Exterior ballistics starts with a projectile traveling at a known speed (initial velocity) and in a known direction. This direction, called the line of fire (LOF), coincides with the center-line axis of the gun bore. Once the projectile leaves the gun, you have no further control over its trajectory. Natural forces, such as gravity, air, wind, drift, and the rotation of the Earth, act on the projectile in flight to alter its trajectory. Therefore, to hit a target, it is necessary to compensate for the effects of these forces by offsetting the LOF of the gun before firing. For example, if it is known that a projectile is going to drift right, the gun should be trained to the left. If it is known that a projectile is going to curve downward, the gun should be elevated.

NOTE

The ultimate purpose of a gun fire control system is (1) to find the correct position for the gun barrel to make the projectile fall where desired and (2) to put the gun in that position.

Figure 10-3 shows the components of a gun. When a projectile and propelling charge are loaded into the gun, the projectile rotating band engages the rifling in the gun bore. The rotating band forms a seal at the forcing cone. When the gun is fired, the expanding

Figure 10-3.-Cross section of a gun.

GRAVITY.- Gravity is a continuous attracting force, acting perpendicular to the surface of the Earth, that tends to pull all objects toward the Earth. Without gravity, a projectile (fired in a vacuum) would continue to travel in the direction it was fired until something interfered with its flight.

Gravity acts on a fired projectile, causing it to begin to fall as soon as it leaves the muzzle of the gun. The projectile, however, is traveling forward and falling at the same time. The projectile has two forces acting on it: (1) the momentum and (2) the pull of gravity. The path of the projectile, as a result of these two forces, is a curved trajectory.

AIR.- When a projectile is traveling through the air, it takes a different path from the one it would follow in a vacuum. In a vacuum, with gravity as the only retarding factor, an angle of departure and an angle of fall of the projectile would be identical (fig. 10-4). The maximum ordinate would be in the exact center of the trajectory.

Figure 10-4.-Vacuum and air trajectories for the same elevation angle.

Air resists the motion of a body passing through it. This resistance is a form of friction that slows the movement of the body. The result is that a certain amount of velocity is being lost for each second of projectile flight. The longer the projectile travels through the air, the slower it goes. Notice the steepness of the descending curve and the location of the maximum ordinate in the air trajectory. These characteristics, as well as the greatly reduced range, are due to air resistance.

Figure 10-5.-Correcting for the effects of wind.

The density of the air determines the amount of resistance the projectile will experience. Air density depends on temperature and barometric pressure, both of which are changing all the time. Dense air will slow a projectile more than thin air. Density also varies at different altitudes, which further complicates the equation.

WIND.- The effect of wind on a projectile in flight is obvious. Depending upon its force and the direction it is coming from, wind can cause a projectile to fall short, overshoot, or fall to the left or right of the target. As with air density, the longer a projectile is in flight, the more it will be affected by wind. The size of a projectile is also a factor: the larger the projectile, the more it will be affected.

True wind is used in all fire control calculations. If the wind is blowing along the LOF, either with or against the projectile, it is called range wind (fig. 10-5). If the wind is blowing at right angles to the LOF, it is called cross wind. Range wind is compensated for by increasing or decreasing gun elevation angle. Corrections for cross wind are made to the train angle of the gun. Normally, however, the wind will be at some angle to the LOF. In that case, the true wind must be broken down to the range and cross wind components (fig. 10-6). This calculation allows for the realized effect of the wind in each direction.

DRIFT.- Naval guns are rifled to give a spinning motion to the projectile. The spinning projectile assumes the properties of a gyroscope. The gyroscopic actions tend to keep the projectile pointed along the trajectory and prevent it from tumbling. These actions make the projectile almost rigid in its trajectory and ensure it will land point first. This rigidity makes the flight characteristics of the projectile predictable.

In addition to this useful effect, gyroscopic action causes the harmful effect of drift (fig. 10-7). Notice that the effect increases with range. Drift is always to the right in a gun with right-hand rifling (the twist of the rifling is to the right from the chamber to the muzzle).

EARTH'S ROTATION.- In our discussion so far we have assumed that the Earth is flat and does not rotate. For ranges up to about 20,000 yards or so, this assumption does not seriously affect our fire control solution. At ranges over 20,000 yards, the rotation of the Earth has a serious affect.

An object in motion above the surface of the Earth tends to turn toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. Corrections are made to the left or right

Figure 10-6.-Readmg true wind.

accordingly. The correction is only made on guns with bores larger than 5 inches.