What’s “RHA” in Penetration Specifications?

Anti-tank, anti-armor, and armor-piercing ammunition needs to have a specification describing its penetration. Now, any scientific test would be buried in disclaimers and details. What muzzle velocity, what distance, what angle, what atmospheric conditions. But there are certain norms.  It’s customary to convert ambient temperature and pressure during the test to an international standard atmosphere, 59ºF and 29.95 inches of mercury. It’s customary to convert slanted armor to its thickness equivalent along the axis of the shot. And it’s customary to describe penetration as distance, millimeters or inches, in a specific medium, RHA.

The Armor of this Russian T-34/76, with armor thickness and obliquity noted. All armor was RHA.

The Armor of this Russian T-34/76, with armor thickness and obliquity noted. On this specific model, nearly all armor was RHA (the 52mm thick turret front may have been cast).

RHA is Rolled Homogeneous Armor and it’s the most common of three types of steel armor that was commonly used in World War II. The others were Cast Homogeneous Armor and Face-Hardened Armor. In general RHA was the gold standard at the time, with CHA and FHA used for specific purposes. There are some terminological differences, of course: the British called RHA machineable armour, because it could be practically cut with machine tools; FHA was very difficult to cut on its armored face, due to heat-treating giving it a very hard, but brittle if overstressed, surface. RHA, conversely, is strong but ductile, which enables it to shuck off more and harder hits. The British breakout of FHA, which Americans call face-hardened armor, is flame-hardened armour.

Panther mantlet penetrated by US 90mm gun in Aberdeen testing.

CHA Panther mantlet penetrated by US 90mm gun at 800m in Aberdeen testing. Only some guns and some specific projectiles could penetrate here.

The US transitioned to mostly RHA early, as did the USSR (all T-34 hulls were entirely RHA, and both RHA weldments and CHA castings were used for turrets). British and German tank production started the war using face-hardened armor, and changed midwar. All armies used cast homogeneous armor for some purposes. For example, the Germans used it in the commander’s cupola and in the mantlet or gun shield of all Panzer V Panther tanks. The US made Shermans with cast turrets and with both cast hulls and welded RHA hulls.  The initial Panther model, Ausführung D, was made with face-hardened armor for the hull and turret (apart from the two cast parts mentioned above). In July 1943, they changed to RHA for the glacis (the upper front plate), and a year later they began using RHA on the sides.

As a rule of thumb, RHA is the best of these steel armors, with the best protection from penetrating and HE attacks, but it has some deficiencies. It is hard to form in anything but flat plates. CHA can be cast in almost any shape.

Heat treating is used to bring RHA to a specific hardness. The hardness of steel armor is measured on the Brinell hardness scale. As a general rule, the thicker the armor the lower the Brinell scale value, and therefore hardness of the metal, will be. The FHA plates of WWI armored vehicles, which were 1/8 to 1/4 inch thick, had a Brinell hardness of 420-650. The RHA for the WWII generation vehicles ranged from 220-390 or so. For example, these are the German specified values (for both RHA and CHA):

Thickness Range (mm) Brinell Hardness

16-30

309-353

35-50

278-324

55-80

265-309

The reason for this decline in hardness with increase in thickness was the state of the production art, and it was fairly universal across the belligerents’ RHA armor. The Russians’ armor was the hardest by Brinell measurement.

Penetration of a Panther glacis. This may have been FHA, judging from the Greens' analysis of these Aberdeen tests.

Penetration of a Panther glacis. This may have been FHA, judging from the Greens’ analysis of this series of Aberdeen tests.

FHA was hardened to a higher level (Brinell in the 500 range), but only a few mm deep. The idea was to have more resistance to penetration on the surface, but more ductility in the rest of the armor to prevent brittleness and fractures, or spalling of chunks off the inside of the armor. Spalling was the kill mechanism of the British HESH (High Explosive Squash Head) round of late-war was designed to produce. The US later produced a version called HE-Plastic or HE-P.

The more you study armor penetration, the stranger it gets. For example, a long rod penetrator like the APFSDS rounds used in modern tank guns can actually perforate armor thicker than it can penetrate, by causing failure in the armor plate; it can also perforate the armor deeper, through that failure mechanism. That’s completely counterintuitive, but penetration and perforation curves from live testing demonstrate it.

Late in the war, shaped-charge warheads became a problem. Using WWII-era understanding of lining materials and explosives, effective shaped charges tended to be larger than most tank main gun calibers. Instead, they were deployed by short-range rockets like the German Panzerschreck and the US 2.36″ rocket launcher, and other infantry weapons, such as Russian drogue grenades, the British PIAT and the Japanese lunge mine (which is exactly what it sounds like, a shaped charge on a stick for a suicidal human attack on tanks. They were used on Okinawa and were made in the hundreds of thousands for the anticipated defense of the home islands).

armor-penetration-table

Penetration curves like this are typical of kinetic-energy penetrators, like these US 90mm shot types. Shaped charges do not depend on kinetic energy for their penetration, and thus, their effect on target is range-independent, as long as the delivery system can deliver the shaped charge to the target. The same shaped charge will work the same in a 3000-m ATGM or at the end of a 1.5 meter lunge mine.

