V.T. bomb fuzes

Two types of V.T. bomb fuzes are in use: the ring type and the bar type. They are similar in operation, but somewhat different in behavior. The ring type is more sensitive to passing targets and generally gives lower bursts upon direct frontal approach to a tar-get. 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 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 effectivels in any bomb with a fuze well that will ac-commodate the Nose Fuze AN-M103; while the ring type, altough it fits the same fuze well, can be used only in the bomb sizes for which it is specified. When it is used in im-proper bombs, very low bursts of three to ten feet will generally be obtained with ring-type fuzes.

Burst height: The size and type of target will affect 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 influence on the fuze. Burst heights over water will be approximately double those over average land. Marshy land will fall 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 releation 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 the increase in burst height will be somewhat less than the height of the ob-jects, 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. V.T. fuzes, and the tables must be followed in predicting burst heights.

The height of release and air speed at time of release affect burst heights by control-ling 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 apporach angle, fairly constant burst heights have been obtained for straight bomb-ing at altitudes of less than 10,000 feet. At altitudes 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 speeds when using ring-type fuzes, be-cause the approach angle will be less vertical.

The air travel necessary to cause a V.T. bomb fuze to arm is designated as S.A.T. (safe air travel). Min. S.A.T., 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. S.A.T. on the fuze. These figures of Min. S.A.T. are obtained from test droppings of representative samples of the lot in 100-pound bombs. All fuzes of the lot will be armed in a spread of 600 feet after Min. S.A.T. That is if a Min. S.A.T. of 3,600 feet were specified on a lot of fuzes, none would be ar-med at 3,600 feet of air travel, and they would all be armed at 4,200 feet. Min. S.A.T. will be longer for larger bombs sizes, because of reduced air-stream velocity over the vanes. These increases are as followns for various bomb sizes: 260-pound AN-M81, 3%; 250-pound AN-M57, 12%; 500-pound AN-M64, 24%; 1,000-pound AN-M65, 33%; 2,000-pound AN-M66, 43%.

Under certain conditions, it is deirable to delay the arming of V.T. fuzes longer than the delay the arming of V.T. fuzes longer than the Min. S.A.T. provided in the fuze. When planes are flying in stack formation with other units of the formation flying 2,000 feet or more below, or with a large fighter cover working below, if the fuze were armed in the normal time and passed near the lower friendly aircraft, influence bursts would occur, causing casualties to friendly craft. To prevent this, mechanical arming delays for at-tachment to these fuzes have been developed. These Air Travel Arming Delays M1 are so installed that they prevent the spring-loaded arming pin from releasing the fuze vanes for a pre-set air travel distance.

Figure 321. Air Travel Arming Delay M1

The start of fuze arming may be delayed by use of this device through air travel dis-tances up to 2,000 feet.

These fuzes are not to be partially pre-armed by turning the vanes to make possible use from lower release altitudes. Casualties ot the carrying aircraft will usually result. Bombs fuzed with V.T. fuzes may be jettisoned safely unarmed, and detonation will not occur.

Salvo release of armed V.T.-fuzed bombs is not recommended, because an early func-tioning of one fuze will cause detonation of all bombs of the group by interaction. These fuzes are designed to function upon a sudden change in their surroundings, and detona-tion of one bomb of a salvoed group will cause them all to function. For the same reason, minimum train spacing should be greater than 50 feet for 100-pound bombs and 100 feet for 500-pound bombs to assure that early functions of one bomb of the group will not cause interaction on other fuzes of the "stick".

Auxiliary safety device for V.T. bomb fuzes: To prevent accidental arming of V.T.-fuzed bombs mounted on exposed wing racks, the additional arming wire assembly, plate, and fahnestock clips shown in figure 322 may be installed on the Fuze M168. For aircraft to be catapulted, the Fuze Protector Mk 1 Mod 0 is used instead, since this device pro-tects the fuze and also secures the arming wires.

Figure 322. Auxiliary Safety Device for V.T. Bomb Fuzes