2006;145:38–58. [58] Kubota N. Propellants and Explosives: Thermochemical Aspects of Combustion. Vol. 15. Wiley; 2002.
6. Conclusion Ultrafine aluminum particles are able to react with inert gases. Consequently, they have the potential to enhance the combustion flame temperature, increase the operating pressure inside the rocket motor. Furthermore they can improve the thermal conductivity of DB propellants. Through these features aluminum particles can alter the combustion mechanism by thinning the dark zone allowing the luminous flame zone to be more adjacent to the burning surface. Ultra-fine aluminum particles positively impacts the burning rate, average operating pressure, total impulse, characteristic exhaust velocity, heat of combustion by 24.6%, 7.6%, 31.6%, 7.8% and 4.6% respectively. Even through being more energetic metalized DB propellants offered accepted thermal stability and thermal behavior to reference formulation. This paper highlights the fact that controlled combustion characteristics and tailored ballistic parameters can be controlled via effective content of ultra-fine aluminum particles. References [1] Sutton GP, B.O. Rocket propulsion elements. New York: editors: John Wiley & Sons; 2001. [2] Dreizin EL. Metal-based reactive nanomaterials. Prog Energy Combust Sci 2009;35(2):141–67. [3] Sutton GPaOB. Rocket Propulsion elements. Seventh ed. New York: John Wiley & Sons; 2001. [4] Yen NH, Wang LY. Reactive Metals in Explosives. Propellants Explos Pyrotech 2012;37(2):143–55. [5] Conkling J, Mocella C. Chemistry of Pyrotechnics Basic Principles and Theory. Second ed. London: CRC; 2012. [6] Bönnemann H, Richards RM. Nanoscopic Metal Particles-Synthetic Methods and Potential Applications. Eur J Inorg Chem 2001:26. [7] Zhi J, et al. Research on the Combustion Properties of Propellants with Low Content of Nano Metal Powders. Propellants Explos Pyrotech 2006;31(2):139–47. [8] Sutton GP, Biblaz O. Rocket Propulsion elements. Seventh ed. New York: John Wiley & Sons; 2001. [9] Koch E-C, Klapötke TM. Boron-Based High Explosives. Propellants Explos Pyrotech 2012;37(3):335–44. [10] Agrawal JP. Pyrotechnics. In: High Energy Materials. Wiley-VCH Verlag GmbH & Co. KGaA; 2010. p. 331–412. [11] Sadek R, et al. Novel yellow colored flame compositions with superior spectral performance. Defence Technol. 2017;13(1):33–9. [12] Mohamed AK, Mostafa HE, Elbasuney S. Nanoscopic fuel-rich thermobaric formulations: chemical composition optimization and sustained secondary combustion shock wave modulation. J Hazard Mater 2016;301:492–503. [13] Vadhe PP, et al. Cast aluminized explosives (review). Combustion Explosion Shock Waves 2008;44(4):461–77. [14] Elbasuney S, Fahd A, Mostafa HE. Combustion characteristics of extruded double base propellant based on ammonium perchlorate/aluminum binary mixture. Fuel 2017;208:296–304. [15] Talawar MB, et al. Emerging Trends in Advanced High Energy Materials. Combustion Explosion Shock Waves 2007;43(1):11. [16] Krier HJ, Peuker JM, Glumac N. Aluminum Combustion in Aluminized Explosives: Aerobic and Anaerobic Reaction. In: 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. Orlando, Florida; 2011. [17] McNesby KL, et al. Afterburn Ignition Delay and Shock Augmentation in Fuel Rich Solid Explosives. Propellants Explos Pyrotech 2010;35(1):57–65. [18] Elbasuney S. Novel Colloidal Nanothermite Particles (MnO2/Al) for Advanced Highly Energetic Systems. J Inorg Organomet Polym Mater 2018;28(5):1793–800. [19] Elbasuney S, et al. Infrared Signature of Novel Super-Thermite (Fe2O3/Mg) Fluorocarbon Nanocomposite for Effective Countermeasures of Infrared Seekers. J Inorg Organomet Polym Mater 2018;28(5):1718–27. [20] Elbasuney S, et al. Super-Thermite (Al/Fe2O3) Fluorocarbon Nanocomposite with Stimulated Infrared Thermal Signature via Extended Primary Combustion Zones for Effective Countermeasures of Infrared Seekers. J Inorg Organomet Polym Mater 2018. [21] Elbasuney S, et al. Chemical stability, thermal behavior, and shelf life assessment of
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Combustion wave of metalized extruded double-base propellants Sherif Elbasuney, Ahmed Fahd
T
