PROPELLANTS
The production of hot gas to lift and move objects, using a pyrotechnic system, began with the development of black powder. Rockets were in use in Italy in the 14th century, and cannons were developed at about the same time. The development of aerial fireworks was a logical extension of cannon technology. Black powder remained the sole propellant available for military and civilian applications until well into the 20th century. A number of problems associated with the use of black powder stimulated efforts to locate replacements
1. Substantial variation in burning behavior from batch to batch. The better black powder factories produced good powder if they paid close attention to the purity of their starting materials, used one source of charcoal, and did not vary the extent of mixing or the amount of moisture contained in their product.
2. Black powder has a relatively low gas output. Only about 50% of the products are gaseous; the remainder are solids.
3. The solid residue from black powder is highly alkaline (strongly basic), and it is quite corrosive to many materials.
"Pyrodex" is a patented pyrotechnic composition designed to fulfill many of the functions of black powder. It contains the three ingredients found in black powder plus binders and burning rate modifiers that make the material somewhat less sensitive and slower burning. A greater degree of confinement is required to obtain performance comparable to "normal" black powder. The advantages of black powder and Pyrodex include good ignitibility, moderate cost, ready availability of the ingredients, and a wide range of uses (fuse powder, delay mixture, propellant, and explosive) depending on the degree of confinement.
As propellant technology developed, the ideal features for a better material became evident
1. A propellant that can safely be prepared from readilyavailable materials at moderate cost.
2. A material that readily ignites, but yet is stable during storage.
3. A mixture that forms the maximum quantity of low molecular weight gases upon burning, with minimum solid residue.
4. A mixture that reacts at the highest possible temperature, to provide maximum thrust.
The late 20th century saw the development of a new family of "smokeless" powders, as modern organic chemistry blossomed and the nitration reaction became commercially feasible. Two "esters" - nitrocellulose and nitroglycerine - became the major components of these new propellants. An ester is a compound formed from the reaction between an acid and an alcohol. Figure 1 illustrates the formation of NC and NG from nitric acid and the precursor alcohols cellulose and glycerine.
"Single base" smokeless powder, developed mainly in the United States, uses only nitrocellulose. "Double base" smokeless powder, developed in Europe, is a blend of nitrocellulose and nitroglycerine. "Cordite," a British development, consists of 65% NC, 30% NG, and 5% mineral jelly. The mineral jelly (a hydrocarbon material) functions as a coolant and produces substantial amounts of CO2 , CO, and H2O gas to improve the propellant characteristics. "Triple base" smokeless powder, containing nitroguanidine as a third component with nitroglycerine and nitrocellulose is also manufactured.
An advantage of the smokeless powders is their ability to be extruded during the manufacturing process. Perforated grains can be produced that simultaneously burn inwardly and outwardly such that a constant burning surface area and constant gas production are achieved.
Nitrocellulose does not contain sufficient internal oxygen for complete combustion to C02 , H2O, and N2 , while nitroglycerine contains excess oxygen. The double base smokeless propellants therefore achieve a slightly more complete combustion and benefit from the substantial exothermicity of NG (1486 calories/ gram).
FIG. 1 The nitration reaction
Shuttle. The pyrotechnic boosters used for these launches typically contain
1. A solid oxidizer: Ammonium perchlorate (NH,,C1O,,) is the current favorite due to the high percentage of gaseous products it forms upon reaction with a fuel.
2. A small percentage of light, high-energy metal: This metal produces solid combustion products that do not aid in achieving thrust, but the considerable heat evolved by the burning of the metal raises the temperature of the other gaseous products. Aluminum and magnesium are the metals most commonly used.
3. An organic fuel that also serves as binder and gas-former: Liquids that polymerize into solid masses are preferred, for simpler processing, and a binder with low oxygen content is desirable to maximize heat production.
A negative oxygen balance is frequently designed into these propellant mixtures to obtain CO gas in place of CO2. CO is lighter and will produce greater thrust, all other things being equal. However, the full oxidation of carbon atoms to CO 2 evolves more heat, so some trial-and-error is needed to find the optimum ratio of oxidizer and fuel.
Propellant compositions are also used in numerous "gas generator" devices, where the production of gas pressure is used to drive pistons, trigger switches, eject pilots from aircraft, and perform an assortment of other critical functions. The military and the aerospace industry use many of these items, which can be designed to function rapidly and can be initiated remotely.