The VT Principle of Fuzing

The VT principle of fuzing makes possible the detonation of explosive ordnance in such a position upon approach to a target that maximum fragmentation damage is obtained. Fuzes utilizing this principle of operation initiate detonation by electrical interaction in-volving closeness of the target, and rate of approach of the fuze to the target, both factors necessarily being present to a certain definite degree before the fuze operates. These fuzes operate entirely automatically, no time setting being necessary or possible. Each fuze is designed to operate at the most effective point for the specific weapon in which it is to be used and is not effective in other weapons.

Two different type of VT bomb fuzes are in production and use; the ring type (see T50E1) and the bar type (see M166). They are similar in operation but somewhat differ-ent in behavior. The ring type is more sensitive to passing targets and generally gives lower burst upon direct frontal approach to a target. The bar type is quite sensitive to targets directly in front of it and correspondingly less sensitive to passing targets. This gives the possibility of selecting a fuze to give burst heights where desired for a specific target. The ring type is excellent for defoliating trees because it is more sensitive to tree tops; it is also good for roof top bursts against tall city buildings. The bar type gives higher bursts over flat ground and tends to pass tall trees or buildings, waiting to burst on approach to the surface.

The bar type fuze can be used effectively in any bomb with a fuze well that will ac-commodate the AN-M103 nose fuze while the ring type, although it fits the same fuze well, can be used only in the bomb sizes for which it is specified. When used in improper bombs, very low bursts, 3' - 10' will generally be obtained with ring type fuzes.

Factors Affecting Burst Height

The burst height that will be obtained depends on certain factors which may be tabu-lated as follows: (1) factors over which user has no control - fuze sensitivity, nature of target, size and height of the target, and; (2) factore over which user has control - size of bomb, height of release, aircraft speed at time of release.

Fuze sensitivity is adjusted in desing and manufacture to give the most effective burst heights for the particular ordnance items for which the fuze is to be used; i.e., a fuze for use against ground targets will have a lower sensitivity than a fuze for use against air-craft because the target is larger and therefore influence the fuze more strongly. No changes in sensitivity can be made after manufacture is complete.

The size and type of target will effect burst heights. A larger target will naturally have a greater effect on the fuze and cause it to operate earlier. A wet target has more in-fluence on the fuze. Burst heights over water will be approximately double those over average land. Marshy land will fail in between these figures. Very dry land or dry sand will cause burst heights about 2/3 as high as average land. Burst heights are normally given in relation to average land unless specifically indicated otherwise.

A mass of tall trees or large buildings will increase burst heights over land in their vi-cinity but hte increase in burst height will be somewhat less than the height of objects, the degree of influence being dependent on the density of the object.

The size of the bomb in which the fuze is used will vary burst heights widely, but in no predictable pattern. Burst heights are tabulated for various bomb sizes with different VT fuzes and the tables must be followed in predicting burst heights.

The height or release and air speed at time of releases affect burst heights by con-trolling the rate and angle of approach of the fuze to the target. In general the ring type fuzes are influenced by angle and rate of approach while the bar type fuzes are rather insensitive to these differences. Bursts with ring type fuzes are generally lower as the angle of approach nears the vertical. However, burst heights of ring type fuzes increase with approach speed up to a certain point. By balancing these factors of approach speed and approach angle, fairly constant burst heights have been obtained for straight bomb-ing at altitudes of less than 10,000 ft. At altitude of release above 10,000 feet, burst heights are reduced somewhat. However, for any given altitude of release on level bomb-ing runs, bursts will be higher with greater plane speed when using ring type fuzes be-cause approach angle will be less vertical.

The air travel necessary to cause a VT bomb fuze to arm is designated as SAT (Safe Air Travel). Min. SAT, the minimum safe air travel, of any fuze in a lot is stencilled as part of the ordnance nomenclature on each fuze of the lot. No fuze will arm at less air travel in any bomb than the figure specified as Min. SAT on the fuze. These figures of Min. SAT are obtained from test droppings of representative samples of the lot in 100 lb. bombs. All fuzes if the lot will be armed in a spread of 600' after Min. SAT. That is, if a Min. SAT of 3600 feet were specified on a lot of fuzes, none would be armed at 3600 feet of air travel and they would all be armed at 4200'. Min. SAT will be longer for larger bomb sizes due to reduced air-stream velocity over the vanes. These increases are as follows or various bomb sizes; 260 lb. AN-M81, 3%; 250 lb. AN-M57, 12%; 500 lb. AN-M64, 24%; 1000 lb. AN-M65, 33%; 2000 lb. AN-M66, 43%.

Under certain conditions, it is desirable to delay arming of VT fuzes longer than the Min. SAT provided in the fuze. When flying stack formation with other units of the formation flying 2000' or more below, or with a large fighter cover working below, if the fuzes were armed in the normal time and passed near the lower friendly aircraft, influence bursts would occour causing casualties to firendly craft. To prevent this, mechanical arming delays for attachment to these fu-zes have been developed. These M1 (T2E1) Air Travel Arming Delays are so installed that they prevent spring loaded arming pin from releasing the  fuze vanes for a preset air travel distance.

Start of fuze arming may be delayed by use for this device through air travel distances up to additional 20,000 feet.

These fuzes are not to be partially prearmed by turning the vanes to make possible use from lower re-lease altitudes. Casualties to the carrying aircraft will usually result. Bombs fuzed with VT fuzes may be jetti-soned safely unarmed and detonation will not occur.

Salvo release of armed VT fuzed bombs is not re-commended because an early function of one fuze will cause detonation of all bombs of the group by interac-tion. These fuzes are designed to function upon a sud-den change in their surroundings and detonation of one bomb of a salvoed group will cause them all to funct-ion. For the same reason, minimum train spacing should be greater than 50 feet for 100 lb. bombs and 100 feet for 500 lb. bombs to assure that early functions of one bomb of the group will not cause interaction on either fuzes of the "stick".