The hardness of armor had much less influence on shaped charge penetration. But as a shaped charge has an optimum standoff distance, detonating it early reduces its ability to burn its way through armor. This led to various kinds of appliqué armor, some factory and some improvised. The Germans were a step ahead here. They had already added stand-off plates called Schürzen to many combat vehicles (including the Panzer III, IV and Panther) as a countermeasure against Russian anti-tank rifles. The Schürzen were homogeneous, but not very hard — only Brinell 105 or so. Schürzen were ineffective against conventional tank and antitank guns, but would sometimes fragment or deflect the steel or tungsten-cored 14.5mm Russian anti-tank rifle projectile, which otherwise could penetrate the side armor of those tanks at close ranges (~100m). The effectiveness of Schürzen against shaped-charged warheads was an unexpected but welcome bonus.

As we said, armor penetration is a weird science. The Schürzen, for instance, had almost zero effectiveness against the 14.5 if struck absolutely square on, at a 90º angle, but got much more effective as the angle increased even a few degrees.

Sources

Farrand, Magness, and Burkins. Definition and Uses of RHA Equivalences for Medium Caliber Targets. Interlaken, Switzerland: 19th International Symposium of Ballistics, 7–11 May 2001. Retrieved from: http://ciar.org/ttk/mbt/papers/symp_19/TB151159.pdf   (That site has the whole proceedings of the symposium).

Green, Michael & Gladys. Panther: Germany’s Quest for Combat Dominance. Botley, Oxford: Osprey, 2012.  (A very worthwhile book, rich in technical detail, with excellent notes and index and a wealth of photographs).

Uncredited, Armor-Piercing Ammunition for Gun, 90-MM, M3. Washington: Office Of The Chief Of Ordnance, January 1945. Retrieved from: http://www.lonesentry.com/manuals/90-mm-ammunition/index.html  (Lone Sentry is a former W4, and is highly recommended for this sort of period material).

Numerous other sources were used en passant but the bulk of the information in this post is from Farrand et. al. and the Greens.

21 thoughts on “What’s “RHA” in Penetration Specifications?

  1. Dyspeptic Gunsmith

    Here’s a question from a civilian completely ignorant of the details of tank armor, but extrapolating from what I know of steel (and other metals/alloys) behavior on a much, much smaller scale:

    When a tank has harder armor plate/castings, and it is hit by a high velocity projectile that perforates (or even almost perfs) the hull, is there spalling or 3-d fracturing of the interior of the armor into the crew compartment?

    What I’m wondering is if crew survivability could be improved by either case hardening the outside of the armor and leaving the interior soft(er) so it has less tendency to shatter off, or by making armor from a sandwich of layers, with an inner layer that isn’t hardened?

    1. Hognose Post author

      Very insightful comment/question, and it shows you have a deep understanding of hardness/toughness/strength/brittleness in the world of steel. Re: the question in the 2nd paragraph, YES. This is a serious problem and is why the long-rod penetrator can puncture steel deeper than it can penetrate. The phenomenon also led to the design of the HESH warhead mentioned above. The whole point of HESH is not to penetrate the armor at all, but to knock a “scab” of steel off the inside face of the armor, which then ricochets around to the detriment of the crew/equipment/ammo inside. I should have mentioned that like a shaped charge, HESH is somewhat velocity-independent. This “scab” factor led to the first use of composites in armor, as a removable “spall liner” inside crew and other penetration-sensitive compartments of armored fighting vehicles in the sixties (maybe the fifties).

      Re the 2 questions in your 3rd and final paragraph. Yes, what you describe is exactly the reasoning that led to FHA. The ductile inner 50mm (or whatever) of the plate backed up its hardened 4mm face. The downfall of FHA, apart from the difficulty of working the hardened face, was that it turns out that hardening to a specific Brinell hardness protects the armor against specific velocity projectiles, but is more vulnerable to a projectile whose velocity falls outside the range. The physics of armor does get very weird, and the deeper you go (conceptually, I mean) the weirder it gets. The ballistics paper is a good place to start digging.

      2nd implied question from last graph, yes, modern armor tends to be a composite of layers. RHA was the state of the art 70 years ago, and is still used for example in light vehicles. But tanks tend to have armor nowadays that incorporates different types of steel and also nonsteel components (i.e. ceramics). Exact composition of modern armors is closely held by the manufacturers and armies that use them. In addition to the shaped-charge threat we now have the Explosively Formed Penetrator threat, something we designed, put in peer-reviewed journals, never manufactured, and first met when we started getting blow’d up with the damnable things.

