Contact or impact fuzes are actuated by the inertial force that occurs when the missile strikes the target. Figure 9-25 illustrates the action of a contact fuze before and after impact. (The booster charge and main charge are not part of the fuze.)

Figure 9-25.-Contact fuze action: A. Before impact; B. After impact.

During the launch and flight of the missile, the plunger remains in the after end of the fuze. When the missile strikes the target, it decelerates rapidly. The inertia of the plunger carries it forward to strike the sensitive priming mixture. The primer detonates and starts a chain reaction by igniting the fuze booster charge which ignites the main charge.

Sometimes a time delay element is used with a contact fuze. This delay permits the warhead to penetrate the target before detonation. Quite often a contact fuze is also used in conjunction with another type of fuze. For example, the main fuze can be a proximity-type fuze. Should it fail to operate as the missile approaches the target, the contact fuze would still function on impact. In this sense, a contact fuze serves as a backup or secondary fuze,

Proximity Fuzes

Proximity fuzes are actuated by some characteristic feature, influence, or property of the target or target area. Several types of proximity fuzes are available. The influence may be photoelectric, acoustic, pressure, electromagnetic (radio and radar), or electrostatic. Each of these influences could be preset to function when the intensity of the target characteristic reaches a certain magnitude.

Proximity fuzes are designed to initiate warhead detonation as the missile approaches or nears the target. The resulting burst pattern occurs at the most effective time and location relative to the target. Designing a fuze to produce an optimum burst pattern is not that easy. The most desirable pattern depends largely on the relative speed of the missile and of the target. Sometimes the fire control computer (during preflight programming) can adjust the sensitivity of the fuze. This action can compensate for varying target speeds and sizes. (We'd want a more sensitive fuze for a small target compared to a less sensitive fuze for a larger target.) Proximity fuzes, therefore, activate the warhead detonating system after computing two factors: (1) the distance to the target and (2) the rate at which missile-target range is closing.

Since a proximity fuze operates on the basis of information received from the target, it is subject to jamming. If jammed, the fuze could become inoperative. The missile would only damage the target if a direct hit (contact fuze) were scored. More seriously, jamming the fuze could result in premature detonation. In that case, the missile has no chance to reach the target. Most proximity fuzes use some means of electronic countermeasure or counter-countermeasure to eliminate or bypass the effects of jamming.

Of all the types of influence available (i.e., photoelectric, acoustic, pressure, etc.), the electromagnetic methods (radio and radar) are most practical. The TDD (fuze) transmits high-frequency energy waves toward the target (fig. 9-26). Some of the waves are reflected from the target. Because the missile is constantly closing on the target, the reflected signal is of a higher frequency than the transmitted signal. The two signals, when mixed, will generate what is called a Doppler frequency. Its amplitude is a function of target distance. When the amplitude reaches a predetermined level, the faze is programmed to operate and warhead detonation is initiated.

Ambient Fuzes

An ambient fuze is one that is activated by some characteristic of the environment surrounding the target. Ambient fazes are used mainly for surface or subsurface applications. A simple example involves a hydrostatic fuze for underwater detonation. In this case, the fuze is basically a depth meter and activates when water pressure reaches a certain amount.

Command Fuzes

A command faze responds to some form of signal from a remote control point. In guided missiles, this type of fuze is often used to order the weapon to self-destruct. The remote control point would be the firing ship. If the trajectory of the missile goes "wild" and/or the flight path endangers friendly forces, the firing ship orders command-destruct.

Other types of self-destroying fuzes are designed to actuate under certain conditions. For example, the missile can "lose sight" of the target. That may occur if some internal component malfunctions or the fire control radar ceases to transmit. Regardless of cause, the missile cannot respond to guidance data. If the

Figure 9-26.-A proximity fuze.

problem cannot be corrected rather quickly, circuits within the missile activate its self-destruct faze.

SAFE AND ARMING DEVICE

The safe and arming (S&A) device is the third element of a warhead. Throughout a guided missile, there are many S&A-type devices. The one we are discussing here is related to the warhead and operates in conjunction with the fuze or TDD.

Faze action may be divided into two phases- functioning and S&A. The functioning process involves initiating payload detonation at the optimum time, thus inflicting maximum damage. The S&A device has a dual purpose. As a safety feature, it must prevent premature initiation of the payload. The safety is effective until a specific signal or series of signals is received. At this time, certain events have occurred in correct sequence or (maybe) a desired time interval has elapsed. In any case, it is now safe to arm the warhead.

For the arming feature, the arming mechanism of the S&A device must actuate. This acuation removes or cancels the safety feature and permits the transfer of energy between the fuze and payload.

Normally, the safety function is accomplished by inserting a physical barrier between the faze and payload. The S&A device thus acts as an open switch until safe detonation can be performed. Armed, the switch is closed and the explosive train is capable of activating.

Study figure 9-27 for a moment. It depicts atypical explosive train for a warhead and illustrates the relationship between the faze (TDD), S&A device, and payload.