IMPACT FIRING FUZES.- Impact firing fuzes (fig. 2-19) function when the rocket strikes a target that offers sufficient resistance to cause crushing or other disarrangement of actuating parts.

All current impact firing rocket fuzes have the same type of safety/arming mechanism. This mechanism consists of an unbalanced rotor, which, under setback forces, drives a gear-train timing system. A given minimum acceleration over a given length of time is required to complete the arming cycle. If rocket acceleration is too low or extends over too short a period of time, the arming mechanism returns to the unarmed condition. The timing mechanism provides a safe separation distance from the launcher before arming.

When located in the nose of the warhead, impact firing fuzes are known as point-detonating (PD) fuzes. If they are located in the base of the warhead, they are known as base-detonating (BD) fuzes. Nose and base

Figure 2-20.-Mk 193 Mod 0 mechanical time fuze.

fuzes function either instantaneously or after a short delay that gives the warhead time to penetrate the target before functioning.

MECHANICAL TIME FUZES.- Mechanical time fuzes (fig. 2-20) function by the action of a mechanical timer. These fuzes contain a safety/ arming device and a clock mechanism. The arming mechanism is similar to those in impact detonating fuzes and requires a minimum acceleration over a given time to complete the arming cycle. Upon arming, the mechanical timer is started, and after a set elapsed time, the fuze initiates the firing train.

The Mk 193 Mod 0 is the only mechanical time rocket fuze currently in use. It is permanently installed in the nose of the Mk 33 Mod 1 flare warhead.

ACCELERATION-DECELERATION FUZES.- Acceleration-deceleration fuzes are similar to impact and time fuzes because they require acceleration for a given time to complete the arming cycle. After the arming cycle is completed and the rocket velocity begins to drop, deceleration causes the fuze to function.

The Model 113A is the only accelerationdeceleration fuze in use by the Navy at this time. It is a base-mounted fuze that is permanently installed in the WDU-4A/A flechette warhead.

PROXIMITY FUZES.- Proximity fuzes, sometimes referred to as VT fuzes (figs. 2-21 and 2-22), initiate by "sensing," usually by electronic means, the presence and distance of a target. Proximity fuzes are primarily used in air-to-ground operations where air bursts above the target are desired. They are not suitable for use against targets that require penetration and detonation within the target for effective destruction.

In general, proximity fuzes consist of an electronics package in the forward end, a thermal battery, a safety/arming device, and an explosive booster in the base. The arming mechanism is similar to those in impact detonating fuzes, and it requires a minimum acceleration over a given time to complete the arming cycle.

Figure 2-21.-Proximity fuze M414A1 or Mk 93 Mod 0.

Figure 2-22.-Proximity fuze M429.

NOTE: Some rocket fuzes designed for use with 2.75-inch warheads can be used with the 5.0-inch warhead by using the BBU-15/B adapter booster (fig. 2-23).

configurations to meet specific tactical requirements. For example, a rocket assembly that consists of a fragmentation warhead armed with a proximity fuze is entirely unsuitable for use against an armored tank or bunker. Likewise, the GP warhead fuzed only with the Mk 191 base fuze is relatively ineffective against personnel or unarmored targets. With each specific type of target, the right combination of warhead, fuze, and motor is assembled from the wide variety of components available.