      1. neutrino_cannon

        Err, pretty sure the US did manufacture EFPs. The various flavors of sensor-fused munition (e.g. BLU-108) use them, and a few ATGMs do as well (e.g. BGM-71F).

      2. Bill K.

        It seems I had read somewhere that the ‘inner liner’ used to slow down and deflect spalling was Kevlar on some fairly modern tanks (Abrams M1A1?) Is this known?

        Secondly, what happens when the angle of the glacis becomes flatter than the included angle of the nose of an incoming projectile? Rather than the 50 degree angle of the T-34 above, if it were 75 degrees, and the nose of an incoming round deviated at 20 degrees from its central axis, it would seem to me that the shoulder, not the tip of the projectile would strike, almost guaranteeing a deflection regardless of underlying armor thickness, no? I’m thinking of how flat the upper works of the Merkava are here…

        1. Hognose Post author

          The effect of angled armor is one reason that, under a ballistic cap, some penetrators have a hard chine or edge.

          Merkava armor was state of the art once, but that was then. The Israelis have lost some tanks and they study each loss.

          Spall liners today are usually Kevlar or another aramid fiber. I’ve seen basalt-fiber composite used for this purpose too, and before aramids were common, glass fiber was used as the reinforcement. There are molded polyurethanes, and spray-ons (that’s what’s on the inside of SAPI plates, and it’s actually applied using the Line-X process, like a pickup truck bed liner.

          Here’s Dupont on Kevlar in spall liners.
          http://www.dupont.com/products-and-services/personal-protective-equipment/vehicle-armor/uses-and-applications/vehicle-armor.html

      3. Y.

        IIRC, the Polish 7.92mm anti-tank rifle also used the spalling effect obtained by firing a very high velocity lead cartridge at the tank..

  2. Scott

    Interesting article, as usual. Small typo you might want to fix: “The more *your* study armor penetration”.

    1. Hognose Post author

      Thanks. It was sooo late already, it didn’t get reread enough before we took it live. Hate that. We’ll fix the error.

      ETA — and we found two more errors when we went to do that. Thanks!

  3. Expat

    “Explosively Formed Penetrator”
    Is that the copper warheads that burn their way through? It always amazed me that molten copper could have that kind of affect on thick modern armor.
    With regards to AR 500 plate: Do you know if it is difficult to work with (cut, weld) like hardened steel is?

    1. neutrino_cannon

      Explosively Formed Penetrators are close kin of more garden variety shaped charges, but instead of forming a jet of metal, they form more a big, fast-moving blob. The fastest moving part of a shaped charge jet moves at mach 30 or so, while EFPs move at a positively crawling mach 6-7.

      For a given size of warhead, shaped charges can potentially penetrate much more than EFPs.

      The tradeoff is that EFPs are far less sensitive to standoff distance than shaped charges are. The very fastest part of a shaped charge jet is moving mach 30, but parts of it are moving only mach 10 or so. Since the whole thing isn’t moving the same speed, the jet stretches and eventually breaks apart. That’s part of why they’re so sensitive about standoff distance.

      An EFP’s penetrator blob stays more or less in one piece, which means you can detonate the things several meters away from the target and still be getting close to optimal penetration performance.

      Incidentally, the liners in most shaped charge jets aren’t molten; this has been confirmed for decades by ultra-fast x-ray photography. The copper in copper-lined shaped charges is quite hot, and my shallow understanding is that there are some rheological considerations; it’s under such an absurd about of pressure that it’s sort of acting like a fluid. However, it’s common knowledge in professional circles that copper jets are still in a solid phase. Journalists keep repeating the “molten jet of metal” or some even more ridiculous lines like “vaporized copper plasma” because they are journalists and indifferent to the truth.

      1. Kirk

        The biggest cause of the misconception about shaped charges and EFP devices somehow “burning” their way through the target is the misguided choice of the word “jet” to describe what the charge liner/EFP is doing as the explosion proceeds. I swear to God, this was one of my biggest irritants whenever I was doing training on these subjects, because the average Joe is just fixated on that whole “burning through” thing. Makes you want to scream, and tear out what’s left of your hair.

        What’s actually happening in a shaped charge is that the blast wave is taking the charge liner, turning it inside out, and then the concentric pressure wave is driving that thing through the target like some kind of high-speed nail. There isn’t really time for it to melt, there at the beginning of the action–The metal is just turned into what amounts to a fluid, and that fluid is kept rigid by the concentric blast wave that is driving the train. Same principle as the hurricane driving a straw through a tree–It’s all about the pressure wave.

