Artillery fuze
An artillery fuze or fuse is the type of munition fuze used with artillery munitions, typically projectiles fired by guns, howitzers and mortars. A fuze is a device that initiates an explosive function in a munition, most commonly causing it to detonate or release its contents, when its activation conditions are met. This action typically occurs a preset time after firing, or on physical contact with or detected proximity to the ground, a structure or other target. Fuze, a variant of fuse, is the official NATO spelling.
Terminology
Munitions fuzes are also used with rockets, aircraft bombs, guided missiles, grenades and mines, and some direct fire cannon munitions.Broadly, fuzes function on impact or at a pre-determined time period after firing. However, by the 18th century time fuzes were aimed to function in the air and in the 1940s proximity fuzes were introduced to achieve a more precisely positioned airburst. Therefore, the terms 'percussion' and 'airburst' are generally used here unless 'time' fuzes are being explicitly described.
Early history
Solid cannonballs did not need a fuze, but hollow balls filled with something such as gunpowder to fragment the ball needed a time fuze. Early reports of shells include Venetian use at Jadra in 1376 and shells with fuzes at the 1421 siege of St Boniface in Corsica. In 1596 Sebastian Halle proposed both igniting the bursting charge by percussion and regulating the burning time of fuzes, but this was considered visionary at the time. These early time fuzes used a combustible material that burnt for a time before igniting the shell filling. The problem was that precise burning times required precise time measurement and recording, which did not appear until 1672. Before this the proofmaster often tested the burning time of powder by reciting the Apostles' Creed for time measurement.It was not until around the middle of the 18th century that it was realised that the windage between ball and barrel allowed the flash from the propelling charge to pass around the shell. This led, in 1747, to 'single-fire' and eliminated the need to light the fuze before loading the shell. At this time fuzes were made of beech wood, bored out and filled with powder and cut to the required length. Experience taught that there was a minimum safe length. In 1779 the British adopted pre-cut fuze lengths giving 4, 4.5 and 5 seconds.
The first account of a percussion fuze appears in 1650, using a flint to create sparks to ignite the powder. The problem was that the shell had to fall a particular way and with spherical shells this could not be guaranteed. The term 'blind' for an unexploded shell resulted. The problem was finding a suitably stable 'percussion powder'. Progress was not possible until the discovery of mercury fulminate in 1800, leading to priming mixtures for small arms patented by the Rev Alexander Forsyth, and the copper percussion cap in 1818. The concept of percussion fuzes was adopted by Britain in 1842. Many designs were jointly examined by the army and navy, but were unsatisfactory, probably because of the safety and arming features. However, in 1846 the design by Quartermaster Freeburn of the Royal Artillery was adopted by the army. It was a wooden fuze some 6 inches long and used shear wire to hold blocks between the fuze magazine and a burning match. The match was ignited by propellant flash and the shear wire broke on impact. A British naval percussion fuze made of metal did not appear until 1861.
There was little standardisation. Well into the 19th century, in British service, virtually every calibre had its own time fuze. For example, seven different fuses were used with spherical cased shot until 1850. However, in 1829 metal fuzes were adopted by the Royal Navy instead of wooden ones. At this time fuzes were used with shrapnel, common shell and grenades. All British fuzes were prepared by cutting to length or boring into the bottom from below. The problem was that this left the powder unsupported and fuze failures were common.
British inventor Colonel Edward Boxer of the Royal Artillery suggested a better way: wooden fuze cones with a central powder channel and holes drilled every 2/10th of an inch. In 1853 these were combined into a single fuze with dual channels, 2 inches long for howitzers and common shell, 1 inch for shrapnel. The holes were sealed with clay, with could be spiked through before loading to select the necessary time delay. There were white and black painted groups of holes for odd and even tenths. The Boxer time fuze used a fuze hole size different to that of Freeburn's percussion fuze, which became obsolete. Freeman's were replaced in army service in 1861 by those designed by Pettman, which could be used with both spherical and non-spherical shells.
