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.

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.

a. Barrel Wear. Gaseous action, propellant residue, and projectile motion wear
away the barrel's inner surface or cause deposits to build up. These conditions result
in lower muzzle velocity, a decrease in accuracy, or both.

b. Propellant Charges. Production variances can cause differences in muzzle

velocity and projectile trajectory. Temperature and moisture in the storage
environment can also affect the way propellants burn.

c. Projectile Weight. The weight of projectiles of the same caliber may vary. The
variance is most noticeable in linked-ball projectiles. These variations do not
significantly influence trajectory.

d. Launcher Tube alignment. Individual rocket launcher tubes are aligned by the
rocket launcher's internal or end bulkhead. However, the precise alignment of each
tube may vary. Because of variances in alignment, the launcher boresight also varies
from tube to tube. Proper boresighting of the launcher should include checking the
boresight of several tubes and selecting the one that best represents the alignment
of the entire launcher.

e. Thrust Misalignment.

(1) A perfectly thrust-aligned free-flight rocket has thrust control that passes
directly through its center of gravity during motor burn. In reality, free-flight
rockets have an inherent thrust misalignment, which is the greatest cause of
error in free flight. Spinning the rocket during motor burn reduces the effect of
thrust misalignment.

(2) Firing rockets at a forward airspeed above ETL provides a favorable relative
wind, which helps to counteract thrust misalignment. When a rocket is fired from
a hovering helicopter, the favorable relative wind is replaced by an unfavorable
and turbulent wind caused by rotor downwash. This unfavorable relative wind
results in a maximum thrust misalignment and a larger dispersion of rockets.

(3) Rockets spin to counteract thrust misalignment. Rockets with MK66 motors
exhibit less dispersion in the target effect area than those with MK40 motors
according to data provided by Rock Island Arsenal.

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.

a. Air Resistance. Air resistance, or drag, is caused by friction between the air and
the projectile. Drag is proportional to the cross-section area of the projectile and its
velocity. The bigger and faster a projectile is, the more drag it produces.

b. Gravity. The projectile's loss of altitude because of gravity is directly related to

range. As range increases, the amount of gravity drop increases. This drop is
proportional to time of flight (distance) and inversely proportional to the velocity of
the projectile. Crew members that fire weapons without FCC solutions must correct
for gravity drop. Table 1 shows gravity drop for different projectiles.

c. Yaw. Yaw is the angle between the centerline of the projectile and the trajectory.
Yaw causes the projectile's trajectory to change and drag to increase. The direction
of the yaw constantly changes in a spinning projectile. Yaw maximizes near the
muzzle and gradually subsides as the projectile stabilizes.

d. Projectile Drift.

(1) When viewed from the rear, most projectiles spin in a clockwise direction.
Spinning projectiles act like a gyroscope and exhibit gyroscopic precession. This
effect causes the projectile to move to the right, which is called the horizontal
plane gyroscopic effect. As the range to target increases, projectile drift
increases.

Table 1. Gravity drop

Projectile Approximate Muzzle Range Approximate

Velocity
(meters) Gravity Drop
(feet per second)
(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

(2) To compensate for this effect in aircraft without FCC solutions, the gunner
increases any correction, such as elevation, depression, or deflection, to hit the
target. To compensate for projectile drift, the gunner establishes combat sight
settings or adjusts rounds toward the target. This compensation is known as
using "burst on target." Figure 1 shows projectile drift.

Figure 1. Projectile drift

e. Wind Drift. The effect of wind on a projectile in flight is called wind drift. The
amount of drift depends on the projectile's time of flight and the wind speed acting
on the cross-sectional area of the projectile. Time of flight depends on the range to
the target and the average velocity of the projectile. When firing into a crosswind,
the gunner must aim upwind so that the wind drifts the projectile back to the target.
Firing into the wind or downwind requires no compensation in azimuth but will
require range adjustment.

3. AERIAL BALLISTICS

a. Common Characteristics. Characteristics of aerial-fired weapons depend on

whether the projectiles are spin-stabilized or fin-stabilized and whether they are fired
from the fixed mode or the flexible mode. Some characteristics of aerial-fired
weapons are discussed below.

(1) Rotor downwash error. Rotor downwash acts on the projectile as it leaves
the barrel or launcher. This downwash causes the projectile's trajectory to
change. A noticeable change in trajectory normally occurs when the helicopter is
operating below effective translational lift.

(a) Although rotor downwash influences the accuracy of all weapon systems,
it most affects the rockets. Maximum error is induced by rotor downwash
when the weapon system is fired from an aircraft hovering IGE, as shown in
Figure 2. Air flows downward through the rotor system and causes the rocket
to pitch up as it leaves the launcher.

(b) When the rocket passes beyond the rotor disk, air flows upward and
causes the rocket to wobble. This air flow causes both lateral (azimuth) and
linear (range) errors.

(c) When the aircraft is hovering OGE (Figure 2) the relative wind strikes the
rocket only from above after it leaves the launcher. This condition decreases
the lateral error. However, the velocity of the rotor downwash increases
because of the additional power required to maintain OGE hover, which may
increase linear dispersion.

