DARPA Armor Example

From the DARPA Defense Sciences Office web page located at:

Technology Thrusts

Materials and Devices for New Military Capabilities

Structural Materials and Components: Personnel Protection

The performance of body armor systems against 7.62 mm (30 caliber) armor piercing weapons has steadily improved due to the use of new ceramic materials and fibers and advances in the synthesis and processing of new armor materials. Nevertheless, a soldier using state-of-the-art body armor would need to wear more than six pounds per square foot of the material in order to stop such a bullet, a load too heavy for effective movement in the field. Unfortunately, significant gains can no longer be made through simple changes in processing techniques or in materials substitution. To spur a breakthrough in body armor performance, DARPA began a program that employs rigorous models and simulations to develop innovative materials/systems designs. The performance goal is to stop a 7.62-mm armor piercing round with 3.5 pounds per square foot of armor. Several promising avenues are being explored in a second phase of this program, including an active armor approach.

From this example we know how much weight of armor is needed to defend against 7.62 NATO AP (armor piercing) rounds. Let us pump some numbers to see how close our system is to reality.

6 lbs / square foot translates to

6 lb/ft2·(1000 gm/kg)÷(2.2 kg/lb)÷(12 in/ft)2÷(2.54 cm/in)2 = 2.94 gm/cm2

This mass of armor is

3.5 · log2(2.94) = +5.45 points

We'll say that this armor is 5.5 points better than a base 1 gm/cm3 thickness of armor.

Consulting the ammunition page for the effects of 7.62 mm NATO AP we find the following:

damage = 2D6+1, piercing = +9, and speed class = 2

Which translates to an attack value of 17 for 50% defense. Since the military would probably use enough armor to stop this round 100% of the time we should add 3.5 points for a total of 20.5 points of defense.

This level of defense minus the 5.5 points for mass beyond the 1 gm/cm3 base is

20.5 - 5.5 = 15 AP

Consulting our armor materials page we find a few modern-day armors with the required 15 AP (for 1 gm/cm3) or better level of defense needed:

MaterialAP @ 1 gm/cm3SpeedSpeed APTotalNotes
organicsspectra fiber mesh20.60-614.6current
spectra fiberglass18.71-315.7current
graphite fiberglass22.61-319.6current
graphite fiber mesh23.50-617.5near future
Fullerene fiber mesh30.50-624.5far future
ceramicsboron carbide (B4C)17.44017.4current
carbon fiber (10%) in B4C20.43020.4near future
carbon mesh (40%) in B4C21.43021.4far future
aluminum/B4C cermet21.62021.6near future
titanium/B4C cermet24.52024.5near future
compositesTi/graphite/B4C+C composite17.34-017.3near future
Fullerene fiber/cermet composite26.42026.4far future

This table excludes materials of an overtly "future-fictional" nature.

Essentially all the materials, flexible or rigid, current and future, listed in this table meet the current protection criteria. The situation changes greatly when the new protection criteria of 3.5 pounds per square foot to stop the same round is used.

The new criteria of 3.5 lbs / square foot translates to

3.5 lb/ft2·(1000 gm/kg)÷(2.2 kg/lb)÷(12 in/ft)2÷(2.54 cm/in)2 = 1.71 gm/cm3

This mass of armor is 3.5 · log2(1.71) = +2.72 points better than the base 1 gm/cm3. Thus we'll say that the new armor is 2.5 points better than a base 1 gm/cm3 thickness of armor. Which leads to a protection criteria of 18 AP including speed class. Of the currently existing armors only the rigid graphite fiberglass makes the cut.

Of the reasonable near-term future armor materials the carbon fiber (10%) in B4C, and the two cermet armor types are sufficiently tough to fulfill the specification. All of these are rigid armor materials. The flexible graphite fiber mesh almost fulfills the specification but not quite.

This isn't too terrible a representation of the way things really are.

Note: a full body armor suit that has an area mass of 6 pounds per square foot will weigh in the neighborhood of 55 kg (120 pounds) while the 3.5 pound per square foot requirement leads to a suit weight of about 32 kg (70 pounds).

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Page first created Friday, April 14, 2000
Page last modified Thursday, July 12, 2001 11:46 PM