Marys Medicine

 

The ways for development of environmentally safe solid composite propellants

Progress in Propulsion Physics 1 (2009) 63-80
DOI: 10.1051/eucass/200901063 Owned by the authors, published by EDP Sciences, 2009 THE WAYS FOR DEVELOPMENT OF ENVIRONMENTALLY SAFE SOLID COMPOSITE PROPELLANTS D. B. Lempert, G. B. Manelis, and G. N. Nechiporenko The paper considers a wide set of issues concerning the creation of highenergetic solid composite propellants causing the minimal polluting e¨ecton the environment. Thereby, the level of toxicity of products of di¨er-ent compositions is discussed and propellants with perchlorates oxidizersare compared with propellants with halogen free oxidizers (mainly, am-monium dinitramide, HMX, CL-20). The main methods for creatingcompositions having a required performance and the highest energeticcharacteristics are also under discussion. The dependences of the spe-ci¦c impulse on the mode of formulation arrangement and on the com-pounds£ properties (i.e., formation enthalpy, density, element content)are demonstrated. The main principles for the maximal use of ener-getic potential of chemical substances are under consideration. Theseare the proper selection of the binder type which would be optimal forthe given mixture of oxidizer with fuel (or energetic) component and theopportunity of using metals and their hydrides (mainly, aluminum hy-dride (AH)). Main obstacles in using di¨erent kinds of compositions, aswell as advantages of speci¦c propellants are under consideration as well.
A special attention is paid to the interrelationship between the energeticparameter and other performances (thermal stability, combustion law,sensitivity, and compatibility).
Low environmental impact of combustion products often becomes a prime con-dition for rocket propellants. What products constitute the environmental prob-lem? Naturally, both the amount of a certain component released and its relativetoxicity should be taken into account. At high toxicity, even trace concentra-tions of supertoxicants (e.g., dioxins) in the combustion products may becomea major problem, whereas less toxic HF, HCl, and even CO become dangerousonly in large amounts. Since a rocket launch is occasional, its environmentalimpact should be assessed di¨erently from that produced by the exhaust of thesame products in continuous industrial processes. The conditions in which a This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, pro- vided the original work is properly cited. Article available at or PROGRESS IN PROPULSION PHYSICS solid propellant is used are also important. For example, the use of berylliumis absolutely unacceptable in the ¦rst stages of rocket launchers, while it can beused in the outer space.
MAJOR TOXIC COMBUSTION PRODUCTSOF SOLID COMPOSITE PROPELLANTSRELEASED INTO ENVIRONMENT Toxic substances cannot be avoided completely. Even CO and NO are toxic andalways form (especially, CO) at equilibrium concentrations at high-temperaturecombustion of CHNO systems. For example, combustion of a simple formulationof 13% hydrocarbon binder (HB) + 20% Al + ADN results in the CO concen-tration at the nozzle exit (at 2200 K) of 9.6 mol/kg = 270 g/kg. Since thelimiting allowable concentration (LAC) for CO is 3 mg/m3 (Russian standard),2.7·109 mg CO released in the course of combustion of a 10-ton propellant chargemust be diluted in 1 km3 to bring it to the LAC.
Similar pattern is observed for 14% HB + 20% Al + ammonium nitrate (AN) (the formulation temperature at the nozzle exit Ta is 1800 K). It is virtuallyimpossible to ¦ght CO, as the shift in oxygen balance and elimination of Al(this will reduce CO concentration as well) result in the dramatic drop of pocketperformance. In addition, one should take into account that after leaving thenozzle, the hot (∼ 2000 K) gaseous products containing CO mix with air and asubstantial part of CO is oxidized to CO2. For this reasoning, this paper doesnot deal with CO as an environmentally harmful substance.
Similar consideration applies to nitrogen oxides. Those are always present in the rocket motor exhaust in the concentrations not lower than the equilib-rium one. In fact, NO concentrations are even higher, since NO, the second(after molecular nitrogen) major nitrogen-containing product of combustion, isquenched due to gas cooling during expansion. The calculated equilibrium NOconcentration at the nozzle exit at ∼ 2100 K comprises 10−8€10−7 mol/kg.
