Operability

Percent Operability. When new, and depending upon the particular model, Navy VT fu-zes have a percent operability of from 65% to 90% normal. They generally are consider-ed unsatisfactory when they fall below 50% normal operability. It should be noted that VT fuzes with 50% normal operability are more effective than normally set time fuzes with 100% operability, because of the inherently greater range dispersion of bursts with the latter. The VT fuze does not cause a normal burst unless a target is within its in-fluence field. Thus, with VT fuzes there is a direct relationship between fire control and the number of bursts. The number of bursts obtained with mechanical time fuzes is inde-pendent of the accuracy of fire. As a result, the word "operability" requires some comm-ent.

The performance of VT fuzes generally is measured in percent of units operating on a water surface at the end of a moderately long flight. Under such conditions, roughly 50% of the failure are prematures. Many of these are otherwise normal units which are trig-gered by some external or internal disturbances before the end of flight, and would have operated normally in the presence of a target prior to the point at which they prematur-ed. Therefore, the number of fuzes normally operable varies with the distance of the tar-get from the firing point. Since only a small portion of the projectiles fired at an enemy plane are at the extreme ranges of VT standard over-water tests, the operability of VT fuzes which come within damage range is almost always greater than quoted performan-ce figures.

Premature Bursts. A premature is any burst caused by a VT fuze which is not the re-sult of proximity to the target or some other radio wave reflecting surface. A large pro-protion of the premature occur upon arming with the first surge of power, as with most electrical equipment.

Duds. A VT fuze which fails to function in the proximity of a target or other radio wave reflecting surface is kown as a dud. Consideration must be given to the arming range and the extent of influence of the particular model before classifying as a dud a failure to burst in the vicinity of a target. Some duds will burst upon impact if there is a sufficient charge on the condenser and the projectile lands in such an attitude that the shock clo-ses the reed switch. This allows the condenser to discharge through the reed switch and the squib. (See fig. 26 p. 37.)

Temperature. Present VT fuzes are operable over a temperature range of 10° F. to 120° F. Operation is best at moderate temperatures (60° F. - 80° F.).

Damage. A blow damaging the nose may affect operability but will not render the fuze unsafe.

Serious Effects on Operability

TRACERS. Tracers cannot be used with VT fuzes. When VT fuzes are fired with missiles containing a tracer, the fuze either premature on arming or remain electrically inoperative until the tracer burns out. (This is caused by the ionited trail from the tracer flame.)

COPPERING. Coppering of 3"/50 guns interferes with VT fuze operability. Decoppering by firing one or more illuminating projectiles after every 50 rounds is recommended.

HOT GUNS. Round wich remain in a very hot gun over 40 seconds generally will result in duds. The heat will melt the potting compound and cause it to flow, thus almost des-troying the resistance of the fuze to the shock of firing.

Moderate Effects on Operability

ATMOSPHERIC EFFECTS. VT fuzes fired during atmospheric electrostatic disturbances have an increased percentage of prematures. Prematures increase with the rate of pre-cipitation during rainfall and in the vicinity of rain clouds. VT fuzes will occasionally func-tion when entering a cloud. Fog and smoke have a lesser effect, but do occasionally cause prematures. These effects are seldom very serious, however; and the ammunition remains effective during all ordinary atmospheric conditions. These statements apply to the present models. Older models were less sensitive and not affected by some of the abover factore.

WINDOW. A very dense infection of window of the proper frequency causes VT fuzes to function. Huge quantities of window would be required effectively to protect a region of significant size.

AERIAL FLARES. VT fuzes will function on aerial flares if they pass within 20 feet; but ex-tensive tests have shown that the ability of influence bursts, or in fact gunfire in gene-ral, to extinguish flares is very slight.

SALVO FIRE. In salvo fire the premature bursting of one projectile will occasuonally actu-ate a closely adjacent VT fuzed missile. Numerous shipboard tests have demonstrated that this effect is seldom serious. However, uniformly better performance occurs in rapid, continuous or ripple alvo fire and this type of fire is recommended in all applications.

VERY NEW OR VERY OLD GUNS. In such guns there is a tendency toward a decreased percentage of normal bursts. In very new guns the decrease in performance is accredi-ted to higher muzzle velocity, greater setback force, and increased spin. In a worm gun, bore enlargement at the orgin results in undue shock and strain when the projectile hits the lands of the rifling.

