BRUCE F. GREEK, Associate Editor in CHARLES F. DOUGHERTY and WALTER Rocketdyne, a division of North America Solid Propulsion Operations, McGregor,
Solid Propellants The very nature of the materials involved in solid propellant compounding and casting defines plant lay out and processing equipment to be used.
SOLID
PROPELLANTS as contrasted with liquid propellants appeared on the ballistic scene soon after gunpowder. As might be expected, their growth, though slow, followed in spurts with almost each war until World IVar 11. During that war and immediately thereafter, use of liquid propellants in large rockets began to grow fast, while solid propellants firmly established their place in smaller rockets from the bazooka size UP. At about this same time, interest also increased in using solid propellants in applications other than missile propulsion. One of the first major uses was jet assisted take off (JrlTO) as an aid in launching heavily loaded jet bombers such as B-47. T o make JATOs in quantity needed for extensive use with the B-47s, the Air Force began a program to develop mass production. Phillips Petroleum, through its subsidiaries, makes ammonium nitrate, synthetic rubber, and carbon black, all important ingredients in many low cost propellants. I t was selected as contractor to re-activate the Bluebonnet Ordnance Works, known now as Air Force Plant 66 a t McGregor, Tex., and to set u p a mass production facility for JATOs. Mass production of JATOs began in 1955. Later North American Aviation and Phillips formed a joint company, Astrodyne, Inc., to operate the production line for JATOs and the nearby pilot plant and test facilities. After producing the JATOs a t various rates, supplies
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were deemed adequate and the mass production line shut down in February 1959. Over 145,000 JATOs were produced. Research and development continued on solid propellants for improved JATOs, boosters, and other applications.
Typical Propellant Formulations Ingredient
Weight Per Cent
CASTABLE PROPELLAXT Liquid Polymer 10.8 Plasticj zer 3.0 Curative 1.o Metal Powder 16.0 Oxidizer 68.0 Catalyst 1.o Antioxidant 0.2 100.0 EXTRUDABLE PROPELLANT Rubber Polymer 12.0 Filler 2.5 Plasticizer 2.5 Curative 0.5 Antioxidant 0.4 Oxidizer 80.0 Catalyst 2.1 100.0 In 1959, North American Aviation purchased Phillips’ share of Astrodyne and incorporated the McGregor facilities into the company’s Rocketdyne
INDUSTRIAL AND ENGINEERING CHEMISTRY
division. Rocketdyne does liquid propellant rocket engine development and manufacturing a t Canoga Park, Calif., and Neosho, Mo. Processing and applied research in solid propellants and development of advanced new solid propulsion systems now is done at McGregor. A411basic research is done in California. -4s originally set up, the development and pilot plant activities concentrated on propellants of the JATO-type which are formed by extruding a mixture of the ingredients. The size of rocket motors using extruded propellants is limited by such factors as viscosity of the materials, particularly the rubbery binder, complexity of propellant grain design, and by economics of manufacture by the powerful mixers and extruders needed. TVhile actual mechanical limits of such machines would permit moderately large extruded propellant grains to be made, practical mechanical limits and safety considerations emphasized the need for another approach to large solid propellants. The next generation of large solid propellant rockets was made by assembling relatively small extruded pieces. Known as modular design, this technique has resulted in such propellant units as the Zel booster made by Rocketdyne. I t launches an F-100 fighterbomber without a conventional runway. The booster produces 130,000 pounds of thrust for 4 seconds. But modular design propellants always retain an important drawback-the sup-
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ports for the individual grains, usually steel spacers and rods, lower effective payload of the rocket or booster. Since still larger solid propellant rocket motors were needed, Rocketdyne scientists began research and development in the use of less viscous castable propellants. Binder materials were selected which could be permitted to remain relatively fluid during mixing and then cast or poured into the rocket case. I n turn the casting process allowed the higher energy oxidizers to be used safely in propellant grains. The results from development of the casting techniques are leading to ever larger rocket motors. Where the largest practical extruded grain reached 16 inches in diameter, cast grains of 100 inches in diameter now are possible. Rocketdyne modified the original pilot plant facilities to permit casting grains ~p to 54 inches in diameter and 13 feet long, weighing nearly 15,000 pounds. Another highly critical component of solid propellant rocket motors is the ignition system. At the McGregor plant Rocketdyne designs, develops, and fabricates igniters tailored to meet specific operating requirements. An igniter must give rapid and reproducible starting to propellant burning with a minimum of explosive shock. Most are electrically initiated, metal-oxidant-type pyrotechnic igniters. However, Rocketdyne is also working on other ignition methods to fit increasingly complex requirements of rocket propulsion systems. The various newer and more powerful types of propellants produced or developed by Rocketdyne have led to extensive changes in manufacturing techniques. Safety considerations in
