Nitrate Ester Flames RUDOLPH STEINBERGER Allegany Ballistics laboratory, Hercules Powder Co., Cumberlaad, Md. Current fundamental research on the flames of nitrate esters and their mechanism of combustion has been studied in an attempt to unravel the sequence of chemical and physical reactions leading from propellant to combustion products. With detailed knowledge changes can be made in the propellant, thus improving its performance. A definite trend away from the traditional burning rate studies i s discernible, increasing emphasis being placed on the details of the physical and chemical structure.
I T R S T E esters, mostly in the form of nitrocellulose arid nitroglycerin, are the major ingi edients of smokeless powder, perhaps the most prominent class of propellants in use today. They can also be used as liquid monopropellants ( 6 ) I n view of their wide applicability, there is ample reason to study their Combustion process in an attempt to unravel the sequence of chemical and physical reactions leading from propellant to combustion products. Detailed knowledge of the process should enable one to introduce pertinent changes into the propellant and thus to improve or tailor-make its performance In this paper the current efforts in this field and the main lines of thought guiding the nork are described briefly. The symposium on “Kinetics of Propellants” ( 7 ) has been chosen as the chronological starting point In general, the woik on nitrate ester flames has followed thc pattern set by combustion studies on more common systems-c.g, fuel-air mixtures. The theoretical basis is the same, and the same variables are of interest, such as: the flame speed-Le., the qpeed a t which the reaction zone moves through the medium; the distribution of intermediate products; and the temperature piofile through the flame. Certain difficulties are imposed by the properties of the materials. For inptance, they are all selfcombustibles and carry with them the danger of detonation. This limits the quantities of material and the experimental arrangements which can be used. Moreover, these esters are usually solids or liquids, a fact which makes for complexity in experimental techniques and theoretical interpretation. A stable, stationary flame such as is very common in the combustion of gaseous fuels-e.g., the bunsen burner-recently has been introduced (3,8, I S , 19) to this field and is still a rarity, difficult to maintain and control. Most of the materials of interest Fill not burn a t atmospheric pressure; thus, it has been necessary to carry out the greater part of the work in pressure vessels, a clumsy procedure at best. All these factors have conspired to reduce the rate of research progress, so that present knowledge of these Annie reactions is very inadequate. Burning Rate Studies Of the parameters available for measurement, perhaps the most easily accessible is the flame speed. I n the case of solid or liquid propellants the related quantity actually measured is the linear burning rate-i.e., the rate a t which the surface of the bnrning propellant regresses normal to itself. This qunntity is the bread and butter of solid propellant development work, and a large amount of data is continually being gathered on the burning rates of various formulations as a function of temperature and pressure. I n general the solid propellants are very complex mixtures, however, and the burning rate data are difficult to interpret in a fundamental way. If liquid nitrate esters are used, greater simplicity results. Adams and coworkers (1-9)have determined the effectsof chem-
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ical parameters on the burning rates of such esters. The liquids mere contained in 3-mm.-i.d. glass tubes, ignited a t the top, and the piogress of the meniscus down the tube was folloived. They found that in a homologous series the burning rate is an exponential function of the heat of explosion. Specific structnral effects are present, however. Thus, moiionitrates have higher rates than dinitrates with the ~ a m eheat of explosion. Nitroalkyl dinitrates burn faster than alkyl dinitrates, which in turn burn faster than nitroalkyl mononitrates. Nitrates containing ether oxygen, such as the polyglycol dinitrates, burn twice as fast as the parent seiies. Through their investigation of the effects of additives they found that the data usually followed the curve of burning rate versus heat of explosion. When the additives R-ere aromatic compounds or amines, however, the resulting buining iates neie higher than would be expected on the basis of heat of explosion. Lead tetramethyl exhibited complex behavior, causing either acceleration or inhibition, depending on the piessure and the concentration. Adams visualizes the burning process as occurring in tliicc stages.
RON02 -+ NO2 KO2
+ organic molecules
+ organic products of ( A ) KO
KO
+ HQ,CO, etc.
(-1)
--f
+ HQ,CO, COz, H20,etc.
+
NZ COI, HzO, etc.
