Heat Rounds and Sabots

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.

  1. Ballistic cap
  2. Chamber reduction – to improve the penetrating beam’s characteristic
  3. Inverse-cone hollow room, covered with thin metal layer, which the penetrator is built from
  4. Fuse
  5. Explosive combat charge
  6. 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.

26 thoughts on “Heat Rounds and Sabots”

  1. ahh, now you’re getting to the very center of my tanker’s heart!

    well written, sufficiently in depth, and you even got the obligatory note about RHA. nice!

  2. mayhap could lead to something about spin stabilized vs fin stabilized??

  3. Thanks for the roundup it meshes well with my new understanding of HEAT rounds and now sabot. I remember they also talked about this counter-countermeasure of HEAT where it has an initial small HEAT to use up the armours reactive explosive defence followed by the main HEAT to actually do the damage. Of course this ups the dollars. So much back and forth countering. I still think the idea of not being hit n the first place like the navy seems to now pray for is a good one. The sabot would be the one using the DU. Uranium is extremely dense (leave tankers comment out) and would pack more punch at the same speed than a sabot of steel. I wonder if it can make hard alloys too or probably they just make the tip very hard.

    As you probably know DU all over the countryside is a popular conspiracy/poison/pollution theory of the left on the military.

  4. DU is popular for penetrators because it has an excellent balance of density and hardness. For a given size penetrator, a DU weighs more than steel or tungsten, and therefore delivers more kinetic energy at impact for a given speed.

    DU is also used in armor. The same things that make it a good penetrator make it a good armor. Most M-1A1s in service are “Heavy Metal” variants with a mesh of DU in the composite armor. Composite as originally designed was made to defeat HEAT rounds. The DU mesh adds a great deal of ability to proof against penetrators.

    As for countermeasures, our tactics, techniques and procedures spend a lot of time focusing on avoiding getting hit. The great standoff range is part of this. In the defense, digging in and careful use of terrain is key. In the offense, using terrain masking is the main technique. One of the primary reasons the designers of the M-1 went with the gas turbine over a diesel of comparable power was the acceleration the GT gave it, giving the M-1 a better chance of dodging shots.

    More to come on the Bushitlerchimpyburton “nuclear war” on innocent kiteflying children by means of DU.

  5. Remember, when disposing of UXO HEAT rounds, always countercharge the main charge (place your explosive near the pointy end, so that it essentially detonates the round backwards) and don’t cast any shadows on the damaged peizoeletric fuze while doing so.

    This PSA brought to you by your friendly, neighbohood sappers.

  6. Never knew that, since I never disposed of any. Tripped over an 82mm mortar round once though.

    But now you reminded me. Danger UXB. BBC miniseries. Not bad at all. Except when the went “Boom!”

  7. The M24 light tank of WWII was actually able to withstand a hit from the German 88 when dressed with sand bags. Also, the British sleeved down the M24’s 47mm gun to 20mm and fired tungsten/titanium slugs. The slugs were able to destroy the heavy tiger tank. The best armor is an incompressible liquid such as water. For all extents and purposes, water is impenetrable by a hypervelocity round and it absorbs heat energy projected by shaped charges. Be that as it may, I am thinking about developing an explosive reactive helmet specifically for the regular military so perhaps they won’t have to have the guard do all the dirty work.

  8. Norm, the M24 had a 75mm gun. And I wouldn’t bet my life on sandbags. Did some survive a hit? Probably. But a full on 90 degree deflection shot from within the lethal range of the weapon? One layer of sandbag equals roughly1/4″ of RHA. It will stop a rifle slug. It will help disperse a HEAT round. It won’t add that much effect against a penetrator as sand acts like a liquid under those circumstances.

    The problem with water is, you need far to much to be practical. If you think water works well against HEAT, you should see what styrofoam does. But you’d need a 20 foot thick block to achieve protection. Water isn’t as heavy as steel, but it is one of the heavier things to lug around for a given volume.

  9. Nice pictures of sabots. Anyone got any data on 1) typical size, 2) radar cross section, 3) how fast the sabot separates from the round after launch? Would appreciate it.

