Home made black powder is used for propellant. It is made from a mixture of potassium nitrate (saltpeter), charcoal (carbon), and sulfur. Commercial black powder is made from a composition of 75% potassium nitrate, 12.5% to 15% charcoal and 10% to 12.5% sulfur. It has been found that any significant deviation from these proportions produce powder which burns more slowly and leaves more residue. The purity, source of chemicals, how finely powdered, amount of moisture present, and the degree of intimacy to which the chemicals are mixed will affect the potency of the powder.
Commercial black powder is prepared in several steps. First the chemicals are mixed together with a small amount of water. Next the mixture goes to the incorporating mill. A stamp mill works vertically pounding the powder in a hollow cavity cut in a slab of granite. A cylindrical section of wood is used to pound the powder. The action is very similar to that of a mortar and pestle.
The wheel mill uses huge granite wheels weighing 8 to 10 tons and rather than pound the powder, they roll it. Much heat is produced in this process and so water is continually added to replace what is evaporated. The powder is kept just moist enough to prevent dust from forming.
This incorporating is the most important part of the process. It mixes the constituents much more intimately than the mechanical mixing process. The powder is then pressed into cakes in a hydraulic press.
Corning or granulating, which is the next step, is the most dangerous. The cakes are crushed and granulated between rollers to form it into particles. The dust created from this process is called meal powder and is the most violent form of black powder and is coveted by fireworks manufacturers. The particles are then rounded, polished, glazed and graded for size.
The grain size controls the speed of burning because it determines the surface area exposed to the flame. The larger the grain sizes the smaller the surface area per given volume of powder there is, and so the longer the burning time is. It takes longer to burn through to the center of the grain. When black powder was used in fire arms, larger grain sizes were used in larger guns so the expanding gases which ejected the projectile would not build up pressure faster than the projectile could be accelerated. Many improvements were made in producing the grains to give more surface area so the powder would burn more rapidly. Special molds were developed that produced hollow hexagonal grains. The outside surface area decreased as it burned but the inside surface area increased so the grain was consumed very rapidly. The special shapes were produced to give high surface area per volume of powder but still have a large percentage of open spaces in and between the grains for the fire to be communicated.
Smaller grain sizes burn more rapidly to a point and then because of decreased voids between grains, they burn progressively slower as the size decreases. Burning is initiated on one surface and is unable to propagate further so burns only at the surface of the pile and burns progressively through the pile as the surface is consumed. In larger grains the fire is spread throughout the pile through the spaces between grains and so nearly the entire pile can be burning at one time. This burning can be so rapid as to be nearly explosive.
In solid propellant rocket engines, the powder is, pressed into the case with a long core. The fuel essentially is one large grain formed by compacting the powder under considerable force. When the fuel is ignited, fire is communicated almost instantly up into the core so the entire inside surface is ignited and the fuel burns towards the outside wall. As it burns the surface area increases and so does the thrust. The thrust starts out low, builds up to a maximum and then drops to zero as the fuel is used up. (see Figure 4) This is the opposite of what would be desired. What is needed is a high initial thrust to quickly accelerate the rocket to a speed where the fins can stabilize it and then drop to a lower sustaining thrust to accelerate the rocket to its maximum speed. This is very difficult to achieve with a core burning rocket but the thrust curve can be improved by using a tapered core. The fuel is used up at the bottom of the taper first and then the burning continues in a fairly uniform conical shape towards the top of the rocket. (see Figure 5) Just keep in mind that the thrust of an engine at any instant is in direct proportion to the surface area burning at that instant.
When preparing black powder at home, normally nothing beyond a standard chemistry lab mortar and pestle is within the means of the experimenter so powder equal in quality to that of commercial powder is quite impossible to achieve. If prepared properly, however, powder of considerable vitality is possible.
All three chemicals can be obtained by mail from laboratory supply companies and of course the larger quantities are less expensive. The lowest purity grade is quite sufficient. The finest particle size available should be used especially for the charcoal. Charcoal available as "dust-air floated" is the best. Saltpeter and sulfur is usually available in most any drug stores, too, but the price is very high. This is often the source used to get started until supplies from other places are obtained.
The cheapest form of potassium nitrate is fertilizer grade. Potassium nitrate is available as fertilizer in 50 or 100 pound bags and can often be found in smaller quantities in garden shops. Potassium nitrate is also available in bulk form as fertilizer and a person may be able to make a deal for a small quantity as a favor. The fertilizer grade is either in granular form or a coarse mesh powder. The granular must be crushed in a mortar and pestle. The coarse powder can be screened through a fine screen to take out the lumps and then used as is. It will dissolve in the water in mixing if done thoroughly.
Sulfur is also used to some extent in agriculture and might be available in garden shops but it might be the most difficult item to obtain at a reasonable cost even though value-wise it should be cheaper than saltpeter. Since it comprises only 10-13% of the total mixture, drugstore or mail order supplies might be acceptable if another supply cannot be easily located.
