Fuzes can be classified by functions as follows: Time fuzes: Mechanical time fuzes (MTFs) function a predetermined length of time after the projectile is fired. The exact time is set before the

projectile is loaded into the chamber by a mechanical fuze setter on the mount. This fuse can also be set with a special fuze wrench. The interval between the instant the fuze is set and the instant the projectile is fired is termed dead time. No matter when, how, or by what it is set, the timing mechanism of a time fuze will not function until the projectile is fired.

Time fuzes for larger caliber projectiles are driven by springs because the relatively slow rotation of these projectiles does not produce enough centrifugal force to run the clockwork reliably. Older time fuzes (no longer in use) consisted of slow-burning powder trains of adjustable length rather than clockwork. The powder was ignited by setback that drove a firing pin into a percussion cap.

Proximity fuzes: Proximity or variable time fuzes (VTFs) are energized after the projectile is fired and function when the projectile approaches closely to the target. Percussion fuzes: Percussion or impact fuzes function either as the projectile strikes the target or after the projectile penetrates. Some fuzes (nondelay type) function immediately on contact with any thin material (for example, the thin sheet metal skin of an aircraft). Fuzes for armor-piercing projectiles, however, always incorporate a slight delay to keep the burster from going off until after penetration. These percussion fuzes can be located either on the nose (PDF) or the base (BDF) of the projectile. Combination fuzes: Combination fuzes incorporate both time and percussion features; that is, the fuze may go off either on impact or after the time set, whichever occurs first.

Auxiliary fuzes: An auxiliary fuze (ADF), as the name implies, operates only in conjunction with other fuzes. In gun projectiles they form part of the explosive train and pass on the explosion initiated by another fuze

Figure 2-7.-Forces that work on fuzes.

(located in the projectile nose) to the main bursting charge.

Proximity fuzes in projectiles are miniature radio transmitters and receivers, powered by tiny battery cells. The cells are activated by setback. When the projectile approaches closely to a target, the radio waves sent out by a transmitter are reflected back to the receiver in sufficient strength to close a circuit that initiates fuze action.

Most projectile fuzes use a small detonating charge to set off the explosive train. These are called detonating fuzes. Some fuzes, however, are called ignition fuzes because they are designed to produce a flame that will set off an explosive sensitive to flame (usually black powder).

In general, proximity, time, and percussion fires are located in the projectile nose. ADFs are located just behind the nose fuze. In AP projectiles (the hardened cap makes no provision for nose fuzes) the fuze is in the base. In some projectiles, to provide greater versatility for selected targets, a nose and a base fuze are provided. The nose fuze can be inactivated at the gun for base fuze initiation. When the nose fuze is activated, the base fuze functions as a backup for greater reliability.

A fuze is intended not only to explode the burster charge at the right time, but it also is intended to prevent explosion at the wrong time. A fuze is armed when it is made ready to function. Before firing, when it is set not to function, it is considered safe.

Fuzes have safety features to protect those who handle ammunition. These safety features may be nullified by the time the projectile reaches the enemy. Some of the features are canceled by hand or mechanically before the gun is loaded. Others depend on the forces developed by the actual firing to arm the fuze. Fuzes that are armed only after the projectile leaves the gun muzzle are called boresafe. Projectiles 40-mm and larger are usually boresafe; projectiles 20-mm and smaller generally are not. This fact is important for you to remember when handling smaller caliber fuzed ammunition.

To illustrate how inertia is used to arm and operate a projectile fuze, let's look at a typical fuze (fig. 2-8). When the gun is fired, the force of setback moves the internal components of the fuze rearward and locks them against movement. As the projectile moves down the rifled bore, it is imparted rotation through the rotating band, creating centrifugal force. The projectile and fuze body travel through the air, meeting resistance and slowing down because of friction. The inertial force of creep frees the internal components for movement. Centrifugal force then moves the two sets of detents outward, unlocking the firing pin and detonator rotor for movement. Centrifugal force, acting on weights in the rotor, causes the rotor to turn until the detonator is in direct alignment between the firing pin and the booster lead-in. Continued centrifugal force maintains the explosive train in alignment. The fuze is armed Upon impact, the firing pin is driven into the detonator, initiating the explosive train through the explosive lead to the booster charge. The booster charge detonates the main burster charge.

Many different arrangements are used to arm both gun projectile and missile fuzes. All use the forces of inertia in one way or another. Some are totally mechanical and some are a combination of mechanical and electrical For further detailed information on fuze arming and operation, see U.S. Navy Ammunition, Historical and Functional Data, NAVSEA SW010-AB-GTP-010. More information on gun ammunition, including explosive charges, projectiles, and fuzes, is contained in Ammunition Afloat, NAVSEA OP 4, and Navy Gun Ammunition, NAVSEA SW030O-AA-MMO-010.