of about 3. T h e smaller the surface area, the less the possibility for increased collisions between molecules of gas and graphite surface. On the basis of this supposition, commercial graphite should encounter increased collisions and a higher reaction rate. This was the case. Summary
Pyrolytic graphite does not oxidize as rapidly as ordinary graphite below 1450’ F., but approaches the oxidation rate of commercial graphite a t 1550’ F. Oxidation of pyrolytic graphite proceeds preferentially in the c direction, contrary to expectation, and may be explained by the available energy associated with the strained condition of the crystal lattice and the exposed edges of the stacked platelets. The break in the Arrhenius plot for pyrolytic graphite occurring at 1550’ F. is attributed to the sudden change in the difference between the air temperature and the graphite surface temperature. The lower activation energy for pyrolytic graphite is associated with a low reaction rate. This anomalous behavior is
attributed to its lower porosity, since a less porous materia1 exposes a smaller surface area for molecular collisions. literature Cited
(1) Blakely, T. H., “Gasification of Carbon in Carbon Dioxide and Other Gases at Temperatures above 900’ C.,” Carbon Proceedings Fourth Conference, Symposium Publications Division, pp. 95-105, Pergamon Press, New York, 1960. (2) Gulbransen, E. A., Andrew, K. F., IND. ENG.CHEW 44, 1034-8 (1952). (3) Heindl, R. A., Mohler, N. F., J . Am. Ceram. SOC. 38, 89-94 11955). \-.--,-
(4) Kuchta, J. M., Kant, A., Damon, G. H., IND.E N ~CHEM. . 44, 1559-63 (1952). (5) , , Levv, M., Tarpinian, A., “Oxidation Resistance of Some Commercial Grade Graphites,” Watertown Arsenal Laboratories, Tech. Rept. WAL TR 825.4/1 (1959). (6) Mauer, F. 4.,Instrumentation 7, No. 4,36-7 (1954). (7) Spolaris, C. N., “Role of Surface Area in the Kinetics 01 Oxidation of Graphite,” U. S. Atomic Energy Comm., Rept. HW-31928 (1954). (8) Tu, C. M., Davis, H., Hottel, M. C., IND.ENG.CHEM.26, 749-57 (1934). RECEIVED for review January 3, 1961 ACCEPTEDSeptember 20, 1961
AMMONIUM NITRATE PROPELLANTS ON A POLYESTER-ACRYLATE BINDER A. 0 . D E K K E R A N D G. A. Aerojet-General Carp., Sacramento, Calif.
Z I M M E R M A N
Ammonium nitrate propellants have been developed to provide a source of noncorrosive and nonerosive primary and secondary power. Castable compositions with good mechanical properties were made with a polyester-styrene-methyl acrylate binder. Castability was achieved, in spite of 82y0 solids loading, b y adjusting the particle size distribution of the solid oxidizer and the viscosity of the binder. Ignition and Adiabatic curing was shown to b e practical combustion were obtained by addition of 2% (NH&Cr*O,. with this propellant. These propellants have found wide application in areas where high performance is not a criterion. They provide clean, relatively inert exhaust and operate at temperatures from -75’ to
+180’ F.
HE DEVELOPMENT OF A COMPOSITE PROPELLANT
based on
TWH4N03 as the oxidizer was initiated in 1949 to provide
,a low-cost, smokeless composition for general use in rocket motors. This accomplishment was based on the earlier development of the KC104 and NH4C104 propellants (7, 3 ) . A family of solid propellants using polyester-styrene-acrylate binders with “,NO3 as the oxidizer was developed and widely applied, particularly in Jatos and gas generators. One application of these propellants is in the Aerojet Jr. Jato motor, an auxiliary or emergency power unit developed for use on small aircraft (Figure 1). Other uses for these compositions have been extensive but are all classified. The use of this oxidizer presented many inherent problems such as hygroscopicity, crystal phase changes, low density, and difficult ignition; however, grains as large as 300 pounds have been made and used successfully in motors a t temperatures
from -75’ to 180’ F. Because of the unique properties of these propellants-Le., an exhaust which is smokeless, noncorrosive, and nonerosive, low burning rate, and low flame temperature-they have filled a need which propellants using other available oxidizers could not provide. However, because of the relatively low energy and density of the oxidizer, they have been limited to uses in which high performance is not required. The problems which must be solved in achieving a successful propellant have been discussed in general terms (#). This report discusses the specific problems solved in achieving a successful XH4N03 propellant. Propellant Composition
A typical polyester-acrylate “,NO3 VOL. 1
NO.
