Used in: 7.2-inch Rocket Head Mk 10 Mod 1 (High Capacity)

Overall dimensions and weight of fuze:

Length

Diameter (maximum)

Body threads

Weight

Applicable specification: (To be prepared)

General arrangement drawing: 440295

Sketch list of drawing & specs: 165126

Explosive components:

A sentsitive primer and detonator assembly consisting of the Primer Mark 102 (mer-cury fulminate) and the Detonator Mk 23 Mod 0 (lead azide primer mixture, lead azide, and tetryl) assembled into a  single case closed by a crimp. The assembly is contained in the shutter between the firing pin guide and the lead-in disc.

A lead-in charge (tetryl) contained in the lead-in disc between the shutter and the magazine.

A booster charge (approximately 12 grams tetryl) contained in the magazine at the aft end of the fuze.

General. Fuze Mk 161 Mod 0 is a base detonating fuze which is armed by the pres-sure of the gases from the rocket motor. The design was adopted from the obsolete Fuze Mk 146 Mod 1 except that the fuze head and base fuze-to-motor adapter are made in one piece.

Use. Fuze Mk 161 Mod 0 was designed to replace the Fuze Mk 146 for use in the 7.2-inch High Capacity Fin Stabilized Surface Rocket (Complete Round) Mk 1 Mod 2. The complete round is called a 7.2-inch demolition rocket which is used for short range land firing from launchers installed on tank turrets. Firing action is instantaneous upon impact with suitable targets.

Functioning mechanism. Two pieces, called the head and body, house the fuze me-chanism. The head is machined out in two bores and each step is supplied with internal threads. The first bore or largest inside diameter takes the place of the motor adapter and the second bore accommodates the diaphragm and plug. The insertion of the dia-phragm and plug into the head forms a gas chamber into which the gases from the rock-et motor flow. The gases flow through the inlet screw after first being filtered by the in-let screen. The outside diameter of the head is supplied with external threads to engage the mating threads in the rocket head. The forward end of the fuze head contains inter-nal threads to accommodate the fuze body.

The body consists of a thick walled steel tube with an external threaded which enga-ges the internal threads in the forward end of the fuze head. The forward end of the body has internal threads into which the magazine is screwed.

Within the body is contained the firing mechanism which consists of an aluminum arm-ing plunger located immediately below the diaphragm. The plunger is held in place by a shear wire inserted through it and extending into two diametrically opposite recesses in the body.

Figure 47 – Base Fuze Mk 161, Cross Section View, Unarmed Position

In the unarmed position the arming plunger holds a locking ball in such a position as to lock the firing pin body and the firing pin in the forward position, compressing the firing pin spring. While in the forward position, the firing pin extends through the firing pin guide and into a cavity in the shutter. This cavity in the shutter is so positioned that when en-gaged by the firing pin the detonator assembly is out of alignment with the tetryl booster lead-in and the firing pin. Rotation of the shutter to bring the explosive elements into line when released by the firing pin is accomplished by torsional force of the shutter spring.

A lead-in disc containing a tetryl lead-in charge to transmit the explosive force from the detonator assembly to the magazine charge, is contained in the fuze body between the shutter and the tetryl magazine charge.

Figure 48 – Base Fuze Mk 161, Cross Section, Armed Position

Arming. The fuze screws into the base of the rocket head. Inasmuch as the fuze head and base fuze-to-motor adapter are manufactured in one piece, it is not neccess-ary to use any seal between the fuze and the rocket head. The exterior surface of the plug is exposed to the forward end of the rocket motor. The fuze arms in two stages:

1. Gases from the rocket motor at pressures ranging from 900 to 2200 psi enter the pressure chamber in the head of the fuze through a small orifice in the inlet screw. Debris from the rocket motor is filtered out by the inlet screen. The small orifice for the entrace of gas delays the build-up of the pressure until approximately half the burning time has passed. Then, when the pressure in the chamber reaches a value of 275 to 325 psi the diaphragm collpases, severing the shear wire, and forcing the arming plunger down. Mo-vement of the plunger releases the locking ball, which, in unarmed condition, locks the firing pin body in place; this allows the firing pin body to move toward the rear under its own inertia force, due to accerleration, and the force of the firing pin spring. The firing pin, which is attached to the firing pin body, is thus withdrawn from the detonator shut-ter, which it normally locks in the safe position.