Thus, the concentration of NO is 8€9 decimal orders lower than that of CO, soeven with the account of LAC, two orders of magnitude lower than that of CO,atmospheric pollution with NO can be considered as minor.
During £50s€£70s of the XX century, an interest arouse to the formulations containing beryllium and its hydride. As a matter of fact, substitution of alu-minum by beryllium brings a substantial increase in the speci¦c impulse. Table 1presents the calculation data on energy properties of the formulations based onHB, oxidizer, and metal. One can see that substitution of aluminum by beryl-lium increases the speci¦c impulse by approximately 20 units. Naturally, thelargest gain is achieved for the formulations with AN which need the additionalenergy to the utmost. It will be seen below that a ratio of components in each SOLID ROCKET PROPULSION Table 1 Comparison of beryllium- and aluminum-containing formu-lations: Isp is the speci¦c impulse at Pc : Pa = 40 : 1; Pc and Pa arethe pressures (in atm) in combustion chamber and at the nozzle exit,respectively; ρ is the density; and E3 ¡ see subsection 4.1 12% HB + 20% Al + ADN 14% HB + 12% Be + ADN composition described in this paper has been chosen based on the following prin-ciples: (a) binder content is not lower than 18€20 %(vol.), otherwise, it would be too hard or even impossible to create the composition; and (b) maximal metal content is not higher than the value when at which the mass percentage of its oxide in the combustion products is higher than 36%€38%,otherwise, a slagging occurs in the course of combustion process.
Finally, the composition meeting conditions (a) and (b) with the highest Ispvalue or highest ballistic eciency has been selected.
Nevertheless, the excessively high toxicity of beryllium and its derivatives (LAC in air is 0.001 mg/m3, i.e., combustion products of each ton of Be-containing propellant contaminate 100 km3 of air) prohibits the use of suchfuels in the ¦rst stages of rocket launchers. Of course, their use in the upperatmosphere and especially in the outer space is well justi¦ed, especially, as theyare more ecient for the higher stages. However, even test ground ¦rings of suchformulations can result in dramatic beryllium pollution.
For combustion of perchlorate-based formulations, hydrogen chloride is a massive product (up to 20€25 %(wt.)). It is an ozone depleting gas. In addition,upon launch or disposal of rockets on the Earth surface, vegetation su¨ers on avast territory (HCl LAC is 5 mg/m3). Hydrogen chloride precipitates mainly ashydrochloric acid directly over the launch site covering the area of 1€2 ha. How-ever, its fallout was detected at a distance up to 7€8 km, and its concentrationin nearby localities sometimes exceeds the LAC by a factor of 20.
The other dangerous combustion products of chlorine-containing formulations are excessively toxic dioxins, the compounds of general formula: PROGRESS IN PROPULSION PHYSICS where R = H or Cl, with 1 to 8 chlorine atoms.
As their LAC in air is 5 · 10−11 mg/m3, a very small amount of dioxins may create a serious prob-lem. Particularly, strong release of dioxins occurs at thrust cuto¨, i.e., when themotor completes its operation. When the concentration of hydrogen becomeslower than that of chlorine, the equilibrium concentrations of dioxins increase bymany decimal orders and constitute major chlorine-containing products.
Thus, the major objective of this paper is to describe a possibility to sub- stantially reduce the environmental danger of solid propellants through the useof formulations based on ammonium perchlorate (AP) and to propose the mostpromising formulations containing neither beryllium, nor §uorine or chlorine.
