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3E1. General

A projectile fuze is a mechanical, electrical, electronic, magnetic (or combination) device which will detonate or ignite a charge in a projectile at the time and under the circumstances desired.

Fuzes may be classified according to function (impact, time, or proximity), the position of the fuze in the projectile (nose or base), type of mechanism or principle utilized (mechanical or VT), and specific action at time of functioning or initiation (ignition or detonation). Figure 3E1 illustrates typical fuzes.

Typical examples of nomenclature for Navy fuzes areas follows:

1. Auxiliary detonating (ADF).

2. Base detonating (BDF).

3. Mechanical time (MTF) or electrical time (ETF).

4. Point detonating (PDF).

5. VT or proximity (VTF).

Point detonating, time, and VT fuzes may all be called nose fuzes because of their location in the projectile. Fuzes are designated as detonating when they contain within themselves a high-explosive charge sufficient to set off a high-order explosion in the burster. Ignition fuzes contain black powder sufficient to ignite the burster of small projectiles. In larger projectiles such fuzes function indirectly through an auxiliary detonating fuze.

3E2. Fuze safety

It is necessary for the safety of personnel that a fuze be made inoperative until the projectile is well clear of the muzzle and of the firing ship. A fuze is said to be armed when its component parts are so arranged that it can operate to set off the next explosive in the chain. It is unarmed when its safety features are so functioning as to prevent its operation.

A satisfactory fuze must meet these requirements:

1. It must be safe to handle; that is, the fuze must not become armed if dropped or joggled.

2. It must be safe within the bore of the gun and for a sufficient distance outside to ensure security of personnel in the vicinity.

3. It must initiate the explosion of the filler at the proper moment, whether on impact or at a specified time.
E. Fuzes and Tracers
3E3. Fuze operation terminology

If fuzes were not equipped with safety features, they would be relatively simple. The need for safety makes for complicated operation, which depends for its initiation on certain forces brought into play when a projectile is fired from a gun. Forces which may be used to operate fuzes are:

1. Setback. The force of inertia which tends to move all fuze parts to the rear as the projectile is initially accelerated in the bore of the gun.

2. Angular setback. The force of inertia, which tends to resist the initial rotational acceleration of the projectile in the gun.

3. Centrifugal force. The continuous force, caused by the rotation of the projectile in flight, which tends to move all fuze parts radially away from the axis of the projectile.

4. Creep. The continuing inertial force resulting from the deceleration of the projectile in flight, caused by air resistance, which tends to move forward the fuze parts not exposed to the air.

5. Impact. The sudden inertia force which tends to move all fuze parts forward when a projectile strikes.

6. Target contact. The rearward movement of a firing plunger or other device when the projectile contacts the target material.

The magnitude of some of the forces is illustrated in the table.
3E4. Auxiliary detonating fuzes

“Aux dets” are used in conjunction with all types of nose fuzes in HC, AA, and AAC projectiles of 3-inch and greater caliber. They are interposed between the nose fuze and the bursting charge of the projectile to provide a heavier shock for detonating the bursting charge. They also act as a safety feature, preventing the projectile filler from exploding in case the nose fuze should be accidentally actuated prior to the arming of the auxiliary detonating fuze.

3E5. Base detonating fuzes

Base fuzes are used alone in armor-piercing and common projectiles. They are used in combination with nose fuzes in such dual-purpose projectiles as AAC and HC. In the latter case their functioning is completely independent of the nose and auxiliary fuses, the former of which may for certain purposes be replaced by a steel nose plug.

All base detonating fuzes function on impact; some, however, incorporate a delay feature. Base detonating delay fuzes function a short time (0.02 to 0.033 second) after the projectile hits the target, thus allowing time for armor penetration. Base detonating non-delay fuzes contain no actual delay element, but a slight inherent mechanical delay provides a time margin sufficient for the penetration of thin sheet metal.

3E6. Time fuzes

In most calibers of gun projectiles, time fuses are clockwork mechanisms used to obtain timed air bursts.

They are used in AA, AAC, AA (non-frag), HC, IlIum, WP, and W projectiles of 3- to 6-inch sizes and in HC projectiles of 8- to 16-inch caliber. There are two general types of mechanical time fuzes: one type depends for its action solely upon centrifugal force; the other is a spring-driven variety. The centrifugal type is less affected by long periods of storage, but the spring-driven fuzes are more satisfactory for use on large projectiles which have slower speeds of rotation. Each type is made in several marks and mods for various calibers.

A highly accurate electric time fuze that can be set very quickly (and thus reduces “dead time” to the vanishing point) is under development at this writing.

3E7. Point detonating fuzes

Point detonating fuzes are designed to function on impact with the target. They have the advantage of being faster acting on impact than base detonating fuses. One group of such fuzes is used in place of mechanical time fuzes in connection with shore bombardment with HC, AAC, and WP projectiles. Other marks of point detonating fuzes are used in 20- and 40-mm projectiles, for which no other type fuzes are provided (20- and 40-mm AP projectiles are solid metal, except for the tracer cavity, and thus unfuzed).

Figure 3E2 shows a sectional view of the 40-mm Point Detonating Fuze Mark 27. This is a simple fuze, an explanation of which will illustrate typical fuze operation and safety features. The fuze is composed of four major parts: the fuze body, the magazine, the firing-pin holder, and the rotor block assemblies. The forward section of the fuze body contains the plastic firing-pin extension or hammer, the stab-type firing pin, detents and springs contained in the firing-pin holder assembly, and the rotor block with rotor, detents, springs, and rotor cover. The magazine is screwed into the after end of the body.
The fuze is designed:

1. To detonate the projectile explosive charge and thereby burst the projectile with high-order detonation instantaneously upon impact.