Historical Changes in Explosive and Propellant Preparation Facilities 1232 Initial development of gunpowder by the Chinese. 1740 Establishment of French and English powder mills. Powder made by hand grinding of ingredients, hand blending, and sifting, then glazing or classifying in vats and kegs. Drying was done by storage in a warm, dry building or by air drying in the sun. Powder-explosive or rocket propellant had little reliability. 1760 Intense military use of rockets began. British cavalry unit decimated in India under barrage of 6- and 12-pound iron barreled rockets. 1801 Sir William Congreve develops highly successful rocket at Royal laboratory, Woolwich, England. 1812 Rockets used extensively in various military actions. French and Du Pont produced quality powder by improved methods. D u Pont used water power to drive crushing, grinding, and classifying equipment. Design of early mills in U. S. provided some measure of safety by including such items as heavy rock and mortar walls as barricades, blow-out roof and wall, and weak wall for blow-out facing river. 1836 Du Pont facilities in U. S. had mushroomed. Development of brown powder and blasting powder were result of Western expansion, War of 1812, industrial revolution. Research was directed to materials and products of increasing energy. 1845 Explosive and propulsion experiments by Schoenbein and Sobrero leads to development of guncotton, and a year later to nitroglycerine. 1860 General Rodman, U. S . Army Ord. Dept., proposes theory of progressive burning and control of burning rate using perforated grains and prismatic forms. 1865 Safe method to produce guncotton by a pulping process or wet pulp method developed. Schultze successfully gelatanized nitrated wood and impregnated it with barium and potassium nitrate to produce the first smokeless powder. Smokeless powder development expands in England, Germany, France, and the U. S. Chemical processing facilities begin to replace powder mills. Equipment to press and extrude perforated grains and prismatic forms were coupled with temperature control, metering, and pumping of sensitive materials in new plants. 1940 Du Pont and other chemical companies developed huge facilities to handle semicontinuous processing of such explosive materials as smokeless powder, TNT, and nitroglycerine for military and commercial use. Solid propulsion theory of stable burning advanced by Von Karman and Malina of Guggenhem Aeronautical Lab and California Institute of Technology. A wide range of rocket and gas generator applications resulted. 19?? New facilities installed to keep pace with trend in propellants toward larger grains and more sensitive and energetic materials.
LOWER FLOOR
This is the proposed layout of the new research facilities
UPPERFLOOR VOL. 52, NO. 12 e
DECEMBER 1960
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PLANT PROCESS SCRlCS
SCREENING WEIGHING
OXIDIZER DRYING GRINDING CLASSIFYING WEIGHING PACKAGING
PROPELLANT MIXING
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PROPELLANT CASTING
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PROPELLANT CURING
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MANDREL REMOVAL
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INGREDIENTS
PROPELLANT TRIMMING
LlOUlD POLYMER CUR AT I YE CATALYST ANTIOXIDANT
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EQUIPMENT
ADHESIVE PREPARATION
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OXIDIZER DRYING GRINDING CLASSIFYING WEIGHING PACKAGING
,t',",',;
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I PROPELLANT MIXING
PROPELLANT FORMING
GRAIN PRE-CURING
FINAL GRAIN CURING
FINAL TRIMMING
PROPELLANT CHEMICALS
RESTRICTOR MIXING
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MOTOR ASSEMBLY
FINAL MOTOR INSPECTION AND LEAK TEST
RESTRICTOR SHEETING MILLING
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ADHESIVE PREPARATION
CASE AND HARDWARE CLEANING
INSULATION
INSULATION PREPARATION
Process diagrams for castable (top) and extrudable (bottom) solid propellants
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INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Staff-Industry particular caused many changes in processing methods. As the trend toward more energetic materials continues, future operations will require additional changes such as: 0 Reducing quantity of material being processed at one time. 0 Further isolating processing equipment and material from personnel and other equipment.
