Rocket basics by Morton-Thiokol 2

Rocket Basics by Morton-Thiokol

 

Sir Isaac Newton


 

Secondary Injection TVC

Secondary injection TVC is accomplished by injecting fluid (liquid or gas) into the main exhaust stream of the rocket motor through ports in the expansion section of the rocket nozzle.

SITVC

The thrust is deflected by the force of the injected fluid and by the imbalance of pressure created in the nozzle.

The total side force produced by secondary injection consists generally of two parts: (1) the force caused by the momentum of the injected fluid; and (2) the imbalance of pressure induced by the injection reacting on an area of the nozzle surface perpendicular to the nozzle centerline.

Because there are several factors which influence the production of total side force during secondary injection into a supersonic nozzle, the location and angle of injection must be determined through experimentation and application of previously developed data.

Types of Control Systems

Power for actuating the control systems is provided by hydraulic mechanical and electromechanical means, with electronic components especially designed for the application providing the required signals for precise positioning of components.

Propellants

The dictionary defines a propellant as a substance that causes an object to move or to sustain motion. Thus, rocket propellants exist for only one purpose, to produce the thrust required to cause a rocket to move from launch site to intended target.

Thrust, as noted earlier, results from the production of hot gases during the combustion (burning) of the propellant and acceleration of those gases to supersonic velocity through a port or nozzle.

The internal combustion engine which powers automobiles, airplanes, and other vehicles obtains oxygen from the air, which is mixed with the fuel in the carburetor in proper proportions so that combustion can take place in the cylinders. Combustion of the propellant in a rocket motor cannot breathe air to obtain oxygen. Consequently, it must carry its own oxygen supply in addition to fuel supply. Separate fuel and oxygen supplies are carried in tanks aboard liquid rockets and these materials are mixed in the combustion chamber. In the solid rockets with which we are primarily interested, however, both fuel and oxidizer supplies must be contained within the propellent charge.

Liquid rocket: Separate oxygen 
supply needed for combustion.
Solid rocket: Both fuel and 
oxygen (oxidizer) contained
in propellant charge.

To assure the release of maximum usable energy during propellant combustion, fuel and oxidizer materials are chosen carefully, and the exact proportions of each which are required for combustion are determined by analysis and experimentation.

The ideal, or best possible, solid propellant has as many of the following characteristics as possible.

  1. High level chemical energy release to produce high combustion temperatures and develop maximum thrust from each pound of propellant (specific impulse) for high performance.
  2. Low molecular weight of combustion products (the lower their molecular weight, the greater the acceleration that can be applied to the gases).
  3. Stability (resistance to chemical and physical change) over a long period of time.
  4. Maximum density so that the greatest amount of energy possible can be packed into every unit of case volume.
  5. Resistance to the effects of atmospheric conditions, such as humidity, heat, cold, etc.
  6. Resistance to accidental ignition from high temperature or impact.
  7. Maximum possible physical strength to withstand the effects of forces imposed by heat and pressure during motor operation.
  8. Very small change in volume with each degree of change in temperature, preferably matching that of the case material, to minimize stresses and strains in the two structures.
  9. As nearly chemically nonreactive (inert) as possible ensuring storage and operation.
  10. Easily produced and with desirable fabrication properties, such as adequate fluidity during casting; easy control of processes such as curing; and minimum volume change (shrinkage) following casting or molding.
  11. Relative insensitivity of performance characteristics and fabrication techniques to impurities or small processing variations.
  12. Predictable physical properties and combustion characteristics (burning rate) not affected appreciably by a wide range of storage and operating temperatures.
  13. Smokeless exhaust gas to avoid deposition of smoke particles at operational locations and detection in military usage.
  14. Readily bondable to metal parts, the application of inhibitors, and to different production techniques; and amenable to use of simple igniter.
  15. Nonluminous, noncorrosive and nontoxic exhaust.
  16. Simple method of preparation, not requiring complex chemical plant installation.
  17. Grain should be opaque to radiation to prevent ignition at locations other than burning surface.
  18. Able to withstand repeated extreme temperature variations prior to operation without physical or chemical deterioration.
  19. Raw materials cheap, safe, and easy to handle and transport.
  20. Burns at steady, predictable rate at motor operating conditions.

Solid Propellant Types

Solid propellants form mono propellant (mono = single) systems in which the oxidizer and fuel components are combined in a single mixture with a liquid material which holds them in suspension. This composition is then cast (poured) into the case, where it is cured to a solid state.

