M734 Fuze - Safety

Safety

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:

1. An axial acceleration pulse similar to the launch inside a mortar tube.
2. Air flow through the nose cone Air-Inlet and Air-Outlet (Figure 3) that is similar to flight.
3. Motion that resembles the trajectory of a mortar shell in flight (on the product improved M734A1 fuze).

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).

• This electrical arming is simply the energizing of the firing circuit to the Microdet used by all three fuze settings.
• Since the apex varies with each type of mortar, the firing angle, and the amount of propellant, a microprocessor in the fuze is used to calculate the time remaining after launch to reach peak height. This is accomplished by monitoring in real time the frequencies of the Doppler radar and the wind driven power supply (Turbine Alternator Assembly in Figure 3) and comparing to a data bank in memory.