Used in: 4.5-inch Rocket Head Mk 3 (High Capacity)

Overall dimensions and weight of fuze:

Length

Diameter (maximum)

Body threads

Weight

Applicable specification: OS 2864

General arrangement drawing:

344523 (Mods 0 & 1)

437988 (Mod 2)

Sketch list of drawnings & specs:

102930 (Mods 0 & 1)

109467 (Mod 2)

Explosive components:

Detonator Mk 23 (azide primer mixture with lead azide and tetryl) housed in the shut-ter.

Booster lead-in (tetryl) housed in the lead-in disc.

Booster charge (9 grams tetryl) housed in the booster magazine.

General. Fuze Mk 137 Mod 0, or Mod 1 or Mod 2, is a setback and air arming, impact firing, rocket nose fuze. The fuze was designed primarily for use in rounds to be launched from small craft in beach barrage actions; however, its use has been extended to many other applications. Fuze Mk 137 arms during flight and fires instantaneously upon impact with suitable targets. Fuze Mk 137 Mod 0 has a propeller with eight vanes. Fuze Mk 137 Mod 1 has a propeller with ten vanes and internal changes. Fuze Mk 137 Mod 2 has the modifications of Fuze Mk 137 Mod 1 plus a rivet head stop on the propeller hub in place of an embossed stop.

Use. Fuze Mk 137 Mod 0, or Mod 1 or Mod 2, is used in the 4.5-inch High Capacity Fin Stabilized Surface Rocket (Complete Round) Mk 1 Mod 0. (See OP 1415). The round is utilized for barrage purposes.

Functioning mechanism. Fuze Mk 137 consists of a brass fuze body which provides housing for the threaded firing pin, the setback block with propeller locking pin, the set-back spring, the firing pin, the spacer sleeve, the shutter with the detonator and locking detent, the lead-in disc with lead-in charge, and the booster magazine with booster charge. The multivane propeller and its hub are mounted externally at the forward end of the threaded firing pin.

The forward section of the fuze body is tapered slightly towards a flat nose surface. At the base of the taper a narrow ledge serves as a mounting support for the propeller guard and as a shoulder for the external installation threads. A deep bore with counter-bore provides the internal housing for the fuze elements. The thin nose wall is drilled and threaded at the center to receive the firing pin and a second hole is drilled off center of accommodate the propeller locking pin. The counterbore at the base of the fuze is threa-ded to engage the booster magazine.

The firing pin is secured to the propeller assembly by means of a cotter pin passing through the firing pin shaft and the propeller hub. Below this hub there is a threaded sur-face on the firing pin which engages the threads in the nose of the body, and beyond the threads a machined shoulder prevents the complete retraction of the firing pin during air arming. The shoulder tapers down toward the base of the firing pin forming a conical surface. An elongated portion of the same diameter as the base of the conical surface extends approximately ½-inch and then tapers to a point. The setback block, a steel inertia collar with an integral propeller locking pin, floats around the firing pin on the set-back spring and bears against the nose wall. The propeller locking pin which locks the propeller extends from the surface of the setback block through the guide hole in the nose wall to engage a rivet or embossed stop in the arming vane hub. The setback spring at its forward end bears against the setback block and at its base the firing pin guide. The firing pin guide, a flat steel disc with the center portion raised to align the setback spring and drilled to accept the pointed shaft of the firing pin, bears against the shoulder provided by the fuze body cavity. The bottom surface of the firing pin guide provides the base for the shutter hinge pin and shutter stop pin. The modified semi-circular shutter houses the detonator and rotates about the shutter hinge pin and under the force of a torque spring. A small spring-loaded detent in the shutter engages a small well in the fir-ing pin guide after arming. A split ring type spacer called a sleeve, bears against the fir-ing pin guide and positions the lead-in disc with its charge. The booster magazine and charge is installed into the base of the fuze body and bears against the lead-in disc se-parated by a thin paper booster disc. A removable safety locking wire which clips over the propeller guard passes through aligned holes in the fuze body and engages a slot in the setback block to lock the setback block and propeller locking pin the forward posit-ion.

Figure 12 – Nose Fuze Mk 137 Mod 2, Cross Section View,
Armed and Unarmed Position

Arming. The safety locking wire is withdrawn prior to the firing of the round. When the rocket is fired, setback force, due to acceleration, forces the setback block rearward and compresses the setback spring. This movement withdraws the propeller locking pin from the engagement with the  propeller hub. The air stream, acting on the propeller, causes it to rotate and thereby unscrew the firing pin forward in the fuze body. The set-back force acts on the setback block during the burning of the rocket propellant then the setback spring returns the block to its original forward position. However, the firing pin has in the meantime unscrewed from the fuze body sufficiently to prevent re-enga-gement of the propeller locking pin with the propeller hub.

Rotation of the propeller continues to unscrew the firing pin until the unthreaded section is reached and the shoulder of the stop on the firing pin contacts the top of the fuze body. Before the firing pin is unscrewed, it holds the detonator shutter to one side so that the detonator is out of alignment with the firing pin and booster lead-in. When the firing pin is unscrewed sufficiently, the shutter is released under influence of the tor-que spring and the shutter is pivoted until it contacts a stop pin the the firing pin guide. At this instant the shutter detent engages the well in the firing pin guide to lock the shutter in aligned position. The detonator is now aligned with the firing pin and the boos-ter lead-in, and the fuze is completely armed. The arming distance of Fuze Mk 137 is ap-proximately 125 feet.

