Propellants Manufacture, Hazards, and Testing

1 Manufacture of Cast Double-Base Propellant R. STEINBERGER

and P. D. DRECHSEL

Downloaded by HARVARD UNIV on June 24, 2013 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0088.ch001

Explosives and Chemical Propulsion Department, Hercules, Inc., Wilmington, Del. 19899

The cast double-base process has been developed into a highly versatile technique for manufacturing solid rocket charges. Propellants manufactured by this process provide a wide range of energies, burning rates, and mechanical properties. Free-standing and case-bonded charges are manufactured routinely for many military and space applications, varying in size from ounces to several tons. The process consists of two basic steps: (1) "casting powder" is manufactured, consisting of nitrocellulose, plasticizers, and any solid ingredients; (2) casting powder is combined in a mold with "casting solvent," consisting of the remainder of the plasticizer. When this mixture is heated to moderate temperatures, a monolithic propellant grain is formed by plasticizer diffusion. A mathematical model of casting and curing has been evolved which relates processing conditions and material properties to the structural behavior of the propellant grain throughout the process.

he cast double-base process is a technique for forming solid propellant charges (grains) based on nitrocellulose as the polymeric binder and nitroglycerin or other high energy liquids as plasticizers. The process consists of two essential steps: (1) Manufacture of casting powder: a product containing all the nitrocellulose and solid ingredients and a portion of the plasticizer is made i n the form of a right circular cylinder approximately 1 mm. i n diameter and length. (2) Casting and curing: casting powder is loaded into a mold; the interstices between the granules are filled with a casting solvent consisting of the remainder of the plasticizer. O n heating to moderate temperatures, interdiffusion of polymer and plasticizer occurs and knits the two-component system into a single monolithic grain. 1 In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


2

PROPELLANTS MANUFACTURE,

HAZARDS, A N D TESTING

Propellant Composition. In principle, any solid ingredient which is chemically compatible with nitrate esters can be introduced into the propellant by way of the casting powder. A n y liquid ingredient used in the formulation must also be a plasticizer or co-plasticizer for nitrocellulose. In practice, three major families of compositions have been developed, differing in the composition of the casting powder (see Table I for typical compositions): (1) Single-base casting powder: the casting powder consists of nitrocellulose, stabilizer, solid additives for ballistic modification, and a small amount of plasticizer. The normal ratio of casting powder to casting solvent, 2:1 by volume, yields a final composition of approximately 60% nitrocellulose. (2) Double-base casting powder: this differs from single-base casting powder i n that a significant amount of nitroglycerin is incorporated i n the casting powder. The resultant propellant is more highly plasticized and more energetic than that made from single-base casting powder. (3) Composite-modified casting powder: significant amounts of crystalline oxidizer and metallic fuel can be incorporated in double-base casting powder, yielding highly energetic propellant compositions. The nitrocellulose is even more plasticized in these compositions than in those made with double-base casting powder. Table I.

Typical Propellant Compositions

Single Base, %

Double Base, %

Ingredient

C.P.

Prop."

CP.

Prop.

CP.

Prop.

Nitrocellulose Plasticizer Ballistic Additives Solid Oxidizer Solid Fuel Stabilizer

88.0 5.0 5.0

59.0 36.0 3.4

75.0 17.0 6.0

50.2 44.0 4.0

30.0 10.0

22.3 32.8

2.0

1.6

2.0

1.8

28.0 29.0 3.0

20.8 21.6 2.5

a

— —

— —

— —

— —

Composite, %

—

—

° Casting powder. Propellant. b

From the manufacturing point of view these three classes of compositions differ mainly in terms of safety precautions, necessitated by incorporating nitroglycerin and/or solid oxidizers and reflected i n equipment differences, remote operations, etc. The drying time for single-base casting powder is longer than for double-base and composite casting powders. History. The cast double-base process was developed under U.S. Government auspices during W o r l d W a r II, the initial work being done by Kincaid and Shuey (7). The process filled a need for rocket charges significantly larger than those conveniently made by the then existing extrusion processes.

In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


1.

