Evolution of Propellants

Propellant materials are the evolutionary product of a basic tenet in weapon technology: energy must be stored for later use. The concept of a propellant is that energy can be stored in chemical form, possibly years before it is ultimately released. This demands a material that is reasonably stable, compact, and portable.

The exact date and place of the invention of black powder, the first propellant, is unknown. It is possible that the development occurred independently at multiple locations over a two- to three-hundred-year time period. It is probable that, given the similarity of formulations, gunpowder was invented in a single location and the technology was exported and spread by early explorers.

It is commonly accepted that the Chinese were among the first to discover that certain materials, blended in the correct proportions and ignited, could propel a rocket or explode to produce a loud report.

A simple rocket is a tube filled with a propellant material, closed at one end and open at the other. The gas produced by the burning propellant spews from the open end and thrusts the rocket in the direction of the closed end. It is simple to imagine that some experimenter conceived the idea of turning the system around. The bodies of ancient rockets likely gave birth to the concept of the first gun barrels.

There are a number of materials that can store chemical energy and convert it to heat and gas for propulsive purposes. Most early experiments focused on blends of charcoal, sulfur, and an oxidizing agent. One of the most abundant, naturally occurring oxidizers is potassium nitrate (KNO ₃), known to the ancients as saltpeter, niter (or nitre), and petral stone, among others.

Early propellants were used for entertainment and religious rites (fireworks), fire starting, and moving material (blasting) for a long time before the concept of a fixed, chemical-filled tube launching a projectile was conceived. The first military use was for blasting through gates and walls. The device used for this purpose was the petard .

Black Powder

The pervasive formula for early propellants was a simple mechanical mixture of charcoal, sulfur, and potassium nitrate, known as black powder. This technology did not arise in Europe until the thirteenth century A.D., although it is likely that the Chinese had launched projectiles with black powder two hundred to three hundred years earlier.

Constituents

Most old formulas called for charcoal made from the wood of certain tree species. Willow wood was highly prized as a source of charcoal. The quality of the charcoal affected many aspects of propellants, including ease of mixture, power, and cleanliness. Varying the ratio of the three components produced different black powders for different purposes. For example, the formula for blasting powder has traditionally differed from that for propellant use. Modern, propellant-grade black powder is seventy-five parts potassium nitrate (KNO ₃), fifteen parts charcoal, and ten parts sulfur.

Manufacturing Process

Black powder makers relied on fine grinding of the components to ensure that they were mixed thoroughly. However, military use of propellant-grade black powder gave rise to a serious problem; dry components could separate during transport over rough roads. This caused a degradation of the propellants power. A short-term cure was to transport the components separately to the battlefield and mix them just before loading. The hazards of a propellant blending operation close to an active line of cannon are easily imagined.

The solution for the separation of the components of this mechanical mixture proved to be the only significant improvement to black powder in its long history. Small amounts of water were added during the milling and grinding operations. Potassium nitrate is soluble in water; adding a small amount of water causes it to become slightly sticky. Then the potassium nitrate can act as a binder to hold the two insoluble components in close contact. The presence of moisture also reduces dust. This largely mitigates the major hazard of blending and grinding as long as an adequate moisture level is maintained. This wetting process is called corning, and the resulting product was called corned powder.

The corned powder is pressed into cakes and allowed to dry. After drying, the cake is broken into granules. All granules contain uniform proportions of the components and maintain those proportions until consumed. Corned powders proved to be much more powerful than the dry mixture, largely due to the incorporation of ingredients. Some cannon that were suitable for the weaker dry-blended powders proved inadequate for corned powder. The improvement in propellant forced an improvement in gun metallurgy.

The granulation process serves another useful purpose; some control of the rate of energy release is afforded by sizing the granules. The granulated pieces are passed through sieves to sort them by size. Larger granules release energy at a slower rate than fine granules. This characteristic means the user can select from several grades to obtain the best velocity from a black powder firearm. Large-bore devices, such as cannon, use coarse granules (up to several millimeters average size). Small-bore devices, such as shoulder arms, use much finer material.

In the United States, sporting black powder is sold by size as indicated by a coding system with the letter F.

For example:

• 1F: coarse, for .69 to .75 caliber muskets
• 2F: medium, for .45 to .58 caliber rifles and muskets
• 3F: fine, for .31 to .45 caliber rifles and most handguns
• 4F: extra-fine, only for priming flintlock arms

Note:

Some have suggested that black powder propels a projectile by a weak but high-order detonation, unlike modern propellants that burn to push the bullet out of the barrel with a progressive increase in pressure. The author has participated in research where black powder was loaded in a cartridge [.45-.70 government cartridge with a 500-grain (32.4 gram) bullet] and then test fired in a modern piezoelectric ballistic pressure system. That research showed the following: Black powder produces time-pressure curves that are remarkably similar to modern propellants, indicating progressive burning and a measured release of energy over time. Black powder in finer granulations produced higher pressures than equal quantities of coarser black powder, indicating the burning rate is controllable by granulation. Black powder peak pressures were as high as 21,000 psi, and roughly equal to modern factory ammunition for this cartridge.

With the improvements of corning and grading, black powder remained largely unchanged until military and sporting arms transitioned to modern propellants at the beginning of the twentieth century.

Disadvantages

Today, black powder is still a useful propellant for specialized purposes. For example, black powder is somewhat easier to ignite than modern propellants; it is commonly used as a booster charge in large-caliber military applications. In spite of the ability of black powder to allow the use of lethal force at a distance, it was not without problems.

• Black powder has a limited amount of total energy per unit of volume. To gain greater velocity, a larger volume of propellant must be used, necessitating a larger cartridge case. With the trend to repeating firearms, large cartridges common to single-shot black powder firearms could not be accommodated.
• In cartridge firearms, velocities seldom exceed 1400 ft/sec.
  (426 m/sec). This again is a limitation of energy content and cartridge size.
• Black powder is volumetrically inefficient. Depending on the grade, granulation, and loading density, gas production is only 45 to 55 percent of the total output. The remaining inert solids, largely dense smoke and fouling, contribute nothing to propulsion.
• Black powder produces a dense smoke cloud. As military tactics shifted from massed troops to individual riflemen, a smoke puff helped the enemy to spot the shooters position.
• Black powder residue is corrosive to steel. Several by-products of black powder combustion are hygroscopic, releasing corrosive compounds when hydrolyzed. This accelerates wear to the firearm.

The goal of researchers became the development of propellants that overcame these shortcomings.

Replica Black Powders

Pyrodex (Reg TM of Hodgdon Powder Company) is a late twentieth century development that is today classed as a replica black powder. It was developed to overcome the high transportation costs and storage limitations of black powder. For such purposes, it falls in the same shipping class as modern propellants.

Pyrodex is formulated to replace black powder on a volumenot weightbasis. In keeping with its replica duties, added materials generate the satisfying puff of white smoke so characteristic of original black powder.

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