LEARNING OBJECTIVE Describe the different types and functions of fuzes used on current 5-inch and 76-mm projectiles.
In chapter 1 you learned that the burster charge of a projectile is relatively insensitive and requires an explosive train. This train begins with a very small amount of sensitive initiating explosive that initiates the chain reaction required to detonate the less sensitive main burster charge.
The component that sets off the projectile bursting charge is the fize. No matter how complicated or simple the construction or function of the fuze is, it always serves the same purpose.
The nature of the fuze mechanism depends, of course, on what type of fuze it is. All fuze mechanisms depend on certain forces either to start their functioning or to keep them functioning. These forces develop when the projectile is fired, when it flies through the air, or at the end of the flight. In the sequence of their development, these forces are called setback, angular acceleration, centrifugal force, creep, and impact. They are worth explaining.
All objects have a property known as inertia. For our purpose we can say that inertia means resistance to change in motion. A moving ship, for example, tends to keep going after the engines have been stopped. It would keep going indefinitely if it were not for the fluid friction of the water and obstacles in its way. By the same reasoning, a ship dead in the water tends to remain so; it takes a mighty effort by its propulsion machinery to get it under way.
In 1687, in a Latin treatise on natural philosophy entitled "Principia," Sir Isaac Newton described this characteristic behavior of material things in the statement of his first law of motion:
"Every body tends to remain at rest, or in uniform motion in a straight line, unless compelled by external force to change."
Why bring up Newton and his laws of motion when we are discussing fuzes? The reason is that every one of the forces that acts on a projectile fuze-from firing to impact-is an effect of inertia.
Let's begin by discussing setback (fig. 2-7, view A). When the propelling charge of the round tires, the fuzes and the projectile are at rest. As the hot gases expand, pressure in the chamber builds up and forces the projectile to move forward. But because of inertia, every particle of the projectile and the fuze tends to stay where it is. The effect is the same as what you feel while riding in a car when the driver stomps on the gas pedal. Your head snaps back as the car jerks forward The same thing happens in the projectile and its fuze, except that the acceleration-and the setback effect-are thousands of times greater. As an example of its application to fuzes, setback is used in mechanical time fuzes to unlock the clockwork mechanism.
Angular acceleration (fig. 2-7, view A) produces an inertia force accompanying the initial rotation of the projectile in the bore of the gun. It is similar in effect to setback, which is the resistance to forward motion, in that it resists the rotational motion of the projectile as it passes through the rifled bore.
As the projectile rotating band is twisted by the rifling of the gun bore, the projectile spins. You know how spinning develops centrifugal force (fig. 2-7, view B),-a tendency to fly directly away from the center of rotation. Centrifugal force is used to operate the clcckwork in most mechanical time fuzes. It is also used to assist in readying (arming) the fuze to function when it strikes or approaches the target.
Creep (fig. 2-7, view C) is another effect of inertia. Like anything else that moves through the air, a
projectile in flight moves against air resistance, which tends to slow it down. Its supersonic speed creates shock waves and turbulence that increase this frictional slowing. This slowing-down effect is applied to the
exterior of the projectile only. The parts inside are not overcoming any air resistance, so they do not tend to slow down. In an automobile, for example, when the brake (simulating air resistance) is being applied lightly, you tend to lean forward. Similarly, movable parts in a fuze tend to creep forward as the projectile plows through the air that slows it down. In many types of fuzes, creep force is used to align the fuze-firing mechanism so that it will function on impact.
Impact (fig. 2-7, view D) is probably the most obvious application of the general principle of inertia to fuzes. When the projectile strikes, it comes to a stop. But the movable parts inside the fuze tend to keep right ongoing. The force developed by impact is used when you drive a firing pin against a percussion cap to initiate the explosive train. Some people think of impact as a kind of creep-but in a very violent form. In principle,
it is true that creep and impact are related, but they are quite different in degree and are used differently in fuze mechanisms. It is best to consider them separately and understand the function of each.