A Delta Dagger, all weather jet interceptor, looses a salvo of 24 folding-fin aircraft rockets. A high-speed camera shows the first 2.75-inch rockets out-distancing the interceptor as others are just leaving firing tubes
For most military uses, solid rocket propellents can now compete with liquids, offering ease of manufacture a n d han dling, a n d reliability
Solid Propellants Vie with H. W . RITCHEY, Thiokol Chemical
Ο OLID FUELS can now compete with
liquids as rocket propellants for most current military applications. Yet, the fundamental basis of operation of a solid propellant rocket engine a n d its importance to the chemical industry are matters which have received little or no attention in chemical literature. Solid rocket propellants may use a wide variety of chemicals. T h e types in use today in this country may be placed in two general classifications: double-base and composite types. Double-base propellants consist of cel lulose nitrate plasticized with some high energy material (usually glycerol trini trate) and other special components to impart such things as combustion sta bility. Composite propellants consist of 78
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a resinous fuel binder that is burned by a crystalline inorganic oxidizing agent. Of the oxidizing agents currently in use, ammonium nitrate is the least expensive so far as raw material costs, but am monium perchlorate possesses marked advantages in respect to density, attain able combustion enthalpy, a n d process ing characteristics. There is a wide range of choice of fuel binders for a composite propellant system. If high performance is to be attained, i t is desirable that no penalty be paid for insulation of t h e pressure vessel (it is necessary to use the cold strength of t h e metal in the design). It is also desirable that t h e pressure ves sel b e filled as completely as possible with propellant and that no weight pen
Corp., Huntsville,
Ala.a
alty be paid for support of the charge against acceleration forces when t h e rocket is launched in flight. These r e quirements are m e t to about the fullest extent possible by the cast-in-place, case-bonded system first introduced in this country by the Jet Propulsion Labo ratories, California Institute of Tech nology. Case-bonding has been accepted b y most agencies engaged in development of high-performance solid propellant rocket engines. It has enabled the solid a
Based on a talk presented at French Lick, Ind., during t h e Commercial Chemical Development Association's symposium on what the rocket and mis siles program means to the chemical industry.
Liquids for Rockets propeîlant rocket engine to become very competitive with the best of the liquid systems for practically all military a p plications. An interesting solid rocket engine application recently released from the cloak of military security is t h e Lockheed X17 Missile. This is a threestage rocket system designed to obtain aerodynamic data at very high velocity and high Reynolds number. The addition of a small fourth stage weighing approximately 80 pounds would permit attaining satellite velocity with a payload of 15 to 20 pounds. The composite solid propeîlant system offers advantages in performance, in ease of manufacture, and in simplicity and reliability of field use; the important characteristics for composite
fuel binders are therefore of very direct interest to the chemical industry. These properties of binders include: • High combustion enthalpy coupled with low molecular weight gaseous combustion products. • High density. • Proper processing characteristics. • Suitability for producing propeîlant with acceptable physical properties. In regard to processing characteristics, a polymer t h a t can be mixed with oxidizer and processed as a semifluid slurry offers great advantages in regard to cost and flexibility. T h e materials should exhibit a p r o p e r "pot life," a n d
subsequently, they must "set" or cure to a solid at a reasonable temperature in a reasonably short period of time. I f a certain binder should show remarkably attractive ballistic and physical characteristics without meeting these processing requirements, these disadvantages might be overcome b y development of different processing procedures and by the use of eq ûpment more adaptable to its particular characteristics. Physical characteristics are of utmost importance in t h e propeîlant used i n a case-bonded system. T h e charge must support its own weight for long periods of time without cold flow and retain flexibility at both high a n d low temperatures. Tensile elongation is an extremely important characteristic; the NOV.