School of Chemical Engineering, Military Technical College, Kobry El-Kobba, Cairo, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Double-base propellant Combustion Burning rate Aluminum Ballistic performance
Aluminum metal fuel, with combustion heat of 32,000 J/g, is of interest as high energy density material. Aluminum is able to react not only with free oxygen but also with inert combustion gaseous products. This study reports on the impact of ultrafine aluminum particles on combustion characteristics of double-base propellant. Metalized double-base propellant formulations were developed by solventless extrusion process. Combustion characteristics were evaluated using small-scale ballistic evaluation test motor. There was an increase in average operating pressure, burning rate, and characteristic exhaust velocity C∗ with aluminum content. Even though there was no oxidizer available for aluminum oxidation; aluminum alone offered 7.6% increase in average operating pressure, 24.6% increase in burning rate, 7.8% increase in C∗. The main outcome of this study is that this superior performance was achieved at 4 wt% solid loading level. Aluminum particles could alter the combustion wave by increasing the combustion flame temperature, average operating pressure inside the combustion chamber, thermal conductivity of DB propellant. All these combustion criteria could decrease the thickness of the dark zone, allowing the luminous flame to be more adjacent to the burning surface; therefore the reaction could proceed faster. Even though the developed metalized DB formulations were found to be more energetic with an increase in calorific value by 4.6%; they demonstrated good thermal behavior to reference formulation using DSC. These findings confirmed that aluminum is an outstanding energetic ingredient for solid rocket propulsion. It has unique ability to react with inert gases generating substantial heat output.
(2)
1. Introduction
2Al(s) + 3CO(g)
The maximum heat of combustion of double-base propellant is generally limited by the enthalpy of formation of reaction products (CO2, H2O) [1–3]. Higher combustion energies and energy density can be obtained by combusting metal fuels, such as Mg, Al, and B [4]. The considerable strength and energetic formation of the metal-oxygen, account for the excellent fuel properties [5]. Metallic and non-metallic fuels can offer massive heat output up to 70 KJ/g (Fig. 1) [4]. Recent developments in the area of metallic fuels have largely focused on ultrafine powder [6–7]. Exhaust toxicity limits the use of beryllium [8–9]. High cost of boron limits its extended applications [5,10]. Aluminum offers many advantages in terms of performance, cost, availability, safety, as well as chemical stability [11–12]. Aluminum, with combustion heat of 32,000 J/g, is the most common fuel in use in the field of rocket propulsion technologies [13–14]. Aluminum is able to react not only with free oxygen; but also it has unique ability to react with combustion inert gaseous products (i. e. CO2, CO, and N2) (Eqs. (1)-(5)) [12,15].
2Al(s) + 3H2 O(g)
Al2 O3 (s) + 3H2 (g) + 866kJ/mol
(3)
2Al(s) + 3CO2 (g)
Al2 O3 + 3CO + 741kJ/mol
(4)
2Al(s) + 3/2O2
Al2 O3 (s) + 1700 KJ/mol
(1)
2Al(s) + N2 (g)
Al2 O3 + 3 C+ 1251 kJ/mol
2AlN + 346 kJ/mol
(5)
Ultrafine-aluminum can imitate this series of exothermic reactions [5,16–17]. Therefore, a large amount of heat would be liberated with minimum consumption of oxygen [18–21]. One of the main features of aluminum is high chemical stability. The surface of aluminum particles is coated with a protective layer of oxide of 5 nm (Fig. 2) [5,22–24]. This tight surface oxide layer (Al2O3) can prevent the inner metal from further oxidation [5]. Consequently aluminum particles can be stored for extended periods without loss of reactivity [25]. All these features can offer aluminum particles an effective role in the development of DB propellants with wide range of burning rate and specific impulse [26–32]. DB propellants have wide applications in tactical missiles due to their superior mechanical properties, aging capabilities, and good operational characteristics [26–27,33–34]. They have recently been used in booster, sustainer, and dual thrust rocket motors [35].
