V.T. Mk 171, Mk 172, and T2004

Rockets uses in

Functioning

Fuzes used with

Arming distance

Launched at 300 knots

Body diameter, inches

Over-all length, inches

10.4

General: The Mk 172 is a V.T. fuze for Navy-type-fin stabilized rockets. The comp-lete round is designated as Rocket Assembly Mk 1 Mod 5.

Description: In appearance, the V.T. Fuze Mk 172 is the same as the V.T. Ring-Type Bomb Fuze T50. The fuze may also be marked as the T2004 for use in Army rockets. In-ternally, the fuzes are also similar, except that a set-back has been added in the V.T. Fuze Mk 172 Mod 0.

Operation: When the rocket is launched, the arming wire is withdrawn from the vane locking pin, which in turn is forced out by its spring to free the vanes. At the same time, the set-back produced by the sudden acceleration forces the hinged inertia weight back against its spring. The hinged inertia weight forces the locking dog from the arming-stem gear sector by means of its lever linkage, freeing the gear train. The vanes rotate, driv-ing the electric generator and the gear train. After approximately 100 vane revolutions, the gear sector on the arming stem has rotated 25 degrees clockwise to move clear of the gear train. As the gear sector clears the gear train, the tension spring snaps it 75 degrees clockwise, where it is detained by the stop pin on the hinged inertia weight. Since the arming stem and detonator rotor are integral with the gear sector, they also move 25 degrees by vane rotation and 75 degrees by spring action. The vanes continue to rotate, driving the generator and the gear train, which is disconnected from the arm-ing stem. As acceleration ceases at the end of burning of the motor, the spring forces the hinged inertia weight forward, pulling the stop on and freeing the gear secor. The tension gear snaps the gear sector 90 degrees clockwise into the armed position, lining up the detonator with the booster lead-in and making electrical connection to the firing circuit. The detonator is locked in position by a spring-loaded detent in the detonator ro-tor. When the electric detonator is connected to the firing circuit, the firing condenser is charged. After 0.7 to 1.4 seconds, the condenser has stored up sufficient power and the fuze is armed. On approach to a target, the V.T. element activates the firing circuit, which discharges the condenser through the electric detonator which initiates the explo-sive train.

Remarks: Each V.T. Fze Mk 172 Mod 0 is shipped as a complete unit including a seal wire and booster safety pin. It will be recalled that the booster safety pin in this applica-tion, like that on the bomb fuzes, gives a visual indication that the detonator rotor is in its original or safe position, if, upon removal of the pin, if can be fully reinserted. If it cannot be reinserted, the detonator rotor will have moved out of position and, hence, the fuze should be disposed of.

The V.T. Fuze Mk 172 Mod 0 has neither self-destructive nor impact-functioning fea-tures. However, if the V.T. fuze is a complete dud, the Base Fuze Mk 157 Mod 0 will cause detonation after impact. While the V.T. Fuze Mk 172 Mod 0 will cause detonation after impact. While the V.T. Fuze Mk 172 Mod 0 is designed primarily for air-to-ground firing, it could be used, though less effectively, for air-to-air firing. In this application, the rocket would have to come within 20 feet of the aircraft target in order to function.

A seal wire through a hole in the arming pin inserted at the factory prevents the va-nes from rotating and insures that the arming mechanism is in the proper position. Any turning of the vanes, unless accompanied by set-back, will cause the rotor system to jam, stripping its gears, so that the fuze will be a dud. For this reason, the fuze cannot be pre-armed.

If the seal wire is broken when the fuze is originally removed from the contai-ner, do not use the fuze. Fuzes in this condition could be disposed of in accord-nance with security regulations by lowering in deep water or by explosibe demo-lition.

High speed rotation of the vanes is necessary to produce the current to fire the fuze. Therefore, the fuze is actually armed only when in flight on the rocket at speeds in ex-cess of 80 knots. Damaged fuzes may hold the electrical charge in the condenser for quite some time. For this reason, damaged fuzes and duds found on the ground should preferably be handled with care.

No disassembly of the V.T. Fuze Mk 172 Mod 0 is authorized by field personnel.

The V.T. Fuze Mk 171, four times as sensitive as the Mk 172, was designed for plane-to-plane firing; but, because of poor results in this technique of firing, few of these fuzes were produced.

V.T. Fuzes Mk 173, Mods 2, 3, 4, and 5 are waterproofed fuzes. Mk 173 Mods 0, 1, 2, and 3 are not under procurement by the Bureau of Ordnance. Mods 0 and 1 had no auxi-liary detonating fuze waterproofing; Mods 2 and 4 are like the Mod 0 in that the Rear Fitting Mk 6 Mod 0 is used. Mods 1, 3, and 5 have the impact detonator. The Mod 5 is the one being manufactured in quantity to supply the fleet.

Figure 159. V.T. Fuzes Mk 171, Mk 172, and T2004

Figure 160. V.T. Fuze Mk 172 Mod 0 - Mechanical Arming Device