(d) High-density altitudes and heavily loaded aircraft further increase linear
dispersion. During IGE and OGE hovering flight, the true airspeed vector of
the helicopter affects the position of rotor downwash and the speed of the
downwash at the rocket launchers. For example, holding a position over the
ground during a right crosswind results in a true airspeed vector to the right
and a shift of the downwash to the left. This shift affects the left rocket for a
longer time during launch than the right rocket. The left rocket also will pitch
up to a higher quadrant elevation and go farther than the right rocket.
Detailed system testing has not shown that differences of QE are required for
right versus left launchers during hover fire.

Figure 2. Rotor downwash error

(e) To prevent a divergence of trajectories, the aircraft can drift with the
wind if the terrain allows. Drifting with the wind allows the aircraft to remain
stable and provides a more consistent rotor downwash for both launchers.

(2) Angular rate error.

(a) Angular rate error is caused by the motion of the helicopter as the
projectile leaves the weapon. It affects most weapon systems. The
exceptions are TOW, Hellfire, and Stinger missiles. For example, a pilot using
the running-fire delivery technique to engage a target with rockets at 5,000
meters may have to pitch the nose of the helicopter up to place the reticle on
the target. When the weapon is fired, the movement of the helicopter
imparts an upward motion to the rocket. The amount of error induced
depends on the range to the target, the rate of motion, and the airspeed of
the helicopter when the weapon is fired.

(b) Angular rate error occurs when aircrews fire rockets from a hover using
the pitch-up delivery technique. Anytime a pitch-down motion is required to
achieve the desired sight picture, the effect of angular rate error causes the
projectile to land short of the target.

b. Spin-Stabilized Projectiles. Certain exterior ballistic characteristics are peculiar

to spin-stabilized projectiles fired from weapons with rifled barrels. These weapons
include the .50-caliber and 7.62-mm machine guns, and the 20- and 30-mm
cannons. When fired in the fixed mode (straight ahead of the helicopter), the
projectiles generally have the same ballistic characteristics as ground-fired weapons.
However, relative wind changes and the velocity of the helicopter increase or
decrease the velocity of the projectile. Ballistic characteristics influencing spin-
stabilized projectiles fired from positions other than a stabilized hover are discussed
below.

(1) Trajectory shift. When the boreline axis of the weapon differs from the
flight path of the helicopter, the movement of the helicopter changes the
trajectory of the projectile. For off-axis shots within ±90 degrees of the
helicopter's heading, trajectory shift causes the round to hit left or right of the
target. To correct for trajectory shift, the gunner leads the target. To lead the
target, the gunner places fire on the near side of the target as the helicopter
approaches. The amount of lead depends on the airspeed of the helicopter, angle
of deflection, velocity of the projectile, and range of the target. Figure 3 shows
trajectory shift. Table 2 shows some examples of how to compensate for
trajectory shift.
Figure 3. Trajectory shift

Table 2. Typical lead angles for a 60-degree deflection

shot at 1,000 meters

Approximate Muzzle Helicopter

Velocity Velocity Lead Angle
Projectile (feet per second) (knots) (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

(2) Port-starboard effect. Trajectory shift and projectile drift combine to

constitute the port-starboard effect. When targets are on the left, the effects of
drift and shift compound each other; both cause the round to move right. To hit
the target, the gunner must correct for both ballistic effects by firing to the left of
the target. When targets are on the right, the effect of projectile drift (round
moves right) tends to cancel the effect of trajectory shift (round moves left).
Therefore, firing requires less compensation. The range and airspeed at which a
target is engaged determine which effect is greater. For example, at ranges less
than 1,000 meters, trajectory shift is greater. The gunner must fire to the right of
the target. At ranges beyond 1,000 meters, the effect of projectile drift is greater
and tends to cancel the effect of trajectory shift.

(3) Projectile jump (vertical plane gyroscopic effect).

(a) When a crew fires a weapon from a helicopter in flight and the weapon's
muzzle is pointing in any direction other than into the helicopter's relative
wind, the projectile will experience projectile jump. Projectile jump begins
when the projectile experiences an initial yaw as it leaves the muzzle. The
yaw is in the same direction as the projectile's direction of rotation. The jump
occurs because of the precession (change in axis of rotation) induced by
crosswind.

(b) The amount a projectile jumps is proportional to its initial yaw. Firing to
the right produces a downward jump; firing to the left produces an upward
jump. To compensate the gunner must aim slightly above a target on the
right of a helicopter and slightly below a target on the left. The amount of
compensation required increases as helicopter speed and angular deflection
of the weapon increase. Compensation for projectile jump is not required
when firing from a hover.

c. Fin-Stabilized Projectiles. The exterior ballistic characteristics affecting fin-

stabilized projectiles are important. They include—

(1) Propellant force. A bullet reaches its maximum velocity at or near the
weapon's muzzle. However, a rocket continues to accelerate until motor burnout
occurs. As the rocket reaches its greatest velocity, the kinetic energy in the
rocket tends to overcome other forces and causes the rocket to travel in a flatter
trajectory.

