The British 2 1 inch Mark IV. torpedo is briefly described below as a typical modern torpedo. (See diagrammatic sketch above.) Head.—Three types are made. (I) The warhead is a thin steel or bronze shell containing 500 lb. of T.N.T. which is detonated on contact by a device, called a pistol, screwed into the nose. (2) The blowing head, used for ordinary practice running, is a steel shell filled with water to make it of the same weight as a warhead and containing a flask of compressed air and the necessary mech anism for blowing the water out at the end of a run so making the torpedo buoyant and enabling it to be recovered. (3) The collision head, used for practice when it is desired to hit a target vessel, is a steel shell partially filled with cork to give buoyancy and brought up to war head weight by filling with water. On im pact with target the fore end collapses thus cushioning the blow on target and torpedo. An indicating light of calcium phosphide is fitted in a pocket to facilitate recovery. These heads are se cured by screws to the airvessel which is made of special forged nickel steel and contains air compressed to 2,500 lb. per sq.in. This air provides the oxygen for combustion of the fuel supplying the motive power. The Balance Chamber is a watertight com partment, with thin steel shell, permanently attached to the air vessel and contains the flasks of fuel and water and the heater apparatus. Fuel and air are burnt in the generator and turn the water into steam there. The steam and products of combustion thus formed pass to the engine, combining to provide the motive power. In this compartment, also, is the depth keeping mechan ism which consists of a pendulum weight so interlinked with a hydrostatic valve that the weight corrects departure from the horizontal, whilst the valve maintains the set depth, their joint action operating the horizontal rudders through the medium of a small air engine termed the servomotor. This combination was one of Robert Whitehead's principal secrets and was responsible for much of the success of his design.
The engine room is the next compartment abaft the balance chamber and together with the buoyancy chamber, to which it is permanently attached, forms what is known as the afterbody. This afterbody is secured to the balance chamber by a number of screwed bolts which allow of the torpedo being parted for exami nation of mechanism in the engine room. The engine room con tains (a) an engine of the four cylinder, single acting, Brother hood type; (b) the steering engine or servomotor; (c) the start ing valve and counter gear; (d) the reducing valve which main tains an approximately constant working pressure at the engine irrespective of air vessel pressure; (e) oil bottles for lubricating working parts; (f) sinking valve. The starting valve is operated
by the "air lever" which projects through the shell of the torpedo and is thrown aft on discharge by a downward projecting bolt in the torpedo tube. In addition to opening the starting valve the air lever releases the gyroscope. The counter is a piece of mechan ism, driven off the engine, for stopping the torpedo at a definite range, or permitting it to run away. Off the counter is driven the ignition gear, whose function is to fire the igniters in the generator at a predetermined moment and start the combustion of air and fuel. The torpedo can be set to float for practice or to sink for action ; in the latter case a small valve is lifted at the end of the run admitting water to the buoyancy chamber, thus sinking the torpedo which might otherwise remain a floating mine.
The buoyancy chamber provides a large portion of the buoy ancy of the torpedo. It consists of thin sheet steel, strengthened up internally to enable it to withstand impulse pressure on dis charge, or immersion at great depth. The engine is secured to the foremost bulkhead of this compartment, and two watertight tubes run through it, one carrying the propeller shaft and the other carrying a rod connecting the servomotor and horizontal rudders. The gyroscope is fixed to suitable brackets in this compartment, and connected to the vertical rudders. This piece of mechanism is a comparatively heavy bronze wheel mounted horizontally and vertically in gimbals. When the air lever is thrown aft, the wheel is spun up to a high rate of revolution, due to the release of a strong spiral spring in torsion, or by air jets playing into pockets formed in the rim of the wheel. At the same time the Whole sys tem is freed from external constraint, and, following the dynami cal laws of rotating bodies, the spinning axis tends to maintain its direction in space. Thus the axis of the spinning wheel provides a datum line, the direction of the gyro wheel on release. Subse quent deviations of the torpedo are corrected by the relative movements of the spinning axis and the fore and aft line of the torpedo. These relative movements control the air supply to a small steering engine which operates vertical rudders, always act ing to bring the torpedo back to its original direction. The gyro scope can be set at an angle to the axis of the torpedo when required. In this case the torpedo turns through this angle before commencing its straight course. The tail carries the horizontal and vertical rudders which are mounted on corresponding fins ; it also contains gearing to enable two four-bladed propellers to be driven in opposite directions off the engine shaft and this ensures that the torpedo keeps its upright condition without heel, the turning effect of one propeller being balanced by the other.