Theoretical analysis of hydrides in solid and hybrid rocket propulsion

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 6 0 e1 7 6 9

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Theoretical analysis of hydrides in solid and hybrid rocket propulsion Filippo Maggi a,b,*, Gabriela Gariani a,b, Luciano Galfetti a,b, Luigi T. DeLuca a,b a b

Politecnico di Milano, 34 via La Masa, 20156 Milan, Italy Aerospace Engineering Dept., Space Propulsion Laboratory, 34 via La Masa, 20156 Milan, Italy

article info

abstract

Article history:

Chemical rocket propulsion can benefit by using hydrides that are able to store high

Received 20 May 2011

volumes of hydrogen at ambient conditions that can be released during combustion. This

Received in revised form

paper offers a theoretical investigation concerning the use of hydrides as additives in

4 October 2011

hybrid fuels and solid propellants. Aluminum hydride is expected to generate interesting

Accepted 5 October 2011

performance gains but lack of commercial availability makes industrial application

Available online 26 October 2011

unfeasible. As a consequence, attention is focused on other simple and complex hydrides used in other fields and readily available. A comparative analysis of theoretical perfor-

Keywords:

mance of gravimetric and volumetric specific impulse, propellant average density, adia-

Solid propellant

batic flame features, and preliminary estimate of exhaust products is conducted. Eight

Hybrid fuel

different hydrides, potentially applicable as replacements for aluminum currently used in

Hydride

solid propellants and hybrid rocket systems are considered.

Theoretical performance

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Thermochemistry

reserved.

Rocket propulsion

1.

Introduction

Hydrogen is very often used in space propulsion systems. It is either burned inside a combustion chamber as a fuel with an oxidizer or used as a propellant, following heating and expansion through a gas dynamic nozzle (thermal electric, nuclear propulsion and so on). When hydrogen is used it is usually stored below its critical point as a cryogenic liquid at 21 K temperature and around 1 bar pressure, otherwise prohibitive tank volumes would be necessary. Use of simple and complex hydrides can be considered as a method of storing hydrogen in a safer, denser and more convenient way with respect to pressurized gas or cryogenic systems and, for this reason, their application in reversible and irreversible hydrogen-storing systems was widely investigated [1e4].

Since the 1960’s, solid propellants incorporating hydrides were widely studied. Alane was the most popular choice [5e9]. The application of hydrides as additives in advanced highenergy propellants is a research topic of current interest because they provide higher regression rates and specific impulse, as well as lower metal agglomeration. Some issues related to availability and stability are still unresolved though [8]. On the other hand, the use of hydrides in hybrid fuels delivers higher theoretical specific impulse. Moreover, it is expected to increase the burning rate, currently the main limitation for this technology [9e11]. Actual serviceability of hydrides in space rockets represents a challenge since aging and stability concerns are not yet fully resolved and shelf-life of final energetic products still represents an uncertainty. Although decomposition studies

* Corresponding author. Politecnico di Milano, 34 via La Masa, 20156 Milan, Italy. Tel.: þ390223998512; fax: þ390223998060. E-mail addresses: [email protected] (F. Maggi), [email protected] (G. Gariani), [email protected] (L. Galfetti), [email protected] (L.T. DeLuca). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.018

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Nomenclature Roman and Greek symbols hri average density, kg/m3 r density, kg/m3 Dhf enthalpy of formation at standard conditions, kJ/ mol g standard gravity acceleration, 9.81 m/s2 Is gravimetric specific impulse, s volumetric specific impulse, kg s/m3 Iv Mm molar mass, kg/mol m mass, kg _ m mass flow rate, kg/s T thrust, N Tc temperature in the combustion chamber, K DV velocity increment for Tsiolkovskii equation, m/s

demonstrate that several hydrides do not decompose at more than 100  C, some of them have a very limited compatibility with common space propulsion ingredients (oxidizer and binder ingredients) [12]. Moreover, a spontaneous dehydrogenation can occur if strict storage conditions are not maintained because these chemicals are highly sensitive to moisture [13,14]. A crystal of lithium-aluminum hydride, after one year of storage in ambient conditions, is shown in Fig. 1 to have dehydrogenated. One final consideration relates to environmental concerns. Even though the use of hydrogen-based compounds is likely to bring benefits to space propulsion performance, a warning was issued about the negative effects of H2 concentrations in the highest part of the atmosphere [15]. An analysis, by Solomon, of the chemistry of ozone depletion at high altitudes, showed that molecules from rocket plume exhaust products were present and involved in the chemical reactions leading to ozone degradation and depletion [16]. Hydrogen is part of this scenario. The quantity of pollutants generated by

