Sectional density | |
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A metal nail has a small cross sectional area compared to its mass, resulting in a high Sectional Density. | |
SI unit | kilograms per square meter (kg/m^{2}) |
Other units | kilograms per square centimeter (kg/cm^{2}) grams per square millimeter (g/mm^{2}) pounds per square inch (lb_{m}/in^{2}) |
Sectional density (often abbreviated SD) is the ratio of an object's mass to its cross sectional area with respect to a given axis. It conveys how well an object's mass is distributed (by its shape) to overcome resistance along that axis.
Sectional density is used in gun ballistics. In this context, it is the ratio of a projectile's weight (often in either kilograms, grams, pounds or grains) to its transverse section (often in either square centimeters, square millimeters or square inches), with respect to the axis of motion. It conveys how well an object's mass is distributed (by its shape) to overcome resistance along that axis. For illustration, a nail can penetrate a target medium with its pointed end first with less force than a coin of the same mass lying flat on the target medium.
During World War II, bunker-busting Röchling shells were developed by German engineer August Cönders, based on the theory of increasing sectional density to improve penetration. Röchling shells were tested in 1942 and 1943 against the Belgian Fort d'Aubin-Neufchâteau^{[1]} and saw very limited use during World War II.
In a general physics context, sectional density is defined as:
The SI derived unit for sectional density is kilograms per square meter (kg/m^{2}). The general formula with units then becomes:
Where:
kg/m^{2} | kg/cm^{2} | g/mm^{2} | lb_{m}/in^{2} | |
---|---|---|---|---|
1 kg/m^{2} = | 1 | |||
1 kg/cm^{2} = | 1 | |||
1 g/mm^{2} = | 1 | |||
1 lb_{m}/in^{2} = | 1 |
(Values in bold face are exact.)
The sectional density of a projectile can be employed in two areas of ballistics. Within external ballistics, when the sectional density of a projectile is divided by its coefficient of form (form factor in commercial small arms jargon^{[3]}); it yields the projectile's ballistic coefficient.^{[4]} Sectional density has the same (implied) units as the ballistic coefficient.
Within terminal ballistics, the sectional density of a projectile is one of the determining factors for projectile penetration. The interaction between projectile (fragments) and target media is however a complex subject. A study regarding hunting bullets shows that besides sectional density several other parameters determine bullet penetration.^{[5]}^{[6]}^{[7]}
If all other factors are equal, the projectile with the greatest amount of sectional density will penetrate the deepest.
When working with ballistics using SI units, it is common to use either grams per square millimeter or kilograms per square centimeter. Their relationship to the base unit kilograms per square meter is shown in the conversion table above.
Using grams per square millimeter (g/mm^{2}), the formula then becomes:
Where:
For example, a small arms bullet weighing 10.4 grams (160 gr) and having a diameter of 7.2 mm (0.284 in) would have a sectional density of:
Using kilograms per square centimeter (kg/cm^{2}), the formula then becomes:
Where:
For example, a M107 projectile weighing 43.2 kg and having a body diameter of 154.71 millimetres (15.471 cm) would have a sectional density of:
In older ballistics literature from English speaking countries and still to this day, the most commonly used unit for sectional density of circular cross-sections is (mass) pounds per square inch (lb_{m}/in^{2}) The formula then becomes:
Where:
The sectional density defined this way is usually presented without units.
As an example, a bullet 160 grains (10.4 g) in weight and a diameter of 0.284 in (7.2 mm), would have a sectional density (SD) of:
As another example, the M107 projectile mentioned above weighing 95.2 pounds (43.2 kg) and having a body diameter of 6.0909 inches (154.71 mm) would have a sectional density of: