V.T. Mk 170 Mods 0 and 1, V.T. Mk 173 Mods 1-5, and V.T. M402

Rocket uses in

Functioning

Over-all length, inches

Materials

Nose section - plastic
Base ring - steel
Body - steel

General: The V.T. Fuzes Mk 170 and Mk 173 Series are for spinner rockets. The Army designation the Mk 170 Mod 1 as the M402.

Bursting heights will depend on the angle of approach to the target and the type of target, as in all V.T. fuze operation. When the rockets are fired at a 15-degree angle from the horizontal, bursting heights over average land targets will be 20 to 60 feet, while bursting heights will be 40 to 80 feet when the rockets are fired at a 45-degree angle of elevation.

Minimum range of the V.T. Fuze Mk 173 in the Navy 5.0-inch S.S. rocket is 2,500 yards, at which point only 26% of the fuzes will operate properly upon approach to the target. Best results are obtained at ranges between 3,000 yards minimum and 5,000 yards maximum - maximum effective range of the rocket. In these limits, 75 to 80% of the fuzes should function properly upon approach to the target.

It has been found necessary to use a very long arming delay on these V.T. fuzes to give assurance that the fuze will not be armed until the after-burning of the rocket mo-tor is no longer a hazard. If the fuze were armed earlier, after-burning would cause the V.T. fuze to function prematurely, thereby wasting the round.

Minimum range of the V.T. Fuze Mk 170 or M402 in the 4.0-inch Army rocket is 2,500 yards, at which point 50 per cent of the fuzes will function properly upon aaproach to the target. Effective range limits are 3,000 yards to 5,000 yards, for full operability. Bursting heights are about the same as for the V.T. Fuze Mk 173 in the Navy rocket.

Operation: After the rocket is fired, when spin has reached 25 to 30 revolutions per second, the centrifugal release plates in the spin breaker swing outboard simultaneously against their hair springs, thereby freeing the release lever. This lever swings outboard, freeing the half-round cam which is rotated counterclockwise by the spring-loaded deto-nator slider, releasing it. The detonator slider is snapped against the fixed striker, firing the detonator. The force of the explosion is exerted against the head of the breaker pin, forcing it upward through a hole in the bottom of the wet energizer and into the electro-lyte vial, breaking it. Centrifugal force distributes the electrolyte, energizing the powder supply. Electrical energy is supplied to the V.T. element.

In the meantime, the rocket has attained its terminal spin velocity of about 140 revo-lutions per second, and the mercury in the mercury switch in the rear fitting is forced outboard through the porous membrane. After a delay of two to eight seconds, depend-ing upon the rate of spin and the temperature, the short circuit across the squib caused by the mercury is relieved and the squib is armed.

When spinning of the rocket starts, the rotor detents of the auxiliary detonating fuze swing outboard, but the rotors are prevented from lining up by their friction against the bottom of the housing, caused by acceleration. When burning is over, the unbalanced rotors swing into position, arming the auxiliary detonating fuze.

When spinning of the rocket starts, the reed contact in the centrifugal handling safe-ty switch is forced outboard, allowing the firing condenser to start accumulating a charge through a high-resistance circuit as soon as the wet energizer is activated. In about five seconds after the beginning of charging, sufficient charge has been accumu-lated to allow firing of the electric detonator and the fuze is armed.

Upon approach to the earth, the V.T. element completes the firing circuit and dis-charges the firing condenser through the electric detonator. The force of the explosion detonates the auxiliary detonating fuze, which initiates detonation of the main charge.

In the V.T. Fuze Mk 173 Mod 1 or the V.T. Fuze Mk 170 Mod 1, if the V.T. element does not function upon approach to the target, the Rear Fitting Mk 10 Mod 0 will cause instantaneous operation upon impact.

Remarks: A rear fitting containing the safety and arming festures common to V.T. fu-zed rounds will be the Rear Fitting Mk 6 Mod 12 in the V.T. Fuze Mk 173 Mod 0 or Mk 170 Mod 0. It contains a mercury switch across the squib, which unshorts as a result of spin, and a centrifugal handling safety switch which allows a charge to be accumulated on the firing condenser only while the round is spinning.

The Rear Fitting Mk 10 Mod 0 is used in the V.T. Fuzes Mk 173 Mod 1 and Mk 170 Mod 1 and, in addition to the components of the standard Rear Fitting Mk 6 Mod 12 described above, also contains a mechanical impact firing feature consisting of a fixed striker and a movable detonator with approprite detents and anti-creep springs to give adequate safety.

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 158. V.T. Fuzes Mk 170, Mk 173, and M402