POSSIBILITIES OF DEVELOPING PROPELLANTSBASED ON AMMONIUM PERCHLORATEWITH SUBSTANTIALLY IMPROVEDENVIRONMENTAL PERFORMANCE At present, AP is the cheapest and most available oxidizer among highly po-tential ones (those based on cheap AN have low density and speci¦c impulse,those based on ammonium dinitramide (ADN) provide high Isp but possesslower density and are substantially more expensive), but whenever a formu-lation incorporates only C, H, O, N, Al, and Cl, the chlorine in the combustionproducts appears in the form of gaseous HCl. Chlorine can be neutralized tosafe solid products only with alkaline metals in quantities no less than equiva-lent to chlorine. This is possible with substitution of AP by lithium, sodium,or potassium perchlorates [1], but lithium perchlorate is excessively hygroscopicand the formulations based on potassium perchlorate have lower energy due tohigh atomic mass of potassium. There is a possibility to prepare a formulationwith equimolar mixed oxidizer AP : NaNO3 (SN).
The data of Table 2 show that at the nozzle exit at about 3000 K, the for- mulations based on the mixed AP : SN oxidizer yield HCl concentration 5 timeslower than the formulations based on AP; with further cooling, this concentra-tion drops due to the reaction of HCl with sodium species (primarily, sodiumvapors) and at 1200€1300 K, the concentration of HCl becomes negligible.
How does introduction of sodium a¨ect equilibrium concentrations of diox- ins? The calculations were performed for equilibrium concentrations of dioxins SOLID ROCKET PROPULSION Table 2 Some formulations based on AP : SN oxidizer. Tc and Ta are the tempera-tures in combustion chamber and nozzle exit, respectively 251 1.84 3610 2520 240 1.76 3000 1900 225 2.05 3650 2610 230 1,96 3665 2990 AP : NaNO3 + 20% HMX + HB + Al 234 1.936 3630 2610 Figure 1 The equilibrium concentration of dioxins in combustion products for theformulations based on AP + SN vs. temperature: 1 ¡ AP : SN = 1 : 0.9; 2 ¡ 1 : 1;and 3 ¡ AP : SN = 1 : 1.1 in the combustion products of a standard formulation Al + AP + HB. The ther-modynamic calculations showed that with the introduction of SN, the contentof dioxins drops dramatically and depending on temperature it comprises from0.5% to 15% of that attained without sodium.
Figure 1 shows the examples of the formulations based on AP : SN, with the molar ratio AP/SN = 0.9, 1.0, and 1.1. In the temperature range below 2500 K,the yield of dioxins drops as the sodium to chlorine molar ratio increases to 1.
Further increase of SN molar fraction in the oxidizer does not lead to reductionin dioxin concentration compared to equimolar AP : SN.
PROGRESS IN PROPULSION PHYSICS Thus, the formulations based on AP : SN mixture provide a possibility of reducing substantially the release of both hydrogen chloride and dioxins to theenvironment.
FORMULATIONS BASED ON HALIDE-FREEOXIDIZERS In the course of development of propellant formulations, not only energy parame-ters and environmental performance discussed afore must be taken into account.
The issues to be addressed also include thermal stability, compatibility of con-stituents, sensitivity to impact and friction, required combustion law, and othersincluding the cost. Each of the halides-free oxidizers discussed below has itsown merits and drawbacks, which must be taken into account when developing acomposition for a particular task (conditions of storage, safety margin, strengthof combustion chamber casing, etc.).
The list of chlorine-free oxidizers presented in Table 3 is exhaustive for prac- tical applications. Possible extensions of this list will be discussed further.
Ammonium-nitrate-based formulations are considered in a complementary paper. Here, it is worth mentioning that AN is the oxidizer with the lowestenergy potential due to its low –H0 value and low density; nevertheless, a proper choice of binder in the presence of metals can narrow the loss for AP-basedformulations.
Starting to discuss the solid propellants based on other oxidizers listed in Table 3, note that it is mainly the oxidizer (depending on its chemical formulationand –H0) that predetermines the energy potential of solid propellant. However, in order to make a formulation where the energy potential of an oxidizer ismaterialized to its best, one must carefully select other components, namely,energetic component (metal, hydride) and binder according to the rule of thumb:the binder should compensate in its chemical composition the disadvantages ofother components.