--f
(B) (C)
.4nalysis of the products indicates that the process goes essentially t o stage B at low pressure. The energy necessary to activate the various reactions is thought to be derived by the conduction of heat from the more advanced, and therefore hotter, stages. Steinberger, Orliclr, and Schaaf (1’7) have carried out some burning rate studies designed specifically to investigate the possible rolc of atomic hydrogen in the flame reaction, TrarionF: deuterated liquid nitrate esters were prepared and their burning rates measured. Deuteration produced lowered rates traceable both t o the reduction of the diffusion rate of atomic hydrogen, which is thought to be an important reagent, and to the decreased rate of breaking a C-D bond as opposed to a C-H bond. Consequently the important reaction involving the original nitrate ester was postulated to be
H.
+ RR’CHOS02
-+
Ha
+ RR’COS02
The atomic hydrogen is thought to be generated in the hot part of the flame and to diffuse back against the gas stream t o react with the nitrate. This scheme emphasizes diffusion of radicals as the principal mode of energy feedback, in contradistinction t o the more usual emphasis on the conduction of heat.
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ROCKET PROPELLANTS I n the case of gaseous nitrate esters the flame speed can be measured by conventional techniques. Adams and coworkers ( 2 , 3) used a cylindrical vessel with central spark ignition t o measure the flame speeds of mixtures of methyl and ethyl nitrates a t low pressures, usually 35 mm. of mercury. On diluting their mixtures with inert gases, they found that the flame speed was lowered roughly in proportion to the molal heat capacities. Their results, interpreted in terms of the thermal theory of Semenov ( I @ , indicated an activation energy of 22 kcal. per mole for the methyl nitrate flame. On this basis, the oxidation of organic molecules by nitrogen dioxide was postulated as the ratedetermining reaction. However, on using this activation energy in an equation derived for the mixtures of methyl and ethyl nitrates, a value of 73 kcal. per mole was obtained for the pure ethyl nitrate flame. No ready explanation for this large value appears t o be available. Wolfhard and coworkers (8,19) have utilized burner techniques in studying flame speeds of methyl and ethyl nitrates. Addition of oxygen or nitrogen dioxide produced no change in flame speed but served t o intensify a n orange afterglow. Added nitric oxide had no effect. Flames of methyl nitrite plus oxygen or nitrogen dioxide were found to be very similar t o those of methyl nitrate, so t h a t a similarity of mechanism is indicated.
Structure of Flames The measurement of burning rates and flame speeds is relatively simple, and i t has been common practice t o carry out such studies under all possible conditions and t o try to fit the data to the various mathematical formulations of the flame reaction which have been proposed ( 6 ) . These formulations are generally based on the conservation equations and the equations of state for the system. They differ in the relative importance ascribed t o the various postulated features of the over-all scheme and in the approximations made t o obtain a solution. Unfortunately, the resulting equations lack in specificity, and it is not possible t o make a critical evaluation of the problem in these terms, It is desirable, therefore, t o investigate the fine structure of the flames in greater detail and to try to determine the important aspects of the reaction by more direct methods. These studies have included temperature profile measurements through the flame, sampling and analysis of gases at various points, and determination of emission and absorption spectra of the flame gases. All three of these methods have been applied to the study of solid propellant flames. Heller and Gordon (9) have described measurements of temperature profiles and product distribution T h e temperatures were obtained by a method developed by Klein and coworkers ( l a ) ,in which a fine wire thermocouple is embedded in a strand of propellant so that the combustion wave passes over the thermocouple as the strand burns. I n addition, Heller and Gordon measured the temperature of the solid surface by allowing a weighted thermocouple to ride down on the surface. The results demonstrate the inherent difficulty of attempting to make such detailed flame structure studies on solid propellants. The entire distance over which the bulk of the temperature rise (and thus the bulk of chemical reaction) occurs is approximately 0.05 inch. More than half of this rise occurs at the surface of the condensed phase, in a semiliquid bubbly layer commonly called the “foam” zone. This region is highly turbulent and cannot be regarded as a unidimensional reaction zone. Therefore, results obtained within it cannot be interpreted in terms of the usual equations of laminar flame propagation. The small region of reaction extending into the gas phase can be expected to carry along a good bit of the foam zone turbulence, and is also suspect. Moreover, this region is inaccessible to sampling probes and spectroscopic light beams, because of its narrowness and the inherent unevenness of the surface. April 1956
