List of British Ordnance Terms - Vent-Sealing Tube

Vent-Sealing Tube

Usually abbreviated to "V.S. tube" or just Tube. This was the traditional, reliable British method of fully igniting powder charges in BL guns to fire projectiles, especially large shells. Briefly, after the powder cartridge was loaded (or even during the loading process), the tube was inserted through a vent in the breech. Early vents were "radial" i.e. at right-angles to the barrel length, bored through the top of the barrel into the chamber; later vents were "axial" through the centre of the breech mechanism and "mushroom" into the chamber. When the breech was closed, one of several methods was then used to trigger the tube, which then sent a powerful flash into the breech. The flash ignited a special "igniter" material in the end of the cartridge, and the igniter in turn ignited the main propellant charge (some form of gunpowder or Cordite). A powerful reliable flash from the tube was required because with bag charges, especially in the stress of combat and/or with variable howitzer charges, it could not be guaranteed that the igniter in the cartridge would be up close to the vent - it may have been pushed in too far, leaving a gap. The Tube was designed to expand on ignition and seal the vent, preventing escape of gas. Tube types :-

• Percussion tube - the tube was inserted in an axial vent in the breech and triggered by a firing pin in a percussion lock in the breech. Single-use. Used with medium-heavy guns and howitzers, e.g. 60 pounder Gun.
• Electric tube - the tube was fired by an electric current from mains or battery. Considered safe, but cumbersome for field use. Common with naval and coast defence guns.

• Friction tube - The tube would have a lanyard attached, with length proportional to the size of the gun, which when pulled caused friction inside the tube which ignited a powder charge, much like striking a match. Single-use. Originally of "Copper" and "Quill" types, replaced by the "T" tube by late 1890s. They were used in great quantitities by field artillery and will be found on old British battlefields up to 1904, e.g. in South Africa. They were inserted in a "radial" vent on top of the breech, or later in axial vents running lengthwise through the centre of the breech such as with the BL 15 pounder. The T design, with the friction wire to which the lanyard was attached running through the crosspiece of the T, ensured that when the lanyard was pulled and the gun recoiled the wire was pulled smoothly out of the T piece without exerting force on the vertical part of the T and hence affecting the gas seal.

From 1904, the new generation of field artillery was QF with propellant in brass cases with self-contained percussion primers, while small naval QF cases had self-contained electric primers. From then on, tubes were used only for guns of 60 pounder (5 inch) and upwards, usually Percussion tubes; and for a few small BL guns such as the 2.75 inch mountain gun, usually Friction tubes. However, Britain entered World War I with many old BLC 15 pounders which continued to require T Tubes until phased out by 1916. To approach a QF rate of fire they used a special "push" version of the T friction tube which was inserted into an axial vent in the breech like a BL percussion tube and fired by a similar mechanism to a firing pin activated by a lever rather than being pulled by a lanyard.

Tubes could also be used with QF cartridges fitted with tube adaptors in place of primers, as with the QF 12 pounder.