E-mail address: [email protected] (S. Elbasuney). https://doi.org/10.1016/j.fuel.2018.10.018 Received 5 February 2018; Received in revised form 25 September 2018; Accepted 2 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
Fuel 237 (2019) 1274–1280
S. Elbasuney, A. Fahd
Fig. 1. Gravimetric and volumetric heats of oxidation for metallic and non-metallic additives.
decomposition reactions occurs very rapidly. IV. Dark zone (induction zone) In this zone, very slowly oxidation reactions occur. This zone length lies in the range 2–20 mm depending on the operating pressure [44–45]. V. Flame zone In this zone, the final combustion products are formed, where they reach the thermal equilibrium state. Aluminum metal fuel can positively impact the combustion flame temperature and increasing the operating pressure. Aluminum particles have the potential to induce highly exothermic reactions in the dark zone (induction zone), decreasing its thickness, and hence could increase the burning rate [46–48]. Furthermore aluminum can enhance the thermal conductivity of DB propellant allowing the combustion process to proceed faster. In this study ultra-fine aluminum particles of 5 µm average particle size were developed by wet milling. The starting aluminum particles (particle size ≤ 32 μm) were dispersed in 250 ml ethanol. The colloidal particles were ground using ball mill Retch 100 for 12 h. Ethanol was employed as the grinding medium in order to dissipate any generated heat (during grinding process) and to protect ultra fine aluminium from further oxidation. The size of developed aluminum particles was visualized using scanning electron microscope SEM, Zeiss EVO-10 by Carl Zeiss Corporation. Colloidal aluminum particles were integrated into DB propellant formulation using screw extrusion process. The impact of Al metal fuel on combustion characteristics as well as ballistic performance, particularly on the operating pressure, burning rate, and C* were evaluated using small-scale ballistic evaluation test motor; which is established as the most representative mean for combustion characteristic evaluation [8]. Aluminum particles demonstrated a dual effect by increasing the average operating pressure inside the rocket motor and increasing the burning rate. These combustion characteristics resulted an enhanced characteristics exhaust velocity C* by 24.6%.
Fig. 2. Aluminium nanoparticles with passivated layer of Al2O3 [12]
The burning rate and specific impulse (thrust per unit weight of propellant) are the main parameters which need to be precisely optimized [8,36–38]. High reaction rates and specific impulse can be achieved by increasing the combustion temperature, an/or operating pressure inside the rocket motor [39–41]. It is widely accepted that the combustion wave of DB propellant consists of five distinctive zones (Fig. 3) [42–43]. I. Heat conduction zone No chemical changes occur at this zone; thermal effect by heat conduction from the burning surface causes temperature increase to the onset decomposition temperature. II. Solid phase reaction zone It is a very thin zone with slightly exothermic reactions. Temperature in this zone is approximately equal to the burning surface temperature (Ts). This zone length is 2–3 mm.