Subscripts and superscripts 0 initial f final f fuel m molar p propellant Acronyms Add additive AP ammonium perchlorate B.P. boiling point CEA chemical equilibrium with applications CCP condensed combustion products HTPB hydroxyl-terminated polybutadiene LOx liquid oxygen

a rocket system depends on thrust (in the order of 105e107 N), meaning that some tons to hundreds of tons of propellant are consumed for one launch. Given the limited number of flights per year presently, a concern about environmental impact may be considered premature, but not from the perspective of future space exploitation for tourism purposes with suborbital flights [17,18]. Table 1 is list of good candidates for space propulsion. The list includes some simple and complex hydrides based on light-weight metals that have a high number of hydrogen atoms per molecule [5,10]. The comparison of hydrogen-storage density (H atoms per cm3) between hydrides and standard storing systems (namely, cryogenic and gaseous hydrogen at 200 bar pressure and 300 K temperature) definitely ranks solid carriers above cryogenic liquids (Fig. 2).

2. Performance of chemical propulsion systems _ gÞ, plays a key role in Gravimetric specific impulse, Is ¼ T=ðm comparing propulsion performance. It relates thrust T and _ and, as a first approximation, is dependent mass flow rate m

Table 1 e Physical and thermodynamic properties of some hydrides.

Fig. 1 e A crystal of dehydrogenated lithium-aluminum hydride after one year of ambient storage.

Name

Formula r, g/cm3 Dhf, kJ/mol Refs.

Aluminum hydride Decaborane Lithium hydride Magnesium hydride Lithium aluminum hydride Lithium aluminum hexahydride Lithium boron hydride Magnesium boron hydride

AlH3 B10H14 LiH MgH2 LiAlH4

1.48 0.96 0.78 1.45 0.92

11.4 28.9 90.7 74.5 113.4

[19, [21, [23, [25, [27,

Li3AlH6

1.13

310.9

[29, 28]

LiBH4 Mg(BH4)2

0.67 0.99

194.4 152.8

[27, 30] [31, 5]

20] 22] 24] 26] 28]

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3. Energetic materials for solid and hybrid propulsion 3.1.

Fig. 2 e Hydrogen-storage density of hydrides. Data are normalized with respect to liquid hydrogen (4.2 3 1022 H atoms per cm3).

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi on combustion chamber properties represented by Tc =Mm [32]. From a propulsion point of view, the requirements of a space mission can be condensed in one quantity called DV which is defined as the maximum vehicle velocity attainable in a gravity-free vacuum after complete propellant combustion [32]. The relation between requested performance and consumed propellant mass is shown by Tsiolkovskii’s equation (Eq. (1)). This is a linear dependence on delivered Is, resulting from the theoretical value with deductions for different kinds of losses. DV ¼ Is g log

Mp þ Mf M0 ¼ Is g log Mf Mf

Description

Composite solid propellants for space applications are a mixture of micrometric oxidizer and fuel powders, bound together by a polymer matrix which confers structural integrity. As a matter of combustion performance, high density and relatively high chemical stability, ammonium perchlorate is the most used oxidizer (60e80% by mass), aluminum represents a nontoxic, stable, and low cost metal fuel (0e20%) while HTPB-based polymer is a common choice for the binder (10e15% as a compromise between mechanical properties and propulsion performance). During propellant combustion, a partially premixed reaction is generated right above the oxidizer crystals and a microscopically diffusive flame involves the decomposition products of the binder. If present, metal powders generate agglomerates that leave the burning surface after a partial combustion, completing their oxidation in the core flow of the rocket (Fig. 3(a)). Pressure dominates the combustion process. Reactions occur close to the burning surface, within a layer a few hundreds of microns thick [35,36]. A common rocket grain for hybrid propulsion contains only fuel components (namely, binder and additives). The oxidizer flows into the combustion chamber through an injector and its mass flux is controlled by a valve. This stream blows over the burning surface of the fuel where it mixes with combustible vapors and particles generated by grain

(1)

Theoretical specific impulse can be computed by thermochemistry while impulse losses derive from two-phase flow nozzle expansion, incomplete chemical reactions, twodimensional flow at nozzle exit, erosion of nozzle throat, and other minor sources. For a standard metalized rocket, about 10% of theoretical Is is not actually available for propulsion purposes, mostly lost by two-phase flow and by consequences of metal agglomeration [33e35]. Another interesting parameter of performance is given by the volumetric specific impulse Iv ¼ hriIs , where hri is the equivalent density of the propellant involved in the combustion, evaluated at storage conditions. Performance increase of an energetic material can be pursued by changing its composition or using additives such as metal powders. In general, oxidation of ingredients with high-energy density releases enthalpy in the combustion chamber, attaining higher flame temperature but also higher molar mass. The current state of the art additive, micrometric aluminum, and, as a research topic, nanometric metal powders or alloys are studied. In this way, gravimetric and volumetric specific impulse, as well as burning rate or density can be enhanced. Additives may also influence rocket combustion stability, aging, propellant mechanical properties, and other macroscopic features [11,32,35].

