GENERAL. The aircraft torpedo is an expensive complicated weapon. The proportion of explosive weight is low. Mass production is lengthly and expensive. In addition present day performances of torpedo engines limit the speed and range of the projectile. Both of these items are essentials for accuracy and safety from antiaircraft fire.

A relatively simple weapon would result were the torpedo engine and the control gear omitted. If this simplified weapon were launched so that the greater portion of the dis-tance to the target was covered through the air, as with an ordinary bomb, the initial speed of launch would be retained over nearly all the range. The projectile would enter the water just short of the target and carry on in the direction of its flight in air by rea-son of its momentum in the same way as does a torpedo. To prevent it from going too deep before detonation, a relatively flat angle of entry into the water is necessary.

Such a weapon was developed in Germany during the closing months of the war, and it was called the Bomben Torpedo. It combines the characteristics of the bomb to travel a long distance in a short time interval with the characteristics of a torpedo in that under-water travel eliminates range errors.

DETAILS. The BT was developed in four sizes: 200 kg, 400 kg, 700 kg, and the 1,400 kg. They all incorporated the same general shape and construction, and were entirely of steel. They were constructed in three pieces; the warhead (two sections) and the tail section. (See fig. 49.) The forward section of the warhead was in the shape of a trunca-ted cone, and the after section of the warhead was cylindrical. The transverse fuze poc-ket was located in the cylindrical section just aft the point where the two sections were welded together. The suspension lug T-type, was secured to the warhead just forward of this weld at the center of gravity.

TAIL SECTION. The tail section was also in the shape of a truncated cone. There were three very large fins spaced 120° apart at the after end of the section. This type of tail provided excellent stability for the bomb while it was in the air. The tail section was se-cured to the after section of the warhead in such a manner that when the missile struck the water, it was jettisoned.

Early in the experiments, a BT 1000 was worked on and this missile had a rocket motor inside the tail section. This idea was soon dropped as it proved impractical for the mis-sile.

FUZING SYSTEM. A magnetic fuze which reacts to the variable magnetic field of a ship is necessary for the most successful position of detonation under the keel of the ship. Work on this aspect of the bomb was found to be far from complete. The susceptibility to disturbances and the reaction capacity of such fuzes had not been investigated throughly either. A magnetic proximity fuze, however, is necessary for greater release ranges and for curved underwater trajectories.

Good detonation positions can be achieved with straight underwater travel if the fuze is set to go of after a specific distance through the water. The angle of entry must natu-rally not be altered as the underwater travel depends on the angle of entry. The time delay set on the fuze can be determined most simply by assuming a constant time for underwater travel.

In designing the fuze system, the following points must be borne in mind. An impact fuze with or without delay is required for direct hits on the ship. Further, the speed and range of release must be functioned very accurately for a preset time as the tolerance of plus or minus 0.1 second can only be achieved with a clock-work fuze. Finally, the tail section must be jettisoned by explosive bolts or by some other adequate method on impact with the water.

UNDERWATER BEHAVIOR. The bomb must in no event ricochet off the water not even in flat angles of entry, but must continue without deviation of its path of entry.

It is known that with ogival noses, as seen in the illustration of the BT 1400, a bomb will ricochet off the water when it strikes at a flat angle. By using a flat nose, as seen in the illustration of the BT 700, or better yet by using a spoiler plate, this ricochet at flat ang-les is definitely avoided. The frontal surface of the spoiler plate is made in the form of a section of a sphere of radius, equivelent to the distance between the surface of the spoiler plate and the bomb center of gravity. As the flow of force is practically perpendi-cular to the upper surface of the body when it is awash, the resulting flow of force must go through the center of gravity and thus it causes no turning moment.

A spoiler plate with the same diameter as the bomb however, has a high water drag. The ideal situation is to have the size of the plate less than the greatest caliber of the bomb body and so shaped that only the spoiler plate and no other part strikes the surface of the water at flat angles of entry.

Figure 49 – 1400-kg Bomben Torpedo