Page:EB1922 - Volume 30.djvu/158

124

to stand up under the impact as well as those of the older form. But by the addition to the older (2 c.r.h.) projectile of a special cap and false ogive, it was found that not only could the excellent ballistic properties of the 7-calibre radius ogive be retained but, on account of the support given by the special cap, the armour-piercing ability of the projectile was increased. Similar experiments with 12-in. projectiles gave the following increases in ranges of the projec- tiles with 7-calibre ogives over those with 2-calibre ogives and blunt caps. The muzzle velocity was about 2,250 ft. per second.

Projectile

Ranges

1st series

2nd series

3rd series

Long Point Blunt Cap

Yd.

5,365 5,020

Yd. 8,900 8,000

Yd. 13,800 11,900

Difference.

345

900

1,900

Percentage increase.

6-8

1 1 -3

16-0

It is to be noticed that the advantage of the long sharp-pointed head increases with the range.

Similar tests made in France, England and Germany, with heads as long as 9-calibre radius ogive, left no doubt that a very sub- stantial increase in range could be obtained by increasing the sharp- ness of the point through use of a longer head. As has been pointed out, the importance of this fact was not fully appreciated until the necessity arrived of obtaining extreme ranges from all guns. When full advantage had been taken of increasing muzzle velocities to the limit of the ability of the guns to withstand the necessary pressures, and means, sometimes improvised, had been used to permit the guns to be fired at the elevation corresponding to the maximum range, there remained only improvement in the projectile to further increase the range.

False Ogives, The hollow extension of the head covering the fuze of field projectiles or the armour-piercing cap of armour-piercing projectiles is called a false ogive. 1 Its use was first suggested in America in 1907 by Capt. W. A. Phillips as an extension for the blunt cap of armour-piercing projectiles. Apart from its effect in lengthening and sharpening the head, and thus increasing the range, no ballistic advantage was claimed for it. ^ts use with an A.P. projectile is shown in fig. 2. As applied to] field shell the false ogive covers the end of the fuze. It is desirable to make the cavity of the shell as large as possible, and so the walls of the shell proper are run forward as far as they can be, and still permit a tap for the fuze of proper size at the front. The head is then completed by screwing the false ogive over the fuze (see fig. 4).

FIG. 4.

The use of a light false ogive throws the centre of gravity backward with respect to the centre of form. This is foundto be of advan- tage in point of ranging power, providing the somewhat greater tendency to initial yaw is sufficiently counterbalanced by increased spin.

Improvements in Form of Base. Experiments to determine the best form of base commenced soon after these remarkable improve- ments in range by changes in the head were obtained. In experi- ments made in 1913 with a 6-pr. gun use was made of ogival and tapered bases coming to a point, the total length of base from the start of the taper being I calibre or less.

The resulting ranges were generally less than those of the square- based projectile of otherwise similar form and weight, and they were less accurate. Bases of i-cafibre length tapered to a cone of 9 were then tried, but seemed to give no better results than the square base. In both these experiments the projectiles had ogival heads of 2-calibre radius. Experiments made in 1915 with 6-in. projectiles of the three forms of base shown in fig. 5, fired with a muzzle velocity of 2,675 ft. per second, at an angle of elevation of 5, gave the following results:

Form of Base

Mean Range

Mean Dispersion

Range

Deflection

A B C

Yd.

8,200

8,440 8,410

Yd. 88 41 52

Yd. 9 7 6

A small increase in range and greater accuracy is shown by the boat-tailed projectiles.

French experiments made in 1914 demonstrated that the form of base to give the best result with any projectile is dependent upon the form of head used, and vice versa. A similar conclusion had been reached elsewhere.

1 Known also as the ballistic cap or " false cap."

FIG. 5.

Influence of Velocity. By comparing maximum ranges obtained with various projectiles and velocities with ranges in vacua, we may obtain a good idea of possible improvements in projectiles.

Fig. 6 shows vacuum range as a function of velocity plotted to a logarithmic scale, together with other lines showing fractional parts of the vacuum range and points showing the maximum ranges of actual guns. The ranges of low-velocity guns fall near the vacuum line, while those of high-velocity guns are much farther away. 2

KO

200 800 4 60780 toOO 2OOO 80OO

MUZZLE VELOCITY- FEET PER SECOND FIG. 6.

6878 lOOOo"

The possible improvement in projectiles to be fired from low-ve- locity guns is, therefore, very much less than in those fired from high- velocity guns. Actual trials show that within reasonable limits neither the form of the head nor that of the base has an important influence on maximum range or accuracy when the velocity is less than 1,200 ft. per second.

Improvements in Rotating Bands. Although the rotating band has performed all its useful functions by the time the projectile has left the gun, it still has to be reckoned with, since it is capable, if improp- erly designed or located, of materially reducing the range and in- creasing the dispersion. If it has a " lip " (called in Great Britain " gas check ") or is thick at the rear, the excess metal will be wiped back on the passage of the projectile through the bore and will form a ragged extension; and when the projectile is free the centrifugal force due to its rotation may be enough to cause this extension to

2 Explanation of fig. 6. Abscissae show velocities in f/s, ordi- nates ranges in yd., oblique lines the % of theoretical vacuum range obtained in practice. Thus the lo-in. gun, which obtains a range of 26,500 yd. with a m.v. of 2,600 f/s in actual practice, would obtain with the same velocity one of 63,000 in vacua. 26,500 is 42% of 63,000 and the projectile is therefore spotted between the 40 % and the 50 % lines.