For example, if oxidizer is rich in oxygen and the formulation contains little or no metal, the binder should be rich in hydrogen; if the formulation containshydride or an oxygen-de¦cient oxidizer (e.g., HMX), the binder must take someoxidizer functions (say, contain functional groups like NO2, NO3), etc.
In this paper, the formulations with di¨erent types of binder are considered, HB, –Hf = −390 kJ/kg, ρ = 0.91 g/cm3, hydrogen content ∼ 12%; poly(methylvinyltetrazol) (PMVT)€(C4N4H6)n, a high-enthalpy polymer with high nitrogen content (∼ 46%), –Hf = +1255 kJ/kg, ρ = 1.28 g/cm3; SOLID ROCKET PROPULSION Table 3 Major halide-free oxidizers Ammonium nitrate, Availability, low cost, Low energy potential because high oxygen content, of the low enthalpy of forma- sucient thermal sta- tion (–H0f = −4514 kJ/kg),bility, good compati- low density (1.725 g/cm3),bility with other com- low combustion rate, phaseponents, low sensitivity transitions in the operational temperature range High energetic poten- Limited supply, high costtial, satisfactory com-bustion law, satisfac-tory thermal stability,compatibility, and sen-sitivityAvailability, moderate Poor combustion law (largecost, high thermal sta- exponent ν in the pressurebility, satisfactory com- dependence rate W = AP ν), relativelyhigh sensitivity, susceptibilityto de§agration-to-detonationtransition High density, satisfac- Same as HMX + high costtory thermal stabilityand compatibility High energy potential High sensitivity, poor com- patibility, lack of mass pro- active binder (AB), a model binder analogous to practical ones, 20% PMVT, ex- tended with a mixture of trinitroglycerol with 2,4-dinitro-2,4-diazo-pentane; –Hf = −757 kJ/kg, ρ = 1.49 g/cm3, oxygen content ∼ 47%; and poly(vinylmethoxydiazen-N-oxide) (PVMDO) (C3H6N2O2)n, –Hf = 0 kJ/kg, ρ = 1.31 g/cm3, a model binder rich in hydrogen (∼ 6%) and oxygen(∼ 31%).
PROGRESS IN PROPULSION PHYSICS Formulations Based on Dinitramide Salts Ammonium dinitramide is a relatively new oxidizer ¦rst synthesized in Russia in1972. During 10€15 years, methods of synthesis, puri¦cation, and stabilizationwere developed, industrial production was implemented, and even practical mo-tors based on ADN-propellants were manufactured [2]. Ammonium dinitramidehas a favorable chemical composition and suciently high –H0 value; hence, the ADN-based formulations have the highest Isp among those based on otheroxidizers (except HNF) listed in Table 3. However, a low density of ADN makesit better suited for the formulations used in the higher stages. On the lowerstages, it loses a bit to AP.
Table 4 presents the calculated properties of the formulations based on am- monium, hydrazonium, and guanidinium salts of dinitramide. Since di¨erentoxidizers possess di¨erent density and the density of propellant (ρ) substantiallya¨ects the ballistics, the formulations were compared not only by Isp but also bythe so-called ballistic eciency (Ei) of the ith stage of a rocket launcher. Theballistic eciency [3] of the ith stage (Ei) is de¦ned as E1 = Ispρ0.6, E2 = Ispρ0.4,and E3 = Ispρ0.2. These exponent values (0.2, 0.4, and 0.6) correspond to three-stage rocket launchers with optimized stage masses and tank volumes of everystage. Using these equations, one can arm that, e.g., if there are two di¨erentpropellants (the ¦rst one possessing Isp1 and ρ1 while the second one possessingIsp2 and ρ2) the usage of the ¦rst propellant in the ¦rst stage would providehigher speed growth than the usage of the second propellant (the propellants Table 4 Energy potential of formulations based on dinitramide salts 246.9 1.625 330.4 299.8 272.1 258.0 1.767 363.0 324.0 289.1 259.5 1.802 369.5 328.4 291.9 260.7 1.704 358.9 322.6 290.0 261.7 1.725 363.0 325.5 291.9 258.3 1.833 371.6 329.1 291.6 259.6 1.870 377.9 333.5 294.2 256.4 1.781 362.5 323.0 287.8 251.2 1.625 336.1 305.0 276.8 251.6 1.742 351.0 314.1 281.1 262.3 1.774 370.0 329.9 294.2 254.2 1.824 364.6 323.3 286.7 SOLID ROCKET PROPULSION used in the 2nd and 3rd stages being equal) if Isp1ρ0.6 2 . Naturally, the exponents for density depend on the volume-to-mass ratio for each stage and candi¨er substantially.
