EFFECTS OF METAL OXIDES ON THE BURNING RATE OF AMMONIUM NITRATE BASED SOLID ROCKET PROPELLANTS

1

XX AIDAA Congress Milano, Italy, 29 June – 03 July 2009

EFFECTS OF METAL OXIDES ON THE BURNING RATE OF AMMONIUM NITRATE BASED SOLID ROCKET PROPELLANTS

Matteo Boiocchi, Laura Merotto, Luciano Galfetti, Giovanni Colombo, and Luigi T. De Luca

Space Propulsion Laboratory, SPLab Dipartimento di Ingegneria Aerospaziale

Politecnico di Milano, Campus Bovisa, Via La Masa 34, 20156 Milano, Italy e-mail: [email protected]

Keywords: Solid Propellants, Burning Rate, Ammonium Nitrate, Metal Oxides.

Abstract. The need to reduce costs and environmental pollution of space launchers suggests to reconsider the use of ammonium nitrate (AN)-based solid propellants. Problems connected with AN phase transitions, low specific impulse and agglomeration phenomena in aluminized propellants are well known peculiarities of this class of propellants, detrimental for solid rocket applications. For this reason phase-stabilized ammonium nitrate (PSAN) propellants were suggested, as well as dual-oxidizer mixtures, in order to overcome the mentioned draw-backs. Aim of this paper is to investigate the possibility to obtain better performance (higher propellant burning rate and lower pressure sensitivity) of AN-based propellants. More than fifty formulations were considered and experimentally characterized in order to optimize the ballistic behavior of AN-based formulations. Natural and synthetic elastomeric binders were used for all the compositions. The effect of several oxides (BaO2, MnO2, PbO2, Sb2O3) was tested. Manufacture procedures were shown to strongly affect the propellant behavior from a ballistic and mechanical point of view. The best results were obtained when BaO2, Sb2O3 and PbO2 were used as additives, allowing a pressure sensitivity exponent in the range 0.43-0.46 over the pressure interval from 20 to 70 bar.

mailto:[email protected]

M. Boiocchi, L. Merotto, L. Galfetti, G. Colombo and L.T. De Luca

2

1 INTRODUCTION Solid rocket motors will play a fundamental role in the future of space exploration: the lev-

el of thrust they can supply, together with their intrinsic simplicity of design if compared to liquid systems, make them still attractive especially for heavy launchers. From this point of view, the need to smooth the cost of these systems looking at new propellant compositions, making use of cheap ingredients, becomes a mandatory goal. A first way is to replace the usu-al Ammonium Perchlorate (AP, NH4ClO4) with Ammonium Nitrate (AN, NH4NO3) [1], [2], [3] as oxidizer. However it is well known that, in terms of performance, pure AN cannot be compared to AP because of the low burning rate one obtains. On the same path, the possibility to replace Hydroxyl Terminated Polybutadiene (HTPB) with new generation binders is cur-rently under investigation at the Space Propulsion Laboratory of Politecnico di Milano (SPLab). This work aims at investigating the performance of AN-based propellants using syn-thetic Polyisoprene [4], [5], that is to say natural rubber, and Hydrogenated Acrylonitrile-Butadiene Rubber as binders, and nano-aluminum as fuel. Moreover, the effect of many addi-tives was analyzed. The final aim of this investigation was to evaluate the possibility to pro-duce cheap propellants without losing too much in performance [6]. For this purpose the use of Nano-Aluminum [7] and additives becomes mandatory, to increase the burning rate or to reduce the ballistic exponent.

2 EXPERIMENTAL SET UP

The burning rate tests were performed in a 0.8 l steel bomb, at constant pressure (provided by a system of electro-valves), under nitrogen inert atmosphere. The sample ignition was de-manded to a Nickel Chrome hot-wire. The burning rate was measured by a non-intrusive opti-cal technique: a high-speed camera allowed to record videos of the combustion and a proprietary software, capable to recognize the burning surface location, allowed an easy measurement of the burning rate. Propellants samples were square in section (5x5x30 mm) and long enough to exclude ignition and extinction transients from the measurement.

