Used in: 5.0-inch Rocket Head Mk 8 (Common)

Overall dimensions and weight of fuze:

Length

Maximum diameter

Body threads

Weight

Applicable specification: OS 3429

General arrangement drawing: 423486

Sketch list of drawnings & specs: 109344

Explosive components:

Detonator Plunger – Sensitive Primer Mk 102 (fulminate of mercury priming mixture); Secondary Primer Mk 101 (fulminate of mercury priming mixture); Detonator Mk 33 (lead azide); detonator plunger load (tetryl); plunger lead-outs (tetryl).

Fuze Body – Booster lead-ins (tetryl); booster (tetryl). Approximately 32 grams.

General. Fuze Mk 31 Mod 0 was originally developed for use as a 4"/50 caliber projec-tile base fuze; however, it has been adopted for use in the spin-stabilized rocket. Fuze Mk 31 Mod 0 is a centrifugal arming, impact firing base fuze which arms completely when the rotational velocity of the rocket reaches the range of 3000 - 4500 rpm which repre-sents approximately 30 feet of travel of the rocket. The fuze will fire reliably upon impact with suitable targets.

Figure 10 – Base Fuze Mk 31 Mod 0, External View,
Full Size (approximately)

Use. Fuze Mk 31 Mod 0 is used in the 5.0-inch Common Spin Stabilized Sufarce Rock-et (Complete Round) Mk 24 Mod 0. (See OP 1415). The round is utilized for bombardment purposes and has a maximum range of approximately 10,000 yards.

Functioning mechanism. The external parts of the fuzes consists of a fuze, body, booster cover, nose cap, and a plunger retaining plug. The steel fuze body is machined to accommodate the auxiliary plunger assembly, the detonator plunger assembly, the two detonator plunger detents, the anti-creep spring assembly, and the entire explosive train from the sensitive primer to and including the two side-wall boosters. The aft end of the fuze body is provided with external threads to allow installation of the fuze into the rock-et head and internal threads to engage the steel plunger retaining plug. The retaining plug integral with the fuze flange is finally secured to the fuze body and held in place by two locking pins peened in position. At the forward end of the body, external threads and a wide shoulder provide for installation of the nose cap which also retains the light metal external booster cover.

The auxiliary plunger, a cylindrical brass inertia block (with a small well at the forward end containing a desiccant mixture), floats on four rows of ball bearings in the auxiliary plunger cavity of the fuze body. The base of the auxiliary plunger bears against the plun-ger retaining plug while the forward end bears against the detonator plunger. The deto-nator plunger houses the sensitive primer, the secondary firing pin, the secondary primer, the spacer, the detonator, the detonator plunger charge, and the booster lead-outs. A shoulder at the after part of the detonator plunger bears against the two spring loaded detonator plunger detents. The forward end of the detonator plunger is guided by the anti-creep spring assembly and bears against the firing pin detents in the nose cap. Two aligning pins extend from the wall of the detonator plunger and ride in slots in the fuze body.

The aluminum alloy nose cap houses the sensitive primer firing pin, the spring loaded firing pin detents, and the two firing locking pins. The firing pin floats against the two fir-ing pin detents and is positioned in its well by the two locking pins. Two diametrically op-posed holes, externally sealed with cover plugs, placed at 90° to and slightly aft of the firing pin detents are for the purpose of locking the detonator plunger in place when firing occurs upon impact. A lock screw or pin is installed in the wall of the nose cap to secure its assembly to the fuze body. The anti-creep spring assembly consists of a light metal tubular inner cup which is lipped at its base to provide a footing for the anti-creep spring, the outer cup which serves as a guide, and the anti-creep spring which bears against the inner cup lip at the aft end and against the outer cup at the fore end. The inner cup is assembled to the detonator plunger at the forward end and is held in position by the sensitive primer holder.

The two boosters and two booster lead-ins are located diametrically opposite in the sidewalls of the fuze body.

