We’ve covered Explosively Formed Projectiles here. EFPs however, are a relatively small slice of the anti-armor pie. Far more common are the HEAT round and the Sabot.
HEAT stands for High Explosive Anti Tank. HEAT rounds are also known as “shaped charges”. Just using high explosives doesn’t do much to penetrate armor. By shaping the explosives in an inverted cone, and usually lining this cone with a copper sheath, the explosive effect can be channeled into a very small spot on the target. This superheated jet of fire and molten copper then “burns” through the armor. While this produces only a small hole in the armor, the fire and copper tend to ignite anything inside the vehicle. Military vehicles are stuffed with flammables (people count as flammables) or explosives, so any penetration of the armor can have devastating consequences.
- Ballistic cap
- Chamber reduction – to improve the penetrating beam’s characteristic
- Inverse-cone hollow room, covered with thin metal layer, which the penetrator is built from
- Explosive combat charge
- Piezoelectric bounce-on fuze initializer
The rule of thumb is that a shaped charge can penetrate six times its own diameter. A 4″ warhead, then would notionally penetrate two feet of armor.* This means that relatively small warhead can provide quite a bit of punch. The first common use of shaped charge warheads was in WWII by the “Bazooka”. To this day, we still see small, man portable rocket launchers all over the battlefield. Common examples are the Soviet designed RPG-7 and the US AT-4. Whereas the RPG-7 is a reloadable launcher, the AT-4 is a disposable, one shot weapon.
Anti-tank missiles also almost always have a HEAT warhead. Since the speed of the warhead on impact makes no difference in penetration, relatively slow (hence lightweight) missiles can be used. In US service, both the TOW missile and the Hellfire have HEAT warheads.
But missiles and rockets aren’t the only place HEAT rounds are used. Tanks can also fire HEAT rounds. In fact, one of the reasons why the M-1A1 Abrams tank has a smoothbore main gun is that HEAT rounds work better when they aren’t spinning rapidly. Instead, they use pop-out fins to stabilize the shell. The original 120mm M830 HEAT round looked like a coffee can with a probe attached.
The probe is designed to detonate the warhead at a set distance from the target, giving the hot jet of gasses space to fully form. To close in, and the jet doesn’t form. Too far out, and the jet loses it’s focus.
In the past few years, the M830 was replaced by the M830A1, with a slightly smaller warhead, but with a proximity fuse to allow it to defeat helicopters and other soft targets.
While HEAT rounds are simple and cheap, and fairly effective, there are countermeasures. A simple, low cost countermeasure is the cage, seen here on a Stryker Infantry Carrier Vehicle.
The cage either prevents the fuse from detonating the warhead, or causes the warhead to detonate too far out to be effective. Explosive reactive tiles can be bolted onto the outside of vehicles as well. These explode when hit by a HEAT round and the explosion disrupts the jet of the warhead.Tanks like the Abrams use composite armor with layers of steel and ceramic to defeat HEAT rounds.
Sabot rounds overcome these weaknesses. Sabots, also know as kinetic energy penetrators, use sheer momentum to penetrate armor. In the early days of armored warfare, AP (armor piercing) rounds were solid shot or had a small bursting charge. Basically, they were just big, hardened steel bullets. But as armor grew thicker, AP rounds lost their ability to penetrate armor. The solution was the long rod penetrator. By making the solid shot thinner but longer, the projectile would maintain the same weight while focusing its impact on a smaller area. This improved penetration. To make this thin, dart shaped projectile fit into a cannon, the dart was wrapped in a “sabot” or shoe. These sabots would fall off as the dart left the barrel.
There are no explosive in the dart. Just the energy transferred from the impact will generate enormous amounts of heat as it slices through the targets armor. Also, chips of the dart and the targets own armor are superheated and sprayed inside the target vehicle, setting afire fuel and ammo. This quickly leads to the catastrophic destruction of the target.
The drawback of a sabot round is that it takes a very big gun (like a 105mm or 120mm) to generate the velocity needed to penetrate armor. This means a big vehicle to mount the gun. The trade off here is that while a heat round can be transported easily, tank takes a lot more logistical support.
Also, while sabot rounds have great penetration, they don’t do a lot of damage outside the immediate area of impact. Sabots are of little use in urban combat such as Iraq. Tank crews there are far more likely to use HEAT rounds to engage the enemy.
*When we talk about armor penetration, we are measuring against a benchmark of Rolled Homogeneous Armor or RHA. Different armors will have different protections. Two feet of aluminum won’t provide as much protection as two feet of RHA steel.