propellant AMT-2011 1
MARCH 1962
23
(Table I) consists primarily of NH4NOa as the oxidize, and a polyester-styrene-acrylate fuel binder with small amounts of polymerization catalysts and ballistic modifiers. Fuel Binder. T h e binder in this propellant is the Genpal A-20 polyester resin, styrene, and methyl acrylate together with the necessary polymerization catalyst (methyl ethyl ketone peroxide) and an accelerator (cobalt octoate or naphthenate). Polyesters were selected because propellants made from them can he cast and cured in large masses a t low temperatures, and they can he readily modified with other monomers to provide a variation in mechanical properties and elemental composition. These materials are also relatively stable and thus offer potentially superior aging characteristics. T h e selection of the ratio of the binder components was based on two considerations: optimizing mechanical properties of the cured propellant and obtaining a high percentage of oxygen in the binder. To achieve castability in the uncured propellant, it is necessary to minimize the solid oxidizer content, which in turn requires that a high percentage of oxygen be in the binder to maintain a carbon-free exhaust. The formulation work toward this goal was guided by a mle-ofthumb approximation that there must he enough oxygen available in the propellant to burn two thirds of the hydrogen to water and all of the carbon to CO. The use of methyl acrylate in the binder makes it possible to achieve the required oxygen content without a deleterious loss in mechanical properties. The fuel viscosity was adjusted by varying the polyester-tomonomer ratios to achieve a castable composition. Oxygen balance was maintained by varying the styrene-to-methyl acrylate ratio while maintaining a polymer system with adequate mechanical properties. As a first approximation for formulation, an empirical extension of the Vand equation (2, 5, 7)for the viscosity of suspensions was used to estimate the maximum concentration of solids permitted in a castable suspension. A graph based on these calculations and simplified for illustration is shown in Figure 2. Polymerization Catalysts. A comparison of the polymerization rates with cumene hydroperoxide (CHP), methyl amyl ketone peroxide (MAKP), and methyl ethyl ketone peroxide (MEKP), accelerated with cobalt naphthenate, is shown in Figure 3 for a typical polyester-styreneemethyl acrylate (GSMA) binder. Methyl ethyl ketone peroxide (MEKP) is the best catalyst because it gives a gel time long enough for processing with a cure that is relatively fast. Cobalt naphthenate or octoate is used to accelerate the rate of polymerization. The addition of small amounts makes it possible to vary the gel time over a period of several hours. Gel times can thus be achieved which are long enough to permit mixing and casting yet short enough to prevent settling of the oxidizer. Lecithin. Lecithin is used in the propellant to reduce the apparent viscosity and thereby improve the castability of the
Table 1.
system. The high solids content of these systems and the accompanying high viscosity necessitates the use of auxiliary aids to improve fluidity. Figure 4 demonstrates the effect of lecithin on propellant viscosity and also shows that minimum viscosity is achieved a t different lecithin concentrations for each of the oxidizer systems used. Oxidizer. The C.P. grade of ammonium nitrate is used to avoid interference of contaminants with polymerization. T h e use of NHhNOa as the oxidizer makes it possible to obtain a smokeless, nonemsive, noncorrosive exhaust, the products of combustion being N2, Hz, HzO, CO, and Con. The upper limit for concentration of NHdNOa is determined by either the flame temperature required or, a t higher concentrations, castability or mechanical properties. Increasing the oxidizer concentration produces an increase in the specific impulse and flame temperature, both of which reach a maximum at the stoichiometric concentration of about 92% NHnN03 in this fuel system. The burning rate also increases with an increase in oxidizer. However, the castability decreases and the mechanical properties of the propellant hecome marginal as the solid oxidizer content is increased. The practical limit of castability is reached a t approximately 82% oxidizer. To achieve a castahle propellant with the high solids content required for the desired performance, it is necessary to use oxidizer of two or more particle size distributions, which are obtained by grinding. By using two particle size ranges in which 70% of the oxidizer is a coarse grind and 30% is a fine