UNO proper shipping name -----Fuzes, detonating
SQ Action --Delay Action
Std OTCM 36841 dtd 1958.
The M51A5 fuze consists of Fuze M48A3 assembled with the M21A4 booster. The fuze PD head assembly contains a firing pin held in position by a firing pin support which prevents initiation of Detonator M24 until impact. The fuze body contains an M1 delay plunger assembly and an interrupter assembly with a setting sleeve which provides a means of setting or selecting fuze PD (Super Quick Action) or delay functioning. The delay plunger assembly includes a firing pin and Delay Element M2. The delay element includes Primer M54, a black powder delay charge and Relay M7. The head assembly is attached to the body by means of the flash tube which also positions the fuze windshield or ogive. The ogive is a thin-walled steel stamping utilized to provide an aerodynamic shape to the fuze. The M21A4 booster consists of a brass booster body having external (male) threads to fit projectiles having 2-inch diameter, 12 threads per inch and internal (female) threads to receive fuzes having 1.7-inch diameter, 14 threads per inch. An aluminum booster cup containing a 340-grain tetryl booster pellet is threaded to the booster body. The M21A4 booster internal configuration is that of an eccentric rotor containing an M17 detonator held in an unarmed (out of line) position by centrifugal and setback lock ins. On firing, the locking mechanisms are released and the rotor becomes aligned with the booster lead charge and the fuze flash tube when set for PD action or the fuze delay plunger relay charge when set for "delay" action.
Upon firing, the combination of setback and centrifugal forces are utilized to arm the fuze. The setback forces retract the booster lock pin allowing centrifugal force to extract the rotor lock pin and permitting the rotor to rotate into an armed position aligning the rotor M17 detonator with the detonation train of the fuze. Simultaneously, centrifugal force will arm the M1 delay plunger of the fuze and retract the flash tube interrupter unless the fuze is set delay, in which instance, the flash tube interrupter will not retract and the flash from the nose superquick element will be prevented from initiating the explosive train of the booster. The fuze is initiated upon impact with the target; the firing pin oft the fuze head assembly is driven into the M24 detonator which flashes through to the M17 detonator activating the lead charge and booster pellet. If set delay, the flash tube is blocked and the M17 detonator is activated by the delay element.
Difference Between Models: M51A5 Mod 3
- USN mod certification only
PD 2.12 lb
Lower limit ------------------------ -80°F (for not more than 3 days)
Upper limit ------------------------ +160°F (for not more than 4 hr/day)
* Packing --------------------------- 8 fuzes in metal container; 2 containers in wooden box
*NOTE: See DOD Consolidated Ammunition Catalog for complete packing data including NSN's.
Shipping and Storage Data:
UNO proper shipping name ----- Fuzes, detonating
Storage compatibility group -----
DOT designation -------------------- DETONATING FUZE CLASS A EXPLOSIVES, DO NOT STORE OR LOAD WITH ANY HIGH EXPLOSIVE.
Detonator M24, Detonator M17, tetryl booster lead charge, and tetryl booster charge
Delay plunger assembly M1 (Delay Element M2, M54 primer, black powder delay charge, Relay M7), Detonator M17, tetryl booster lead charge, and tetryl booster charge.
Bore safe only. Premature functioning can occur when fuzes are fired in heavy rainfall.
TM 9-1300-251-20 SC 1340/98-IL SB 700-20
Std OTCM 36841 dtd 1958.
Point detonating fuzes of the M78 series are constructed especially for use in spotting and destruction of concrete targets. The fuzes are used with HE projectiles fired from guns and howitzers in calibers 75mm through 8-inch, except 175mm.
The fuze has a solid hardened steel body with an ogival nose. A well in the base houses a firing pin and an inertial-type delay plunger mechanism containing a detonator. The delay plunger in each type is locked by two spin- activated, spring-loaded plunger pins. All M78 series fuzes are equipped with Booster M25, designed solely for this fuze. The booster has an externally threaded body containing a delayed arming mechanism, Detonator M17, and tetryl booster lead charge. The delayed arming mechanism is an eccentric, spin-activated rotor containing the detonator. In the unarmed position, the detonator is out of line with the flash hole and the rotor is locked by a spring-loaded centrifugal lock pin, which is in turn locked by a setback pin. The base of the booster is an aluminum cup threaded onto the body and containing a 340-grain tetryl booster charge. As issued, Booster M25 is packed and shipped with, but not attached to, the fuze.
Upon weapon firing, setback force withdraws the setback pin from the lock pin. As the spin rate of the projectile increases, centrifugal force withdraws the two plunger pins from delay plunger Assembly Ml in the head of the fuze, thus arming the delay plunger. Simulta neously, centrifugal force withdraws the lock pin, permitting the rotor to turn on the pivot pin until Detonator M17 is aligned with the flash hole in the booster top cover. The rotor is locked in the armed position by the centrifugal lock pin for the remainder of the flight. This delayed arming of the booster mechanism provides bore safety. Upon impact, Delay Plunger M1 is driven forward by inertia into the firing pin to initiate the explosive train.
□NOTE: See DOD Consolidated Ammunition Catalog for complete packing data including
Difference Between Models:
Fuze M78 has a delay plunger with a single 0.025 second delay. Fuze M78A1 is supplied with a non-delay Plunger Assembly M1, or a 0.025 second delay Plunger M1. Fuzes preset for non-delay are intended primarily for spotting, and are identified by a white-painted nose.
Firing: Lower limit -Upper limit Storage: Lower limit
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