2. Theoretical evaluation of combustion characteristics
III. Fizz zone
Thermochemical methods have been widely accepted as a valuable fast tool to predict the combustion characteristics of different solid
This zone is adjacent to the burning surface; where a series of 1275
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Fig. 3. Distinctive combustions zones of DB propellant with associated chemical reaction [11]. Table 1 The chemical composition of the investigated DB propellant formulations. Formulation
NC (wt %)
NG (wt %)
Stabilizers (wt %)
Other additives (wt %)
Al (wt %)
F0 F1 F2 F3
56.20 55.07 54.51 53.95
28.55 27.98 27.69 27.41
2.1 2.06 2.04 2.02
13.15 12.89 12.76 12.62
0 2 3 4
(Reference) (2 wt% Al) (3 wt% Al) (4 wt% Al)
propellant compositions [12,49–50]. The impact of Al metal fuel on the combustion characteristics of DB propellants particularly the combustion temperature (Tc) and specific impulse, were evaluated using thermodynamic code named ICT (Institute of Chemical Technology in Germany, version 2008). This code is based on the chemical equilibrium and steady-state burning model; which is based on two methods of calculation: frozen equilibrium and shifting equilibrium. The employed frozen equilibrium model is based on the assumption that the composition is invariant (the product composition at the nozzle exit is identical to that in combustion chamber). The chemical compositions of the investigated DB propellant formulations are listed in Table 1. Thermochemical calculations demonstrated an increase in the combustion temperature and specific impulse with the increase of aluminum metal fuel content. Fig. 4 demonstrates the theoretical impact of aluminum particles on specific impulse and combustion temperature of DB propellant. The great impact of aluminum particles on specific impulse and combustion temperature could be assigned to the exothermic oxidation of aluminum with combustion heat of 32,000 J/g. This criteria could enhance the combustion flame temperature and increase the operating pressure inside the rocket motor [35,51]. Aluminum can also enhance the heat transfer from the burning zone to the adjacent layers of unreacted composition due to its high thermal conductivity [52]. Thus Al has the potential to speed up the burning rate of DB propellant formulations.
Fig. 4. Theoretical impact of aluminum particles on combustion characteristics of DB propellant.
aluminum particles are represented in Fig. 5. MDB propellant formulations, based on different percentages of Al metal fuel (Table 1), were manufactured by screw extrusion. Screw extrusion technique emphasizes mixing of different ingredients to molecular level, good homogenization, high density, and dimensional stability [33]. This technique includes many stages in consequence for instance: blending, rolling, grinding, granulation, and finally extrusion to obtain grains of desired shape and dimensions (Fig. 6). The maximum solid loading level was limited to 4 wt% to secure DB propellant formulations with good homogeneity and integrity (free from defects and cracks). For each investigated formulation, a tubular specimen of accurate dimensions 29 mm OD, 10 mm ID, and 200 mm length was manufactured for combustion characteristics evaluation using small-scale ballistic evaluation test motor. 4. Combustion characteristics of metalized DB propellants
3. Manufacture of tested MDB propellant formulations
Small-scale ballistic evaluation motor is widely accepted to measure combustion characteristics of solid propellants during static firing tests [8,33]. The burning rate and the specific impulse are the main parameters which need to be precisely optimized and evaluated during
Colloidal aluminum particles with an average particle size of 5 µm were developed by wet milling. SEM micrographs of developed 1276
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Fig. 5. SEM micrographs of aluminum particles developed by wet milling.
Fig. 6. Schematic diagram for solventless extrusion process.
Fig. 8. Practical static fire test using small scale rocket motor.
combustion process to meet specific propulsion requirements (booster, sustainer, or dual thrust rocket motor). Combustion characteristics of metalized DB propellant formulations were evaluated using one-inch test motor (33 mm ID and 225 mm length) (Fig. 7). The pressure (p) inside the combustion chamber was recorded with time (t) during static firing process. The moment of static fire test is represented in Fig. 8. On the basis of p(t) curve, the average operating pressure, burning time, burning rate, and characteristic exhaust velocity can be evaluated [53]. All developed metalized DB propellant formulations exhibited ideal stable burning during the static firing process. It is obviously clear that there is an increase in the average operating pressure and burning rate with Al content (Fig. 9).