Thus, in solid propellants it does not appear feasible t o analyze the primary reaction zone. Heller and Gordon carried out mass sampling studies which bear this out. They withdrew samples through fine probes and analyzed them by mass spectrometry. Their analyses a t the point of closest approach to the surface show that the first stage of reaction, which yields essentially nitric oxide, carbon monoxide, hydrogen, etc., has already gone t o conipletion. As the probe is moved away from the surface, no fnrther change in composition is found until a point is reached, perhaps 10 mm. from the surface depending on the pressure, where the reaction goes through a second stage. The final products are formed here, and a visible flame is produced. From the point of view of over-all rate this secondary flame seems to be unimportant. Therefore the inability to trace the development of the primary reaction represents a serious limitation. Spectroscopic investigations on solid propellants have encountered these same troubles and have had limited success. Crawford and coworkers ( 4 ) have used a rapid-scanning infrared spectrometer in conjunction with a slowly moving slab of propellant to trace the concentration of species like carbon monoxide and carbon dioxide through the flame. Rekers and Villars (14,18)have used visible and ultraviolet spectroscopy on burning strands rendered stationary by means of a feed motor monitored by photoelectric cells. I n this way they have detected certain radicals such as Cz. As in the case of burning rate studies, liquid nitrates offer certain advantages over the solids in flame structure investigations. Hildenbrand and coworkers (10, 11 ) have measured temperature profiles by an adaptation of Klein’s technique for solid propellants, allowing the liquid to burn down a glass tube and over a thermocouple which was strung across the tube. The progress of the burning surface over the thermocouple was recorded photographically, and a point on the temperature profile was established which corresponded t o the position of the liquid-gas interface. According t o this technique the surface temperature is in the neighborhood of 200’ C. and varies with the pressure and with the material under study. Steinberger and Carder ( 1 6 ) used similar techniques but found surface temperatures not appreciably higher than ambient. This discrepancy is at present unresolved. It may be traceable to the slight differences in experimental technique; the latter authors, for instance, operated generally at higher pressures and at lower rates of travel of the liquid surface. The situation is aggravated by a certain amount of foaming at the surface, especially at low pressure, and by the unfortunate fact t h a t the thermocouple does not pass cleanly from liquid to gas, but rather pulls up a film of liquid for an appreciable distance. Therefore the interpretation of the experimental data is not so clear-cut as might be desired. T h e results have been used, however, to propose mechanisms whereby the phase change from liquid t o gas occurs. Hildenbrand postulated simple vaporization and thermal decomposition; Steinberger favors a chemical attack by free radicals on the nitrate ester with the generation of volatile products. Potentially the most useful approach t o the study of flame structure in liquids is probably the one being pursued by Powling ( I S ) , who has established stationary flat flames of ethyl nitrate at atmospheric pressure and has probed these as to both temperature and composition. Unfortunately the greater part of the main reaction zone, as in the case of solid propellants, is too condensed and too close to the liquid surface to be accessible for detailed study. Nevertheless, substantial changes in the concentration of intermediate products have been detected as a function of distance from the liquid surface, and these have been interpreted by means of a radical chain mechanism involving NOz, CHB,and CH,O as chain-propagating radicals. I n an effort t o extend the primary reaction zone sufficiently to allow detailed analysis, the present author is carrying out similar flat flame studies on ethylene glycol dinitrate a t subatmospheric pressures. Preliminary data show that a t atmos-