Propellant specialists divide solid propellants into two classifications - double-base and composite. These classifications refer to the physical and chemical characteristics of the propellants, as well as to the types of materials used in their manufacture. The double-base (also called homogeneous) type propellants use nitrocellulose (guncotton) and an energetic plasticizer to cause it to dissolve and then harden into a solid form. Each molecule of this final material contains the fuel and oxygen required to sustain combustion (burning). The chemicals used in the manufacture of this type of propellant are unstable, usually nitrocellulose and or nitroglycerin. Each of these materials contains the necessary fuel and oxygen in its individual molecules to burn or explode without coming into contact with any other material.

Composite propellants are composed of separate fuel and oxidizer materials, neither of which will burn satisfactorily without the other. Mixed together and combined with a liquid which later is cured to solid form, they form a compound in which fuel and oxidizer are adjacent to each other in close enough contact to assure efficient combustion. Composite propellants generally are composed of finely ground chemical oxidizers and inorganic fuels (made from noncarbon-containing material) which are suspended in a binder of rubber-like material which also serves as a fuel. Most composite propellants also may use a variety of chemicals to increase or decrease the burning rate (to control) hot gas production rate), provide better physical properties than are obtainable with the basic binder, or to regulate chemical reactions for better control during the manufacturing process.

Double-Base Propellants

The nitrocellulose used in the manufacture of double-base propellants, is prepared by combining cotton (cellulose fuel) with nitric acid (an oxidizer), which produces the single chemical nitrocellulose. Addition of nitroglycerin (a shock sensitive high explosive) to the nitrocellulose causes a physical reaction which partially dissolves the latter and causes it to swell, then to gel or solidify the nitroglycerin. In the process, the nitroglycerin is made less sensitive to shock. Certain other chemicals may be added to control the speed of gelling (solidification), speed or slow the burning rate, and improve physical characteristics or other properties of the propellant.
Cellulose Fuel
Oxidizer
Nitrocellulose
Nitroglycerin


 

Double-Base (Homogeneous)
Each molecule contains both fuel and oxidizers. Chemical reactions solidify the propellant and additional materials improve its physical properties.

During storage, nitrocellulose decomposes slowly but steadily by releasing oxides of nitrogen. The rate of decomposition is accelerated by the presence of these oxides. Certain materials called "stabilizers" can be combined with the oxides to remove them, thus slowing the rate of decomposition and considerably extending the useful life of the propellant.

Materials used to improve the mechanical properties and extrusion (pressure forming) characteristics of double-based propellants are known as plasticizers. They may be either explosive or nonexplosive materials.

Because of the tremendous heat developed during combustion, high temperature radiation may be transmitted deep within the propellant unless darkening agents such as carbon black or lampblack are added to make the penetration impossible. This process prevents below-surface ignition, which could cause uncontrolled burning with an undesirable increase in chamber pressure.

Other chemicals may be added to improve the burning rate of other performance characteristics of the double-base propellant. These are known as ballistic agents.

Still other additives may be used to reduce the temperature of combustion gases as a means of controlling chamber pressure, or to reduce the hygroscopicity (moisture absorbing characteristics) of the propellant. And finally, various liquid ingredients may be added to the basic materials to serve as solvents for the purpose of speeding or improving the reduction of the components to liquid form.

Composite Propellants

The separate fuel and oxidizer components used in the production of composite propellants must be mixed together in the proper proportions to assure complete combustion, since neither will sustain combustion without the other. The chemical oxidizers and inorganic fuels are ground to fine, power-like states, mixed together, then added to a liquid material (binder) which holds them in suspension while it cures to a solid state. The final composition also may include a variety of chemicals to increase or decrease the burning rate, provide better physical properties than are obtainable with the basic binder, or improve processing quantities.
Separate, but adjacent fuel and oxidizers form an evenly dispersed compound throughout the binder.
Chemical additives regulate and improve its properties.

Binders. Both natural and man-made materials have been used as binder-fuels for solid propellant grains, including asphalt, and synthetic liquid prepolymers. All of the organic prepolymers have rubbery properties following cure and form a strong matrix (binding structure) within which the inorganic fuels and chemical oxidizers are solidly bound.

The most commonly used liquid prepolymers cure up much like rubber. An excellent example is the curing of the white of an egg, which solidifies and becomes rubbery when heat is applied. The egg white also acts as a binder in a cake, holding the other ingredients together through development of a long, chain like molecular structure.

