M734 fuze

The M734 Multi-Option Fuze shown in Figure 1 is a rangefinder and collision detection system used on 60mm, 81mm, and 120mm mortar shells as a trigger to detonate the shells at the most damaging heights of burst when combating four types of battlefield threats:

This integration of four functions into a single fuze reduces the logistics and cost to support mortar crews on the battlefield.

A typical mortar firing procedure is for a Squad Leader to select a target and call for one of the four fuze settings. A Gunner sights the mortar onto the target and an Ammunition Bearer sets the fuze. An Assistant Gunner drops the shell into the tube upon a command to fire from the Squad Leader.

Tools are not required to install or set the fuze. It is adjusted by hand, even with Arctic mittens, simply by rotating the top of the fuze clockwise until a 3-letter engraving is above an index line. Additionally, the setting can be changed any number of times without causing damage to the fuze. The four engravings around the circumference of metal housing of the fuze have the following meanings for detonation height:

In all four settings, the high explosive in the mortar shell is detonated by a cascading explosive train of 4 increasing energies within the fuze. These are the Microdet Electric Detonator, the Explosive Lead, the Explosive Booster, and the Delay Primer Assembly (Figure 3) functioning as follows:

Fuzes assembled by the manufacturer are preset to PRX and stockpiled on mortar shells for immediate use. The fuze is safe to handle, however, because the two detonators are mounted in a Safety and Arming (S&A) Assembly that holds them 180 degrees out of alignment with the Explosive Lead and Booster (Figure 3). The events required to rotate the explosive train into alignment and generate power for the fuze electronics cannot be accomplished by accident or deliberately by a vandal because three actions difficult to simulate must be applied in rapid succession:

Axial acceleration and wind stream forces combine to arm the fuze 100 meters or more from the launcher. This mechanical arming is accomplished by a torsion spring rotating the detonators 180 degrees into an explosive train alignment as soon as the spring is unlocked by the acceleration forces depressing a Zig-Zag Setback Device (Figure 3) and the wind stream forces unscrewing a Jackscrew (Figure 4) locking device.

This delay in mechanical arming after two independent features of gunfire is a basic safety requirement called "dual-safing." An unprecedented third safety factor incorporated as a product improvement in the M734A1 fuze was to delay the electrical arming of the PROX, NSB, and IMP settings beyond 100 meters out to the highest point of mortar flight (apex).

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