Figure 11 – Base Fuze Mk 31 Mod 0, Cross Section View,
Assembled and Firing Position

Arming. Fuze Mk 31 arms when the rocket reaches a rotational velocity within the range of 3000 to 4500 rpm which represents a distance of travel of approximately 30 feet. At this point under centrifugal force the firing pin detents and the detonator plun-ger detents move out against their respective load springs and allow both the detonator plunger and the auxiliary plunger on its ball bearings to float against the anti-creep spring.

Firing. Upon impact and retardation of the rocket head, the inertia of the auxiliary plunger and the detonator plunger forces them forward to compress the anti-creep spring and impings the sensitive primer against the sensitive primer firing pin to pierce and fire the sensitive primer. At this point the detonator plunger is forward and the boos-ter lead-outs in the detonator plunger are aligned with the booster lead-ins in the fuze body. As the sensitive primer fires, the expanding gas passing through small ports in the primer and firing pin container creates an internal pressure which bulges the wall of the inner anti-creep spring cup at the points adjacent to the two locking holes in the nose cap. The bulging of the inner anti-creep spring cup locks the detonator plunger in the forward and assures complete alignment of the explosive train. The shear wire securing the plunger or secondary firing pin is sheared under the gas pressure and the plunger or secondary firing pin is forced into the secondary primer. The functioning of the second-ary primer fires the detonator which fires the detonator plunger load and the two booster lead-outs, the two booster lead-ins, and the two booster which initiate the main explo-sive charge in the rocket head. The fuze fires with a slight inhercent mechanical delay peculiar to all inertia plunger type base fuzes (0.003 seconds approximately).

Acceptance test data. Random sample live-loaded fuzes from each lot are assembled in 4"/50 caliber HC projectiles and fired an accordnance with the ballistic acceptance tests in Table 2.

Table 2

Rounds

Velocity

Obliquity

Plate

2

2200 f/s

45°

3/8" m. s.

0°

5th lot thereafter)

ricochet angles

Detonation of tested samples must take place at not more than four feet behind the plate when the projectile is fired at 2200 f/s velocity and not more than two feet at 1000 f/s velocity. For the plate impact test, an overall perfromance of 75% high-order functioning is required for acceptance of each lot. For the water impact test, satisfac-tory performance, when combined with the plate impact test, shall be 80% high-order functioning. A premature functioning of any fuze shall cause rejection of the lot.

Sensitivity limits. Fuze Mk 31 Mod 0 will function reliably upon impact with targets offering sufficient resistance such as ground, plate, or wood and upon impact with water at angles of fall of 5° or greater. Tests in rockets have indicated that the fuze will funct-ion on 85% of hits on 1/8-inch mild steel plates at angles obliquity upon to 55° when fired in the 5.0-inch Rocket Head Mk 8. On special treatment steel of thickness of one inch or greater, rocket head breakup, generally occurs.

Detonator safety. Fuze Mk 31 Mod 0 is detonator safe. The detonator lead-out and the booster lead-ins are not aligned until the detonator plunger moves forward upon im-pact after the detonator plunger detents have moved out under centrifugal force.

During shipping and stowage. Fuze samples from production lots are subjected to rough handling tests which are considered to be more severe than conditions encounte-red in normal stowage and shipping. Failure to pass these tests shall be cause for reject-ion of the lot.

General. Fuze Mk 31 Mod 0 is unsafe to handle after the round has been launched. Any dud around utilizing this fuze should be disposed of by gently lowering it, base down, into deep water or by Explosive Ordnance Disposal Personnel in accordance with existing instructions. No attempt should be made to remove the fuze from the round.

Disassembly. Disassembly of this fuze or the removal of the fuze from the rocket head is not permitted except at authorized activities when directed by the Bureau of Ordnance.

The rocket head loading activity installs the Fuze Mk 31 Mod 0 in the rocket head and assembles the gas check gaskets. Before assembling the rocket head to the rocket mo-tor, be certain that the fuze is present, gas checked, and properly installed into the rocket head.

Packing. Fuze Mk 31 is shipped assembled into the rocket head.

Marking. The fuze is marked at the base end of the plunger retaining plug with the mark, mod, and lot number, the manufacturer's initials or symbol, the initials or symbol of the loading facility, the month and year of loading, the inspector's symbol, and the an-chor stamp.