        Too many people conceptualize what is going on as some kind of super-thermite device, burning through the target. This causes problems because they then don’t understand why the charges have to be set up a certain way, and why you have to pay such careful attention to things like stand-off, and making sure that the charge remains stable and properly oriented during the blast. Screw up that stuff, and you’ve got huge issues making it work. There’s nothing like hauling a metric s**t-load of 40-lb shaped charges in your ruck somewhere to hell and gone across the countryside, then watching your leadership screw up the placement and securing of the damn things, subsequently leading to the enraging consequence of having to dig the charge holes by hand in a tearing hurry with an e-tool. If you’ve got no reference for trying to get through your typical gravel road surface with an e-tool, just take my word for it: It’s a bitch.

        There came a time in my young life as a combat engineer where I realized I had to learn all this esoteric crap, if only in self-defense. If you don’t understand what you’re doing, life is extremely rough, and exponentially more work.

  4. Stefan van der Borght

    Oooh, WM has broached a good topic this time. Personal interest recently led me back to Bisalloy, a most unpleasant alloy to work, but with some interesting properties…it loves to work harden. Couple that with spaced, sloped armour techniques, spall liners, and analysis of immune zones based on the ballistics of the weapons likely to try to defeat it, and one can enhance certain scenarios to suprising levels of success (factoring in time, of course…).

    I wonder how accurate are the equivalents sometimes listed for composite armours in RHA…I guess there’s a certain amount of wishing/hoping, alchemy and lying thrown in. Personally, the avoid/evade and strike first options seem so much more attractive than having to rely on a plate of some sort; but, it’s nice to have that extra leeway if one has the option. We used to rib the armour guys that they drove around the battlefield in a mobile oven looking for someone to light it. Now, even the grunts wear vests and plates and plastic hats one can’t cook or wash in.

    So how about Krupp Cemented, and the other weird words one comes across. Dagnabbit, the box is open once more, and I’m gonna have to go look again. The superplasticity of ultra-high carbon steel at certain temps is an avenue I’d like to explore…mainly because it only requires iron, carbon and heat; but in what mixture? Still tending in the direction of “it’s the man, not the machine”, because the weapon always wins; and being a cowardly weakling I’d rather avoid my enemy’s strength and hit him where it really hurts with what puny power I can muster.

  5. ernie

    So then you inspired me to go reading on Chobham Armor.
    http://en.wikipedia.org/wiki/Chobham_armour

    And I came away with this tidbit:

    The concept of ceramic armour goes back to 1918, when Major Neville Monroe Hopkins discovered that a plate of ballistic steel was much more resistant to penetration if covered with a thin (1–2 millimetres) layer of enamel.[18][19]
    This bears further experimentation on my part using Krylon and a AR500 plate.

    1. whomever

      “his bears further experimentation on my part using Krylon …”

      I’m guessing that the enamel they are talking about is the glassy enamel used on cookware, not Krylon. Think of the blue/black speckled turkey roasting pan, but thicker.

    2. Kirk

      Here’s a quick hint: Jet formation/propagation seems to be affected by density variation between layers. You pick this up through observation of the varying efficiency with which the standard charges penetrate various targets.

      In other words, when you go to use a shaped charge against particular target types, the charge is more effective against homogenous ones than against ones that have a bunch of different layers whose density changes between each one.

      Example: Using a shaped charge on solid rock will get you a nice even hole. Same-same on clay, or some sorts of consistent topsoil. When you go to do a hole on a surfaced road, where there’s a layer of concrete over compacted gravel over sand, or something? The hole isn’t as deep, nor as clean. The absolute worst results come when you’re using the things on something like the glacial moraines around Fort Lewis, where the ground is basically a little topsoil over a mixed layer of small rocks and sand/soil. You take a 40-lb charge out there and set it up as scientifically as possible, and about all you’re going to accomplish is loosening the soil/rock matrix for digging. The individual rocks seem to act as deflectors, or something.

      I don’t know for sure what’s going on with a lot of the stuff I observed, but I’m going to suggest that the reduced effect from penetrating the various layers has to do with the varying densities bleeding off energy from the jet as it passes each one. If you go digging through that stuff, what seems to be happening is that there’s a bit of a pause at each layer where the energy has spread out along the top of the new layer, right before the jet penetrates the next one. I suspect that this is why the layered overhead cover which we put on top of fighting positions is more effective when it’s layered as opposed to simply being homogenous. You get better results from having, for example, a layer of wood, a layer of rock/gravel/soil, and then a layer of sand or pure soil than you get from having a more-or-less homogenous cover of simply soil and some timber/plywood.

      Can’t define it or explain it, but that’s what one observes when you go back and look at the results of things.

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