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phere the primary reaction zone is only 1 mm. thick, a n indication of the experimental difficulties to be expected even under the most favorable circumstances. Low pressure spectroscopic studies have been carried out by Wolfhard and coworkers (8) on methyl nitrate flames. Techniques include the establishment of flat flames on cylindrical burners, slot burners, and circular sintered stainless steel burners, as well as the use of twin slot burners for diffusion flames. Both absorption and emission spectra have been obtained. Where possible t h e results have been correlated with observations on flames supported by the oxides of nitrogen (20). An interesting result is that in a methyl nitrite-oxygen diffusion flame all the methyl nitrite disappears before the onset of the first luminous zone. This fact has been used to emphasize the importance of so-called pre-flame reactions. The following mechanism is proposed :
+ NO,
LIeOX02---c 1 I e O
NO, Me0
+ O2
---c
CO
+
co + Hz
KO
+
‘/’~02
+ HB+ (organic products ?) 1/202
+ ‘/zO,
-
COS
H20
At low pressure nitric oxide is not reduced, and a t atmospheric pressure a direct reaction between M e 0 and NO is postulated. The distinguishing feature of this mechanism is the importance attached to the reactions of molecular oxygen, as opposed t o the more usual suggestion of direct reactions between oxides of nitrogen and oxidizable materials. Low pressure flat flames of gaseous nitrate esters, such as those used by Wolfhard, appear well-suited for probing studies. They are stationary, reasonably extended, and accessible for study all the way from the unburned t o the burned side. They offer the closest approximation available in the nitrate ester field t o the sort of flames found so uPeful in the study of hydrocarbon combustion. However, no such detailed investigations have been reported to date. Conclusions
From the foregoing it is clear that, although nitrate esters are very old materials in the propellant field, knowledge of the details of their combustion is exceedingly meager. Even the basic steps of the chemical reaction are in doubt. Moreover, any sub-
COURTESY OLE“
stantial increase in knowledge will require the utmost extension of the finest analytical tools available. Burning rate studies offer some hope of progress, provided they are very carefully designed t o test specific points. Somewhat greater promise, perhaps, is given by investigations of the fine structure of the flames as regards t o temperature and composition. The results of such studies can form the basis of mathematical formulations which are more refined than those presently available and which may help greatly in unraveling this complex problem. Literature Cited (1) Adams, G. K.. Parker, W. G., Wolfhard. H. G., Discussions Faraday Soc. 14, 97 (1953). (2) Adams, G. K., Scrivener, J., “Fifth Symposium on Combustion,” p. 656, Reinhold, h-ew York, 1955. (3) Adams, G. K., Tt’iseman, L. A., “Selected Combustion Problems,” p. 277, Butterworths, London, 1954. (4) Dickson, A. D., Rotenberg, D. L., Crawford, B. L., IKD.ENG. CHEM.48,759 (1956). (5) Doumani, T. F., Coe, C. 8 . (to Union Oil Co. of Calif.), U. S. Patent 2,645,079 (July 14, 1953). (6) Evans, 11.W., Chem. Reus. 51, 363 (1952). (7) Gibson, R . E., Crawford, B. L., Jr., others, J . Phys. & Colloid Chem. 54, 847 (1950). (8) Gray, P., Hall, A . R., Wolfhard, H. G., private communication; Proc. R o y . SOC.( L o n d o n ) A 232, 389 (1955). (9) Heller, C. X., Gordon, A. S., J . P h y s . Chem. 59,773 (1955). (10) Hildenbrand, D . L., Whittaker, A. G., Euston, C. B., I b i d . , 58, 1130 (1954). (11) Hildenbrand, D . L., Whittaker, A. G., I b i d . , 59, 1034 (1955). (12) Klein, R., hlentser, hl.,Elbe, G. von, Lewis, B , J . Phys. & Colloid Chem. 54, 877 (1950). (13) PonImg, J., “Selected Combustion Problems,” p 389, Butterworths, London, 1954. (14) Rekers, R . G., Villars, D. S.,Rev.Sci. I n s t r . 25, 424 (1954). (15) Semenov, K. K.,Progi. P h y s . Sci. (U.S.S.R.) 24, S o . 4, 433 (1940) [Translation, S a t ’ l Advisory Committee on heronautics Tech. Memo No. 1026 (1942)l. (16) Steinberger, R., Carder, K. E., J . Phys. Chem. 59, 225 (1955). (17) Steinberger, R . , Orlick, C. A., Schaaf, V. P., J . Am. Chem. SOC. 77, 4748 (1955). 118) . , Villars. D. 8.. “Selected Combustion Problems.” D. 387. Buttermorths, London, 1954. (19) Wolfhard, H. G., Fuel 34, 60 (1955). (20) Wolfhard, H . G., Parker, W. G . , “Fifth Symposium on Combustion,” p. 718, Reinhold, Kew York, 1955. RECEIVEDf o r re?iew December 30, 1955. ACCEPTED January 17, 1956. The Allegany Ballistics Laboratory is a facility owned by the U. S. Kavy and operated by Hercules Powder Co. under Contract NOrd 10431.
L . M A R T I N GO. HUOHES AIRCRAFT GO.
Matador surface-to-surface turbojet-powered missile on its mobile launching stand; solid propellant booster rocket i s used to a i d take-off
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Falcon air-to-air guided missile is small and light enough to be easily lifted b y two men
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