It is apparent that Al metal fuel had a dual effect by increasing the average operating pressure inside the rocket motor and increasing the burning rate. This effect was illustrated by the increase in the combustion temperature as well as the increase in thermal conductivity of DB propellants which accelerate the heating of the next burning surfaces [48]. The main combustion characteristic parameters, including burning rate, average operating pressure, and total impulse (IPT) of the developed formulations, are summarized in Table 2. The total area under the P-t curve (IPT) was calculated to retrieve the characteristic exhaust velocity using Eq. (6).
C =
IPT At Wt
Fig 7. Schematic for employed small-scale ballistic evaluation test motor during static firing test. 1277
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Fig. 9. Pressure-time curves of developed aluminized MDB propellants. Table 2 Results on ballistic (performance) parameters of reference DB propellant formulation and MDB developed propellant formulations using the one-inch small-scale ballistic test motor.
Fig. 10. Impact of Al metal fuel on MDB characteristic exhaust velocity & burning rate.
where: C*, IPT, At, Wt are the characteristic exhaust velocity, the total impulse, throat area, and propellant mass respectively. Al particles demonstrated dramatic increase in burning rate as well as C* (Fig. 10).
F3 with 4 wt% Al offered enhanced burning rate and C* by 17.26% and 10.52% respectively. Aluminum particles are able to react not only with free oxygen but also they are also able to react with inert 1278
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Fig. 11. Flame behavior of double base propellant under different pressure values [56–57].
Fig. 12. DSC thermograms for metalized DB propellant to reference formulation.
can decrease the dark zone thickness allowing the combustion flame to become more adjacent to the burning surface (Fig. 11). Fig. 10 demonstrated how aluminum particles can alter the combustion wave of DB propellant by increasing the combustion flame temperature, thermal conductivity of DB propellant, and average operating pressure inside the rocket motor. All these criteria could result in a decrease in the dark zone thickens allowing the luminous flame zone adjacent to the burning surface [58].
Table 3 Auto-ignition temperature, Calorific value, and DSC outcomes of metalized DB propellant to reference formulation. DSC results
F0 (Reference) F3 (4 wt% Al)
Total heat released [J/g]
Max. peak temperature [°C]
1591 1655
194.48 191.85
Calorific value (Qv) kJ/g
Ignition temperature [°C]
Density [g/cm3]
3891 4071
171 172
1.612 1.619
5. Thermal behavior of metalized DB propellant The impact of ultrafine aluminum particles on the thermal stability as well as thermal behavior of DB propellant was evaluated. The autoignition temperature was measured using cook off test; the calorific value was evaluated using bomb calorimeter; thermal behavior was investigated using DSC. While metalized DB propellants exhibited an increase in calorific value; they exhibited onset decomposition temperature similar for reference formulation. This thermal behavior was further verified using DSC (Fig. 12). Aluminum particles did not greatly impact the maximum peak temperature; but greatly impact the total heat released by 65 J/g. The
decomposition gaseous products and add much more heat to the combustion process [54–55]. Aluminum particles, with high heat of combustion and excellent thermal conductivity, have the potential to positively impact the burning rate [35]. The high heat output of ultra-fine aluminum particles could inhanced the increased the average operating pressure inside the rocket motor. The operating pressure and flame temperature are the most effective factors to control the combustion mechanism [48]. It has been reported that the increase in operating pressure inside the rocket motor 1279
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main thermal properties including calorific value, auto ignition temperature, and DSC outcomes of developed metalized DB propellant to reference formulation, are tabulated in Table 3. While, metalized DB propellant formulations were found to be more energetic; they exhibited similar thermal behavior to the reference formulation. 4 wt% Al offered enhanced combustion heat (C*) by 10.5%.