Fig. 3 e Flame structures typically present in solid and hybrid rockets.

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decomposition, and places the flame inside a macroscopically diffusive and turbulent boundary layer [37]. With respect to composite solid propellants, pressure influence is rather limited and the reaction zone can be up to few millimeters thick, changing along the grain according to boundary layer development (Fig. 3(b)).

3.2.

independent variables. Binder fractions were derived as a percentage of the total. For hybrids, the variables were represented by the share of additive with respect to solid fuel and the oxidizer-to-fuel mass ratio. At least 10% of binder was imposed. Average density, combustion mixture properties, volumetric specific impulse, and exhaust products were also computed.

Theoretical performance evaluation 4.1.

Theoretical analysis is performed on both solid propellants and solid fuels. The investigated composition of propellants comprises ammonium perchlorate (AP), HTPB-based binder (HTPB) and an energetic additive (Add). Solid fuels are composed by HTPB-based binder and an energetic additive, assuming liquid oxygen (LOx) as oxidizer stored as cryogenic liquid at 90 K temperature. Physical and thermodynamic data are listed in Table 2. Combustion chamber and nozzle expansion are modeled through a thermochemical approach based on Gibbs free energy minimization. This technique is fairly common for complex trade off studies of system wide performance, thereby circumventing the uncertainties of chemical kinetics and fluid dynamics at the cost of some restrictive hypotheses. Main assumptions consist of chemical equilibrium in the combustion chamber as well as during the gas dynamic expansion through an ideal nozzle (shifting equilibrium model). Reported data are obtained by using the NASA CEA code which implements the methodology of Gordon-McBride [39e41]. Minimization of Gibbs free energy allows the evaluation of combustion chamber properties (namely, adiabatic flame temperature, molar mass, species molar fractions, specific heats and so on). The same properties are also available along the nozzle down to the exhaust, where propulsion parameters are computed. All data from this theoretical study are exempt from losses that are typically present in real systems such as boundary layer, multiphase flow expansion, two-dimensional exhaust, kinetic delay, or throat erosion.

4.

Results for optimal formulations

The analysis scrutinized both solid propellants and hybrid fuels containing the additives listed in Table 1. In order to get a meaningful comparison between different performance data, chamber pressure was uniformly set to 70 bar and expansion ratio to 10. The nozzle was presumed to discharge in vacuum assuming thermochemical equilibrium during expansion. An automated procedure was built up for the definition of the optimal formulations, targeting the highest gravimetric specific impulse. In the case of solid propellants, mass fractions of AP and additive were considered as

Table 2 e Properties of ingredients common to all formulations. Name

Formula

AP NH4ClO4 LOx (B.P. 90K) O2 HTPB C7.075H10.65O0.223N0.063

r, g/cm3 Dhf, kJ/mol Ref. 1.95 1.14 0.92

295.8 13.0 58

[22] [22] [38]

Solid propellant performance analysis

Solid propellants performance is significantly improved with the incorporation of energetic fuel additives. Gravimetric specific impulse increases from 274 s to almost 288 s with the sole addition of aluminum. Gain in volumetric specific impulse is also attained due to increased density [32]. When most of the hydrides listed in Table 1 are used as a drop-in substitute of metal fuel, gravimetric specific impulse is further increased. Relevant data are reported in Table 3. In some cases, even better performance would have been reached if the binder were progressively reduced, down to an unrealistic zero-fraction optimum. Specific impulse data are normalized with respect to baseline AP/HTPB formulation and compared in Fig. 4. Aluminum hydride performs better than any other additive, LiAlH4 closely follows. With respect to nonmetalized compositions, they both gain more than 10%. If aluminized compositions are chosen as a reference, gravimetric specific impulse gain for the hydrides of aluminum and lithiumaluminum is about 6e8%. Other additives under investigation do not demonstrate breakthrough performance increase and, in the cases of lithium hydride no gain is attained. On the other hand, all tested hydrides, with the sole exception of AlH3, result in a reduction of the volumetric specific impulse. Standard aluminum-based composition features higher values than other additives as a consequence of consistent propellant density decrease. LiAlH4 and LiBH4 represent exemplary cases. A comprehensive comparison is reported in Fig. 4 where data are normalized with respect to a baseline AP/ HTPB propellant formulation. Flame parameters of the reaction products are reported in Fig. 6(a). With respect to standard aluminized propellants, hydrides release hydrogen. In general, resulting flame

Table 3 e Optimal compositions for solid propellants as Is. Ingredient fractions are given by mass. Baseline formulation consists of AP/HTPB. If necessary, minimum binder content was enforced to 10%, reporting in parentheses the unconstrained optimum. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Iv Add. AP/Add./Binder, % rp Tc =Mm Is Baseline Al AlH3 LiAlH4 Li3AlH6 B10H14 MgH2 LiH Mg(BH4)2 LiBH4

