Introduction

U.S. Navy VT fuzes are designed primiarily for use in antiaircraft fire to produce automa-tic detonation of the projectile in close proximity and proper relation to the target, so that the maximum amount of damage will be inflicted on the airplane and its operating personnel by the fragments from the bursting projectile. Extensive tests and combat use for Army VT fuzes designed primiarly for howitzer use established the devasting effect of proximity fuze air bursts on light equipment and especially on exposed and entrenched personnel. VT fire also can be very effective against certain types of shipping.

Antiaircraft

Limitations. Certain features and conditions limit, to a degree, effectiveness of VT fuzes in antiaircraft fire. A distance of travel along the trajectory, called the arming range, is required for the fuze to become fully armed. In the various fuzes this distance ranges from 500 to 1500 yards (1800 to 3300 yards for Mk 173) and is required for safety rea-sons. The fuze, being unarmed, will not operate against targets within the arming-range distance.

During firing on a flat trajectory against low-flying planes, the sensitivity of the fuzes is decreased by the wave-suppression feature. This means that the projectile must pass nearer the target in order for the VT fuze to cause detonation. In effect, the size of the target is reduced, and fewer bursts will be obtained with the same accuracy of fire.

It should be remembered that a full sensitivity it is possible to obtain bursts at miss dis-tances too great to yiled any significant probability of usefull effect on the target. Loss of these bursts does not necessarily imply a loss in damage done to the target.

In salvo fire the premature bursting of one porjectile will occasionally actuate a closely adjacent VT fuzes projectile. The result is a somewhat increased percentage of prema-tures salvo fire. Numerous tests have demonstrated that this effect is seldom serious. However, unformly better performance occurs in rapid, continuous, or ripple salvo fire. This type of fire is recommended in all applications, including shore bombardment.

Rain storm, electrical disturbances in the atmosphere, fog, and heavy smoke also affect VT fuzes to s slight degree, and somewhat poorer performance can be anticipated when such conditions are encountered. Present VT fuzes operate in the temperature range 10° F. to 120° F. Operation is best at moderate temperatures (60° F. to 80° F.).

Tracers cannot be used with VT fuzes. When tracers are used, the fuzes are caused either to "premature" on arming or to remain electrically inoperative until the tracer burns out. This is caused by the ionized trail from the tracer flame. The same effect is present in rockets; hence the long arming time for rocket VT fuzes.

When VT fuzed ammunition is fired, falling projectiles present the danger of influence bursts in the midst of firendly ships, personnel, and equipment. A self-destructive ele-ment is being developed for VT fuzes; and, if it is incorporated in the fuzes, it will have considerable value in reducing this danger. This would permit more active and unrestric-ted fire against planes attacking a large task group or attaching shipping in the vicinity of friendly territory.

Effectiveness. Experience demonstrates that VT fuzes make heavy A.A. guns several times as effective as if time fuzes were used exclusively. Even a two-fold increase in ef-fectiveness is, of course, equivalent to twice as many guns bearing on the target.

No special technique is required in firing VT fuzed projectiles at aircraft. Fire is directed at the target in the best manner possible with whatever fire-control equipment is avail-able. No setting of VT fuzes is possible or necessry with current types of VT fuzes, and they are always "on" ??illegible word?? range. They will detonate automatically if they come sufficiently close to the target. The radius of operation would be equal to the lethal distance in the ideal VT fuze.

The radius of operation against aircraft depends on various factors, among which are size of the airplane, aspect which the airplane presents to the approaching projectile, rate of approach, particularly type of fuze being used, etc. The average radius of opera-tion of current VT fuzes is about 75 feet, varying somewhat with the factors mentioned above.

A good fire-control solution is especially well repaid in the case of VT fuzes. The nearer the projectile passes to the airplane, the higher will be the operability of the fuzes. Few twin-engine bombers will survive more than two or three influence bursts of five-inch projectiles within the optimum sensitivity pattern; and occasionally one is sufficient to produce immediate crash damage. The damage to an airplane and to its operating per-sonnel of a three-inch burst is roughly one-third of that of a five-inch burst, similarly lo-cated.