2. When the rocket accelerates, the inertia of the shutter forces it back against the firing pin guide, compressing the shutter spring and engaging the shutter locking pin in a hole in the firing pin guide. About midway during acceleration, the firing pin is withdrawn from the shutter; but the shutter, held by the shutter locking pin, remains locked in the safe position.

When the acceleration is over, the shutter spring forces the shutter foward against the lead-in-disc, disengaging the shutter locking pin from the hole in the firing pin guide. The shutter spring then swings the shutter into the armed position (detonator assembly in line with firing pin and lead-in) where it is locked by the detent.

Firing. The fuzes fires on impact with any material offering sufficient resistance. Un- der the force of impact the inertia of the firing pin body drives the pointed firing pin for-ward piercing and initiating the sensitive primer in the detonator assembly. The detonator assembly initiates the tetryl lead-in and the tetryl booster.

The fuze functions with a slight inherent delay, equivalent to the time required for the firing pin body and firing pin to move forward to strike the primer.

Acceptance test data. Acceptance tests have not yet been established duo to the fact that the  fuze was not produced for service issue.

Sensitivity limits. Firing test with the Fuze Mk 146, which has the same functioning mechanism as the Fuze Mk 161, in the 7.2-inch demolition rocket, indicated that the fuze functions on both ground and water impacts, but does not function consistently at very low angles of impact.

Detonator safety. Fuze Mk 161 Mod 0 is detonator safe. In the unarmed position the detonator assembly is out of alignment with the other increments of the explosive train. Should the detonator assembly function prematurely, the force of the detonation will be dissipated upward through a hole in the firing pin guide and away from the balance of the explosive train.

Because of the delay in admission of gas from the rocket motor to the pressure cham-ber, the first stage in arming does not occur until approximately one-half the burning is cover. Because of this delay, if the rocket motor blows up before it leaves the launcher, the fuze should not arm. Arming is not completed until after acceleration has dropped to a value corresponding to 9 g. The burning distance, and therefore the arming distance, will vary with the temperature. On the average, arming is completed within 0.1 second after the end of burning. Approximate calculated arming distances for the 7.2-inch demo-lition rocket are:

0

70

135

65

50

35

During shipping and stowage. The light weight aluminum arming plunger and the 0.057-inch phosphor-bronze shear wire make the fuze safe from arming by accidental dropping in a full weight projectile from heights up at least 40 feet.

General. From an examination of the exterior of the fuze, it is impossible to tell whet-her or not the fuze is armed. If, for any reason, it is throught that the fuze may be arm-ed, it should be treated as an armed and very sensitive fuze. (No attempt should be made to remove the fuze from the rocket body.) The complete fuzed round should be disposed of by gently lowering it, tail first, into deep water, or by Explosive Ordnance Disposal Personnel in accordance with existing instructions.

Disassembly. Disassembly is not permitted except at authorized activities when di-rected by the Bureau of Ordnance.

The fuze is shipped installed in the base of the rocket head. A safety wire is not nec-cessary and no preparations are required to ready the fuze for use. A shipping protects the exposed end of the fuze and the threads of the fuze-to-motor adapter. The shipping plug should be kept in place until the round is assembled and should be promptly replaced if the round is disassembled.

Packing. Fuzes will be shipped assembled in the rocket heads and protected by a shipping plug which screws into the fuze-to-motor adapter.

Marking. The fuze is stamped with the mark, mod, lot number, date of loading, the manufacturer's initial or symbol, and the inittials or symbol of the loading facility.