89/0/11 67/22/11 62/28/10 (59/40/1) 60/30/10 (58/42/0) 61/29/10 (73/27/0) 75/15/10 (82/17/1) 57/33/10 (57/38/5) 87/3/10 (88/5/7) 60/30/10 (58/42/0) 59/31/10 (64/36/0)

1736 1836 1624 1347 1475 1532 1591 1685 1390 1143

10.68 10.92 11.87 11.84 11.04 10.86 11.10 10.85 11.04 11.10

274.3 287.8 310.7 307.7 290.6 287.7 289.8 276.1 293.8 294.0

4.76  5.28  5.05  4.14  4.29  4.42  4.61  4.65  4.08  3.36 

105 105 105 105 105 105 105 105 105 105

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Fig. 4 e Solid propellant performance comparison. Optimal Is and Iv normalized to nonmetalized baseline formulation (AP/ HTPB mass fraction 89/11).

temperature and molar mass are both reduced, in some cases even below nonmetalized compositions. Benefits on gravimetric specific impulse already highlighted in this section come mainly from increment of the Tc/Mm ratio.

4.2.

Hybrid fuel performance with LOx

Unlike solid propellants, generalized performance gain is not attained when a polymeric hybrid fuel is enriched by hydrides. Optimal compositions that maximize specific impulse are reported in Table 4 along with relevant O/F ratio, fuel and average propellant density hri (weighted density of fuel and oxidizer). Performance data are contrasted to pure HTPB baseline fuel in Fig. 5 while flame temperature and molar mass are plotted in Fig. 6(b). With respect to the initial list of additives, LiH, MgH2, Al and Li3AIH6 are dropped since they reduce the gravimetric specific impulse and their optimal formulation would be coincident with the baseline. Nevertheless, the specific ingredients may be included for practical reasons based on testing experience rather than performance optimization, such as regression rate or average density increase [11]. By focusing on gravimetric specific impulse data, we can determine that hydride additives do not confer groundbreaking benefits. Is gain is in the order of þ5% or less for most of the results, when compared to baseline data. Aluminum hydride addition creates the best combination of propulsion performance and density increase, delivering þ15% of

volumetric specific impulse. LiBH4 also achieves a comparable increase of gravimetric specific impulse but suffers from limited density, decreasing hri. In general, boron-based hydrides demonstrate promising results thanks to low molar mass of combustion products with the sole exception of decaborane. Finally, it is worth mentioning that a 10% minimum binder content was enforced for LiBH4 and Mg(BH4)2 formulations. Even better performance would have been obtained for an unrealistic formulation without HTPB.

5.

Results for sub-optimal formulations

When a formulation is under development, specific impulse is not the sole parameter to optimize. In different applications, other properties may have higher priority with respect to theoretical performance (namely, burning rate, density, limited flame temperature, low-signature exhaust, mechanical and aging properties, or other). For example, one important limit of hybrid fuels is represented by low mass burning rate and the use of additives is now more focused on the increase of ballistic properties, even at the expense of gravimetric specific impulse. In this direction, aluminum as a nanometric powder is extensively studied [11]. Other issues may concern some additives (namely, cost, availability, chemical compatibility or detriment of mechanical properties) thus imposing an upper limit to the effective quantities. As an example, it is worth mentioning that hydrides may react with

Fig. 5 e Hybrid fuel performance comparison. Optimal Is and Iv compared to baseline formulation and burning condition (pure HTPB and O/F mass ratio 2.2).

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a

1765

In this section performance data are presented for increasing fractions of additives in the range of 5 and 30% with respect to propellant mass (solid propulsion, Fig. 7(a)) and to fuel mass (hybrid propulsion, Fig. 7(b)). Results are given for those compositions that maximize the gravimetric specific impulse and, if necessary, minimum amount of binder mass fraction is enforced to 10%. All data are normalized to the respective nonmetalized baseline compositions (AP/HTPB for propellants and pure HTPB for hybrid solid fuels). Hydrides of lithiumaluminum and of aluminum represent the best performers. Considering solid propellants, gravimetric specific impulse progressively increases with the amount of additive, up to a gain

a

b

b Fig. 6 e Flame properties of optimal compositions.

hydroxyl groups of binder polyols [14]. For this reason the compounding process with HTPB binder may be subjected to limitations, in some cases. However, according to the author’s knowledge relevant information is not available in the literature about long term stability and aging.