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Solid propellant slurry pours from mixer
What About Fuels? In a liquid rocket engine, fuel is sometimes delivered to the combustion chamber under pressure produced by pressurized tanks. In higher performance rockets, it is delivered by a turbopump system. The problems of pumping several hundred pounds of fuel per second, metering the oxidizer and combustible fuel in the proper ratio, and mixing these in the combustion chamber to attain sufficiently complete combustion ordinarily require complex and costly mechanical equipment. Mechanical complexity introduces other factors such as the tendency toward higher costs, difficult handling problems, and a requirement for relatively long lead times for checking system prior to firing. In the solid propellant rocket engine, the propellant is properly mixed and "injected" into the combustion chamber at the manufacturing plant. A composite type of solid propellant is processed as a slurry in the manufacturing plant and cast directly into the pressure vessel. The charge burns on all the exposed inner surface of a specially shaped propellant cavity. Since burning occurs from the inside outward, the flame does not contact the walls of the pressure vessel and no penalties in weight or volume are paid for support of the propellant.
propellant charge is exposed to tensile stresses that increase as the temperature of the rocket is lowered. This increase is caused by differential thermal expansion between case material and propellant material. A typical propellant may have a thermal coefficient of expansion approximately 10 times that of steel. Thus, when the charge is cooled, it is subjected to tensile stresses concentrated at various points within the charge. The stress concentration factors depend on geometric configuration. Combustion chamber pressure also causes tensile stress in the propellant. When the charge is ignited, most of the pressure load is carried by the walls of the vessel which then expand appreci80
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JATO gets a B-47 off the ground in a hurry
How Is Rocket Fitted to Need? There are many requirements that may be placed on a rocket engine that may be required in any given application. For example, in a JATO used to boost an aircraft up to flight velocity, pound-seconds of impulse per dollar may be the most important requirement. Since the device finishes its job at a relatively low velocity, light weight is not required. For missile applications, on the other hand, it is usually a prime necessity that the rocket be as light as possible for a given total impulse and that the velocity obtained during rocket operation be as high as possible. For high performance applications, the rocket velocity in free-space conditions is proportional to the specific impulse of the fuel, and the ratio of the initial and final weight of the system. And in high velocity applications, a 1% improvement in propellant mass ratio may produce three or four times more increase in velocity than a 1% improvement in propellant specific impulse ( thrust X burning time/propellant consumed ).
ably under the high working stresses introduced by the internal pressure. Here's how various temperatures affect tensile properties of a typical propellant composition: • Maximum tensile elongation drops off at lower temperatures. • At a given temperature, the elongation tends to decrease as the strain rate is increased. • Under the application of low loading rate tensile stresses, the material may lose most of its tensile elongation in the range of —30° F. • A t —10° F., tensile elongation disappears when the strain rate is increased by a factor of 100.
Although hydrocarbons and inorganic oxidizers are currently receiving wide application in the composite solid propellant field, other materials show theoretical promise. No effort should be spared to improve propellant specific impulse. A number of materials, based on boron hydride chemistry, show higher combustion enthalpy. Presumably such materials offer one approach toward attainment of higher propellant specific impulse. Although boron hydrides have been considered primarily as being applicable only to liquid fuel engines, some of the materials are solid and could find application in solid propellant rockets. The fundamental requirements for
Engine design and fuel determine performance Static firing aids development of Redstone H o w Is P e r f o r m a n c e Predicted? A solid propellant rocket engine, when properly designed and manufactured, is stable in operation and predictable in performance. If the propellant is prop erly compounded, the rate of regression of the surface a t any point will be equal to the constant Kx times the pressure to the nth power, where η is less than one. In any given design 'where the burning surface is fixed, the mass rate of generation of working fluid will b e equal to the burning surface multiplied by propellant density and the linear burning rate. Gas discharge through the nozzle is linear with pressure, once the n o z z l e throat area and the thermodynamic constants o f the gas are fixed. It is obvious that where these two curves cross, a stable operating pressure of the system i s attained, e.g., at a higher pressure, fluid would b e discharged faster than it is generated. In any given design, the operating pressure can be changed either \yy changing the propellant burning rate, the burning surface area, or the nozzle throat area. Choice of geometry of burning surface permits a wide range of preprogrammed thrust-time characteristics to match needs of a given system.