(2) center of gravity. Unlike a bullet, the CG of a rocket is in front of the

center of pressure. As the rocket propellant burns, the CG moves farther
forward. The rocket's fins cause the center of pressure to follow the CG.

(3) Relative wind effect. When a helicopter is flown out of trim, either
horizontally, vertically, or both, the change in the crosswind component deflects
the rocket as it leaves the launcher. Because the rocket is accelerating as it
leaves the launcher, the force acting upon the fins causes the nose to turn into
the wind.

(a) A horizontal out-of-trim condition results when a pilot tries to align the
sight on the target during a crosswind by cross-controlling, or slipping, the
helicopter. For example, a pilot flies at 100 knots and maintains 10 degrees
out of trim with a quartering crosswind component of 10 knots. This
condition causes the rocket to turn into the relative wind after leaving the
tube. As the velocity of the rocket increases and the motor burns out, the
crosswind component decreases. After the motor burns out, the rocket drifts
with the air mass (real wind). If the pilot is unable to align the helicopter into
the wind, the gunsight must be corrected upwind. While firing from a hover
or during slow flight, the pilot must make a downwind correction because the
rocket will turn into the wind.

(b) A vertical out-of-trim condition results from an improper power setting.

This condition creates a vertical relative wind on the rocket during launch,
causing the rocket to turn into the wind. If the pilot fires the rocket while
applying power (as in a climb), the relative wind will be from above. The
relative wind will cause the rocket to hit beyond the aiming point. To
maintain a vertical trim condition, the pilot must maintain a constant power
setting that will produce the desired airspeed and altitude.

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.

a. Impact Fuzes. Impact fuzes activate surface and subsurface bursts of the
warhead. The type of target engaged and its protective cover determine the best
fuze for the engagement. Engage targets on open terrain with a superquick fuze that
causes the warhead to detonate upon contact. Engage targets with overhead
protection, such as fortified positions or heavy vegetation, with either a delay or
forest penetration fuze. As shown in Figure 4 these fuzes detonate the warhead after
it penetrates the protective cover.

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

b. Remote Set or Variable Time Fuzes. Timed fuzes produce airbursts and are
most effective against targets with no overhead protection. Flechette, smoke, and
illumination warheads incorporate a timed fuze, which depends on motor burnout.
The range for this type of fuze is fixed. Remote range-set fuzes are in use for high
explosive, multipurpose submunition, smoke, illumination, and chaff warheads. The
range is variable for this type of fuze and can be set by the crew in the AH-1E/F, AH-
64, and OH-58D (KW).

c. Wall-In-Space Fuze.

(1) Multipurpose submunition warheads provide a large increase in target

effectiveness over standard unitary warheads. The MPSM warhead helps to
eliminate range-to-target errors because of variations in launcher/helicopter
pitch angles during launch. The M439 fuze is remotely set from the aircraft with
range (time) to the target data.

(2) Once fired, the initial forward motion of the rocket begins fuze time. At the
computer-determined time (a point slightly before and above the target area),
the M439 fuze initiates the expulsion charge. The submunitions eject and each
ram air decelerator inflates. Inflation of the RAD separates the submunitions,
starts the arming sequence, and causes each submunition to enter a near
vertical descent into the target area. Figure 5 shows the wall-in-space concept.

Figure 5. Wall-in-space concept

d. Surface Conditions. The surface of the target area (such as sand, rocks, or
vegetation) affects the lethality of the projectile. If superquick fuzes are used against
targets covered by heavy foliage, they will function high in the tree canopy but will
be ineffective at ground level. However, the same fuze would be effective against a
target area with a sandy surface. To get maximum effectiveness from the warhead,
use the proper fuze for the surface condition.

e. Warheads. The type of target to be engaged determines which warhead to use. A

large variety of warheads are available. The factors of METT-T help determine the
proper mix of warheads for the particular mission.

f. Angle of Impact. The altitude from which the projectile is fired and the range to
the target determine the angle of impact and fragmentation pattern. Weapons fired
with a high angle of impact produce fragmentation patterns that are close together. A
projectile fired from NOE altitudes at the midrange of the weapon forms an elongated
pattern with the projectile impacting at shallow angles. As the range increases, the
impact angle of the projectile increases. The length of the fragmentation pattern
decreases while the width increases. Figure 6 shows the angle of impact.
Figure 6. Angle of impact

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.

a. Vibrations. Because mounts for weapons are fixed to the helicopter, vibrations in
the helicopter transmit through the mounts. These vibrations affect azimuth and
elevation.

b. Sights. The condition of the sights and the accuracy of their alignment with the
bore axes of the weapons cause a displacement of the dispersion pattern of the
projectiles.

c. Boresight. Proper boresighting of aircraft weapons is critical to accurate fires.

Improper boresighting is a factor in dispersion differences between like aircraft.

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Updated: 12 January 2008 Born on 01 March 1999