Proximity fuzes – V.T.

V.T. fuzes are automatic proximity fuzes designed to cause detonation of a projectile at a most advantageous distance from its target. Operating with equal effectiveness in daylight or at night, they require no setting or adjustment before using, eliminating time-of-flight error. They are electrical fuzes which activate the auxiliary detonating fuzes after electrically integrating two factors: (1) nearness to an object, and (2) rate of ap-proach to the object. Both conditions must be fulfilled to a definite degree before an electric impulse in the fuze will discharge a charged condenser through an electric deto-nator, called a "squib".

V.T. fuzes may be employed for antiaircraft purposes of for barrage fire against land or surface targets where air bursts will be effective against personnel and unarmored structures and equipment.

V.T. antiaircraft fuzes are designed to detonate the projectile at the most advanta-geous point upon approach to an aircraft, if they pass close enough to insure good pro-bility of lethal fragmentation. The fuzes, moreover, will not normally function until the target is within the umbrella-shaped cone of fragmentation of the round. An advantage of V.T. fuzing for A.A. work lies in its sensitivity to the presence of its target and the re-sulting increase of effective target area. The V.T. fuze comprehends a medium bomber as a target about 7,000 square feet in area. The plane presents and actual surface of about 200 square feet.

Wave suppression: One disadvantage encountered with early Mods of the V.T. Fu-zes Mk 32 and Mk 45 was their sensitivity to waves or choppy water, causing either pre-mature or non-functioning errors when fired at low elevations at targets near the water- A wave suppression feature has been incorporated in later models of V.T. fuzes, to elimi-nate this disadvantage. This feature also makes the fuze less sensitive to large targets, such as land or water, so that detonation will not occur until the fuze is within 10-20 feet of the surface.

Tracer influence: A second disadvantage in the use of V.T. fuzes is that no tracer may be employed, since the burning tracer will influence the fuze to cause functioning upon arming, or to prevent functioning until the tracer has burned out.

Prematures: All V.T. fuzes are subject to random premature bursts along the trajec-tory after amring but before approaching a target. This makes the fuze somewhat dan-gerous to use for bombardment fire to cover landing opperations, as the prematures will occur over our own forces. However, other advantages of V.T. fuzes for this type of fire are deemed to outweigh this danger. Normal bursts against land installations will be ob-tained between 10-30 or 50-200 feet above the target respectively, for fuzes with and without the wave-suppression feature. The fuze will automatically adapt itself to varying terrain features.

Safety features: Because of the number and variety of safety features incorporated, V.T. fuzes are among the safest in use as regards handling, bore safety, and freedom from muzzle bursts. In addition to the bore and muzzle safety provided by the auxiliary detonating fuze, a centrifugal clock is incorporated in the V.T. Fuzes Mks 32 and 40. The V.T. Fuze Mk 45 and later models contain mercury short circuits across the squib in place of the centrifugal clockwork mechanism. These mercury unshorters are so designed that handling, tumbling, or shock will not cause them to open. Centrifugal force caused by the projectile rotation must be applied for 0.2-0.4 seconds, depending on the fuze, before the mercury shorts can be opened. Also, these models incorporate a centrifugal switch which prevents charging of the firing condenser except when the round is rotating at or above a certain rate. This is called the "centrifugal handling switch".

All V.T. projectile fuzes, except the V.T. Fuze Mk 32, are powered with reserve batte-ries, wet batteries with the active ingredients contained in a glass ampoule until the round is fired. Upon set-back, the ampoule is broken, and the battery is automatically activated.

The battery ampoule is the weakest part of the fuze from the handling standpoint. While the complete round may be dropped 40 feet on armor plate without making it un-safe to fire, the fuze cannot be expected to function porperly. Battery ampoule in the V.T. Fuze Mk 53 will withstand a four-foot drop against armor plate in any position with-out breakage or impaired functioning. Ampoules in the V.T. Fuzes Mk 32 Mod 30 and Mk 40 may be broken by a two-foot drop on armor plate. If the ampoule is broken a consi-derable time before firing, the round will probably not function. If the ampoule breaks less than 30 seconds before firing, normal functioning may be expected.

V.T. fuzes operate effectively over a temperature range of 10° to 120° F. Outside this range, a higher percentage of malfunctioning will occur.

Storage: Exposure to high humidity conditions in storage reduces the service life of these fuzes. The V.T. Fuze Mk 32 requires special treatment, as it is not a reserve-bat-tery fuze. The V.T. Fuzes Mk 32 Mods 1-20 & Mod 40 are re-energized by special Bureau of Ordnance personnel about every six months.

Supply: V.T. fuzes are supplied in specially cavitized projectiles for Naval use, and the fuzes cannot be interchanged with nose time or point detonating fuzes. V.T. fuzes are ballistically similar to nose time fuzes, so no corrections need be made in practical fire control when V.T. fuzing is employed. Although all V.T. fuzes have formerly been supplied in complete rounds, the V.T. Fuze Mk 58 will be issued to replace previously fit-ted V.T. Fuzes Mk 45 Mod 12 in 3"/50 A.A. projectiles.