The prepolymers used as binders in composite propellants are initially liquid, so that the fuel and oxidizer can be blended in more easily prior to the start of polymerization (cure). The polymers cure to an irreversible solid form by cross linking (formation of long chain-like molecules linked together).

Asphaltic Oil Types. The first material used as a binder for solid propellants was asphalt, which was used in the first jet assisted take-off (JATO) units for aircraft near the end of World War II. Asphalt occurs naturally and is a by-product of the distillation of certain crude oils. It has several disadvantages for rocket applications, however, including: formation of thick black smoke combustion, a melting point near that of water (212 F), and brittleness at low temperatures unless mixed with oils.

Elastomers. These are the rubber-like natural and synthetic materials which have found the widest application in the modern (50's - 70's era) solid rocket propellants. The first material used as a binder was a liquid polysulfide prepolymer, first produced commercially as a solid elastomer in 1932 by Thiokol Chemical Corporation. Its application in "GALCIT" solid propellant by the Guggenheim Aeronautical Laboratories of the California Institute of Technology marked the beginning of the modern era in solid propellant, since it made possible production of large (up to 260 inch diameter) solid rocket motors.

Other liquid prepolymers which have been and are being used in large rocket motors include polybutadiene-acrylic and acid-acrylonitrile prepolymer, and carboxyl terminated polybutadiene. Each of these materials provides reproducible physical characteristics, and all have excellent aging and chemical resistance characteristics.

Additives. In order to obtain specific properties, several special ingredients may be added to composite propellants, including: oxidants, antioxidants, and curing and burning rate catalysts.

In other instances, an increase in the burning rate may be desirable, and this can be obtained by adding chemical agents known as burning rate catalysts which cause an increase in the combustion reactions.

Oxidizers. A number of inorganic chemicals can be used to supply the oxygen required to support the combustion of metallic and other fuels in solid propellants. The amount of oxygen provided by each oxidizer depends upon its molecular structure, and certain performance characteristics may restrict applicability to various degrees.

In general, the perchlorates: potassium, ammonium, lithium, sodium, and nitronium - have more oxygen in their structure than do the nitrates: ammonium, potassium, and sodium, although availability of oxygen is not the only consideration when choosing the oxidizer to be used.

Some of the perchlorates produce hydrogen chloride, a highly corrosive gas which forms hydrochloric acid when mixed with water, and other chlorine compounds. The exhaust gases of these oxidizers not only are toxic, but highly corrosive to many materials. With the exception of ammonium and nitronium perchlorates, all form a dense smoky exhaust because potassium perchlorate and sodium chloride are white powders. Ammonium and potassium perchlorate are only slightly soluble in water, so they can be used in propellants which are exposed to moisture. The oxidizing potential of the perchlorates is generally high, and because of this fact they are often found in propellants of high specific impulse. All perchlorate oxidizers are potential explosives, but use of high purity material, special crystal processing techniques, and careful handling make processing of high energy propellants using these oxidizers possible. Nitronium perchlorate, the most powerful of the perchlorates, is very sensitive and can be detonated readily.

Of the three nitrates of interest, two (potassium and sodium nitrate) produce undesirable smoke because of the solids formed in the combustion products. Ammonium nitrate, widely used as a fertilizer, has the big advantage of producing a smokeless, relatively nontoxic exhaust. With the lowest oxidizing potential of all the materials used, however, ammonium nitrate is suitable mainly for low performance, low burning rate applications.

Metallic Fuels. Powdered aluminum is the metallic fuel most widely used in solid rocket propellants. The addition of approximately 15 percent aluminum powder to solid propellants seems to help in three ways: (1) by causing an increase in combustion temperature and thus an increase in thrust produced per pound of propellant (specific impulse); (2) by increasing the density of the propellant; and (3) by exerting a dampening effect on unstable burning which might develop at motor operating pressures and temperatures.

Beryllium causes not only an increased combustion temperature, but also produces lower molecular weight exhaust products; consequently, it can increase the theoretical specific weight impulse by 5 to 10 percent. The reaction products containing beryllium are extremely toxic, so special safety precautions are required.

Since metals lighter in weight than aluminum (lithium, beryllium, sodium, and magnesium) tend to be expensive, dangerous to handle in powder form, and highly active chemically, aluminum has been adopted as the workhorse metallic fuel in solid propellants.

What's in the future?

Solid propellant technology is a continuously changing field in which even higher performance is sought and attained. Many of tomorrow's materials have yet to be developed or tested.
 

Update: The last two sentences reflect the future as well as the past. Thiokol is continuously developing new materials to improve its products and meet the needs of the future.

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