high impulse, low molecular weight, and high flame temperature, are suffi cient clues to point the way toward the clioice of the ultimate "molecular" rocket fuel. However, mechanical properties, density, chemical stability under storage, and processing charac teristics must not be forgotten in pur suit of the impulse goal. Since research must b e supported either by profits from sales or by gov ernment subsidy, the size of the chemi c a l market in the missile and rocket field becomes a matter of direct interest to management. Production estimates of trie various missile systems are classi fied and cannot b e revealed in open literature. The outward limits of the
W h e r a ^ C a n Research Help? The propellant specific impulse can be improved within certain limits. A higher combustion enthalpy of the propellant-oxidizer mixture ordinarily results in higher attainable propellant specific impulse. This is not universally true, however, since propellant specific impulse is fundamentally proportional t o the square root of combustion chamber temperature divided b y the molecular weight of the combustion products. Specific impulse is also dependent on engine design since it is also a function of the ratio of internal pressure to external pressure and/or nozzle dimensions. These latter two characteristics are secondary factors under the control of the rocket designers, while the combus tion temperature a n d molecular weight are factors inherent in the chemical make u p of the propellant. The ratio of initial to final weight can be improved by reducing the weight of inert hardware components of the engine. Approaches to this problem consist of using material with high strength-to-weight ratios, de signing for high working stresses, and putting as much propellant as possible in a given volume.
propellant market, however, can b e de fined by t h e application of a few simple concepts. At the present rate, the level of effort i n the guided missile field will soon reach $3 billion per year. By no means is all of this going into produc tion of missiles. Much of it is going into such things as development and
THE COVER. A " N i k e - H e r c u l e s / ' t h e A r m y ' s n e w surface-to-air g u i d e d missile, sits o n a launcher a t W h i t e Sands Proving G r o u n d .
procurement of real estate for launch i n g sites. It is not possible to state spe cifically what portion of this fund is used for production procurement. Of this unknown portion, however, it is possible to arrive at a third factor rep resentative of the dollar volume that might be channeled into rocket engine propellants. It would be most unusual if propel lant materials cost approached any where near 5% of the cost of a finished missile. In many cases, a figure of 0.5% is much more representative of t h e proper ratio. Thus, it is a reason able guess that $30 million represents a top annual figure for the value of rocket fuel materials going into production NOV.
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missiles. The fuel materials in research and development testing would add somewhat to this number; in fact, for certain large liquid engine propulsion systems, it is possible that research and development tests may use much more propellant than "production" missiles. It should be emphasized that these ra tios apply only to materials—not to the completed rocket engine ready for as sembly into a guided missile system. The relatively low ratio of propel lant dollar gross to total guided missile gross is quite likely to increase in the future. Even relatively minor improve ments in propellant specific impulse and in the propellant "mass ratio" may permit a tenfold saving in missile weight for accomplishment of a given mission. Thus, it is distinctly possible that a propellant costing 10 or 15 times that of present propellants would, in the end, prove to be a more economical propellant for a given application. By aggressive research and ingenious de velopment, it should be possible for the chemical industry to increase its slice of the guided missile market. The magnitude of the effect of these factors may be demonstrated by con sidering the ultimate missile—one de signed to escape the earth's gravitational field. Here the missile must be de signed to achieve kinetic energy equiva lent to the earth gravitational potential. This requires a velocity (at die earth's surface) of about 37,000 feet per sec ond. A higher theoretical velocity must be attainable to compensate for losses introduced by atmospheric drag and gravitation. Velocity is calculated by the equa tions: ν = I s p X 32.2 X 2.303 X log W^/Wo, where ν is the velocity in crement per stage; I s p is propellant spe cific impulse; W t is initial missile weight and W 2 the weight at stage burnout. The following table forcibly demonstrates the merits of propellant energy and efficient hardware "when ap plied to an escape-velocity missile.
Sidewinders, air-to-air missiles fueled with solid propellants, are pushed toward a Navy F-J-3 Fury fighter for loading at Naval Air Station, Miramar, Calif. What the table means is this: If you have a propellant with a specific im pulse of 195 and a propellant mass ratio of 0.85, you require 3000 pounds of take-off weight per pound of payload to reach escape velocity. If you can improve this figure to a propellant with a specific impulse of 2 8 0 and the propellant mass ratio of 0.92, then you only require about 1O0 pounds of take off weight per pound of payload to reach escape velocity. This gives a tremendous improvement in the size of the missile required to do the job. It would thus be possible to use very ex pensive fuels and still come out ahead of the game.
What It Takes to Escape Earth's Field Stage to Load Ratio
lto4
82
Ratio of Propellant Weight to Engine Weight
Specific Impulse
195
Ratio Take-off Weight toPayload
Stages to Get Escape Velocity
0.85
3000
5
lto4
225
0.92
600
4
lto4
280
0.92
100
3
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