To date, ADN is the most ecient oxidizer for solid propellants (especially, for upper stages) among the industrially produced ones. The formulations with ABand boron can provide the highest ballistic eciency, as boron-based propellantshave no condensed combustion products and hence, experience no two-phaselosses of speci¦c impulse (this losses are typical of Al-based formulations).
Formulations Based on Organic Oxidizers The available choice of candidate inorganic oxidizers suitable for propellants islimited indeed. Those include onium nitrates, perchlorate, and dinitramide salts.
Salts of other acids are either less energetic or unstable (when the acid is weak).
The chemistry of organic oxidizers provides a wider choice.
Formulations Based on HMX RDX and HMX are the well developed commercially produced explosives. HMXtakes over RDX in energy, as with the same element content and approximatelyequal –H0 (314€334 kJ/kg) HMX has a substantially higher density (1.91 vs.
1.80 g/cm3). Therefore, this subsection deals only with HMX. HMX is not usedas an individual oxidizer in solid propellants. Its advantage of high –H0 is combined with a disadvantage ¡ it is not suciently rich in oxygen. For thisreason, it should be used only with a highly active binder (Table 5).
Another drawback of HMX (typical also of other CHNO oxidizers) is that the formulations based on it have a high value of exponent ν in the combustionlaw W = AP ν . Therefore, systems with high HMX content are dangerous for Table 5 Energy potential of formulations based on HMX and AB PROGRESS IN PROPULSION PHYSICS possibility of de§agration-to-detonation transition or failure of motor casing dueto uncontrolled pressure rise in the combustion chamber. At the same time, useof up to 25% HMX in combination with another oxidizer is relatively safe andfor certain systems, this secures improved energetic parameters (e.g., when APis the major oxidizer). But this is not a common rule; ADN does not requireHMX. In order to secure a favorable combustion law for the formulations basedon HMX and an AB, one usually introduces a small amount of AP (5%€10%)even at the expense of a small drop of energetic parameters.
Formulations Based on High-Enthalpy Organic Oxidizers Recently, a number of publications has been devoted to synthesis and investiga-tions of properties of other organic compounds with high oxygen content and rela-tively high –H0. Those are mainly the compounds based on high-enthalpy struc- tures: strained carbon cycles (cubane), acetylene derivatives, nitrogen-containingheterocycles (aziridines, furazanes, furoxanes, triazoles, tetrazoles, etc.).
Recently, CL-20 has been studied extensively [4]; this is a high-density high- enthalpy compound (–H0f ∼ 840 kJ/kg, ρ = 2.04 g/cm3, see Table 3). However, authors£ analysis has shown that despite these advantages over HMX, CL-20is more ecient than HMX only in ¦rst stages of multistage rocket launchers(exclusively due to higher density). Because of a lower hydrogen fraction (almosttwofold as compared with HMX), CL-20 is virtually equivalent to HMX in thesecond stage and less e¨ective in the third stage.
In early 1980s, octanitrocubane (–H0 = +250 kcal/kg, ρ arose much hope. Much e¨ort was devoted to synthesis of this oxidizer. However,the attainable energy performance of octanitrocubane (Table 6) may not payo¨the bitter problems encountered in the synthesis. Not so high ballistic eciencyof octanitrocubane stems from the absence of hydrogen.