Changes in Extrudable Propellant Processing
Though a t a disadvantage for large high energy applications, extruded propellants have properties that still make them useful. Besides use in smaller sized rocket motors, extruded propellants are finding more use in gas generators for powering auxiliary motors in large rockets, some of them with main motors liquid fueled. The latter use stems from their generally lower burning rates, low burning temperatures, and noncorrosiveness. Other general advantages of extruded propellants include low cost materials, excellent physical properties, good reliability over a wide range of temperature and weather conditions, and clean burning. These propellants burn with exhaust temperatures in the 1800' to 2000' F. range. Demands for higher energy led to use of perchlorate oxidizers in solid propellants. Processing these materials initially required minor changes in equipment since they are more sensitive than the nitrates. Also, safety for personnel and equipment caused some changes in processing operations. Environmental control became more important for the higher energy materials. Both temperature and humidity during processing had to be controlled more completely and to closer limits. More vessels, mixers, and storage containers were jacketed. The variable temperature water system for these jackets was extended, and control made more sensitive. Vacuum operation also was extended to additional steps. Rocketdyne uses steam ejectors to achieve 0.25 to 0.5 inch of H g pressures. The pilot plant communications system received revisions to make it more complete and faster acting. Under the communications system, Rocketdyne includes the detection and counter-measures devices for fire. Throughout the facilities, heat-actuating devices (HAD) sense abnormal temperatures. They trigger the deluge fire water system and signal the central control section a t the security station. While the HADs have been improved, Rocketdyne is aiming to cut the reaction time to 50 milliseconds. Normally, HADs may have
This drawing shows the over-all layout of the plant and the position of each process ing section
a n allowable reaction time of u p to 5 seconds. Instrumentation plays a very important part in controlling processing and in safety at the pilot plant. Tempera-
ture measurement has been extended to cover seals, shafts, and mixer bowls as well as water jackets, dryers, and curing ovens. I n the case of extrusion, pressure, extrusion rate, die design, and
The Quest for Higher Specific Impulse Specific impulse, simply defined as the thrust per pound o f propellant that burns per second, i s the major measure of a propellant's effectiveness. Other properties, such as physical strength, are important, but none are as vital as specific impulse. Like mountain climbing with its various base and supply camps at increasing altitudes, the development of solid propellants that have higher and higher specific impulse will come in steps. At present, the extruded propellants fall into the first base camp and they have specific impulses in the 160 to 200 range. The other base camps are roughly: 0 245-250 Specific Impulse-Rocketdyne is now conducting research and development in new cast propellants that reach this general level. Flexibility and other physical properties of propellants o f this impulse generally equal the good properties of the extruded propellants. 0 255-260 Specific Impulse-Now in laboratory development stages, propellants of this level will probably contain light metal hydrides and compounds containing nitro groups. Some loss in physical properties may occur. 275-280 Specific Impulse-At this level rubbery or plastic binders will no longer be useable. Here the handling situation i s comparable to trying to get dry sugar and salt to stick together as they are mixed. Potential ingredients include metal hydrides, fluorine-nitrogen compounds and exotic perchlorates. Encapsulation appears useful in handling such reactive materials to make good solid propellants. 0 300 Specific Impulse-The theoretical maximum for solid propellants is determined b y free energy calculations. To reach this summit will require extensive basic research into synthesis of many compounds.
VOL. 52, NO. 12
DECEMBER 1960
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R E M O T E CONTROLLED M O B I L E HANDLING EQUIP
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U T I ~ I T I E S8 CONTROLS
\ EQUIPMENT 8 CONTROL AREA
BY-PASS AND SURGE A R E A
Here i s a typical testing cell showing the remote control arrangements
T -!he
Rocketdyne Quickmix Process In 1959, Rocketdyne, under an Air Force contract, initiated development of a continuous process to mix solid propellant ingredients. The process, known as Quickmix, mixes propellant ingredients faster, safer, with more uniformity, and at lower cost than conventional batch mixing processes. Briefly, the Quickmix process operates like this. First the oxidizer and all other solid ingredients including metal powders are each dispersed in a carrier which has a relatively low density and high volatility. Carrier and propellant components are mutually insoluble. These streams are brought together in a small iet mixer. Mixing is essentially instantaneous, retention time in the mixer being only a fraction of a second. As it comes out of the mixer, the carrier i s largely separated from the propellant b y gravity. Vacuum casting, the final step, removes the remaining carrier. Solid ingredients are dispersed in the carrier using a centrifugal recirculating pump. Liquid materials such as the binder are pumped directly to the mixer. COST. Low equipment cost i s a major advantage of the Quickmix process. For example, a conventional batch mixing system would require six 200-gallon mixers or equivalent to have a 5000-pound-per-hour capacity and would cost about three times as much as a Quickmix plant. SAFETY. The Quickmix process has inherent safety advantages. Only a small amount of mixed propellant i s in the system a t any one time. When suspended in the liquid carrier the propellant raw materials are less subject i o friction, impact, and energy absorption which could initiate fire or exp losion. Mixing energy requirements are very low as compared to conventional batch mixers. These advantages may become processing requirements later as highly sensitive, high energy propellants are developed. Special safety features are provided where required, such as seal flush systems for pump seals. UNIFORMITY. The Quickmix process has the usual advantages of a continuous mixing process. Handling the propellant ingredients and propellant as slurries makes the process more adaptable to continuous metering, monitoring, and other phases of continuous processing as compared to handling solids a s such. Human error i s a t a minimum because operator adjustments of processing are few. FUTURE. Field loading of rocket engines appears as one attractive advantage of the Quickmix process. Continuous processing in the field combines safety, low cost, uniformity, and versatility. It avoids the problems of transporting the finished rocket-which in the future may weigh several hundred tons and be extremely large dimensionally-to the launching site. The same Quickmix unit can convert quickly from one propellant formulation to another and move from site to site to make propellant for a variety of rockets. Rocketdyne built a pilot-sized Quickmix plant with a 500-pound-per-hour capacity which was mounted on an 8 X 35 foot trailer. Improvements led to the design of a 1000-pound-per-hour unit that will fit in the same space.