3 INGREDIENTS AND PREPARATION OF SAMPLES For this work purposes many different propellants were produced and analyzed in our la-

boratory. Each propellant is made up of an oxidizer (Ox), a fuel (F), a polymeric binder (PB) and an additive (Ad). The complete list of the ingredients considered in this work, together with their main properties, are summarized as follows:

Oxidizers (Ox): Ammonium Nitrate (NH4NO3, AN); Potassium Nitrate (KNO3, KN). Fuels (F): Aluminum Powders (Al_01_b; Al_02_c; Al_04_b). Polymeric binders (PB): Polyisoprene; Hydrogenated Acrylonitrile-Butadiene Rubber

(HNBR Therban KA). Additives (Ad): Potassium Dichromate (K2Cr2O7, PD); Ammonium Dichromate

((NH4)2Cr2O7, AD); Potassium Permanganate (KMnO4, PP); Manganese Oxide (IV) (MnO2); Iron Oxide (III) (Fe2O3); Lead Oxide (IV) (PbO2); Antimony Oxide (III) (Sb2O3); Barium Ox-ide (IV) (BaO2); Magnesium Hydride (MgH2); Hexamethylenetetramine (C6N4H12, HMTA).

All additives were utilized “as received” without further purification. Besides the sieving of the AN and KN powders, the first step of the propellant manufactur-

ing was the binder preparation: to permit an easier workability, both kinds of rubbers were subdued to a swelling treatment in mineral oil. Two different procedures were used: in the

https://www.researchgate.net/publication/280830100_Dual-Oxidizer_Solid_Rocket_Propellants_for_Low-Cost_Access_to_Space?el=1_x_8&enrichId=rgreq-c2d89b22-dfc0-40d7-b1a0-aaf7026b3a31&enrichSource=Y292ZXJQYWdlOzI4MDkzOTMzNztBUzoyNjMxMTg5MTU4OTUyOTZAMTQzOTc0Mzg0NjkxNA==


3

first one the rubber was cut in very small pieces and the swelling was demanded totally to the effect of the oil diffusion inside the rubber. In the latter it was strongly fastened by adding a volatile solvent, whose swelling power is much higher than that of mineral oil. This second procedure was completed by a thermal extraction of the total amount of volatile solvent add-ed.

The advantage of the second procedure is its ability to enhance homogeneity, repeatability and workability of the binder to the high swelling performance of the volatile solvent. Two solvents were chosen for their volatility and chemical compatibility with the ingredients: cy-clohexane and acetone, respectively for polyisoprene rubber and HNBR rubber, have been used for this work.

The mixing of the ingredients was performed in three different ways: o Mechanical Mixing (MM): performed by a mechanical stirrer. o Heavy Load Mechanical Mixer (HLMM): performed by an industrial mixing machine.

This procedure did not prove to be effective with HNBR, as the propellants obtained could never be ignited.

o Post-Swelling (PS): a further swelling is performed in presence of the other components, thus obtaining a compact compound through evaporation of the volatile solvent. A com-plete extraction of the solvent was necessary in order to permit a correct curing process of the rubber by vulcanization with Sulfur.

Sulfur and the vulcanization accelerant (zinc N-pentamethylene dithiocarbamate) were added by mixing to the other chemical substances to obtain an homogeneous compound. The mixture obtained was then placed in a teflon mould, and submitted to a thermal treatment (at 90°C for 14 hours) that permitted the vulcanization of the rubber [8].

4 RESULTS AND DISCUSSION At the beginning of the experimental campaign three different propellants, with pure AN as

oxidizer and Polyisoprene as binder, but with different additives, were investigated. The addi-tive percentage was in the order of 5% (but at the same molar metal content), except in the case of IS-01, which was additive free being the reference propellant. Tested compositions are summarized in Table 1.

Ox F PB Ad Preparation IS_01 AN Al_04_b Polyisoprene - PR IS_02 AN Al_02_c Polyisoprene BaO2 MM IS_03 AN Al_02_c Polyisoprene PbO2 MM IS_04 AN Al_02_c Polyisoprene Sb2O3 MM

Table 1: Details of the composition for the Polyisoprene-based propellants. First set of propellants considered in this study.