There is a variety of onium salts of organic acids-oxidizers. Among those, there are many onium salts of primary nitramines. These compounds are readilyavailable; as a rule, their synthesis is not complicated.
Table 6 Energy potential of formulations with octanitrocubane SOLID ROCKET PROPULSION Table 7 Energy potential of formulations based on salts of methylenedinitramine Binder %(wt.) % % 0 206.2 1.50 1710 263.0 242.5 223.6 0 221.9 1.52 2130 285.3 262.4 241.3 (NH4)2CH2(NNO2)2 PMVT 0 246.2 1.62 2590 328.8 298.6 271.1 0 258.0 1.67 3150 351.0 316.7 285.9 0 10 245.2 1.58 2330 322.6 294.4 268.7 0 219.5 1.64 1910 295.4 267.5 242.3 0 233.9 1.67 2270 318.2 287.2 259.2 (N2H5)2CH2(NNO2)2 PMVT 0 251.7 1.73 2640 349.7 313.4 280.9 0 265.2 1.78 3180 374.8 334.0 297.6 0 10 252.2 1.70 2350 346.7 311.8 280.4 The energy potential of the formulations based on ammonium and hydrazo- nium salts of methylenediniramine are presented in Table 7. One can see thathigh properties can be attained using hydrazonium salt; however, stability ofthis salt is insucient for its use as a component in solid composite propel-lants.
Additionally to that, almost all hydrazonium salts manifest remarkably strong impact and friction sensivity, whereas hydroxylammonium salts are toohygroscopic, have poor compatibility with other components of solid propellants,and often insuciently stable. The stability of hydrazonium and hydroxylam-monium salts depends on the strength of the acid and its activity as an oxidizer.
Thermal stability of the salts grows with increasing the acid strength and re-ducing its activity as an oxidizer, as thermal decomposition of the onium saltsproceeds via proton transfer and the formation of the free base in equilibriumconcentration. The base either reacts with free acid or decomposes itself. Hy-droxylammonium perchlorate (HAP), the salt of a very strong perchloric acid,manifests insucient stability to be promptly used as a component for solidcomposite propellant.
Using special methods, by achieving a shift in the equilibrium to formation of a little excess of free acid, one can substantially increase its thermal stability [5].
In the series NH+ 5 X−, and NH3OH+X−, ammonium salts are the most stable, hydrazonium are less stable, and hydroxylammonium salts are the leaststable. The ¦rst reason is that alkalinity of onium base decreases substantiallyfrom ammonium to hydroxylammonium. The second, the reducing activity ofthe base increases in the same series while thermal stability of the base as acompound drops.
Table 8 shows the formulations with ammonium and hydrazonium salts of nitroform. The higher energy parameters of hydrazonium salts are obvious.
PROGRESS IN PROPULSION PHYSICS Table 8 Energy potential of formulations with nitroform salts 3190 1980 353.7 316.2 282.7 3180 2140 357.0 317.4 282.2 3660 2430 371.3 329.8 292.9 3150 1750 346.7 311.1 279.1 3880 2740 378.5 332.9 292.8 3570 2450 369.6 326.8 289.0 3350 2000 359.9 319.6 283.9 3540 2310 368.7 326.0 288.3 3230 1870 362.2 323.9 289.6 3250 2200 369.3 327.6 290.6 3590 2330 377.2 335.4 298.2 3420 2100 369.2 329.7 294.5 3810 2610 386.1 339.9 299.3 3460 2280 374.9 332.2 294.4 3340 1940 368.3 327.1 290.5 3520 2280 376.4 333.2 295.0 Recently, a number of papers have been devoted to studies of possible ap- plication of high-enthalpy CHNO oxidizers, e.g., from the series of substituted¦ve-member N-heterocycles (pyrrole (I) → pyrazole (II) → 1,2,3-triazole (III)→ tetrazole (IV ) [6, 7].
where X, Y are NO2, C(NO2)3, or other oxidizing groups. These compoundsmay be interesting as components of solid composite propellants only with alarge number of constituents incorporating oxidizing groups. In the series frompyrrole to tetrazole, the –H0 value of such oxidizers increases and so does oxygen balance. The derivatives of furozanes, furoxanes, and other ¦ve-, six-member N-heterocycles can also make potential oxidizers (Table 9).
