Page:EB1922 - Volume 30.djvu/159

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stand out perpendicularly to the projectile and thus immensely increase the air resistance. Conditions are not improved by the partial breaking-off of this extension or by its incomplete forma- tion in the gun. It is just this irregular form and size of this exten- sion or " fringing " of the rotating band which makes it a possible source of great inaccuracy. Whether fringing actually takes place depends not only on the design of the band but also on the velocity of rotation of the projectile and the thinness and length of the extension formed at the rear of the band. By taking all these points into consideration it is possible to make a design which will give no trouble from fringing. But, apart from the effect of fringing, the rotating band may materially increase the resistance if improperly located. While it is desirable from other considerations to have the rotating band as near to the base of the projectile as possible, it is found that a better position for range and accuracy, even if a square- based projectile is used, is ] /2 calibre or more from the base. Simi- larly, if a boat-tailed base is used, the range and accuracy are both reduced if the band is placed just at the beginning of the taper. It should be at least 1/8 calibre forward of this position.

Double and even triple rotating bands close together at the rear are sometimes used, the idea being that this construction will make the band more efficient as a gas check and that fringing is less marked than for a single rotating of the necessary width. Bands near the bourrelet have also been used. A more recent development is the use of a copper bearing band at the bourrelet.

Optimum Weight of Projectile. The question of the weight of projectile to be used with a gun of a given calibre frequently arises. Other considerations besides that of ballistics affect the answer. There is a practicable limit to the pitch of rifling, which has been fixed at about one turn in 15 calibres for low-powered guns, and one turn in 20 calibres for higher-powered guns. With some such limit in pitch of rifling, projectiles cannot be made more than about 5 calibres in total length and retain the necessary factor of stability in flight. There is thus formed a certain upper limit of length and weight. If the projectile is shortened below this limit and the weight reduced, we may assume that, with the use of a suitably quicker powder, the same muzzle energy and conse- quently a higher muzzle velocity may be obtained; but while the higher muzzle velocity would tend to increase the range, the smaller weight and ballistic coefficient would tend to reduce it. It is evi- dent that for each gun there is some weight of the projectile, called the " optimum " weight, which will give the greatest range, assum- ing the muzzle-energy constant.


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The weights of similar projectiles vary with the cube of the calibre. Similar projectiles for different calibres, being the same length in calibres, are of equal stability providing the pitch of rifling is the same. The weights of the optimum projectiles vary about with the square of the calibre, if based on uniform muzzle velocities in different calibres. For high-powered guns of calibres roughly below 5 in. the optimum weight is greater than the usually accepted weight based on similarity, and for larger calibres it is less. The optimum weight of projectile for any gun and muzzle velocity may be readily worked out by the methods of exterior ballistics, by assuming several different weights of projectiles and

working out the maximum ranges on the basis of equal muzzle energies. Fig. 7 illustrates the maximum ranges to be expected from a 75-mm. gun under the assumption of equal muzzle ener- gies. It is to be noted that the optimum weight increases with the muzzle energy and that the range changes only slowly as we pass from the optimum weight. In the case of large-calibre naval or coast-defence guns a reduction in weight of projectile tends toward optimum, that is, toward increase in range; but the reduced weight and increased velocity of the projectile lead to greater losses of energy in flight, resulting in a smaller striking energy at a given range. (W. H. T.)

MANUFACTURE OF SHELL

The material of which a projectile may be made depends largely on the functions required of it.

Cast iron is brittle, more or less hard, with low elasticity, practically no ductility, and low tenacity; consequently this material is of no value for a shell which is required to do heavy work at the end of its flight or to promote a good explosive effect, and is somewhat risky when required to stand the shock of discharge from a high-velocity gun. Cast iron, however, is fusible and easily worked, and therefore cheap; it is consequently sometimes used for practice shot with reduced propellant charges. In the World War it was used for certain chemical shell where the chemical content was liable to attack steel, and especially by the Germans as a substitute for steel when the latter could not be had in sufficient quantities; but its use for projectiles is almost entirely confined to such. Wrought iron has a fair tenacity and a good ductility, but it is quite superseded by steel which can be manufactured as easily and cheaply.

Steel possesses the characteristics of elasticity, ductility and tenacity, and is sufficiently hard to enable it to withstand the stresses and shocks a modern projectile is required to sustain. Forged steel 1 is fibrous in molecular structure, and is improved by forging, which increases the tensile strength and minimizes the chance of porous metal remaining; the more work put into the forging, the better the quality of the finished material as measured by its tensile strength in the direction of the forging. Cast steel is crystalline in molecular structure and much harder than forged steel and has less ductility and tenacity; it must always be annealed after it has been allowed to cool after casting, in order to dissipate the uneven molecular stresses set up during cooling. In the case of steel for projectiles the composition includes from '3S% to 0-7% of carbon and small percentages of nickel, manganese, and silicon. With cast steel, the walls of a shell cannot be so thin as with forged steel because the material is not so good and there always is a risk of blow-holes and porous metal being present.

The chemical composition of the steel for shells is generally as follows :

Carbon. Silicon. Manganese. Sulphur* Phosphorus*

Composition : Per cent

H.E. Shell

Shrapnel

Armour- piercing Shell (t)

0-5 o-35 0-4 to i-o 0-08 0-08

o-75 0-3

I-O

0-04 0-06

0-5 to 0.75

o-5 1-25 0-08 0-08

Tensile strength. Yield point.

35-49 tons /in 2

19 tons

(Light shr.) 56 tons /in. 2 (Heavy do.) 38 tons /in. 2 (Light shr.) 36 tons (Heavy do.) 24 tons

38 tons/in. 2 24 tons

low as possible.
 * The sulphur and phosphorus are deleterious and should be as

t Steel for A.P. shell should have a higher percentage of carbon in order to give harder material.

1 The term " forged steel " is still used but the process of forging under a hammer has been discontinued for some time, the hydraulic press being used instead. The hydraulic press is said to work the mass more uniformly than does the hammer, while hollow-forging on a mandril has the same advantage over solid-forging. Forging should cease at a temperature of about i,2OOF., for if continued below this temperature, the metal tends to become " hammer hard " and internal strains are introduced.