No Effect on Operability

DEGAUSSING AND DEPERMING. No special precautions are necessary with respect to VT fuzes during a deperming operation or while the degaussing gear is energized.

SHIPBOARD RADIO AND RADAR. Present Navy equipment in these categories should have no effect on U.S. Navy VT fuzes.

Arming Range

Arming range is the distance along the trajectroy that the missile must travel before the fuze is capable of operation in the presence of a target. It is especially important during antiaircraft use and in consideration of danger to surrounding personnel and gear. The arming range for the various Navy VT fuzes is given in Appendix B, page 39.

The missile cannot arm until several actions take place within the fuze. The time required for these actions determines the arming range and is influenced by a number of factors, which may be classified under two general headings:

1. Internal design features

2. External factors

Internal Design Feature

ACTIVATING TIME OF BATTERY. The activating time of the reserve energizer is the time required for setback and centrifugal forces to break the ampoule and distribute the elec-trolyte, and for voltages to build up to maximum. At moderate temperatures full power is available within 0.1 seconds (for 3"/50 gun) to 0.25 seconds (for 5"/38 gun). This time interval varies inversely with the spin frequency of the missile, since distribution of the electrolyte is dependent upon the centrifugal force produced by the rotation of the pro-jectile.

CONDENSER CHARGING TIME. The condenser charging time is the time necessary for the firing condenser to become charged through a fixed resistor and the mercury unshorter switch. A charge ample for firing the squib usually is available within 0.2 to 0.5 seconds (somewhat longer in Mk 173) after full power is received from the energizer. The rate of charging is determined by the characteristics of the circuit, which are designed to meet the tactical arming requirements of the particular model.

MERCURY SWITCH UNSHORTING TIME. The unshorting time for the mercury switch is the time necessary through the porous wall, thus unshorting the squib resistor. In VT Fuze Mk 53 Mod 3-6, this occurs about 0.6 seconds after firing. This time interval varies inver-sely with the spin frequency. It is selected to match the over-all arming time of the con-denser.

REED SWITCH UNSHORTING TIME. The unshorting time for the reed switch is the time necessary for the spin forces to open the reed switch, thus unshorting the firing conden-ser. This occurs immediately after firing and does not affect the fuze arming time. The switch remains open as long as the spin remains greater than some specified value. Thus it is potentially a self-destrcution factor, but in present fuzes the missile-usually comple-tes its trajectory before the rate of spin decreases to the specified value.

AUXILIARY DETONATING FUZE ARMING TIME. The arming time for the auxiliary detonating fuze is the time necessary for the rotor detents to be thrown outward by centrifugal force, permitting the two rotors to rotate until the explosive train is aligned. This occurs within a few feet of the gun muzzle.

FAULTY CONSTRUCTION. Delay in arming of the fuze may result from faulty construction or wiring of one or more of the components of the fuze. This may lead to a condition where the thyratron condenser circuit fires continously, preventing the condenser from storing up a sufficient charge to detonate the squib. This condition may be temporary, so that the fuze may be operable during the latter part of its flight; or the condition may persist throughout its entire flight, leading to duddage.

External Factors Affecting Arming Range

FUZE TEMPERATURE. Arming time increases with the drop in temperature. At low tempe-ratures the increased viscosity of the electrolyte and slower electrochemical reactions increase the energizer activating time. With the 5"/38 gun the arming time may increase to approximately 1.0 second at 10° F., the lower temperature limit for use the VT fuzes.

GUN EFFECTS. The arming time of the VT fuze may be affected by instable flight of the missile. Instability of the projectile, caused either by characteristic yaw or by side-slap in a worn barrel, will affect performance of the fuze. In such cases, mechanical distortion of fuze parts and erratic activation of the battery may occur, with subsequent delay in arming. Yaw may produce battery noise which, in turn, may trigger the thyratron, caus-ing a premature.

Activation of the reserve energizer is dependent upon the breakage of the ampoule and the spin of the missile. The breakage of the ampoule containing the electrolyte for the battery must be complete to insure rapid activation of the battery. A considerable num-ber of ampoules and breakers have been developed so that they will break satisfactory in a particular gun. The activation time of a particular type of battery is less when fired in a high-spin gun than if it is fired in a low-spin gun.