The final Boxer time fuze, for mortars, appeared in 1867 and the army retained wooden fuzes although the navy used metal ones. There was a similar American wooden fuze. However, in 1855 Armstrong produced his rifled breech loading gun, which was introduced into British service in 1859. The problem was that there was little or no windage between the shell and the barrel, so the propelling charge could no longer be used to ignite the fuze. Therefore, a primer was added with a hammer suspended above it, such that the shock of firing released the hammer, which initiated the primer to ignite the powder time-train. Armstrong's 'A' pattern time fuze was introduced to British service in 1860, while the shorter Borman fuze was used in the United States.
The introduction of rifled breech loader guns led to non-spherical projectiles, which landed nose first. This enabled percussion nose fuzes, but they had to cope with the spinning shell and centrifugal forces. This led, by about 1860, to percussion fuzes with a direct action firing pin and detonator and a magazine to boost the detonators sufficiently to initiate the shell's main charge.
Armstrong's time fuze designs evolved rapidly. In 1867 the F pattern was introduced; this was the first 'time and percussion' fuze. Its percussion function was not entirely successful and was soon replaced by the E Mk III fuze. Made of brass, it contained a ring of slow burning composition ignited by a pellet holding a detonator cap that was set back onto a firing pin by the shock of firing. It was the prototype of the T&P fuzes used in the 20th century, although initially it was used only with naval segment shells, and it took some time for the army to adopt it for shrapnel.
Description
Since the second half of the 19th century, most artillery fuzes have been fitted to the nose of the projectile. The base of the fuze is screwed into a recess, and its nose is designed to conform to the shape of the shell's ogive. The depth of recess can vary with the type of shell and fuze. Artillery fuzes were sometimes specific to particular types of gun or howitzer due to their characteristics, notable differences in muzzle velocity and hence the sensitivity of safety and arming mechanisms. However, by World War 2, while there were exceptions. Most fuzes of one nation could be used with any artillery shell of that nation if it could be physically fitted to it, although different army and navy procurement arrangements often prevented this. The exceptions were mortar bomb fuzes, and this continues.An early action in NATO standardisation was to agree the dimensions and threads of the fuze recess in artillery projectiles to enable fuze interchangeability between nations. Modern artillery fuzes can generally be used with any appropriate artillery shell, including naval ones. However, smoothbore mortars constrain the choice of safety and arming mechanisms because there is no centrifugal force and muzzle velocities are relatively low. Therefore, shell fuzes cannot be used with mortar bombs, and mortar fuzes are unsuitable for the higher velocities of shells.
The fuze action is initiated by impact, elapsed time after firing or proximity to a target. In most cases the fuze action causes detonation of the main high explosive charge in a shell or a small charge to eject a carrier shell's contents. These contents may be lethal, such as the now-obsolete shrapnel shell or modern sub-munitions, or non-lethal such as canisters containing a smoke compound or a parachute flare.
Fuzes normally have two explosive components in their explosive train: a very small detonator struck by a firing pin, and a booster charge at the base of the fuze. This booster is powerful enough to detonate the main charge in a high-explosive shell or the ejecting charge in a carrier shell. The two charges are typically connected by a 'flash tube'.
The safety and arming arrangements in artillery fuzes are critical features to prevent the fuze functioning until required, no matter how harsh its transport and handling. These arrangements use the forces created by the gun or howitzer firing – high acceleration and rotation – to release the safety features and arm the fuze. Some older types of fuze also had safety features such as pins or caps removed by the user before loading the shell into the breech. Defective fuzes can function while the shell is in the barrel – a 'bore premature', or further along the trajectory.
Different fuze designs have different safety and arming mechanisms that use the two forces in various ways. The earliest 'modern' fuzes used wire sheared by the shock of firing. Subsequently, centripetal devices were generally preferred for use with low-velocity howitzer shells because the set-back was often insufficient. However, late 19th- and 20th-century designs used more sophisticated combinations of methods that applied the two forces. Examples include:
- Centripetal force moving a bolt outwards, which allows another bolt to move backwards by inertia from acceleration.
- Inertia from acceleration overcoming the pressure of a retaining spring to release a catch that allows an arm, plate, segmented sleeve or other bolt to move outwards either by centrifugal force, or spring in the case of mortars.
- Centripetal force causing a plate holding a detonator to swing into alignment with a firing pin.
- Centripetal force causing a barrier plate or block to overcome a spring and swing out of the channel between the firing pin and detonator or between the detonator and the booster.
- Rotation causing a weighted tape to unwind from around a spindle and free the firing pin hammer.