Table 4 e Optimal compositions for hybrid solid fuels as Is. Baseline fuel consists of pure HTPB without additives. If necessary, minimum binder content was enforced to 10%, reporting in parentheses the unconstrained optimum. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Tc =Mm Is Iv Add. Mass fr., % O/F rf hri Baseline AlH3 LiAlH4 B10H14 Mg(BH4)2 LiBH4

0% 64% 77% 63% 90% (100%) 90% (100%)

2.2 920 1061 1.0 1214 1176 0.9 920 1013 1.9 945 1064 1.3 983 1066 2.0 688 935

12.45 12.85 12.52 12.78 12.63 13.16

323.6 340.5 333.4 334.8 336.4 342.5

3.43  4.00  3.38  3.56  3.59  3.20 

105 105 105 105 105 105

Fig. 7 e Gravimetric specific impulse of sub-optimal formulations normalized with respect to the baseline.

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of about þ12% with respect to baseline composition. As a comparison, metallic aluminum delivers a þ5% which is comparable to the result obtained by other hydrides. Only lithium hydride leads to specific impulse losses for the investigated range. For hybrid rockets, only some hydrides show increasing trends but rather limited (less than þ3%). Metallic aluminum as well as the hydrides of magnesium, lithium, and Li3AlH6 all resulted in a decrease of gravimetric specific impulse. As already mentioned, hydride density depresses volumetric specific impulse (Fig. 8). As a result of the presence of a solid oxidizer, propellants have a high average density, even

a

b

for the baseline composition. When part of the oxidizer is replaced by low-density hydrides (e.g., lithium boron hydride), rp and volumetric specific impulse are reduced. An exception is AlH3 which has the highest density among the hydrides under investigation. For both solid propellants and hybrid fuels, only aluminum and its hydride lead to a 6%e12% benefit. Variation induced in hybrid rockets by other additives is limited to changes of a few percent. Combustion of fuel and solid propellant additives may form condensed combustion products (CCP) which take part in the expansion through the gas dynamic nozzle, impairing propulsion performance. Simple empirical correlations between theoretical and delivered specific impulse exist in the literature, in the fashion of functions depending on CCP mass fractions as well as specific experimental data on agglomerate size [33,34,42]. Full application of such formulas is beyond the scope of this work and this analysis is limited to the thermochemical prediction of condensed combustion products. Agglomeration behavior of hydrides is still an open issue and few investigations have been carried out up to now. However, considering some data available on aluminum hydride, smaller agglomerate size can be expected for the specific additive if compared to standard aluminum [9]. From the thermochemical point of view, the investigation on CCP production shows that aluminum hydride produces not only the highest increase in specific impulse but also comparable amount of condensed alumina with respect to metallic aluminum (Fig. 9). In contrast, all other additives feature reduced or, in some cases, zero condensed products, thus limiting one of the parameters that drive two-phase losses. Finally, data show that the use of hydrides in solid propellants results in performance increase as of gravimetric specific impulse up to þ12% with respect to nonaluminized baseline (about 8% higher than aluminum-based propellants), whereas benefits for hybrid rockets are somewhat limited and then only for some selected ingredients. The situation changes for volumetric specific impulse due to the decrease in density of solid propellants. Lastly, it is interesting to note that some additives do not generate condensed combustion products, according to theoretical predictions and, in general, even for these novel energetic formulations, the amount of CCP in solid propulsion systems is higher than in hybrid rockets.

6. Emission from hydride-based energetic materials: a preliminary assessment

Fig. 8 e Volumetric specific impulse of sub-optimal formulations normalized with respect to the baseline.

Combustion products discharged by airbreathing and rocket systems have been the subject of past and current critical reviews [16e18,43]. Environmental concern on space launches stems from the fact that pollution is mostly released in the upper atmosphere (stratosphere and mesosphere) where the ozone layer, located at an altitude of about 20e30 km, may suffer from depletion. The Propulsion and Energetic Panel (PEP) of AGARD, a NATO organization, discussed this subject during a symposium held in 1994 and came to the conclusion that the level of pollution attributed to space activities was negligible, for the time being [44]. The resolution developed in the mid nineties referred to a limited number of launches using solid and liquid rockets, which is still the situation currently. Nevertheless,

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a

b

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throat and frozen chemistry in the divergent of the nozzle, for a matter of slower kinetics [46]. Results from this simplified approach are useful for cross comparison among different formulations but do not supply exact results for those reactions where kinetic processes dominate, such as carbon black generation. Computations were performed on reference formulations containing 20% by mass of additive. Molar fractions of predicted exhaust composition are summarized in Fig. 10(a) and (b) respectively for solid and hybrid propulsion. When the baseline solid propellant is considered, hydrochloric acid, carbon monoxide and dioxide, and water represent about 80% of exhausted moles of gas. Incorporating any kind of additive (whether hydride or not) increases the molar fraction of molecular hydrogen while nitrogen oxides and OH radicals are produced in limited quantities. Major exhaust products do not interact with ozone but some molecules are precursors of its depletion. HCl represents a long term reserve of chlorine that continually accumulates in the atmosphere, subject to decomposition through photochemical process or in the plume. Hydrogen and carbon monoxide are responsible for afterburning in the wakes of rockets where exhaust products contact atmospheric oxygen, raising local temperatures and forming nitrogen oxides. Previous investigations revealed that local plume effect on ozone may be severe, leading to complete depletion however natural levels are readily restored by atmospheric recirculation. As reported in the open literature some metallic atoms, such as sodium and magnesium, contained in the molecules of hydrides also have neutralizing or scavenging effects on the acid content of the

a

Fig. 9 e Condensed combustion products of sub-optimal formulations at the throat section. Baselines are exempt from CCP.