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temperature are critical. Mixer power input has become important in controlling batch mixes. Recording watt meters are used to monitor mix r operation. The higher energy materials also led to greater intraline distances and changes in personnel limits. For example, in one instance, the distance from a mixer to its control room was increased four times. Rooms where hazardous operations take place are designed to relieve a t two places in case of explosion -top and a side. The side blow out panel faces a n earthen barricade to protect other operations from damage by a possible incident. Wherever possible, only one operation is done a t a time per location. Along with these steps to increase distances and to increase protection, quantities of materials in a single location have been reduced. More and more operations are remotely controlled. I n those operations where personnel are needed, unobstructed exits are always maintained. Personnel limits per room have been reduced.
Cast Propellant Processing
Development of cast propellant grains brought special problems to the pilot facilities. Much of the equipment used in processing extruded propellants was needed and usable for cast grains. Problems of size of finished propellant grains-deformation under the grains’ own weight-and resistance to tearing or cracking during temperature cycling and firing were solved by changes in formulation and by new materials such as the polybutadiene binders used in Rocketdyne Flexadyne propellants. These and other developments required new capability beyond that added for processing higher energy materials. Equipment changes and additions were extensive for some processing steps. However, in the batch mixing steps, present mixers were modified only slightly to handle cast propellants. When a mixer handles extruded propellant materials: hydraulic power is needed to confine ingredients within the blade action area and keep the heavy lid tight; when mixing material for a cast propellant, a light, vacuum held cover is all that is needed. Generally, both cast and extruded propellant materials are mixed under vacuum. The major additions to the pilot plant facilities for cast grains were the casting pits and assemblies, auxiliary equipment, and tooling. Both attendant and remote controlled pouring and handling can be done. Hoists available can remove finished motors from the casting assembly, carry any grain cast, and also handle mandrels used to make the center core in the propellant grain. The working room is air conditioned and de-
b This maze of pipe and conduit emphasizes the intraline distances required between operating areas and control rooms in the pilot plant facilities. The pipes carry variable temperature water used to control heat in mixers, extruders, and the like
An operator prepares to sift and classify ammonium perchlorate oxidizer. The unit has a capacity of 1500 pounds per hour
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A This rotary vacuum dryer i s being prepared for loading ammonium nitrate oxidizer. It has a capacity of 1600 pound and will dry about 1000 pounds per hour Ingredients for a castable propellant are loaded into a 100-gallon mixer. In the conversion of this mixer from extruded propellants to castable products, the heavy lid, hydraulically held in place, was replaced with a light weight lid and the motor size for the mixer was reduced
Operators load a Banbury mixer for masterbatching of synthetic rubber for manufacture of extruded nitrate propellants. The mixer i s jacketed with variable temperature water
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Safety in a Solid Propellant Plant Effectively, Rocketdyne's Solid Propulsion Operations encounters the same conditions as found in an explosives manufacturing operation. As a general rule Rocketdyne follows building and intraline distances for Class 9 explosives in the Ordnance Safety Manual. In addition it has its own set of safety regulations. Examples of these regulations include: b Foreign objects eliminated from propellant ingredients. Before, during, and after various grinding steps, oxidizers and other materials are screened. Sometimes successive screening i s done. After grinding, screening, and drying, containers are sealed. b Equipment cleaned on regular schedule as dependent on processing. Solvents and detergents are selected for a cleaning job so that they will not sensitize any material that might be left behind or produce toxic products. In some cases a mixer may b e only scraped clean between batches of the same formulation. Complete cleaning i s done between batches of dissimilar formulations.