  10. hmm…a typical sabot dart is usually about .8 to 1.2 inches or 2 to 3 cm in diameter and about 50 cm or 20 inches in lenght or longer. I have heard of test done in the late 70s through the 80s and into the early 90s were small live stock were put in the hulls and turrets of old tanks. Then according to the discussions I was in on these tanks were shot with APFSDS and APFSDSDU rounds. Besides the ability of the DU rounds to ignite and mushroom less than other material I heard from these test that a vaccum of sorts similar to that caused in an EF 4 or 5 tornado is present when the dart punches through the one side of a hull or a turret and exits out the other. This vaccum said to be caused by the hypervelocity of the round is also said to suck anything not tied down inside the fighting compartment of a tank through the exit hole not matter how big it was before being forced through. I am just wondering how much truth is in that. Because it was said after the tank battles of the First Iraq war and those in the early Second Iraq that this situation was why so few Iraqi tank crew survived the war.

    1. Well, I actually meant the M136, which is the licensed built US version of the Swedish Bofors AT4. But in the US Army, no one calls it the M136, we all called it the AT4. Pretty much the same thing, except the instructions are printed in English.

  11. “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.”

    not correct

    there are IFV’s with calibre 30mm that can fire APFS capable of penetrating most medium armor

    1. Well, the M242 Bushmaster 25mm gun on the Bradley fires APFS and APFSDS rounds. And they’ll penetrate “armor” to a certain degree. That is, they’ll defeat the armor of light AFVs such as BMPs and BTRs.

      Believe me, I know.

      But to penetrate the armor of a main battle tank using sabot rounds requires a rather significant cannon, such as a 105mm. And mounting a 105mm gun imposes a weight cost. It may be worthwhile to pay that cost. But there’s no practical way to give dismount infantry that capability. That’s why light infantry anti-armor weapons use HEAT warheads.

      Both approaches to defeating armor have their strengths and weaknesses. That’s why we still see both used on the battlefield today.

    2. I’ve never actually fired the M919. But I do know some tracks fired enough M791 APDS-T to penetrate the flanks of Type 59 tanks in Iraq. So that should be pretty easy for the M919, at least at short range.

  12. Hi, regarding a 105mm recoilless rifle in the 1982 time frame, here is the scenario, a heat round was fired up a sloping hill at a very jagged rocky position, would it be possible for it to have deflected off the rocks and gone down the reverse slope and exploded, and when it exploded would it produce mainly blast injuries, as two people take the impact, those in almost the immediate area are blown over,, . any help appreciated

    1. DU is used due to its density, its basic physics. A denser metal (DU which is roughly 2.5x denser than steel) striking a less dense metal (Steel, Aluminum, etc) will cause the less dense metal to be damaged or penetrated easier in this case. The velocity of the penetrator causes an extreme amount of friction while passing through the less dense metal generating immense amounts of heat.This extreme amount of heat causes the metals, including the DU to effectively liquefy spraying the inside of the turret with molten metal. The molten metal finds a combustible, usually fuel or the ammo and detonates it. T-72’s and older model Russian tanks had low crew survivability because the ammunition was stored in a circle literally around the turret. This is why in photos and videos you see those tanks with the turrets blown clean off the hull.

      In regards to the penetrators over penetrating armor and sucking everything out, this is mostly true, I’ve seen the effect. The only times it happened were largely on BMP-1’s and BMP-2’s with very thin armor. Stuff will get sucked out the other end including people but they were so badly charred from the round entering they were more than likely incinerated and dead before actually being pulled out the rather large hole on the other side. I’ve never seen over penetration on a main battle tank to the same degree, a small burned hole on the other side but no debris outside of said hole or anything being pulled out, that was exclusively on BMP’s.

      I’m currently on an M1A2 SEP V2 right now, and while i can’t give out full capabilities such as optics, actual effect ranges etc I can say this much. The M1 series of tanks was primarily designed as a defensive tank up until the SEP V1 and V2 versions which are now hunter/killer tanks. Believe me when I say our 72 ton tank is not effective at ducking and dodging rounds, don’t necessarily have to be, we attack head on exposing only the frontal armor which provides the most armor and deflection capability. Short of a few weapon systems in the world, there isn’t too much out there that can defeat the armor and we or allies own the large majority of them.

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