The steady burning rate results are shown in Fig. 1.

https://www.researchgate.net/publication/239140938_Vulcanization_of_cis-_and_trans-polyisoprene_and_their_blends_Cure_characteristics_and_crosslink_distribution?el=1_x_8&enrichId=rgreq-c2d89b22-dfc0-40d7-b1a0-aaf7026b3a31&enrichSource=Y292ZXJQYWdlOzI4MDkzOTMzNztBUzoyNjMxMTg5MTU4OTUyOTZAMTQzOTc0Mzg0NjkxNA==


4

Figure 1: Burning rate results for the Polyisoprene-based propellants: effects of metal oxides. First set of propellants considered in this study.

The effect of metal oxides as burning rate modifiers is clearly pointed out comparing the

curves of IS_02, IS_03 and IS_04 with IS_01: a strong effect is noticed for PbO2 and Sb2O3 [9] (at 70 bar a burning rate increase of near 150 % was gained). The effect on the ballistic exponent is not equally evident (11% higher when using Sb2O3).

An opposite effect is shown by the propellants series reported in Table 2.

Ox F PB Ad Preparation

IS_05 AN Al_01_b Polyisoprene - HLMM IS_06 AN Al_01_b Polyisoprene MnO2 HLMM IS_07 AN Al_01_b Polyisoprene MnO2+PD HLMM

Table 2: Details of the composition for the Polyisoprene-based propellants. Second set of propellants considered in this study.

The effect of MnO2 on the burning rate reduction is evident (20% at 70 bar), even when

coupled with PD. Moreover, inserting a small amount of PD causes a significant ballistic ex-ponent increase (21% higher for IS_07 than IS_06, see Fig. 2).


5

Figure 2: Burning rate results for the Polyisoprene-based propellants Effects of metal

oxides. Second set of propellants considered in this study. In order to explain the different ballistic behavior found for the investigated propellants,

some XRD measurements were carried out for a group of propellants. For each propellant formulation, the amount of metal oxide (MeOx) introduced in the formulation is known (Meintroduced). XRD measurement allows to determine the amount of metal Me remaining in the condensed residues collected after the propellant combustion (Memeasured).

The ratio between Mintroduced and Mmeasured is then related to the metal boiling point (bp) and to the ballistic exponent obtained from the burning rate measurements. Meintroduced/Memeasured appears to increase as bp decreases. On the other hand, n decreases with decreasing bp. It is apparent that the physical nature of the metal oxide used to fill a propellant is strictly related to the propellant pressure sensitivity: metals having lower boiling points give propellants with lower n (lower sensitivity to pressure).

Table 3 reports the metal boiling point, the ratio Meintroduced/Memeasured and the ballistic ex-ponent for a significant group of the investigated formulations.

Propellant Metal

(Me) bp

[K] Meintroduced/Memeasured n

IS_06 Mn 2097 2,8 0,68 IS_02(*) Ba 1600 - 0,42 IS_03 Pb 1740 42,0 0,44 IS_04 Sb 1635 106,9 0,47

Table 3: Metal oxide additives: boiling points, XRD measurements results, ballistic exponent of the corresponding propellant.


6

To explain the inverse proportionality of Meintroduced/Memeasured with the metal boiling point bp, one must take into account the physical phenomena involved. If the flame temperature is lower than the metal boiling point bp, a high amount of Me will result in the condensed com-bustion residues, hence giving a low Meintroduced/Memeasured ratio; if the flame temperature is higher, the opposite trend will be observed.