STEINBERGER A N D DRECHSEL

Double-Base Propellant

3

The early work was done with single-base casting powder i n producing cartridge grains for such applications as aircraft JATO's, sounding rockets, and guided missiles. The success of the cast double-base approach was attributed in great part to the development of a ballistically attractive family of "plateau" propellants (8, 9) which utilized various lead salts of organic acids (e.g., lead stearate) to create low temperature coefficients and low sensitivity of burning rate to pressure. As the need for higher energy became pressing, double-base casting powders were developed which maintained the excellent ballistic properties of the earlier propellants. Moreover, since these compositions were more heavily plasticized, they were more suitable for case-bonded applications in which the propellant is cast directly into the motor and bonded to the wall; such applications demand a low modulus of elasticity and a high strain capability in preference to high strength. Case-bonded doublebase propellants were first used i n space launch vehicles—e.g., Project Vanguard and Scout. Another substantial increase i n delivered energy was obtained b y incorporating substantial amounts of solid oxidizer (e.g., ammonium perchlorate) and metallic fuel (e.g., aluminum) i n the casting powder. The resulting family of composite-modified double-base ( C M D B ) propellants has found widespread use i n ballistic missiles and space motors. Accompanying the development of propellants has been an evolution of more complex grain designs demanding ever-increasing sophistication in tooling and casting techniques. Early examples of cast double-base propellants were straight cylindrical charges cast into cellulose acetate inhibitors and featuring star-shaped perforations for maintaining a constant burning surface (Figure 1). T o accommodate the requirement for high thrusts and short burning times of larger charges with propellants having low burning rates, the multiperforated charge design evolved. In principle, this consisted of concentric rings of propellant connected by struts; i n practice, it involved a complex array of arc-shaped mold cores held in place by appropriate base plates and spiders, still inside a cylindrical inhibitor tube. A t the same time, the slotted tube design was introduced for longer burning applications. The straight cylindrical configuration was appropriate for cartridgeloaded motors—i.e., motors which permit the separate manufacture of propellant grains and subsequent loading into the chambers. More efficient designs, i n terms of volumetric loading and mass fraction of propellant, are possible i n case-bonded motors. This approach was first used with cast double-base propellants i n the Altair motor for Project Vanguard, Scout, Delta, and other space missions. I n such designs, it became appropriate to fill both domes of the motor with propellant and to work through relatively small openings at either end of the motor case


4

PROPELLANTS

MANUFACTURE,



for introducing mold components and propellant. Star perforations and slots were still used for ballistic control; however, the geometric design problem shifted from a predominantly two-dimensional to a threedimensional one. A n added degree of complexity was introduced with the development of multinozzle configurations (typically four), demanding appropriately shaped channels leading to each nozzle.

Figure 1.

Propellant grain design

Manufacture of Casting Powder Casting powder consists of right circular cylinders with a length-todiameter ratio ( L / D ) of approximately unity; the casting powder comprises the polymeric binder (nitrocellulose), all other solid ingredients, and a fraction of the plasticizer and stabilizer intended for the final composition. The manufacturing process borrows heavily from that used for smokeless powder intended for guns, differing only where the incorporation of new ingredients demands special techniques. Process Steps. Casting powder manufacture is basically a plasticsforming operation, utilizing a volatile solvent to impart mobility to the polymeric binder. The process is shown schematically i n Figure 2 i n terms of the major equipment used. F o r discussion, the process may be divided conveniently into the following operations:


1.



5

(1) Mixing. A l l ingredients are blended i n the presence of solvent to produce an extrudable dough. (2) Granulating. The dough is shaped into right circular cylinders by extrusion into strands which are then cut. (3) Drying. Volatile solvent is removed by exposure to a circulating hot fluid.


(4) Finishing. The granules are glazed with graphite, blended into large homogeneous lots, and screened to remove clusters, dust, and foreign material. Many variations are practiced in each of these steps, depending on the composition, the facilities available, and the individual experience of the manufacturer. Since space is not available to treat each of these variations here, we shall describe a particular set of techniques used at one plant. For a more complete treatment of smokeless powder manufacturing techniques, we refer the reader to Refs. 2 and 3. M I X I N G . Nitrocellulose is introduced into the process as alcohol-wet material in a lumpy condition. Depending on the needs of the subsequent process, it may be subjected to agitation in heavy duty mixers to reduce the size of the lumps.

For convenience in introducing nitroglycerin, a solution of nitroglycerin in a volatile solvent (typically acetone) is blended with the nitrocellulose i n a Schrader bowl, a low shear mixer with large clearances. This is called premixing. The resulting premix serves as a feed stock for the final mixer, a heavy duty, horizontal, sigma-blade mixer. The mixer is jacketed to provide for circulation of hot or cold water during various stages i n the mixing operation.

BLENDING

Figure 2.