future trends foresee the use of high-energy density materials for solid rockets, the commercial development of hybrid technology and an increase in the number of launches due mainly to the expected fast growth of space tourism [45]. In this respect, combustion of metal hydrides inside novel energetic materials represents a new potential source of pollution and should be investigated. A preliminary evaluation of exhaust products was performed, using chemical equilibrium. As suggested by the AGARD Symposium, the simplified Bray method was applied assuming local thermochemical equilibrium down to the nozzle

b

Fig. 10 e Exhaust products for formulations containing 20% by mass of additive.

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exhaust [38]. By virtue of magnesium based hydrides, for example, part of the hydrogen chloride is converted into magnesium chloride. Also lithium-based hydrides are effective in removing almost all HCl. Lithium belongs to the same group as sodium and a similar behavior may be expected by capturing hydrochloric acid and promoting the formation of lithium chloride, a stable salt soluble in water. The combustion of lithium based additives also releases some amount of lithium hydroxide, a stable poisonous compound, from the nozzle. For instance, a solid propellant containing 20% of lithium-aluminum hydride (both LiAlH4 and Li3AlH6) exhausts respectively 2e4%, by mass, of LiOH while the corresponding hybrid fuel composition reaches about 4e8%. Generally speaking, hybrid rocket exhaust composition is influenced by the choice of both the oxidizer and the solid fuel. In the cases presented, liquid oxygen is chosen as a reference oxidizer. With respect to solid propellants, it is possible to generate chlorine-free exhausts, reduced quantities of molecular hydrogen and higher fractions of CO, favored by high temperatures. Major combustion products are almost unchanged by additives. Use of hydrogen carriers inside the solid fuel formulation increases hydrogen content but the molar fraction does not exceed 20% of the whole exhaust products. Emission of fine condensed products was already addressed in the previous section in connection with performance losses. Aerosols may also have an important role for heterogeneous ozone-depleting reactions in the atmosphere, though this has not been fully assessed. Past works report that atmospheric solid suspensions may foster photochemical decomposition of hydrochloric acid [16,47]. For the compositions discussed here the main condensed exhaust constituents contain metal oxides, boron nitride, boron oxide and a minor fraction of other molecules, depending on the nature of the additives. Alumina is known to perform a catalytic decomposition on ozone but looses its reactivity after a limited time [48]. One more recent work by Ross et al. focused on N2O hybrid rockets and on their future commercial activity, raising a warning about the role of carbon black on global atmospheric recirculation and on temperature and ozone distribution [18]. Finally, hybrid rockets with some specific additives (namely, decaborane and some hydrides of lithium and boron) may be good candidates for the development of low-signature propellants because the presence of condensed combustion products makes the exhaust plume visible.

7.

Conclusions

This paper presented an overview of a selection of solid hydrogen carriers used as energetic additives for solid and hybrid space propulsion units. Performance parameters typical of chemical propulsion (such as gravimetric and volumetric specific impulse, fuel density and combustion mixture properties) were compared for optimal and suboptimal formulations. Notwithstanding the superior level of theoretical gravimetric specific impulse granted by hybrid rockets, the combustion of hydrides leads to generalized performance gain only for solid propellants, when compared to the respective baselines. In fact, only a limited number of hybrid fuel additives, out of the tested group, delivers

a performance increase of gravimetric specific impulse, and even then it is limited to a small percentage increase. For a matter of density, only pure aluminum and aluminum hydride grant a considerable increase of the volumetric specific impulse. Turning to solid propellant performance analysis, a comparison with aluminized compositions shows that only the use of aluminum hydride and lithium-aluminum hydride (LiAlH4) results in a significant gain in gravimetric specific impulse while other additives lead to moderate benefits or even a decrease. Moreover, the lower density of hydrides causes the volumetric specific impulse to be generally lower. In terms of flame properties, hydrogen release in the combustion chamber causes a reduction in the adiabatic flame temperature as well as the molar mass. Regarding the theoretical analysis of exhaust products based on equilibrium chemistry, limited changes can be ascribed to the addition of hydrogen carriers to solid propellants or hybrid solid fuels. In the former case, molecular hydrogen becomes one of the main constituents at the expense of water vapor; in the latter case mixture composition is not altered, with carbon monoxide and water the main components. A remarkable effect on solid propellant exhaust products occurs with the addition of lithium-based hydrides where almost all hydrochloric acid is scavenged and transformed into lithium chloride. Nevertheless, stable poisonous molecules may be generated by the combustion of lithium (namely, lithium hydroxide) and may accumulate in the atmosphere. Along with the analysis of pollutant emission, this paper also considered the generation of condensed combustion products, which have an important role in relation to specific impulse losses. In this respect, boron and lithium-based additives have positive effect due to the expected reduction of exhaust CCP content. Finally, we should remark that the theoretical predictions shown in this paper did not consider the actual feasibility of formulations that may encounter manufacturing problems with current compounding procedures and ingredients. This assessment requires a detailed case by case investigation. This kind of experimental work is currently underway.

