This chapter is a summary of the safety features of VT fuzes and precautions to be ob-served in handling, stowing, and using the fuzes.

Safety Features

Energizer. The energizer remains inert until the ampoule is broken as a result of setback forces and the electrlyte distributed by spin forces imparted when the projectile is fired from the gun.

Charging Resistor. The charging resistor slows up the charging of the firing condenser until the projectile has traveled several undred yards from the muzzle of the firing gun.

Reed Switch. A small reed-type spin switch provides a short circuit across the firing condenser which prevents the firing condenser from becoming charged in case of break-age of the ampoule previous to firing. The switch is opened by spin forces imparted to the projectile and the fuze when fired. Hence this switch is a handling safety device.

Mercury Switch. The mercury switch (two switches in parallel in some fuzes such as Mk 53) short out the firing squib until the switch is opened by centrifugal force imparted when the projectile is fired.

Auxiliary Detonating Fuze. In the auxiliary detonator, safety is provided by the fact that two rotors must be in line before the booster charge can be fired. The rotors are held out of line by detents until freed by centrifugal force when the prjectile is fired. See OP 1212, Projectile Fuzes.

Arming Range. The distance the projectile travels along its trajectory until it becomes electrically operative is called the arming range. In different fuzes this distance varies from 500 yrads to 1500 yards (1800 to 3300 yards for Rocket Fuze Mk 173). This allows the projectile to reach a safe distance from the firing ship before it can explode, without danger to the firing ship or its personnel.

Handling Precautions

VT fuze should be given the same care in handling that is ordinarily given to all ammuni-tion. Direct blows on the nose may result in breakage of the plastic nose, but have no effect on the safety of the fuze.

The ampoule of electrolyte in the energizer may be broken by more than a two-foot drop on armor plate. Breakage of the ampoule does not impair the safety of the fuze, and or-dinarily no ill effects result if it is broken in the loading tray within 30 seconds of being fi-red. A dud will usually result, however, if the electrolyte enters the energizer more than 30 seconds before the round is fired.

Stowage

Deterioration of the VT fuze leads to a decrease in operability. Present models of fuzes, if exposed to spray, water, high humidity, and high temperatures will deteriorate. Cons-tant effort is being directed toward the development of a completely moisture-proof fuze. They are designed to withstand temperatures from 0° F. 10 120° F. Best stowage is in a cool, dry magazine. Brass waterproofing caps on some early issues of 5"/38 rounds provided protection against moisture and spray. They were not removed until it was ne-cessary for firing.

Loading

VT fuzed five-inch projectiles should be passed through the ammunition hoists in the u-sual manner. Early fuzes were subject to occasional nose breakage upon removal from the hoists in the mount. Breakage of this type is seldom encountered with modern fuzes. Accomplishment of NAVORD ORDALT 1788 on the fuze pot, or setting of the fuze setter in manual at "safe" or 30 seconds, greatly minimizes the likelihood of nose breakage. The fixed setting can be employed only when 100% VT fuzes are being employed in any par-ticular mount. Ships with open mounts and multiple fuze pots alongside the gun need not pass VT fuzed rounds through the fuze pot operation unless it is desired to do so in the interest of unifirm handling procedure.

Mk 58 fuzed 3"/50 projectiles must not be used with any knife-tyoe fuze setter which will cut or crack the plastic nose.

Disposal of Damage Fuzes

Damaged fuzes shall be disposed of by dropping the fuzed projectiles over the side in deep water in accordnance with latest instruction for disposal of ammunition. No VT fuze shall be disassembled or in any manner broken down for inspection by any but authorized representatives of the Bureau of Ordnance.

Disposal of Fired Duds

Fired duds laying on the ground are to be given the circumspection accorded to land mi-nes. A VT fuze in the ground or lying on the ground or lying on the ground is very unlikely to operate by influence even if undamaged, but an occasional one will detonate if distur-bed. Almost all wet energizer fuzes are inoperative within a few minutes after being fired, and all such fuzes are entirely inert with a week.

Old fuzes with dry energizers remain dangerous if disturbed, for indefinite periods. Reco-vered Fuzes Mk 32 have been known to detonate as long as one year after being fired, even through the nose was smashed.

All such duds should be disposed of by competent bomb disposal personnel.

Degausing and Deperming

VT ammunition need not be removed from a ship during degaussing, depermining, etc. These operations have no effect on the fuze.

The frequency of VT fuzes is selected so that shipboard radio and radar equipment should have no effect on the operation of safety of U.S. Navy VT fuzes.

Firing Precautions

VT fuzes are subject to some prematuring after arming. These prematures and influence bursts of projectiles falling among friendly ships, personnel, or equipment present a haz-ard in VT fuzed fire. In shore bombardment, fire should clear crests by 500 feet if occu-pied by friendly troops, because of the danger of an influence burst.

In 5"/51 guns it was necessary to fire Fuzes Mk 32 and Mk 53 Mod 0 at a reduced charge of 2600 f/s. Mk 53 Mods 1-6 may be fired at full charge. Bombardment velocity of 1200 f/s is not satisfactory with any VT fuzes yet in production. Full charge should be used.