ALLEN R. DESCHERE Redstone Arsenal Research Division, Rohm & Haas Co.,Huntsville, Ala.
Applied Research and Product Development for Rocket Propellants Much progress has been made since rockets became an important military weapon, but greater achievements in propellants are ahead
APPLICAT~ON
and development work in the rocket propellant field differs from similar work with other products mainly in the rather dangerous nature of the materials being handled. This discussion deals primarily with the field of solid propellants in which, in contrast to most liquid propellants, the oxidizer and fuel to react for energy release in the rocket are intimately mixed. In many solid propellants the fuel is a binder for a crystalline oxidizer, while in the nitrated type the oxidizer is chemically linked in the propellant. In either case, reaction
takes place under heat and pressure to give high temperature gsseous products. The primary difficulty then, is that the propellant is capable of combustion or even detonation under special circumstances during many stages of its process without the presence of air. Safety is the first consideration. For a product to be competitive, however, many other requirements must be satisfied. The necessary properties fall into certain general groups, and the specific values needed vary somewhat from application to application. Such
Research Starts with Numerous Tests.
groupings include : energy released per unit weight and volume of propellant, burning rate and its variation with temperature and pressure, physical properties and their variation with temperature, ease of ignition both when desired and when not desired, property changes which take place during storage and handling under various conditions, and methods of installing propellant in rocket motor. This list is not exhaustive and is probably similar in its own way to lists that can be made up for other products.
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1. When new propellant components or propellant compositions are being developed in the chemical laboratory, one of the first tests to be made is that of detonation o f a small quantity by impact. A standard impact testing machine drops a weight from varying heights onto a small quantity of material of interest; the height a t which 50% of the specimens fire gives a good indication of the relative safety in handling the material. If the material is highly sensitive, it must be handled with great care or rejected as too unsafe for development (below).
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2. Because the propellant is seldom one component alone, a few ounces must be mixed and prepared for more extensive laboratory tests. If the composition
is doughy or viscous, a small mixer is typically used. More fluid materials can be mixed in glassware. Milling is sometimes used. VOL. 49, NO. 9
SEPTEMBER 1957
1333
Still more tests in the lab
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e
4. Strands are then dipped in inhibitor to prevent burning along the outer surface (above).
3. The propellant is made into strands about I/’q inch in diameter and 8 inches long for preliminary investigation of burning properties. Doughy materials may be extruded (left) or pressed into strand-shaped cavities. Propellants which polymerize from the liquid form can be poured into molds.
5. Fuze wires are inserted and the strand is placed in a burner, where it can be burned cigarette fashion by igniting at one end by a hot wire. This bomb is pressurized with nitrogen and brought to temperature in a barricaded room before the burning is started. Instrumentation is provided for rapid determination of the rate of burning. A glass window in the bomb can be used to permit visual observation (below).
6. Tensile strength, elongation, and creep properties at various temperatures
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INDUSTRIAL AND ENGINEERING CHEMISTRY
are also important for propellant applications (above).
. . . followed by pilot plant research
When all metal parts are ready, the propellant itself can be made up. If it if a dough or a slurry, mixers of the BakerPerkins type may be used. If it is rubbery or plastic before it is placed in the motor, “powder mills” somewhat like mills for rubber may be used. The propellant may be extruded to shape in a manner similar to that used in strand formation. Many solid propellant materials are liquid or thixotropic and can be poured or “cast” into molds and then cured in place. I n general, these operations are hazardous and wherever possible, they are controlled remotely. If the propellant is formed outside the motor, it must then be assembled in the motor by fastening or holding it in place. If the propellant is cast in place, the mandrel must be removed, in which case the propellant is ready for the finishing steps. Installation of the igniter and assembly of the nozzle and other parts to the motor put the motor into its final shape for placement in conditioning ovens or boxes until ready for firing.
9. Before propellant ingredients are mixed and put in motors. design studies must be made of the form and shape the propellant is to have for burning in the motor. I t is usually no more possible to substitute one propellant for another in a
single shape, form, or motor size than to substitute one polymer for another in a synthetic rubber composition. Each material has its own peculiarities and characteristics and must be installed to do the job most effectively (above). VOL. 49, NO. 9
SEPTEMBER 1957
1335
Finally, performance is checked in a rocket motor
10. and 11. Motors, nozzles, mandrels, extrusion dies, and other metal parts must be designed and fabricated. Motor metal parts, if made with care? can often be used repeatedly in the development program (above, left and right)
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12. The early tests and many of' the later tests take place on a "static range." This test range is instrumented to measure thrust, pressure: and other quantities which may be of use in studying the performance of the propellant in a rocket motor. The motor is generally mounted in yokes butted up against a thrust gage and with pressure gages attached at appropriate points (right).
13. Elaborate instrumentation is used to measure all details of the rocket's performance swiftly and accurately. The traces obtained from the instrumentation are reduced by computers or electronic data processing equipment and then are evaluated (right).
The final test of a propellant takes place on the flight range. Here the development motors are fired and flown with dummy or complete missile systems, so that they undergo all the forces and conditions which they will be expected to meet in field use. Most materials
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INDUSTRIAL
tested are discarded some.ivhere along the way, but a few develop to provide a steady advance Stiff competition exists among development agencies. to the benefit of the defense effort. Much Progress has been made since rockets became an important military weapon a
A N D ENCINEERINO CHEMISTRY
few years ago, but it is the firm belief of all those involved in this field that therr are greater achievements ahead in propellants. Commercial
Chemical
Development
Association, French Lick, Ind., May 14, 1957.