Firing. On impact, the firing pin is forced back to shear the light screw threads in the nose wall of the fuze body and to pierce and fire the detonator which in turn fires the lead-in charge and the booster charge.

Acceptance test data. Completely live-loaded random sample fuzes from each lot shall be installed in suitable live-loaded rockets and the rockets fired for high oder funct-ioning on ground or water impact. All rockets shall be fired from a launcher which is set at an angle of elevation of approximately 45 degrees. Acceptance is based on a cummu-lative test plan designed to accept (95% of the time) lots in which 95% of the fuzes function satisfactorily: lots poor than 95% will be subject to rejection with lots 82% effective being rejected 90% of the time. Based on wartime production acceptance re-cords, Fuze Mk 137 gave better than 97% perforance.

Sensitivity limits. Fuze Mk 137 will function instantaneously superquick upon impact with water, plate, or ground targets at angles of fall of 15 degrees or greater.

Detonator safety. This fuze is detonator safe. In the unarmed position, the detona-tor is held out of alignment with the explosive train. Should the detonator function pre-maturely while the fuze is unarmed, the force of the detonator action will be dissipated forward through a hole in the firing pin guide and away from the retaining explosive com-ponents. The fuze is not armed until the setback force withdraws the propeller locking pin and the propeller rotates and withdraws the firing pin to a point where the detonator shutter is released.

During shipping and stowage. Sample fuzes from each production lot are subjected to rough handling tests which are considered to be more severe than conditions encoun-tered in normal stowage and shipping. Failure to pass these tests is cause for rejection of the lot.

Fuze Mk 137 is provided with a safety locking wire which passes through holes in the fuze body to lock the setback block in the forward position so that the propeller locking pin engages the propeller hub. The locking wire also engages the propeller to prevent it from rotating. The propeller guard gives protection to the propeller during normal hand-ling.

General. If the fuze becomes accidentally armed, whether assembled into the rocket or not, no attempt should be made to unarm it. If the fuze is armed, turning the propeller counterclockwise will cause the firing pin to pierce the detonator and initiate the booster and main charge. The fuze shall be considered armed if the propeller is out of engage-ment with the propeller locking pin so that the propeller is free to rotate. Armed fuzes must NOT be installed in rounds for any purpose. To dispose of a fuze or fuzed round, the fuze propeller should first be carefully taped and the fuze or fuzed round, as the case may be, disposed of by gently lowering it, base down, into deep water or by Explo-sive Ordnance Disposal Personnel in accordance with existing instructions.

Disassembly. Disassembly of this fuze is not permitted except at authorized activi-ties when directed by the Bureau of Ordnance.

Examine each fuze before installation in the rocket to check if the propeller has been unscrewed so that the forward tips of the propeller blades extend out beyound the plane of the rim of the propeller guard. If the fuze is found in this condition, it should be consi-dered armed; the propeller should be taped so that it cannot be moved; and the fuze should be disposed of by methods described under paragraph 5, Disposal and Servicing.

After each fuze has been examined as above, and the propeller has been found to be below the rim of the propeller guard, remove the safety locking wire and try to turn the vane in a clockwise direction (when the fuze is viewed from the nose) to see if the vane is locked by the vane locking pin. Do not turn the vane more than one-half a turn. If the propeller can be turned, consider the fuze armed and dispose of it. If the propeller cannot be turned in the clockwise direction, try to turn it in the opposite direction. If the propeller cannot be turned, replace the safety locking wire, and the fuze is safe for use. If the propeller can be turned counterclockwise, turn it one-half a turn and see that the propeller locking pin snaps into the nearest hole in the propeller hub. Then replace the safety locking wire, and the fuze is safe for use or stowage.

Note: The sealed metal containers in which these fuzes are shipped should not be opened for the sole purpose of checking the fuze. Rather, the checking should be done at the time the sealed cans are opened preparatory to installing the fuze in the rocket body.

After the above examination, proceed as follows:

1. Remove the shipping plug, shipping plug gasket, and paper tube from the rocket body. Inspect the liner and clean if necessary. Be certain that the auxiliary booster is present in the fuze seat liner.

2. Examine the fuze for damage. If defective, (but not armed) repack, seal container with tape, and return to an ammunition depot.

3. With the fuze gasket in place, screw the fuze securely into the nose of the rocket with the proper fuze spanner wrench.

4. As the rocket is placed in the launcher, remove the safety locking wire.

5. If, for any reason, it becomes necessary to remove the rocket from the launcher, the safety locking wire should immediately be replaced, and the fuze may then be remo-ved from the rocket.

Packing. Each fuze is packed in a hermetically sealed can; 48 cans are contained in a wooden box 23¼ inches by 15 inches by 8 3/8 inches which represents a total of 1.7 cu. ft. The weight of the fuzes and packing is approximately 72 pounds.

Marking. The fuze is marked or stamped to show the letters NF, the mark, mod, and lot number, the manufacturer's initials or symbol, the date of manufacture, the initials or symbol of the loading facility, the month and year of loading, the anchor stamp, and the inspector's initials.