Acknowledgements This work was partially supported by CNES (Centre National d’E´tudes Spatiales) Launchers Directorate under contract No. 4700024752/DLA094 and No. 4700028003/DLA094. The authors wish to thank Mr. Claude Cole and Mr. Hugh McSpadden for their careful review of the manuscript.

references

[1] Sakintuna B, Lamari-Darkrim F, Hirsher M. Metal hydride materials for solid hydrogen storage: a review. International Journal of Hydrogen Energy 2007;32:1121e40. [2] Devarakonda M, Brooks K, Ronnebro E, Rassat S. Systems modeling, simulation and material operating requirements for chemical hydride based hydrogen storage. International Journal of Hydrogen Energy; in press.

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[3] Biniwale RB, Rayalu S, Devotta S, Ichikawa M. Chemical hydrides: a solution to high capacity hydrogen storage and supply. International Journal of Hydrogen Energy 2008;33:360e5. [4] Yamamoto H, Miyaoka H, Hino S, Nakanishi H, Ichikawa T, Kojima Y. Recyclable hydrogen storage system composed of ammonia and alkali metal hydride. International Journal of Hydrogen Energy 2009;34:9760e4. [5] Sarner SF. Propellant chemistry. Reinhold Publishing; 1966. [6] Brew WP, Price RM. Polyurethane solid propellant binder, US Patent No. 4165247, 1979. [7] Consaga JP. Bonding agent for composite propellant, US Patent No. 4944815, 1990. [8] Chan M, Johnson CE. Evaluation of AlH3 for propellant applications. In: DeLuca LT, editor. Proceedings of 8-IWCP, International Workshop on rocket propulsion: present and future; 2002. Paper No. 33. [9] DeLuca LT, Rossettini L, Kappenstein C, Weiser V. Ballistic characterization of AlH3-based propellants for solid and hybrid rocket propulsion; 2009. AIAA Paper 2009-4874. [10] Calabro M. LOX/HTPB/AlH3 hybrid propulsion for launch vehicle boosters; 2004. AIAA paper 2004-3823. [11] Risha GA, Evans BJ, Boyer E, Kuo KK. Fundamentals of hybrid rocket combustion and propulsion [chapter 10], Metals, energetic additives, and special binders used in solid fuels for hybrid rockets, pages 413e456. In: Progress in Astronautics and Aeronautics, vol. 218. AIAA; 2007. [12] George L, Saxena SK. Structural stability of metal hydrides, alanates and borohydrides of alkali and alkalieearth elements: a review. International Journal of Hydrogen Energy 2010;35:5454e70. [13] DeLuca LT, Galfetti L, Severini F, Rossettini L, Meda L, Marra G, et al. Physical and ballistic characterization of AlH3based space propellants. Aerospace Science and Technology 2007;11(1):18e25. [14] Cotton FA, Wilkinson G, Murillo CA, Bochmann M. Advanced Inorganic chemistry. 6th ed. Wiley; 1999. [15] Tromp TK, Shia R, Allen M, Eiler JM, Yung YL. Potential environmental impact of a hydrogen economy on the stratosphere. Science 2003;300:1740e2. [16] Solomon S. Stratospheric ozone depletion: a review of concepts and history. Reviews of Geophysics 1999;37(3):275e316. [17] Hawks Clark W. Environmental effects of solid rocket propellants, perceptions and realities. volume AGARD-CP559 of AGARD Conference Proceedings. In: Environmental Aspects of rocket and Gun propellants. NATO; 1995. [18] Ross M, Mills M, Thooley D. Potential climate impact of black carbon emitted by rockets. Geophysical Research Letters 2010;37(L24810). doi:10.1029/2010GL044548. [19] Turley JW, Rinn HW. The crystal structure of aluminum hydride. Inorganic Chemistry 1969;8(1):18e22. [20] Sinke GC, Walker LC, Oetting FL, Stull DR. Thermodynamic properties of aluminum hydride. The Journal of Chemical Physics 1967;47(8):2759e61. [21] Kasper JS, Lucht CM, Harker D. The crystal structure of decaborane, B10H14. Acta Crystallographica Nov 1950;3(6):436e55. [22] Chase Jr MW. NIST-JANAF themochemical tables. Journal of Physical and Chemical Reference Data 1998;9 [Monograph]. [23] Messer Charles E. A survey report on lithium hydride. Technical Report NYO-9470. Atomic Energy Commission (USA); 1960.  [24] Gunn SR, Green LG. The heats of formation at 25 of the crystalline hydrides and deuterides and aqueous hydroxides of lithium, sodium and potassium. Journal of the American Chemical Society September 1958;80(18):4782e6. [25] Ellinger FH, Holley Jr CE, McInteer BB, Pavone D, Potter RM, Staritzky E, et al. The preparation and some properties of magnesium hydride. Journal of American Chemical Society 1955;77:2647e8.