Also, in the case of the airplanes flying low to the water, the height of the target above water and the state of the sea affect the operating radius. The first fuzes issued were only partially effective against low-flying airplanes. All current Navy VT fuzes contain a wave-suppression feature (WSF), which discriminates between the influence effect of the rough surface of the sea and the influence effect of an airplane. The operating radi-us of WSF fuzes is automatically adjusted to a smaller value along low trajectories and upon approach to the sea. All WSF fuzes are fully effective against planes at high alti-tude, as well as reasonably effective against planes flying low over the water. Some bursts, close to but beyond the plane, and therefore ineffective occur in the latter case.

Any evaluation of the radio of VT fuzes to time fuzes used in antiaircraft fire should take into account the tactical situation, the number of guns in a group, and the total size of the battery which can be brought to bear.

Under average circumstances it is recommended that 50% to 75% of five-inch and three-inch rounds be VT fuzed. Time fuzed bursts in the general vicinity of the target to other ships and to the automatic weapons, in deterring an attack and influencing its ac-curacy, and in permitting adjustment of a grossly inaccurate solution.

Individual ships will, of course, evaluate these various factors differently; but 50-75% VT fire is believed to be a good figure.

Under unusual conditions, such as in night action, a higher ratio of VT to time fuzes might be desirable, possibly up to 100% VT fuzes.

Navy ships, especially carries, prefer 100% VT fire against diving targets, where the vir-tues of time fuzes have little or no application.

Theoretical advantage ratios of VT fire over time-fuzed fire have been arrived at from analyses of proving ground and other special firing. These theoretical advantage ratios are, in general, confirmed by analysis of combat reports. The table below shows this ad-vantage in five-inch fire. Two typical fire-control situations are considered, (1) one-half the rounds pass through a circle, centered on the target, with radius of 100 feet, (2) one-half the rounds pass through a circle, centered on the target, with radius of 200 feet.

1.

2.

With smaller shooting errors, the advantage ratio is greater.

The difference between ballistics of VT fuzed and mechanical time fuzed ammunition is indistinguishable with practical fire control, and need not be considered.

Shore Bombardment

Limitations. Certain factors limit the use of VT fuzes for bombardment. Current VT fuzes are not operable at usual bombardment velocities, of 1200 f/s, but must be fired at full charge. Therefore, at ranges under 6000 yards, the trajectory is so flat that serious range error in burst occurs unless firing against observe slopes. Above 6000 yards, this effect is not serious.

Clearance of crests on which friendly troops are located should exceed 500 feet to pre-vent the possibility of a burst over the crest.

Rough terrain, jungle, and swamps will cause some increase in the burst height.

Due regard must be given to the effect of random prematures along the trajectory when firing over boat lanes, friendly ships, and friendly troops. Most spontaneous prematures occur short of 3000 yards, and the danger from random pre-matures beyond this range is rather slight.

Effectiveness. Navy VT fuzes function at about the same height over flat land of any type as over the sea. Typical average heights of operation are 10 feet at 6000 yards range and 25 feet at 12,000 yards range.

VT fuze air bursts "follow" the terrain, and ar ranges over 6000 yards the range disper-sion of the bursts is only slightly greater than the fall of shot with A.A. Common ammuni-tion.

Use Against Ships

The danger area which large ships present to VT fuzes is roughly twice that presented to impact fuzes. Air bursts over ship can be expected to have an effect on light topside equipment and on exposed personnel on the weather decks, but light splinter shields are sufficient to stop must of the fragments. Comman and A.A. Common ammunition with im-pact fuzing is much more effective in disabling and sinking ships.

Against motor torpedo boats, personnel barges, and other light craft, current VT fuzes with WSF are recommended for all ranges.

Targets for Anitaircraft Gunnery Practice with VT Fuzes

VT characteristics for various standard targets:

Targets of small influence radius are considered unsatisfactory, since they present such a small target area to a VT fuze as to give no true indication of the results to be expec-ted when firing against a full-scale plane.