The influence of the gasified metals on the overall flame structure, and thus on the ballistic exponent, is a very complex phenomenon. With increasing the flame front temperature, an increased concentration of the radicals in the flame front zone near the burning surface is ob-served. A flame rich in radical species is pressure sensitive, so its ballistic exponent n is high. If the metal boiling point bp is low, vaporization and radical species depletion is easier, and therefore their molar concentration will decrease. To decrease radical fraction means to re-duce the ballistic coefficient n. That is why n exponents for IS_02, IS_03 and IS_04 are high-er than that of IS_06: lower Ba, Pb and Sb boiling points make possible a more complete transition in a gas state than Mn will do. On the other hand, if Pb vaporization is larger than that of Mn, one can expect to find a lower amount of Pb than of Mn in condensed combustion products: this is the interpretation concerning Meintro/Memeasured ratio.

When aluminum was totally replaced by Magnesium Hydride, a huge augmentation of burning rate (150% higher at 70 bar), together with a significant reduction of the ballistic co-efficient (35%) was obtained, due to the high reactivity of Magnesium and Magnesium Hy-dride. Lower effects (burning rate doubled) were reached with partial replacement of ammonium nitrate with HMTA [10] (Table 4 and Fig. 3).


IS_08 AN - Polyisoprene MgH2 MM IS_09 AN Al_01_b Polyisoprene HMTA MM

Table 4: Polyisoprene propellants, Magnesium Hydride and Hexamethylentetramine compositions. Third set of propellants considered in this study.


7

Figure 3: Burning rate results for the Polyisoprene-based propellants Effects of MgH2 and C6N4H12. Third set of propellants considered in this study.

The following series of propellants (Table 5, Fig. 4) shows the effects of a partial replace-

ment of AN with KN [11]: the effect of a higher amount of KN on the ballistic coefficient increase is definite (90%), leading to very high burning rate at high pressure (50% higher at 70 bar). The burning rate is further increased (25% at 70 bar with respect to IS_11) with the introduction of PD (IS_12), obtaining a desirable reduction of the ballistic exponent (decrease of 25% with respect to IS_11) . An even higher burning rate (120% at 70 bar) is obtained by means of a complete replacement of Al_04_b aluminum powder (IS_11) with the finer Al_02_c powder (IS_13).


IS_01 AN Al_04_b Polyisoprene - PR IS_10 AN+10% KN Al_04_b Polyisoprene - PR IS_11 AN+15% KN Al_04_b Polyisoprene - PR IS_12 AN+15% KN Al_04_b Polyisoprene PD PR IS_13 AN+15% KN Al_02_c Polyisoprene - MM IS_14 AN+10% KN Al_02_c Polyisoprene - MM IS_15 AN+10% KN Al_02_c Polyisoprene - PS

Table 5: Details of the composition for the Polyisoprene-based propellants. Effects of oxidizers: AN and KN.

Fourth set of propellants considered in this study.


8

Figure 4: Burning rate results for the Polyisoprene-based propellants: effects of oxidizers: AN and KN. Fourth set of propellants considered in this study.

A further step concerning tests of Polyisoprene rubber-based propellants, involved the in-

fluence of the manufacture procedure: two propellants (IS_14 and IS_15) with the same nom-inal composition but different manufacture procedure (Post Swelling and Mechanical Mixing) were compared. The effect seems to be negligible; moreover, a further confirmation of the ballistic coefficient augmentation (20% for a 5% increase in KN) was observed (IS_13), be-cause of a higher amount of KN,.

As far as the Therban KA rubber is concerned as binder, the positive effect of PP on the ballistic behavior of pure AN-based propellants is clear (reduction of 9% of n, while the burn-ing rate is 60% higher), especially if compared to PD (20% higher n compared to TK_1, for a slight burning rate increase, 10%).


TK_1 AN Al_04_b HNBR - PS TK_2 AN Al_04_b HNBR PD PS TK_3 AN Al_04_b HNBR PP PS

Table 6: Details of the composition for the Therban -based propellants.

PD: Potassium Dichromate (K2Cr2O7); PP: Potassium Permanganate (KMnO4,) Fifth set of propellants considered in this study.


9

Figure 5: Burning rate results for the Therban-based propellants of Table 6. Fifth set of propellants considered in this study.

