The main requirements of propellants are:
- Stability: The propellant should not easily ignite if it is shaken or accidentally dropped.
- Long shelf life
- Less residue: Burning black powder leaves a lot of residue behind in the barrel, which means that the user has to spend time cleaning the barrel often. Newer powders leave very little residue behind.
- Uniformity: The propellant should be manufactured so that it provides a similar amount of propulsive force, even if one batch of propellant is manufactured at a different time from another batch, or by another manufacturer. This way, the firearm's range and accuracy are not compromised when firing cartridges manufactured at different times and places.
- Low explosive power: This one is a bit hard for newbies to understand. Basically, the propellant should burn pretty violently when lit, but not quickly enough for the speed of the flame front to exceed the speed of sound (otherwise, it is classified as a "high explosive".) The idea is that the burning propellant generates pressurized gas, which expands out and pushes the bullet out at speed. The expansive forces are also acting on the inside of the barrel (which is made of iron or steel), which has a certain elasticity. The forces acting on the barrel shouldn't apply force at a rate greater than what the barrel's elasticity can handle, otherwise the barrel will explode. Therefore, a good propellant should be explosive enough so that the maximum speed of the bullet can be achieved, but not explosive enough to convert the firearm into a hand-grenade! This is why people don't use TNT or RDX in modern propellants! In some cases, people use high-explosives (e.g. nitroglycerine) for propellants, but they add enough stabilizers to slow down the burn time of the high-explosive so that it can be safely used as a propellant.
In the next few posts, we will study the history of propellants and their manufacturing techniques.