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CHAPTER 4

BALLISTICS

Ballistics is the science of the motion of projectiles and the conditions that influence that motion. The four types of ballistics influencing helicopter fired weapons are interior, exterior, aerial, and terminal. Each type produces dispersion, which is the degree that projectiles vary in range and deflection about a target.

4-1. INTERIOR BALLISTICS

Interior ballistics deal with characteristics that affect projectile motion inside the barrel or rocket tube. It also includes effects of propellant charges and rocket motor combustion. These characteristics affect the accuracy of all aerial-fired weapons. Aircrews cannot compensate for these characteristics when firing free-flight projectiles. The characteristics of interior ballistics are discussed below.

4-2. EXTERIOR BALLISTICS

Exterior ballistics deal with characteristics that influence the motion of the projectile as it moves along its trajectory. The trajectory is the flight path of the projectile as it flies from the muzzle of the weapon to the point of impact. Aerial-fired weapons have all the exterior ballistic characteristics associated with ground-fired weapons. They also have other characteristics unique to helicopters. The characteristics of exterior ballistics are discussed below.

Table 4-1. Gravity drop

Projectile

Approximate Muzzle

Velocity

(feet per second)

Range

(meters)

Approximate

Gravity Drop

(mils)

7.62 mm

2,800

1,000

7

.50 cal

2,900

1,000/1,500

9/18

20 mm

3,380

1,000/1,500

9/21

30 mm

2,640

1,000/2,000

15/60

40 mm

795

1,000

87

4-3. AERIAL BALLISTICS

Figure 4-1. Projectile drift

Figure 4-2. Rotor downwash error

Figure 4-3. Trajectory shift

Table 4-2. Typical lead angles for a 60-degree deflection
shot at 1,000 meters

Projectile

Approximate Muzzle
Velocity
(feet per second)

Helicopter
Velocity
(knots)

Lead Angle
(mils)

7.62 mm

2,800

100

51

.50 cal

2,900

100

49

20 mm

3,380

100

47

30 mm

2,640

100

64

40 mm

795

100

182

4-4. TERMINAL BALLISTICS

Terminal ballistics describes the characteristics and effects of the projectiles at the target. Projectile functioning, including blast, heat, and fragmentation, is influenced as described below.

Figure 4-4. M433 multioption fuze/2.75-inch high-explosive warhead

Figure 4-5. Wall-in-space concept

Figure 4-6. Angle of impact

4-5. DISPERSION

If several projectiles are fired from the same weapon with the same settings in elevation and deflection, their points of impact will be scattered about the mean point of impact of the group of rounds. The degree of scatter (range and azimuth) of these rounds is called dispersion. The mean point of impact with respect to the target center, or intended air point, is an indication of the weapon's accuracy. Both dispersion and accuracy determine whether a particular weapon can hit an intended target. Firing rockets at maximum ranges decreases range dispersion and normally increases accuracy. The reverse is true with other weapon systems; that is, as range increases, dispersion increases and accuracy decreases. Dispersion is caused by errors inherent in firing projectiles. These errors are influenced, in part, by the factors discussed in the ballistics paragraphs. In addition, they may be influenced by the vibrations in the mount and condition of the sighting systems.