But the most astonishing part of the design was the length of the gun, which worked out to 225 feet. An enormous powder-chamber would have to be used, so that the powder gases would keep speeding up the shell until it reached the required velocity at the muzzle. The weight of the barrel alone was estimated at 325 tons.
It would have to be built up in four sections screwed together and because of its great length and weight it would have to be supported on a steel truss. The gun would be mounted like a roller lift-bridge with a heavy counter-weight at its lower end so that it could be elevated or depressed at will and a powerful hydraulic jack would be required to raise it.
The recoil of a big gun is always a most important matter. Unless a gun can recoil, it will be smashed by the shock of the powder explosion. Usually, heavy springs are used to take up the shock, or cylinders filled with oil in which pistons slide. The pistons have small holes in them through which the oil is forced as the piston moves and this retards the gun in its recoil. But this "super-gun" was designed to be mounted on a carriage running on a set of tracks laid in a long concrete pit. On the recoil the gun would run back along the tracks, and its motion would be retarded by friction blocks between the carriage and the tracks and also by a steel cable attached to the forward end of the carriage and running over a pulley on the front wall of the pit, to a friction drum.
The engraving facing page [68] gives some idea of the enormous size of the gun. Note the man at the breech of the gun. The hydraulic jack is collapsible, so that the gun may be brought to the horizontal position for loading, as shown by the dotted lines. The cost of building this gun is estimated at two and a half million dollars and its 400-pound shell would land only about sixty pounds of high explosives on the target. A bombing-plane costing but thirty thousand dollars could land twenty-five times as big a charge of high explosives with far greater accuracy. Aside from this, the gun lining would soon wear out because of the tremendous erosion of the powder gases.
THE THREE-SECOND LIFE OF A GUN
Powder gases are very hot indeed—hot enough to melt steel. The greater the pressure in the gun, the hotter they are. It is only because they pass through the gun so quickly, that they do not melt it. As a matter of fact, they do wear it out rapidly because of their heat and velocity. They say that the life of a big gun is only three seconds. Of course, a shell passes through the gun in a very minute part of a second, but if we add up these tiny periods until we have a total of three seconds, during which the gun may have fired two hundred rounds, we shall find that the lining of the barrel is so badly eroded that the gun is unfit for accurate shooting, and it must go back to the shops for a new inner tube.
ELASTIC GUNS
We had better go back with it and learn something about the manufacture of a big gun. Guns used to be cast as a solid chunk of metal. Now they are built up in layers. To understand why this is necessary, we must realize that steel is not a dead mass, but is highly elastic—far more elastic than rubber, although, of course, it does not stretch nor compress so far. When a charge of powder is exploded in the barrel of a gun, it expands in all directions. Of course, the projectile yields to the pressure of the powder gases and is sent kiting out of the muzzle of the gun. But for an instant before the shell starts to move, an enormous force is exerted against the walls of the bore of the gun, and, because steel is elastic, the barrel is expanded by this pressure, and the bore is actually made larger for a moment, only to spring back in the next instant. You can picture this action if you imagine a gun made of rubber; as soon as the powder was fired, the rubber gun would bulge out around the powder-chamber, only to collapse to its normal size when the pressure was relieved by the discharge of the bullet.
Now, every elastic body has what is called its elastic limit. If you take a coil spring, you can pull it out or you can compress it, and it will always return to its original shape, unless you pull it out or compress it beyond a certain point; that point is its elastic limit. The same is true of a piece of steel: if you stretch it beyond a certain point, it will not return to its original shape. When the charge of powder in a cannon exceeds a certain amount, it stretches the steel beyond its elastic limit, so that the bore becomes permanently larger. Making the walls of the gun heavier would not prevent this, because steel is so elastic that the inside of the walls expands beyond its elastic limit before the outside is affected at all.
Years ago an American inventor named Treadwell worked out a scheme for allowing the bore to expand more without exceeding its elastic limit. He built up his gun in layers, and shrunk the outer layers upon the inner layers, just as a blacksmith shrinks a tire on a wheel, so that the inner tube of the gun would be squeezed, or compressed. When the powder was fired, this inner layer could expand farther without danger, because it was compressed to start with. The built-up gun was also independently invented by a British inventor. All modern big guns are built up.