Table 9 Energy potential of formulations based on high-enthalpy organic oxidizers PROGRESS IN PROPULSION PHYSICS A POSSIBILITY TO SUBSTANTIALLY ENHANCEENERGY POTENTIAL OF SOLID COMPOSITEPROPELLANTS THROUGH INTRODUCTIONOF ALUMINUM HYDRIDE A possibility to use AH (AlH3, −12 kJ/mol, ρ = 1.48 g/cm3) as an energeticcomponent instead of aluminum was studied from early 1960s. Indeed, AHplays not only the role of energetic component, but also gas-generating one;this provides a possibility to increase Isp dramatically. The eciency of AH,however, is materialized only when an AB is used. Therefore, the extensive searchaimed at synthesis and manufacturing of binders incorporating large fraction ofnitroether-, nitro-, or nitramine groups was performed in parallel with the searchof optimal ways for AH synthesis and improvement of its operational properties.
The initial AH-containing compositions were really created and manufactured insmall amounts of AP as oxidizer; further, several years later, the ones with ADNwere created as well. Table 10 shows the eciency of AH on the example offormulations with ADN. Note that the binder volume fraction is also presentedin the table. Since AH particles constitute a very ¦ne fraction, there must beat least 25 % by volume of binder for 25 %(wt.) of AH in the formulation (whilefor aluminum-containing formulations, the minimum volume fraction of binderis 18%€19%). For a lower content of binder, it is virtually impossible to preparea propellant with satisfactory mechanical and rheological properties.
Table 11 presents the data for AH-containing formulations with di¨erent oxidizers and AB. It shows that the use of AH allows one to overcome to acertain extent the disadvantages of an oxidizer; the formulations based on ANdo not lose much to those with ADN, HAP, or HNF. Presently, the major factorlimiting wide use of AH is its high cost.
Table 10 Energy parameters of formulations 25% AH + binder + ADN dependingon binder type and content SOLID ROCKET PROPULSION Table 11 Energy parameters of formulations with AH, AB, and di¨erent oxidizers THE WAYS TO SECURE NECESSARYCOMBUSTION PROPERTIES OF SOLIDCOMPOSITE PROPELLANTS A solid propellant can be used in practice only when a proper combustion lawis secured, i.e., the combustion rate must equal a required value for a prescribedpressure within a chamber, the exponent ν in the formula relating the combustionrate to pressure (W = AP ν) must belong to a certain range (in the majority ofcases, 0.3 < ν < 0.4 is optimal).
The combustion mechanism of solid composite propellants cannot be satis- factorily formulated and described without a preliminary description of the com-bustion mechanism of oxidizers. The latter mechanism is primarily governed byexothermic decomposition and sublimation (evaporation) of the substance. Thisprovides a controlling and stabilizing action on the formulation combustion as itenters the overall heat balance at the combustion surface. The combustion ratecan be varied not only via oxidizer decomposition accelerated with a catalyst butalso by reducing oxidizer volatility (e.g., due to introduction of alkaline metalsalts).
The kinetics plays an important role in the combustion mechanism of solid composite propellant; important is the ratio of oxidizer decomposition rate to therate of fuel oxidation at the initial stages of combustion. For a suciently stableoxidizer (e.g., AP), a nonvolatile fuel can get partially oxidized in condensedphase and thus contribute to the heat balance; this has a stabilizing e¨ect oncombustion and enhances the combustion rate, compared to pure oxidizer.
PROGRESS IN PROPULSION PHYSICS In the case of readily decomposing oxidizer (e.g., ADN) and a relatively thermally stable fuel, the conversion rates for both components can have sucha ratio that, owing to heat consumption for heating and gasi¦cation, the fueloxidation does not occur in the condensed phase and, as heat §ux from thegaseous phase is insucient, the combustion rate of the formulation becomeslower than that for a neat oxidizer.