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6. Conclusion Ultrafine aluminum particles are able to react with inert gases. Consequently, they have the potential to enhance the combustion flame temperature, increase the operating pressure inside the rocket motor. Furthermore they can improve the thermal conductivity of DB propellants. Through these features aluminum particles can alter the combustion mechanism by thinning the dark zone allowing the luminous flame zone to be more adjacent to the burning surface. Ultra-fine aluminum particles positively impacts the burning rate, average operating pressure, total impulse, characteristic exhaust velocity, heat of combustion by 24.6%, 7.6%, 31.6%, 7.8% and 4.6% respectively. Even through being more energetic metalized DB propellants offered accepted thermal stability and thermal behavior to reference formulation. This paper highlights the fact that controlled combustion characteristics and tailored ballistic parameters can be controlled via effective content of ultra-fine aluminum particles. References [1] Sutton GP, B.O. Rocket propulsion elements. New York: editors: John Wiley & Sons; 2001. [2] Dreizin EL. Metal-based reactive nanomaterials. Prog Energy Combust Sci 2009;35(2):141–67. [3] Sutton GPaOB. Rocket Propulsion elements. Seventh ed. New York: John Wiley & Sons; 2001. [4] Yen NH, Wang LY. Reactive Metals in Explosives. Propellants Explos Pyrotech 2012;37(2):143–55. [5] Conkling J, Mocella C. Chemistry of Pyrotechnics Basic Principles and Theory. Second ed. London: CRC; 2012. [6] Bönnemann H, Richards RM. Nanoscopic Metal Particles-Synthetic Methods and Potential Applications. Eur J Inorg Chem 2001:26. [7] Zhi J, et al. Research on the Combustion Properties of Propellants with Low Content of Nano Metal Powders. Propellants Explos Pyrotech 2006;31(2):139–47. [8] Sutton GP, Biblaz O. Rocket Propulsion elements. Seventh ed. New York: John Wiley & Sons; 2001. [9] Koch E-C, Klapötke TM. Boron-Based High Explosives. Propellants Explos Pyrotech 2012;37(3):335–44. [10] Agrawal JP. Pyrotechnics. In: High Energy Materials. Wiley-VCH Verlag GmbH & Co. KGaA; 2010. p. 331–412. [11] Sadek R, et al. Novel yellow colored flame compositions with superior spectral performance. Defence Technol. 2017;13(1):33–9. [12] Mohamed AK, Mostafa HE, Elbasuney S. Nanoscopic fuel-rich thermobaric formulations: chemical composition optimization and sustained secondary combustion shock wave modulation. J Hazard Mater 2016;301:492–503. [13] Vadhe PP, et al. Cast aluminized explosives (review). Combustion Explosion Shock Waves 2008;44(4):461–77. [14] Elbasuney S, Fahd A, Mostafa HE. Combustion characteristics of extruded double base propellant based on ammonium perchlorate/aluminum binary mixture. Fuel 2017;208:296–304. [15] Talawar MB, et al. Emerging Trends in Advanced High Energy Materials. Combustion Explosion Shock Waves 2007;43(1):11. [16] Krier HJ, Peuker JM, Glumac N. Aluminum Combustion in Aluminized Explosives: Aerobic and Anaerobic Reaction. In: 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. Orlando, Florida; 2011. [17] McNesby KL, et al. Afterburn Ignition Delay and Shock Augmentation in Fuel Rich Solid Explosives. Propellants Explos Pyrotech 2010;35(1):57–65. [18] Elbasuney S. Novel Colloidal Nanothermite Particles (MnO2/Al) for Advanced Highly Energetic Systems. J Inorg Organomet Polym Mater 2018;28(5):1793–800. [19] Elbasuney S, et al. Infrared Signature of Novel Super-Thermite (Fe2O3/Mg) Fluorocarbon Nanocomposite for Effective Countermeasures of Infrared Seekers. J Inorg Organomet Polym Mater 2018;28(5):1718–27. [20] Elbasuney S, et al. Super-Thermite (Al/Fe2O3) Fluorocarbon Nanocomposite with Stimulated Infrared Thermal Signature via Extended Primary Combustion Zones for Effective Countermeasures of Infrared Seekers. J Inorg Organomet Polym Mater 2018. [21] Elbasuney S, et al. Chemical stability, thermal behavior, and shelf life assessment of
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