1769

[26] Bogdanovic B, Bohmhammel K, Christ B, Reiser A, Schlichte K, Vehlen R, et al. Thermodynamic investigation of the magnesiumehydrogen system. Journal of Alloys and Compounds 1999;282:84e92. [27] Zuttel A, Wenger P, Rentsch S, Sudan P, Mauron Ph, Emmenegger Ch. LiBH4 a new hydrogen storage material. Journal of Power Sources 2003;118:1e7. [28] Løvvik OM, Opalka SM, Brinks HW, Hauback BC. Crystal structure and thermodynamic stability of the lithium alanates LiAlH4 and Li3AlH6. Physical Review B 2004;69(134117):1e9. [29] Ehrlich R, Young AR, Rice G, Dvorak J, Shapiro P, Smith HF. The chemistry of alane. XI. a new complex lithium aluminum hydride, Li3AlH6. Journal of American Chemical Society 1966;88(4):858e60. [30] Orimo S, Nakamori Y, Eliseo JR, Zuttel A, Jensen CM. Complex hydrides for hydrogen storage. Chemical Reviews 2007;107:4111e32. [31] Konoplev VN, Bakulina VM. Some properties of magnesium borohydride. Seriya Kimicheskaya (Russian Chemical Bulletin) 1971;1:159e61. [32] Sutton JP, Biblarz O. Rocket propulsion elements. 7th ed. New York, NY, USA: Wiley; 2001. [33] Reydellet D. Performance of rocket motors with metallized propellants (AGARD PEP WG-17). AGARD AR-230; 1986. [34] Maggi F, Bandera A, Galfetti L, DeLuca LT, Jackson TL. Efficient solid rocket propulsion for access to space. Acta Astronautica 2010;66(11e12):1563e73. doi:10.1016/j.actaastro.2009.10.012. [35] DeLuca LT, Galfetti L, Colombo G, Maggi F, Bandera A, Babuk VA, et al. Microstructure effects in aluminized solid rocket propellants. Journal of Propulsion and Power 2010;26(4):724e33. [36] Beckstead MW. Solid propellant combustion mechanisms and flame structure. Pure and Applied Chemistry 1993;65(2): 297e307. [37] Gariani G, Maggi F, Galfetti L. Numerical simulation of HTPB combustion in a 2D hybrid slab combustor. Acta Astronautica 2011;69(5e6):289e96. doi:10.1016/j.actaastro.2011.03.015. [38] Kubota N. Propellants and Explosives: thermochemical Aspects of combustion. 2nd ed. Wiley-VCH; 2007. [39] Gordon S, McBride BS. s. Technical Report RP-1311, Computer program for calculation of complex chemical equilibrium compositions and application. NASA Reference Publication; 1994. [40] S. Gordon and B.J. McBride. CEA: chemical equilibrium with applications. downloadable software. http://www.grc.nasa. gov/WWW/CEAWeb/. [41] Glassman, Sawyer RF. The performance of chemical propellants. AGARDograph 1970;129. [42] Hermsen RW. Aluminum oxide particle size for solid rocket motor performance prediction. Journal of Spacecraft and Rockets 1981;18(6):483e90. [43] Nojoumi H, Dincer I, Naterer GF. Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft propulsion. International Journal of Hydrogen Energy 2009; 34:1363e9. [44] Environmental Aspects of Rocket and Gun Propulsion, AGARD-CP-559, AGARD Conference Proceedings, 1995. [45] Ziliotto V. Relevance of the Futron/Zogby survey conclusions to the current space tourism industry. Acta Astronautica 2010;66(11e12):1547e52. [46] Tishin P, Alexandrov EL. The impact of space launchers on the earth ozone layer. volume AGARD-CP-559 of AGARD Conference Proceedings. In: Environmental Aspects of rocket and Gun propellants. NATO; 1995. [47] Pueschel RF. Stratospheric aerosols: formation, properties, effects. Journal of Aerosol Science 1996;27(3):383e402. [48] Sullivan RC, Thornberry T, Abbatt JPD. Ozone decomposition kinetics on alumina: effects of ozone partial pressure, relative humidity and state of film oxidation. Atmospheric Chemistry and Physics Discussions 2004;4(2):1977e2002.