CASTING POWDER

Casting powder manufacturing process


6

PROPELLANTS MANUFACTURE,


The mixer serves three purposes. First, it blends all the ingredients to provide uniform distribution in the final propellant. Second, it provides time, heat, and contact for solvation of all or part of the nitrocellulose by the volatile solvent and plasticizer. Third, it provides mechanical energy to disrupt nitrocellulose fibers and expose them to solvation. Solvated nitrocellulose is the matrix which bonds the rest of the material together and eventually gives strength and elasticity to the finished propellant. A t this stage many parameters control the uniformity and subsequent extrusion and handling characteristics of the product—e.g.,


(1) Solvent composition. Mixtures of ether or acetone and alcohol are commonly used as solvents. (2) Solvent amount. The amount is largely determined by the amount of nitrocellulose i n the composition. Figure 3 provides an example of the effect of solvent composition and amount on density. SOLVENT LEVELS , % X

O A

£

26.1

(STANDARD)

29.1 32.1

1.910

! 1.900

1.890

50:50

60 : 40 (STANDARD)

70 : 30

ACETONE-to-ALCOHOL

80:20

RATIO

Figure 3. Effect of processing solvent on density of casting powder (3) Order of adding ingredients. It is usually advantageous to provide for partial solvation of nitrocellulose first, thus establishing a viscous matrix which helps distribute the remaining ingredients. (4) Temperature. Temperature is maintained just above the boiling point of the solvent to displace air without excessive loss of solvent. (5) Time. A n example of the breakup of agglomerates oxidizer as a function of mixing time is given i n Figure 4. (6) Direction of blade rotation. constant.)

of solid

(Blade speed is normally kept

(7) Solvent removal. When dough is oversolvated for any reason, the mixer l i d is opened and/or air is circulated over the mixer charge to remove excess solvent.


1.



7

Quantitative techniques are not commonly used for characterizing the dough at the conclusion of the mix cycle. Appearance of the mix and manual evaluation of consistency form the basis for determining whether the dough is mixed adequately and suitable for extrusion. Some progress has been made, however, i n the use of a test pressing. In this test, a portion of the charge is loaded into a press, extruded at a standard rate, and the resulting pressure noted. This technique has been quite successful in providing mix-to-mix uniformity once the proper test pressure has been established empirically for a particular formulation.


Z30
.... —

y

) y

y

y

y

\r< y

^

40

60

PERCENT OF

Figure 7.

y

LOT A

80 IN

100

BLEND

Burning rate of casting powder blends

1.310

30 60 BLENDING A N D

Figure 8.

90 120 150 1 80 G L A Z I N G TIME, MINUTES

Effect of tumbling time on screen loading density

A n important side effect of the glazing and blending operations is an increase in S L D , traceable to a general smoothing of the granule surfaces by attrition. A typical set of data is given in Figure 8.



12

PROPELLANTS M A N U F A C T U R E ,


Finally, the casting powder is screened to remove clusters and foreign material and then packed into containers typically holding 1 5 0 lbs. Casting Powder Parameters. The steps in the foregoing description of casting powder manufacture are designed to yield certain characteristics in the final product. The properties desired in casting powder arid the significance of these properties are described below. D E N S I T Y . The density of the casting powder should be the highest possible, normally higher than 9 7 % of theory. Deviations arise from two sources: voids and volatiles. Voids represent air which may not disappear during final processing of the propellant and may, therefore, lead to pits or porosity in the grain. The resulting ballistic effects can be severe, depending on motor design and inherent ballistic properties of the propellant. Mechanical properties w i l l also be degraded. Volatiles, such as moisture and processing solvent, are always present to a certain extent. Complete elimination by drying is not feasible. Volatiles act as plasticizers and coolants, and their presence is taken into account in the normal evaluation of a propellant. Excess amounts, however, w i l l alter mechanical properties and curing characteristics, paralleling the effect of excess inert plasticizer. Minor ballistic changes w i l l also result. M O I S T U R E A N D V O L A T I L E S . These are measured as independent quantities, largely as an indication of the efficiency of the drying process. Possible undesirable side effects were mentioned above. As nitrocellulose is slightly hygroscopic, casting powders w i l l often pick up small percentages of moisture from the atmosphere (see Figure 9 ) . Normal total volatile contents range from 0.5 to 1 . 0 % . i

^

0.80

O

40%

[

0.60 0.40

Propellants Manufacture, Hazards, and Testing

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