The combustible casings used in modern tank gun ammunition are indeed based on nitrocellulose, but they are engineered to function in a very specific way. The main purpose of these casings is to hold the propellant charge while being able to burn up almost completely upon firing, leaving minimal residue inside the gun barrel and preventing extraction of a spent cartridge case, which speeds up the firing process and reduces weight.
Here’s a more detailed explanation of how these semi-combustible casings are made and how they work:
What Is Nitrocellulose and Why Is It Used?
Nitrocellulose, sometimes called guncotton, is a chemically treated form of cellulose. As you described, it is created by reacting cellulose fibers (from plant materials like cotton or wood pulp) with a mixture of nitric acid and sulfuric acid. The acids replace hydrogen atoms in the cellulose’s hydroxyl groups with nitrite groups, creating nitrate esters. This gives nitrocellulose its combustible properties.
- Highly Flammable: Nitrocellulose burns rapidly and cleanly, making it ideal for use in ammunition where controlled combustion is required.
- Formable and Processable: Nitrocellulose can be processed into various forms, from powders used as propellants to more solid shapes that can be turned into sheets or molded forms.
How Are Combustible Casings Made?
The nitrocellulose used in tank ammunition casings is typically combined with binders and other additives to create a stable, semi-rigid material that can be molded or pressed into shape. This process is different from simply using raw nitrocellulose fibers and involves several stages to ensure that the resulting material is both structurally sound and burns effectively during firing.
- Nitrocellulose-Based Material:
- The main body of the casing is made from a nitrocellulose-based material, which is combined with plasticizers and binders to create a solid form that holds its shape under stress.
- The nitrocellulose is usually mixed with a polymer or other resin to form a composite material. This process is somewhat analogous to making fiber-reinforced plastics, where a fibrous material is bound together with a polymer matrix.
- Molding and Pressing:
- The composite is pressed or molded into the shape of the casing, which can have a cardboard-like or fiberboard texture but is much more dense and rigid.
- This process ensures that the material is sturdy enough to contain the propellant charge securely, while still being combustible when the round is fired.
- Combustion Properties:
- The binders and additives are selected so that the entire casing will burn up efficiently when exposed to the high-pressure and high-temperature environment inside the gun’s chamber during firing.
- The result is that the main body of the casing burns away, while the metal base (typically made of brass or steel) is ejected or left behind, minimizing the need to extract an empty case manually after firing.
Is It a Polymer-Bonded Propellant?
While it is not exactly the same as polymer-bonded explosives (PBX), which are used for certain types of solid propellants, the manufacturing process does share similarities:
- The nitrocellulose acts as the primary combustible component.
- It is bonded with polymer materials that give the casing its rigid structure while maintaining the ability to burn effectively.
- The process results in a strong, semi-rigid structure that holds the shape of a casing but can fully combust under the conditions present in a tank gun’s chamber.
Structure of Semi-Combustible Ammunition
Modern tank rounds like those used in the 120 mm smoothbore guns on the M1 Abrams or Leopard 2 feature a two-part design:
- Combustible Casing: The main body that holds the propellant charge. This burns up almost entirely upon firing.
- Metal Base: Typically made from brass or steel, the base provides a solid seal at the chamber end and holds the primer. It is often the only part left after firing, which can be extracted or left to fall out.
Benefits of Combustible Casings
Using a semi-combustible casing offers several tactical advantages:
- Reduced Weight: The lack of a full metallic casing reduces the weight of each round, which means tanks can carry more ammunition.
- Simplified Loading Process: The absence of an entire spent cartridge to extract makes the loading cycle faster, allowing for a higher rate of fire.
- Cleaner Chamber: Because the main body of the casing burns away, there is less debris left behind, reducing the risk of jams and the need for clearing spent casings.
Summary
The semi-combustible casings in modern tank ammunition are made using a nitrocellulose-based material mixed with binders and plasticizers. The resulting composite is rigid enough to hold the propellant and withstand handling but is designed to burn completely upon firing, leaving only a small metal base. This design balances the structural needs of the round with the ability to burn away cleanly, providing a lighter, more efficient ammunition solution for modern tanks.