Charcoal can be made if a little extra work is not too objectionable. The source and degree to which it is pulverized will make the most difference in the performance of the finished powder. There are some differences in the impurities present and quality of the charcoal that comes from different types of wood. Any charcoal can be pulverized and used but it should first be filtered through a piece of silk cloth such as is used on model airplanes. A nylon stocking is not sufficient. The holes are too large and vary depending on how much it is stretched. One method that is fairly quick and easy is to use charcoal briquettes and an old blender or osterizer. The briquettes are first put in a piece of canvas preferably or a piece of old denim (which gets holes too easily) and pounded with a wooden or plastic mallet on a concrete walk or floor until they are broken in small pieces. The charcoal can then be put through a quarter inch mesh hardware cloth. What does not go through should be repounded. The rest can be put in an old blender on high speed for several minutes. Don't use a good blender because the charcoal dulls and erodes away the blades quite rapidly. If the motor is lugged or the speed changes, there is too much load on the motor and it could be burned out quickly. Remove some of the charcoal and try again. Let the blender set a few minutes after it is shut off to let the dust settle. The charcoal dust is very messy and will cover everything so this is a good outside job. A good way to separate the dust is to use a large coffee can with a plastic lid. Cut the top of the lid out leaving the edge and about a quarter inch ring. Use this to hold a piece of silk on the can. Dump the charcoal out of the blender into the coffee can and secure the silk to it with the plastic ring or string, wire or large rubber bands. The best way I found to collect the dust without getting it all over is to use a large plastic bag or a dry cleaners bag tied at one end. The plastic bag is held tightly to the silk end of the coffee can and the dust shaken into the bag. The dust can then be transferred to another container with a lid. What didn't go through the filter can be returned to the blender. The dust thus obtained is very fine and works extremely well in gun powder. The charcoal must be as fine as dust to make good powder. This is the main key in the manufacturing process at home.
A composition of 68% saltpeter, 22% charcoal, 12% sulfur has been found to work well with charcoal obtained from charcoal briquettes. This could possibly be because there are binders or other impurities in the charcoal or possibly for some other reason. It is important that you experiment with small quantities of different compositions to find what works best with the particular chemicals you are using. Start with the 75-15-10 combination of potassium nitrate, charcoal and sulfur, respectively, and compare other mixtures to it. Use small identical quantities and ignite them one at a time. Compare the effect. The test should be done on a clean flat surface. A good powder should produce few "pearls" or small balls of residue on the surface after the burn. Make sure that you use the same weight each time, each composition has the same percentage of water by weight, and the shape of the pile is about the same. Too much water in the powder will slow the burning rate and cause more pearls to result.
All mixing proportions are by weight. This is the only way to get consistent accurate measures. Some form of balance is a necessity. 1% of the weight being weighed is a sufficient degree of accuracy. A home made balance is described later if a purchased one is not available. However, this is one item that I consider a worthwhile investment if this or similar activities are to be pursued to any extent.
To prepare the black powder, the potassium nitrate must be pulverized in a mortar and pestle if it is in granular form or lumpy. Sift it through a fine mesh screen (finer than ordinary window screen) and then weigh out the proper portion and put it in a glass, plastic, or stainless steel bowl. Weigh the proper portion of sulfur being sure to deduct the weight of the weighing container. Take a portion of your weighed and screened saltpeter and mix it up thoroughly with the sulfur and pass the mixture through the screen once more. Mixing the two together before screening the sulfur will prevent the sulfur from recaking after screening before they are mixed and will prevent unmixed sulfur pockets. Rub through the screen any sulfur lumps that did not pass through. Mix all the saltpeter and sulfur thoroughly. Now very carefully spoon out and weigh the proper amount of charcoal dust. If it is handled and mixed slowly it will not fly around too much. Mix all three ingredients very thoroughly. Passing them all through a screen once or twice more will help break up pockets of unmixed chemicals. Finally, weigh out about 3% to 5% the total powder weight of water and mix into the powder. This will seem like a very small amount of water for the amount of powder but it does not take much to moisten it. The mixture should now be worked thoroughly--the more the better. Repeatedly mashing the powder against the side of the bowl with an old spoon and working the water evenly into the powder is the operation that takes the place of the commercial incorporating mill. You don't have to spend hours doing this but several minutes would be very beneficial. To most effectively test the powder, it should be dry. It can be passed once more through a screen to fluff it up before setting it aside to dry. A thermostatically controlled food dehydrator can speed the process if you are in a hurry. If you are going to load the powder in rocket engines, it must be done while still moist or if dry, must be remoistened with 3%-5% water. If it is to be stored before being loaded, the powder should be dried first and then stored in an airtight plastic container.
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