The joint effect of KN and additives is summarized in Fig. 6: the effect of 15% of KN is to

increase the burning rate (55% at 70 bar) but, unfortunately, a ballistic exponent increase (6%) is observed. A combined effect of burning rate increase (155% at 70 bar) and ballistic exponent reduction (9%) can be gained by means of the additives use, with the PP being the more effective.


TK_4 AN+15% KN Al_04_b HNBR - PS TK_5 AN+15% KN Al_04_b HNBR PD PS TK_6 AN+15% KN Al_04_b HNBR PP+PD PS TK_7 AN+15% KN Al_04_b HNBR PbO2 PS TK_8 AN+15% KN Al_04_b HNBR PP PS

Table 7: Details of the composition for the Therban-based propellants. Sixth set of propellants considered in this study.


10

Figure 6:. Burning rate results for the Therban-based propellants of Table 7. Sixth set of propellants considered in this study.

From the results obtained, some conclusions can be drawn. A direct comparison of the propellants behavior seems to suggest the following trend: adding the propellant compound with metal oxides of low fusion and ebullition temperatures (such as PbO2, BaO2 and Sb2O3), a value for the ballistic exponent in the range 0.43 - 0.48 is obtained, while when using Man-ganese and Chromium compounds the experimental value of n becomes higher, in the range 0.7 - 0.8. Moreover, only the oxides of the first group allow a burning rate increase. This result can be explained considering that oxide particles in the flame region immediately close to the burning surface, where temperature is still relatively low, are in a strongly reduc-ing environment because of aluminum and the continuous polymer degradation to smaller agents as aldehydes and ketones. Metal oxides reduce freeing the metals, which vaporize at lower temperatures, intercepting the radicals (OH, H, O, CHx), which are responsible of the propellant high pressure sensitivity. This clarifies the role of vaporized metals in combustion regularization and stabilization. This mechanism implies that oxides and metals, characterized by a low fusion and ebullition temperature, are more effective in capturing the radicals, thus lowering the value of n. This is confirmed by XRD analysis of the combustion residues, which shows how the metal fraction almost completely disappears for the PbO2, BaO2 and Sb2O3 based propellants, while it survives for MnO2 based propellants in the form of alumi-nates.

The same mechanism is proposed to explain the effect of aluminum powders size on the reduction of the ballistic exponent: in previous works carried out at SPLab [12] it was pointed out that finer Al particles lead to an increase of the burning surface temperature, causing a more effective decomposition of Potassium Nitrate in Potassium Oxide [13] and, finally, in metal particles. Finally, the positive effect of Magnesium Hydride can be easily explained if


11

one considers its decomposition in metallic Magnesium and active Hydrogen [14], which re-acts with the oxygen coming from AN decomposition. As far as the comparison between the two kinds of rubber is concerned, one can immediately see the increase in burning rate ob-tained by replacing the Polyisoprene rubber (propellant IS_11) with Therban KA ® (propel-lant TK_2). Currently, investigation of Therban XQ ® is in progress at SPLab, together with different metal oxides.

5 CONCLUSIONS AND FURTHER DEVELOPMENTS • AN-based propellants, added with different metal oxides, were investigated in this study

in order to obtain a better performance of this class of propellants (higher propellant burning rate and lower pressure sensitivity). More than fifty formulations were consid-ered and experimentally characterized in order to optimize the ballistic behavior. Natural and synthetic elastomeric binders were used for all the compositions. The effect of sever-al oxides was tested.

• Manufacture procedures were shown to strongly affect the propellant behavior from a ballistic as well as mechanical point of view.

• The best results were obtained when BaO2, Sb2O3 and PbO2 were used as additives, al-lowing a pressure sensitivity exponent in the range 0.43-0.46 over the pressure interval from 20 to 70 bar.

• To explain the experimental observation a reaction mechanism is proposed, which clari-fies the role of vaporized metals in combustion regularization and stabilization.

• The obtained results suggest to further investigate this field.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the important support of Prof. Febo Severini for helpful discussions.

REFERENCES [1] G. Singh, I.P. Singh Kapoor, S.M. Mannan and J. Kaur. Studies on Energetic Com-

pounds Part 8: Thermolysis of Salts of HNO3 and HClO4. Journal of Hazardous Mate-rials, A79 (2000), 1-18.