grind and in which the fine oxidizer has an average particle size not greater than approximately one sixth that of the coarse material, it is possible to obtain a high bulk density for the oxidizer. Only a small quantity of uncured binder is thus required to make the propellant castable. Still further improvements can be made by introducing a third distinctly different particle size. The advantages of a bimodal blend of oxidizers is shown in Figure 5. Ammonium dichromate is used to modify the ballistic and combustion characteristics of the propellant and is necessary to obtain ignition and sustain combustion (6). Manufacturing Processes a n d Control In-plant processing of raw materials is limited to grinding
NHdVOa and distilling the methyl acrylate. The NHtN03
Composition of AMT-2011 Propellml
wt.% NHlNOs
Genpol A-20 polyester resin Methyl acrylate Styrene Methyl ethyl ketone peroxide Cobalt octoate (1% in styrene) Lecithin (10% in styrene) (NHJrCrtO,
24
72.79 9.79 12.22 2.22 0.49 0.25 0.25 1.99
100.00
I a E C P R O D U C T RESEARCH AND DEVELOPMENT
Figure 1. Emergency power for small aircraft is furnished by Jr. Jato rocket engine nacelle installation on Beech Model 50 airplane
POLYESTER RESIN
Figure 2. Nomograph guides selection of initial propellant compositions to provide fuel composition and viscosity necessary to achieve "smokeless" propellant at a given oxidizer concentration Dotted lines indicate constant fuel viscosity; solid line of wing shows calculated curve for maximum percentage solids allowed for castability as a function of fuel viscosity. All formulations in region below "smokeless and castable" solid line will be processable and free of carbon in the exhaust
is dried a t elevated tempexatures, then ground in standard commercial grinders to achieve the desired particle size distribution; it is delivered to the mixing stations in sealed hoppers for introduction into the mixer. The methyl acrylate is distilled to remove the inhibitor used to prevent polymerization during shipment. The liquid ingredients, except the catalyst, are added to a standard internal sigma-blade mixer, and the oxidizer is discharged continuously and remotely into the mixer through a brush feeder outlet in the hopper. The ballistic modifiers are introduced during the course of oxidizer addition. After all the oxidizer has been added and mixed into the fuel, the polymerization catalyst is added and mixed into the propellant under vacuum of 5 to 10 cm. of Hg to eliminate air entrapment. The propellant is then cast into molds under vacuum. All of the above operations are perlormed in a controlledhumidity atmosphere to prevent moisture pick-up in the propellant. The mixing temperature is controlled at 70' + 5' F. to prevent premature polymerization and reduce volatilization of the fuel components. T o avoid the loss of volatile fuel ingredients during mixing, the vacuum line is fitted with a condenser to return vaporized components to the mixer. The castings, or grains, are then placed in conditioning rooms to polymerize or cure. The curing schedule of the propellant, dictated by the polymerization reaction, is designed to prevent cracking of these large castings from excessive strains (induced by expansion and shrinkage) and to obtain optimum mechanical properties. Since the polymer-
12 A
I
Ill
1%
CHP (0.528 Md XI, 0.011% Co
0 l % M A K P ( 0 . 3 5 8 M o l W , OOiI%Co
ELAPSED
TIME
I
, MIN.
Figure 3. Polymerization rate of polyester-styrene-methyl acrylate fuel at 75" F. as determined by shrinkage, showing effect of various peroxide catalysts VOL. 1
NO. 1
MARCH 1962
25
release and the thin polymer-rich skin, which would interfere' with ignition. Then the grain3 are usually inhibited with an inert coating on one or more of the surfaces, leaving only the desired initial burning area uncoated. The inhibitor is composed of polyester and styrene with an inert filler and is potted into place. The finished grain is then ready to be loaded into the motor chamber. Process and Quality Control. Rigid controls are exercised to ensure that a reliable and consistent product is produced. The raw materials must meet chemical, physical, and usespecification standards. The process controls exercised are : adjustment of the propellant gel time, guided by prior fuel gelation tests; particle size analysis of the oxidizer grinds; moisture determination of the fuel ingredients and oxidizer; measurement of the uncured propellant density; analyses of the uncured propellant; and measurement of gelation time. Final acceptance is based on performance in small-scale o r full-scale static test firings.
