The M734 multi-option fuze is a rangefinder and collision detection system used on 60mm, 81mm, and 120mm mortar shells as a trigger to detonatethe 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.
Settings
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 three-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:
DLY = Delay after impact of 0.05 seconds in the fuze explosive train before the shell detonates.
In all four settings, the high explosive in the mortar shell is detonated by a cascading explosive train of four increasing energies within the fuze. These are the Microdet electric detonator, the explosive lead, the explosive booster, and the delay primer assembly functioning as follows:
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 assembly that holds them 180 degrees out of alignment with the explosive lead and booster. 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:
An axial acceleration pulse similar to the launch inside a mortar tube
Air flow through the nose cone air-inlet and air-outlet that is similar to flight
Motion that resembles the trajectory of a mortar shell in flight
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 and the wind stream forces unscrewing a jackscrew 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 shell flight.
Power supply
The wind stream in flight provides both the mechanical power needed to arm the S&A and the electrical power needed for the fuze electronics. There is a system of components used in the M734 to capture and regulate air flow within the fuze and convert a portion of the air power to mechanical and electrical power before exiting the fuze. Since arming is required to occur after a flight of 100 meters for three mortars that have a wide range in launch velocities, the rpm that releases the jackscrew at the slowest launch velocity must increase in direct proportion to any increase in launch velocity. The turbine, however, will tend to spin faster than desired, so, to prevent early arming, three governors are used to reduce the spin: Once the air flows from the tips, the air outlet directs the exhaust into the atmosphere at an angle oblique to the external wind stream. The resulting turbulence degrades the accuracy of flight toward the target, so the exhaust is directed onto a vertical metal fin that guides the flow into the external wind stream. The performance of the turbine alternator is unaffected if the mortar shell encounters a tropical rainstorm while en route to the target.
History
The M734 fuze was developed at the Harry Diamond Laboratories for the 60mm lightweight company mortar system, which now is managed by the Armament Research Development and Engineering Center Fuze Division. It was determined to be suitable for army use in July 1977 and accorded type classification standard. To demonstrate readiness for transition into full rate production by the Armament Munitions and Chemical Command, ARDEC/HDL managed the first production for war reserves as well as the construction of automated initial production facilities for mobilization readiness. HDL awarded three competitive contracts in 1978/79: Eastman Kodak for the fuze assembly and IPF, Motorola for the amplifier assembly and IPF, and Alinabal for the alternator assembly and IPF. After successful first article inspection and production lot acceptance tests, transition was completed in March 1983. AMCCOM performed all procurements for stockpile with technical support by ARDEC. The Army Mortar Plan issued in 1985 expanded use of the M734 fuze to 60mm, 81mm, and 120mm mortars. Improvements in the fuze reliability and performance by ARDEC engineers led to production of the M734A1 fuze manufactured by L-3 FOS.