[2] V.A. Babuk, A. Glebov, V.A. Arkhipov, A.B. Vorozhtsov, G.F. Klyakin, F. Severini, L. Galfetti, and L.T. DeLuca. Dual-Oxidizer Solid Rocket Propellants for Low-Cost Ac-cess to Space. In-Space Propulsion, 10-IWCP edited by L. T. De Luca, R.L. Sackheim, and A.B. Palaszewski, Grafiche GSS, Bergamo, Italy, paper 15, Nov 2005.

[3] A. Vorozhtsov, V. Arkhipov, S. Bondarchuk, N. Popok, G. Klyakin, V. Babuk, V. Kuz-netsov, E. Sinogina, L.T. DeLuca, and L. Galfetti. Ballistic Characteristic of Solid Pro-pellant Containing Dual Oxidizer. European Conference for Aerospace Sciences (Eucass), Moscow, Russia 2005.


12

[4] D. Maghetti. Caratterizzazione sperimentale di propellenti solidi compositi con matrice a base isoprenica. Tesi di Laurea in Ingegneria Aerospaziale, Politecnico di Milano, Dipartimento di Energetica, SPLab, Milano, 2004.

[5] J. E. Mark, B. Erman, and F.R. Eirich. Science and Technology of Rubber. Academic Press, 1994.

[6] M. Boiocchi. Indagine sperimentale di propellenti innovativi a base di nitrato di ammo-nio e leganti elastomerici: formulazione e caratterizzazione balistica. Tesi di Laurea in Ingegneria Aerospaziale, Politecnico di Milano, Dipartimento di Energetica, SPLab, 2007.

[7] L. Merotto, Caratterizzazione chimico-fisica di polveri nanometriche di alluminio e cor-relazione con le proprietà balistiche di propellenti solidi alluminizzati. Tesi di Laurea in Ingegneria Aerospaziale, Politecnico di Milano, Dipartimento di Energetica, SPLab, Milano, 2005.

[8] P. Boochathum and W. Prajudtake. Vulcanisation of Cis and Trans-polyisoprene and their Blends: Cure Characteristics and Crosslink Distributions. European Polymer Journal, 37, 2001.

[9] S.R. Jain and C. Oommen. The Influence of Flame Retardants on the Thermal Stability of CIS-1,4-Polyisoprene. Journal of Thermal Analysis, Vol. 53, 1998.

[10] S. Cudzilo, P. Kohlicek, V.A. Trzcinski, and S. Zeman. Performance of Emulsion Ex-plosives. Combustion, Explosion, and Shock Waves, Vol. 38, No. 4, pp. 463-469, 2002.

[11] C. Oommen, and S.R. Jain. Phase Modifications of Ammonium Nitrate by Potassium Salts. Journal of Thermal Analysis and Calorimetry, Vol. 55, 1999, pp. 903-918.

[12] M. Mileo. Influenza della Composizione sulla Temperatura di Superficie di Propellenti Solidi per Propulsione Spaziale. Tesi di Laurea in Ingegneria Aerospaziale, Politecnico di Milano, Dipartimento di Energetica, SPLab, 2005.

[13] L.Y. Kashporov, L.A. Klyachko, N.A. Silin and E.S. Shakhidzhanov. Burning Mixtures of Magnesium With Sodium Nitrate. I. Burning Velocity of Two-Component Mixtures of Magnesium with Sodium Nitrate. Combustion, Explosion and Shock Waves, Vol. 30, No. 5, 1994.

[14] K. Bohmhammel, B. Christ and G. Wolf. Kinetic Investigations on the Basis of Iso-thermal DSC Measurement of Hydrogenation and Dehydrogenation of Magnesium Hy-dride, Thermochimica Acta 310 (1998), 167-171.

Documents

EFFECTS OF METAL OXIDES ON THE BURNING RATE OF AMMONIUM NITRATE BASED SOLID ROCKET PROPELLANTS