IO000
BIMODAL OXlMZ W
In
5
n
0
2
100
5
I
I
I
t
7
,
I
U 10
1
I
I
l
1
I
I
[L
I
1
1
I
I
Table II. Mechanical and Physical Properties of AMT-2091 AX Propellant Autoignition temp. 330' F. Impact sensitivity (Bureau of Mines) 200 kg. cm. Density 0.056 lb./cu. in. Tensile strength, 60" F. (av.) 270 p.s.i. Elongation, 60' F. (av.) 11%
ization reaction is exothermic, the simplest and most reliable method of following the reaction is to follow the heat rise in the grain: The grains begin cure at 70' F. and are subsequently moved to 180' F. for 24 hours to complete the cure. The move from the 70' F. room is made after the propellant has reached its exotherm peak, as determined by a thermocouple appropriately placed in the grain, normally about 72 hours. Adiabatic curing of the propellant is also practical in large grains. After curing, the grains are removed from the molds and further processed as required for the end use. The grains are normally machine finished to remove traces of the wax mold
Thermodynamic Properties of AMT-2091 Propella nt Theoretical specific impulse at 1000 193 lb. force sec./lb. mass p.s.i.a.a Measured specific impulse at 1000 190 lb. force sec./lb. mass p.s.i.a. 2240' F. Theoretical isobaric flame temp. 20.0 Molecular weight of gases, M Effective K = @/Cv 1.24 Theoretical mass flow coefficient, C, 0.0082 lb. mass/lb. force sec. Experimental mass flow coefficient, 0.00813 lb. mass/lb. force C, sec. Table 111.
a
Shifting equilibrium.
: W
n 6 00 4 00
I /
! I
1
I
!
/FINE
I
1
-f-
I
>r c_
v)
0
8> I-
z
W
a U n n
U
20 I
COARSE 100 0 FINE
1
I
I
1
I
I
90 IO
80 20
70
60 40
SO
30
40 60
SO
I '50 70
I
20 80
I IO
90
I 0 100
Figure 5. Effect of blends of fine and coarse NH4N03 on viscosity of a propellant containing 64.0 vol. % solids and 0.025 wt. % lecithin (fuel viscosity = 20 cp.) at 17.2-second shear rate 26
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
0.15
f 0.10
-c- 0.06 L
. )
w
sa 0.04
500 800 lo00 20oo CHAMBER PRESSURE Pc P.S.I,A,
4Ooo
Figure 6. Burning rate of NHdN03 propellant, AMT-2091, at various temperatures and pressures
Properties
Mechanical and Physical. These propellants are rugged and relatively insensitive to shock and rough handling. They have been successfully fired in grains as large as 300 pounds, to +I8Oo F. Their over temperature ranges from -75' mechanical properties, along with their inherent shrinkage in cure, limit their general use to free-standing grain configurations. The properties of a typical propellant, AMT-2091 AX (similar to the AMT-2011 propellant except that it contains 75y0oxidizer), are given in Table 11. Ballistic and Thermodynamic. The properties peculiar to the use of these compositions as propellants in rocket motors are their ballistic and thermodynamic properties. The theoretical thermodynamic properties are obtained from calculation of specific impulse (Table 111). Ballistic parameters are best shown in graphical form (Figure 6).
Acknowledgment The work of numerous coworkers a t Aerojet included in this report is acknowledged-in particular, that of G. W. Batchelder (currently a t Douglas Aircraft Co., Santa Monica, Calif.), J. S. Billheimer, G. W. Chew, E. Mishuck, and K. H. Sweeny. literature Cited (1) Brown, C. O., Dekker, A. O., Aerojet Gencral Corp., Sacramento, Calif., unpublished work, 1961. 2) Geckler, R. D., Sweeny, K. H., J. Appl. Phys. 25, 1135 (1954). 3) Hepner, F. R., Dekker, A. O., Aerojet General Corp., Sacramento. Calif.. unpublished work. 1961. M'Isduck, E.', Carleton, L. T., IND. ENG.CHEM.52,754-60,1960, 5) Sweeny, K. H., 135th Meeting, ACS, Boston, Mass., April 1959. (6) Taylor, J., Sillitto, G. P., Third Symposium on Combustion, Flame and Explosion Phenomena," pp. 572-9, Williams & Wilkins, Baltimore, Md., 1949. (7) Vand, V., J.Phys. €3 Colloid Chem. 52, 277-314 (1948). RECEIVED for review March 22, 1961 ACCEPTEDSeptember 19, 1961
I I"'
VOL.
1
NO. 1
MARCH 1962
27
..