2. To ensure safety and prevent detonation of the projectile when fired in a gun or in normal flight until detonated by impact.

Operation. On examining figure 3E2 the student will see that the firing pin cannot move aft (to the left in the diagram) because the firing-pin detents prevent it from doing so. Also, the detonator is not in line with the firing pin, so that, if the firing pin should somehow move aft, it would strike the rotor body and not the detonator. If the detonator should explode, it would not detonate the booster. The rotor, and consequently the detonator, is held in the unarmed position by the rotor detents, which fit into the rotor body as shown in the diagram. The rotor body also contains two lead counterweights.

When the projectile is fired, the rifling in the gun rotates the projectile. As the projectile spins, centrifugal force causes the firing-pin detents to move outward, freeing the firing pin. The rotor detents also move outward, freeing the rotor. Centrifugal force also acts upon the lead counterweights in the rotor body, tending to move them outward. This causes the rotor body to rotate, bringing the detonator into line with the firing pin and lead-in. The fuze is now armed.

Upon impact, the firing pin is rammed aft, striking and exploding the detonator. The detonator explodes the booster, which in turn detonates the burster charge of the projectile.

3E8. VT fuzes

The radio proximity or VT fuze is used in all of the types of projectiles which can use mechanical time fuzes except illuminating and window (which are not supposed to be exploded in the immediate vicinity of a target). The VT fuze is a self-contained, radio-controlled fuze capable of transmitting pulses of radio energy, and of receiving a portion of these pulses which may be reflected by a target. The fuze fires when the returning signal is of sufficient strength, due to proximity to the target, to trigger the firing circuit. Essentially, the fuze is an extremely rugged radio transmitting and receiving station, which fits into the nose of a projectile and is so compact that it displaces a volume less than half of an ordinary pint milk bottle.

See figure 3E3.
The principle of its operation can best be illustrated by describing the firing of a typical VT-fuzed projectile. Not only are VT fuzes as rugged as most fuzes, but they have been provided with reliable safety features. As a result, in safety of handling, safety in the bore of the gun, and freedom from muzzle bursts, they are as safe as any fuzes used by the Navy.

At the instant the projectile is fired, a tiny wet battery that furnishes energy to the fuze begins to be activated. The shock of fire breaks a small glass vial filled with liquid electrolyte. Centrifugal force in the rotating projectile causes this liquid electrolyte to flow toward the outside of a cylindrical cell through a stack of thin ring-shaped plates that have been carefully insulated from each other. Contact between the electrolyte and the plates makes the battery electrically active. Within a half second after the battery has become active, it has charged a firing condenser with electricity. Once this condenser is charged and a mercury safety switch has been opened, the projectile is “armed”, and ready to detonate when a target influences it to do so. All this has been accomplished by the time the projectile has traveled four or five hundred yards.

As the projectile speeds through the air at a rate of approximately 2,600 feet per second, a radio oscillator sends out electromagnetic waves or impulses at the speed of light. These impulses will be reflected back to the oscillator by any target that gives a radio reflection, such as metal objects, water, or earth.

At first the projectile is so far from the target that these impulses are not returned in any strength. But as the projectile approaches closer to the target the oscillator receives ever stronger reflected impulses. These incoming impulses interact with outgoing impulses to create a “ripple pulse” which is amplified by vacuum tubes. If the projectile comes within a radius of about 75 to 100 feet of its target, this “ripple pulse” becomes powerful enough to trigger a thyratron tube which acts as an electronic switch. This releases the electrical energy stored in the charged condenser which, in turn operates an electrical primer. The primer consists of a small container of black powder which is fired by an electrical current passed through a resistance wire. The blast from this primer operates an auxiliary detonating fuze, which sets off the main explosive charge in the projectile.

This fuze, the result of Navy research, has been regarded as one of the greatest forward steps in recent ordnance development. It eliminates completely the tremendous problem of fuze setting in order to time the burst at a point in the trajectory where lethal damage results to a fast-moving target. This allows maximum concentration on accurate tracking and solution for the correct trajectory, which is necessary because the projectile must be placed within 100 feet of the target for the fuze to function.

VT-fuzed ammunition is very effective on exposed personnel and lightly armored targets ashore. It is also well adapted for harassing and interdiction fire to deny the enemy the use of, but not destroy, bridges and other works which our own forces may later require. No matter what the topographic configuration, the fuze will detonate at that designed point in its flight in close proximity to a reflecting mass, such as the earth or trees, where fragmentation blankets a maximum effective area. The introduction of this fuze in the European campaign of World War II by United States Army artillery had a tremendously demoralizing as well as destructive effect on enemy ground troops.

3E9. Tracers

It is sometimes advantageous to follow a projectile in flight. For this purpose a tracer body is installed in the base or as an extension to the base, of the projectile. It contains a pyrotechnic mixture designed to burn with a definite color during all or a specific part of the projectile’s flight. Standard tracer colors in the Navy are red or white in AA projectiles and orange (for night tracers) in AP and common projectiles. The tracer is ignited by the heat or pressure of the propelling charge.

In 40-mm projectiles, tracers perform the special function of setting off the burster charge at the end of the tracer burning period. This is accomplished simply by allowing the inside end of the tracer to have direct access to the main charge. The advantage of a self-destructive feature in AA projectiles, which might otherwise land and burst on own ships or installations, is obvious.