M1 Delay For M739 Fuze

Fuze Detonation Grenade

Type Classification:

M739 - Std A MSR 02756077 December 1974,

- Std B MSR 08826010 August 1982, M739A1 - Std A MSR 08826011 August 1982.

Use:

Point Detonating Fuzes M739 and M739A1 are selective superquick and 0.05 second delay (M739) or auto-delay (M739A1) impact fuzes designed for use in all standard HE artillery 4.2 inch Mortar, 105mm through 8-inch Howitzers and 175mm Guns.

Description:

The M739 series fuzes are the latest improved version of the selective impact fuzes. The fuze body is a one-piece design of solid aluminum and has a standard 2-inch threaded base to match projectile nose and fuze cavity. The fuze consists primarily of five (5) modular subassemblies: (1) crossbar and holder assembly (2) firing pin and detonator assembly, (3) setting sleeve assembly, (4) M1 Delay Plunger Assembly (M739), or Impact Delay Module Assembly (M739A1), and (5) the safe and arming assembly.

The crossbar and holder assembly is a rain insensitive sleeve that allows firing in heavy rain with reduced probability of down-range premature functioning. The assembly is in the nose section of the fuze and consists of five (5) crossbars which break up raindrops and foliage and thus reduce fuze initiation sensitivity without affecting ground or target impact sensitivity.

The firing pin and detonator assembly is located below the rain insensitive sleeve and provides the superquick action on impact. The firing pin is held in position by a firing pin support which prevents initiation of the M99 Stab Detonator until impact.

The setting sleeve assembly (interrupter) is located in the side of the fuze body extending through the flash path of the M99 Detonator and thus provides selection of a PD mode which does not interrupt the flash from the detonator; or a delay mode which prevents the detonation flash from initiating the explosive train.

The M1 Delay Plunger Assembly is located in the rear portion of the M739 fuze and provides a 0.050 second fuze initiation delay for target penetration when the setting sleeve is set "delay", When not set "delay", the M1 delay plunger provides a back-up and graze action function for the superquick setting.

The M739A1 fuze contains an Impact Delay Module (IDM) assembly instead of the M1 Delay Plunger Assembly. The IDM provides fuze initiation delay based upon the completion of mechanical actions caused by projectile deceleration and will function immediately after passing through the target. Function occurs when a spring loaded firing pin is released. There are no explosive components contained within the IDM.

The safe and arming (S&A) module is below the delay assembly. It contains a rotor with a M55 detonator, an escapement to prevent the detonator from aligning with the explosive firing train until safe arming distance is achieved, both setback and spin locks to prevent accidental arming prior to firing, The explosive lead when initiated will detonate the booster pellet made of 22 grams of Composition A5 which is held by an aluminum booster cup assembled into the base of the fuze.

Functioning:

Condition as Issued:

In the firing pin and detonator assembly the firing pin is held over the SQ detonator by a collapsible support. The setting sleeve assembly interrupter blocks the flashhole between detonator and S&A assembly.

The S&A assembly is not armed since the M55 detonator which is contained in the S&A rotor is held out of axial alignment by a setback pin and spinlock detents.

The delay assembly is not armed because the detents hold the plunger from moving forward and beginning the sequence of events required for function.

Prior to Firing:

For delay action the setting sleeve must be turned clockwise so that the slot is pointed toward "Delay". This keeps the flashhole blocked regardless of the interrupter position, The setting sleeve may be returned counterclockwise to the "SQ" setting at will.

For super quick (SQ) action, the selector normally requires only inspection to assure that the slot of the selector sleeve is pointed toward the "SQ" mark, A coin, screwdriver or tip of the fuze wrench M18 may be used to turn the slot to the desired setting,

Action Caused by Setback and Spin in Firing the Projectile:

In the interrupter assembly centrifugal force moves the interrupter outward. When the setting sleeve is set for "SQ" the interrupter unblocks the flashhole in its move outward.

In the delay assembly centrifugal force moves each detent outward and locks each detent in the outward position by means of the centrifugal plunger pin lock.

In the S&A assembly the setback pin is disengaged from the rotor and the spinlocks move outward under centrifugal force allowing the rotor to turn and carry the M55 into line with the flashhole, This arming action is briefly delayed by a runaway escapement. The arming distances for associated cannon and mortar systems are given in the tabulated data. The rotor is held in its armed position by the rotor lock pin.

Action in Flight:

The plunger restraining spring in either the M739 or M739A1 delay assembly holds the plunger rearward.

When fired in rain the crossbars, after erosion of the nose cap, serve to break up raindrops and prevent functioning of the superquick detonator. Excess water is expelled through the holes in the crossbar holder assembly due to centrifugal force created by the spin of the round.

Action Upon Impact:

When the projectile hits a soft impact surface, the material ruptures the nose cap and then flows between the crossbars to strike the firing pin. If the projectile hits masonry or rock, the entire crossbar holder assembly will drive the firing pin into the SQ detonator.

For delay setting, the solid structure of the fuze body protects the delay assembly so that it will function after penetrating the target. Within the delay assembly the plunger travels forward upon impact, The M739 Fuze contains an M1 Delay Plunger Assembly and when the plunger travels forward an explosive delay element is carried by the plunger into a stationary firing pin held by the Ml housing thus initiating a timed delay function. The M739A1 Fuze contains an impact delay module and when its plunger travels forward a series of mechanical actions are initiated culminating in the release of a spring loaded firing pin propelled into the M55 detonator contained in the S&A.

In normal functioning with superquick action, the delay action has no effect, and the superquick detonator will have fired the detonator M55 in the rotor and the S&A assembly before the delay assembly can complete its action. However, should the SQ action fail, the projectile will function with delay action rather than become a dud,

Tabulated Data:

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