WAVE EFFECTS ON ARMING TIME. When a VT fuzed missile travels along a very low tra-jectory over waves, the fuze may receive a succession of signals. In a rough sea the amplitude may be sufficient to fire the thyratron circuit. With the reed switch open, the firing condenser might be discharged repeatedly, preventing the accumulation of enough charge to detonate the squib and resulting in a dud. In the case of calmer waters, of higher trajectory, late arming or early bursts may occur. Influence of wave effects on ar-ming of the VT fuze has been greatly reduced by the introduction of fuze model (with a wave-supression feature) which automatically reduce sensitivity to the type of signal generated And received over a wave surface, but which retain their sensitivity to signals received from an airplane target.

ATMOSPHERIC EFFECTS ON ARMING TIME. Atmospheric effects, which affect operability, and especially electrostatic disturbances, may produce a succession of signals similar to wave effects described above, with similar results on the VT fuze.

Sensitivity

The maximum sensitivity of a VT fuze ultimately depends on the minimum positive ampli-fier out put voltage necessary for the thyratron control grid to allow the condenser to discharge through the squib. In a given fuze circuit this corresponds to a detected sig-nal of an optimum frequency, amplitude, and phase.

For practical use, the specific conditions for a detected signal that will cause the fuze to detonate must be translated into a distance from the target. Thus the sensitivity of a VT fuze is defined as the average maximum radial distance from a target at which a fuze that is operating properly will fire. An average value must be used because the actual value varies with target size, shape, and material; orientation of target and shell; and rate of approach of target and shell. The actual value also varies from fuze to fuze of the same model, because of inherent difficulties in the manufacturing process. The aver-age values are given in Table 1, Appendix B.

VT fuze with the wave-suppression feature automatically decrease in sensitivity in the presence of spurious signals caused by such factors as the presence of ocean waves near the missile, atmospheric disturbances in the electrostatic class, or stray internal voltages. The reduction on sensitivity on such occasions reduces the number of malfunc-tions that might otherwise occur.

Life

The effective life of a fuze is considered over when the percent operability of the lot falls below 50% normal. The limiting factor on present fuzes has been found to be the dete-rioration of electrical components in the presence of high temperatures and especially of high humidity. Deterioration takes place in all climates, but is accelerated in regions of high temperature and humidity.

Life of VT fuzes is considerably prolonged by stoarge at 40°-50° F. under dry conditions and avoidance of exposure to salt spray. Under optimum conditions the average operabi-lity of current model fuzes is expected to decline slowly to about 50% normal after two years, with the remainder mainly duds.

Increasing deterioration with age has little effect on sensitivity, but the arming range gradually increases, thus eventually causing duds.

Refuzing

Because of the deterioration of VT fuzes, it may become desirable to refuze projectiles with new fuzes when the old ones have deteriorated to such a point that their effective-ness is greatly reduced. At the present writing there are no fuze-cavity liners in the pro-jectiles and the fuze body is in direct contact with the explosive filler; therefore great care must be exercised in the refuzing operation to minimize the danger of explosion or fire. Refuzing is to be performed by authorized U.S. Navy Ammunition Depots.

The refuzing operation is to be accomplished in the most suitable location, removed from other explosives and vital installations. Only those persons essential for the work shall be in the vicinity, and the smallest number of rounds practicable shall be exposed.

To remove the fuze, a special wrench made to fit the slots in the steel nose ring of the fuze is used. After careful unscrewing and removal of the fuze, the fuze cavity is tho-roughly inspected. Any loose grains of explosive are removed, and the screw threads are thorough cleaned. No metallic tools are used in the cleaning operation, because of the danger of sparks.

Before the new fuze is inserted, its threads are inspected and the given a coat of luting compound. The fuze is then carefully screwed into the projectile and thoroughly tighte-ned with the special wrench.

Precautions

Avoid producing sparks.

Clean fuze cavity and threads thoroughly to avoid ignition of explosive grains by friction of the screw threads.

Keep area clear of unnecessary personnel Expose only the smallest practicable number of round.

Be certain that the new fuze is the proper one and of a size to fit the fuze cavity.

Old fuzes are to be disposed of by dumping in deep water.

Fuze-Cavity Liner

At the present time, work is in progress on design of a fuze-cavity liner for projectiles to accommodate VT fuzes. When these fuze-cavity liners are incorporated in the project-iles, it is planned to stow a certain percentage of a ship's allowance of projectiles with-out VT fuzes installed. The VT fuzes will be hermetically sealed in metal cans to prevent deterioration. The cans will remain sealed until it is necessary to fuze additional projecti-les in order to maintain a ready ammunition supply.

When this plan is inaugurated, instructions for fuzing will be issued by the Bureau of Ord-nance.