Mk 162 and Mk 166

Rockets uses in

Functioning

Arming distance, feet

Over-all length, inches

Body diameter, inches

Mk 166 - 2.125

General: These fuzes were designed primarily to be used against marine targets. They will not detonate immediately after impact with water, but will allow the rocket to continue on its underwater trajectory. If the rocket strikes the hull of a ship above water or under water, the fuze will detonate the rocket after penetration of the hull is comple-ted. If the rocket misses the ship, however, the fuze fires after approximately 150 to 200 feet of underwater travel. Upon penetration of very heavy fortifications, the fuze does not function after fixed pyrotechnic delay but automatically varies the delay in firing me-chanically, so that it does not fire until penetration of the target is completed or, if the target is too heavy, until the rocket stops. It is in this manner that the fuze is discrimi-nating.

Operation: The operation of this fuze is divided into five stages: (1) gas pressure ef-fect; (2) rotation cauisng alignment of firing train in a vertical plane; (3) creep; (4) im-pact; (5) firing.

First stage - After the round has been fired, gases from the burning motor enter through the inlet filter, pass through the orifice in the inlet screw, and exert sufficient force to open the inlet valve. The gases accumulate and build up pressure in the upper chamber. Because of the differential pressure between upper and lower chambers, the gases seep through a small orifice in the baffle cup into the lower chamber. Upon the completion of burning of the motor, the pressure of the gases in the upper chamber is above the remaining motor pressure, and therefore forces shut the inlet valve. The gases from the upper chamber continue to seep into the lower chamber, tending to equalize the pressure between chambers. When the pressure in the lower chamber is sufficient, the diaphragm collapses, forcing the arming sleeve forward and shearing the shear wire.

Second stage - The rotor, which has been kept from turning by the shear wire, is now free to rotate under the force of the rotor spring. The detonator plunger is attached to the rotor through a detonator-plunger pin, which rides in a vertical groove in the rotor. The trigger block likewise is attached to the rotor through trigger-block rotating pins. Thus, as the rotor turns in a clockwise direction, so do the detonator plunger and trigger block assemblies. Rotation continues for 90°, at which point the rotor stopped by the rotor stop pin. This aligns the firing train in a vertical plane. The grooves in the detonator plunger are now aligned with two stop pins.

Third stage - Upon the completion of burning of the rocket motor and completion of the rotation of parts discussed in the second stage, the force of creep causes the deto-nator plunger and trigger block assembly to move forward, the rotor being held in position by the retaining ring. The trigger block continues to move forward until it engages a shoulder in the fuze body. As the detonator plunger is attached to the trigger block by four firing balls, further movement of the detonator plunger also ceases as the trigger block engages the fuze body. The firing train is prevented from getting out of line in a vertical plane by the stop pins which ride in groove in the detonator plunger and prevent further rotation of the detonator plunger. Up to this point of the operation, the lead-outs are not yet in complete alignment with the lead-ins in a horizontal plane.

Fourth stage - The force of impat is sufficient to free the detonator plunger from the trigger block by camming the firing balls inward. The detonator plunger then moves all the way forward, aligning the two lead-outs with the two lead-ins. The detonator plun-ger is locked in the aligned position by two detents. Thus the firing train is locked in alignment in both vertical and horizontal planes. The trigger-block locking balls are cam-med into the recess between trigger block and detonator plunger by the action of the trigger-block spring. The trigger block is now free from the trigger latch, allowing the trigger-block spring to act on the trigger block. The fuze is now fully armed.

Fifth stage - The fifth stage of the operation occurs when the force of deceleration drops to a value below the strength of the trigger block spring. The trigger-block spring now forces the trigger block aft, thus presenting a recess to the firing balls. The firing balls are cammed into the recess by the action of the firing pin spring, freeing the sensi-tive-type firing pin. The cocked firing pin is driven into the primer, which sets off the fir-ing train.

The Base Fuze Mk 162 is similar to the Mk 166 except for the following:

1. Thread diameter of 2.75 inches instead of 2.125 inches.

2. An inlet shield is issued instead of an inlet disc.

3. The trigger spring of the Base Fuze Mk 166 is about 50% stronger than in the Mk 162.

4. The Base Fuze Mk 162 has a percussion-type firing pin.

5. The orifice in the baffle cup of the Base Fuze Mk 162 is smaller than that of the Mk 166.

Remarks: Should a dud occur after the round has been fired, the fuzes may be ex-tremely sensitive because of the spring-loaded trigger block and the cocked firing pin and should not be disturbed of jarred in any manner.

Figure 167. Navy Base Fuze (Rocket) Mk 166 (Assembly)

Figure 168. Navy Base Fuze (Rocket) Mk 166 (Details)