The same mechanism accounts for the anomalous dependence of the com- bustion rate on the particles size for this type of oxidizer, the combustion rategrows with increasing oxidizer particles. Therefore, the use of suciently ther-mally stable components bearing highly reactive functional groups, the latteroxidizing fast and with high heat e¨ect, provides ecient way to enhance thecombustion rate.
At all stages of propellant combustion, the heat and mass transfer between the components a¨ect both the combustion rate and its dependence on externalconditions and intrinsic features of the formulation (particles size, reactivity).
The interplay of these factors and reaction kinetics determines combustion pro-ceeding in kinetic, di¨usion-kinetic, heat exchange, or autonomous regime. Theways to control these regimes include variation of components dispersion and alsohigher homogeneity of propellant by combining in one phase or one componentthe features of both oxidizer and fuel (e.g., guanidinium nitrate or guanidiniumnitramide) or oxidizer, fuel, and catalyst (e.g., ferricinium salts).
These ways also include surface covering of oxidizer particles with the ¦lms, which have a catalytic, inhibiting, or highly energetic e¨ect. The combustionrate depends on chemical reactions of oxidation and decomposition proceedingin both condensed and gaseous phases. The nature of intermediate productsof combustion (decomposition) is also important. For example, some chlorineoxides, which are the intermediates in combustion of AP, are active oxidizers,whereas the decomposition products of AN and certain organic oxidizers aremuch less chemically reactive.
The rate of condensed-phase reactions governs the combustion rate for for- mulations based on low-volatile compounds (primarily, inorganic ion salts: AP,ADN). This rate, in its turn, depends on the temperature of the surface, whichis controlled by the ratio of evaporation and decomposition rates [8]. For theabove mentioned compounds, this ratio is of such a value that ν in W = AP ν iswithin 0.3 to 0.5.
For volatile organic compounds, the condensed phase evaporates completely at relatively low temperature, where thermal decomposition has not occurredyet and gas phase reaction at the maximum combustion temperature controlsthe process (combustion mechanism described by Belyaev€Zel£dovich theory). Inthis case, the combustion rate strongly depends on pressure (ν ≈ 1). The case ofnot very stable and relatively volatile organic compounds like HMX, RDX, etc. isan intermediate one. Here, heat transfer from gaseous phase becomes noticeable.
For such systems, ν ≈ 0.5€0.7. Therefore, for the formulations with high HMX SOLID ROCKET PROPULSION content, one must introduce some small concentrations of AP (5%€10%) in orderto bring ν down.
CONCLUDING REMARKS The combustion products of the formulations containing beryllium and/or APare most harmful for the environment.
The environmental impact of propellants based on AP can be substantially reduced through the use of the mixed oxidizer, equimolar mixture of AP withsodium nitrate. This, however, brings down the energy potential of the propel-lant.
The major pollutant in the combustion products of chlorine-free formulations is carbon monoxide and its formation is unavoidable. Nevertheless, fast dilutionof gaseous products in atmosphere and partial oxidation with air rapidly bringCO concentration to admissible values.
The energy potential of propellant components can be materialized in optimal formulations, e.g., with a choice of a proper binder for a particular oxidizer,presence or absence of metal or hydride.
When developing new propellant formulations, from the very start, one should consider for what class of rockets this propellant is made for; the for-mulation should be optimized for particular case with the ballistic eciencydepending on both speci¦c impulse and density.
Use of aluminum hydride provides the way to substantially enhance energy potential of propellants; it should be employed with ABs enriched in oxygen.
The development of new propellant formulations requires consideration, in addition to high energy potential of a formulation, of a number of other proper-ties (appropriate combustion law, sucient thermal stability, compatibility, lowsensitivity to friction and shock, etc.).
1. Lempert, D. B., G. N. Nechiporenko, G. P. Dolganova, and L. N. Stesik. 1997. En- ergetics of low pollution solid propellants. Chem. Phys. Rep. 16(9):1629€41.
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