b Spills, chips, and the like are disposed of immediately. Extensive dust and chip collecting devices are used in propellant manufacture and their effectiveness routineiy checked. Depending on (heir nature, the materials collected b y these devices are recycled or removed for burning. b General housekeeping goes on continually. Excess tools and equipment are not allowed to accumulate. In mixing, extruding, and other areas only required equipment i s permitted. Before mixing or other processing steps begin, a tool check i s made for that area.
Storage techniques depend on the quantities involved. Large quantities, usually more than several hundred pounds of potentially sensitive propellant materials, are stored in igloos away from the pilot plant area. Small quantities are stored in surge buildings in the pilot plant area. Limits have been established for quantities in a given area taking into account the materials and the intraline distances involved.
humidified to a standard 30 grains of water per pound of air. Sometimes, an inert gas is used to pressure the castable propellant from the casting can to the rocket case. Future Solid Propellant Processing
Solid propellants a t the developmental level change in some way almost every day. More powzrful materials are but one phase of the development programs. Nozzels, insulation, and controlled thrust devices are other areas of special interest. But it is the materials that affect future processing operations most strongly. New techniques will be required to handle high-energy binders which contain such reactive groups as the nitro and nitramine radicals. Changes will also be needed in using the metal hydrides and the borons. Oxidizers such as the light metal perchlorates must receive very special considerations. There are the beryllium compounds whose toxicity is recognized, but many details of its toxicity remain unknown.
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These generally more hazardous materials mean more of the same to future processing-more isolation, smaller quantities processed in a single location at a given time. and more automatic operations. .4t the McGregor pilot plant facilities, several additions are planned for the immediate future. The first involves more isolation-removal of curing ovens to a separate building that will become the cure building. 3'0 special construction problems are expected for this building or its equipment. The second addition will be a propellant rework facility. Here small quantities of propellants may be salvaged. These propellants would include both those of a batch that have been subjected to environmental testing and not test fired, and those that failed to measure u p to some specification during their manufacture. A third addition will be facilities for development work on welding around loaded propellant cases. This work involves solid propellants in gas generator applications.
INDUSTRIAL AND ENGINEERING CHEMISTRY
In the more distant future, the need for more isolation and still smaller quantities of materials may be offset to a limited extent by desensitization for processing. Encapsulation is an irnportant area of research. The coating materials p ill be limited to those which will serve as fuel or binders. Large scale equipment for encapsulation thus will be part of future facilities. Still loiver personnel density seems certain. This will mean more use of closed circuit television and more remote-handling equipment. Allied will be more extensive and more sensitive instrumentation and controls. Temperature control becomes more critical. Precise flow control in continuous mixing processes will be needed. Electronic data processing will be a part of both experimental and production work. Reproducibility of grain composition will improve accordingly. In scveral ways, some present equipment \vi11 become unsuitable for processing the sensitive materials now in the laboratory or yet unsynthesized. Mixing, for example, may have to be done ultrasonically or by flow techniques. I n all equipment, friction and impact will be avoided. Waste disposal \Till become much more important. Methods will have to be found to react toxic materials at the time of preparing the propellant, and toxic by-products converted to reasonably safe compounds for disposal. As the trend to higher energy material continues, it brings new raw material supply problems. Where large supplies of ammonium nitrate are available now because of the large demand for the material as a fertilizer and in other chemical products, in the future supplies of such materials as lithium perchlorate, beryllium compounds, and triamino guanidine may have Lo be provided by the propellant manufacturer. Here economics, influenced by such factors as processing and use aL a single location and estimared life before obsolescence, loom important. Bzfore looking too far ahead, many present propellants in larger and small grains are still needed. Minor modifications of present formulations extend the value of such propellants. Use of solid propellant gas generators to power auxiliary systems in large rockets and in boosters is expected to expand. Effective thrust variation over a 707" range of nominal thrust already has been achieved by Rocketdyne. Other modifications in propellant design are expected to extend this range. Controlled shut doivn and re-start are long-range research goals. However, these design and use possibilities are not expected to requier very extensive further modification to Rocketdyne's present pilot plant facilities.
