The validity of RHA equivalent

[DavidDCM]

Short question, I got one quick example (numbers are completely made up):

 

Round A can penetrate 1000 millimetres rolled homogenous armor.

Round B can penetrate 1100 millimetres RHA.

But:

Round A succeeds to penetrate the armor array of Tank XYZ

Round B fails to penetrate the armor array of Tank XYZ

 

→ Could such a result actually happen in reality? That a round which is weaker on paper comes out to be stronger against a particular, "real" armour array? If so, then what does that say about the validity of "RHA equivalent" armour values? Isn't that a flawed concept nowadays?

 

Note:

We assume that no luck is involved, let's just say the shots were fired under controlled test conditions (Each round was fired against a new and undamaged target. Distance, temperature, wind etc. was all the same).

 

Thank you.

Edited December 3, 2011 by DavidDCM

[Lieste]

<br>Short question, I got one quick example (numbers are completely made up):<br><br>Round A can penetrate 1000 millimetres rolled homogenous armor.<br>Round B can penetrate 1100 millimetres RHA.<br>But:<br>Round A succeeds to penetrate the armor array of Tank XYZ<br>Round B fails to penetrate the armor array of Tank XYZ<br><br>→ Could such a result actually happen in reality? That a round which is weaker on paper comes out to be stronger against a particular, "real" armour array? If so, then what does that say about the validity of "RHA equivalent" armour values? Isn't that a flawed concept nowadays?<br><br>Note: <br>We assume that no luck is involved, let's just say the shots were fired under controlled test conditions (Each round was fired against a new and undamaged target. Distance, temperature, wind etc. was all the same).<br><br>Thank you.<br>

<br><br><br>Target material and arrangement have strong effects on the relative performance of rounds of different design or material quality.<br><br>Paraphrasing, Arrow posted that the NATO triple heavy was a tough target, not because it was very heavy, but because poor metallurgy in the penetrator would result in the round fragmenting and losing penetration within the array, while a good penetrator would pass through more or less as if it were a simple RHA target.<br><br>For 'similar' rounds attacking 'similar' arrays the comparison is still useful, and the use of RHAE allows fairly safe relative comparisons - and overmatch requirements for good probability of defeat.<br><br>For different attack mechanisms, and defeat arrangements, the results will not be directly comparable. eg HEAT and APHE rounds. <br><br><br><br> Edited December 3, 2011 by Lieste

[DavidDCM]

Thank you for your fast answer

 

So an armour value estimate can only be taken as a very vague hint, even if it comes from an educated source?

 

A statement like "Tank XYZ offers protection like 800mm RHA against KE" implies that every KE round that fails to penetrate 800mm RHA would also fail to defeat the tank, whereas a KE round that penetrates 800+ RHA would succeed. But isn't it far too simple to equalize a plain, monolithic and predictable target material like RHA with such complex arrays as are used today? Couldn't there be a KE round that penetrates more than 800 mm of plain RHA but would still fail against our tank, whereas another KE round might only manage to penetrate 750mm RHA but would still work succesful against this particular tank?

 

Of course, all KE/APFSDS rounds are broadly similar. Big darts of metal that hammer into the target at great speed. But even within this class isn't there already so much variation that any general statement like the one above is shaky at best and utterly useless at worst? Speed, length, mass, internal material composition can vary quite a bit between different types of KE ammunition, even from the same generation, am I correct?

[Lieste]

Thank you for your fast answer

 

So an armour value estimate can only be taken as a very vague hint, even if it comes from an educated source?

 

A statement like "Tank XYZ offers protection like 800mm RHA against KE" implies that every KE round that fails to penetrate 800mm RHA would also fail to defeat the tank, whereas a KE round that penetrates 800+ RHA would succeed. But isn't it far too simple to equalize a plain, monolithic and predictable target material like RHA with such complex arrays as are used today? Couldn't there be a KE round that penetrates more than 800 mm of plain RHA but would still fail against our tank, whereas another KE round might only manage to penetrate 750mm RHA but would still work succesful against this particular tank?

 

Of course, all KE/APFSDS rounds are broadly similar. Big darts of metal that hammer into the target at great speed. But even within this class isn't there already so much variation that any general statement like the one above is shaky at best and utterly useless at worst? Speed, length, mass, internal material composition can vary quite a bit between different types of KE ammunition, even from the same generation, am I correct?

 

Yes.

 

I suspect that a lot of the public data are based on deliberately misleading publications like the frankly awful General Dynamics efforts - where by care(less?)ful selection of axes they suggest a 75% improvement in performance, (IIRC it was penetration at range) where the approximate (but fairly accurate) performance was more like 92% retained vs 89%

 

I am always suspicious of any claim of more than a 2-3% improvement without a tangible increase in the obvious parameters that drive performance of similar magnitude (eg a long rod with 20% more length - I'd be reasonably happy that 22% improvment in performance is possible... but claims of 30%+ are implausible, even with the modest changes in velocity that are seen from time to time.

 

As armour estimates are at least partially 'fudged' to align with 'known' penetrator performance (and vice versa) the tendency to 'round everything up' and to slightly over-egg the biggest and best, leads to rapidly escalating public consumption/amateur estimates (IMO). I'd note that my own estimates for long-rod penetration using the Odermatt equation are all much lower than commonly quoted values for rounds where dimensions are reliable. With this in mind, I'd also reject any estimate of armour which can be penetrated using current weapons of above ~700mm, and be sceptical of anything above 650mm - of course if it cannot be penetrated then it theoretically could take any value you like - but values near 1000mm seem improbable and unnecessary to me.

 

At least part of the 'problem' might be treating the penetration as invariant and modifying the armour effectiveness or 'value' - I'd prefer an invariant armour value based on mass, with the modifier applied to the penetrator efficiency/effectiveness in response to the array materials and arrangement.

 

EG a heavy spaced array might be ~300mm, but reduce penetration of 'an APFSDS capable 600mm of RHA and marginal penetration of the array' by 50% - In my mind this better represents the relative merits better than

a heavy spaced array of ~600mm effectiveness being marginally penetrated by 'an APFSDS with penetration ~600mm'

 

YMMV, and I might have misinterpreted some of the literature and/or misremembered.

[Mobius]

You also would have to factor in country and year. Different penetration criteria for different country and what year. It seems that armor base quality has changed over the years. What was RHA armor in 1960 is not the same RHA in 1990.

[Guest Jason L]

You have to remember that the RHAe empirical system is based around trying to reconcile the thickness/mass equivalency of various designs and materials in a manner which allows for simple direct comparison. In that respect it's a very good system if the estimations are done intelligently. The caveat of course is that you are trying to distill a huge amount of physics and material science into a very, very "compact" simplification and so you're bound to lose accuracy.

 

Good estimates are pretty good, but the stuff in the public domain is obviously highly variable and the methologies used behind closed doors to get Te and Tm efficiencies are very different than the amateur/public consumption stuff mostly because having actual data points makes a huge difference. Stuff in the public lit tends to be more scienc-y too and configured as such rather than just trying to provide empirical databases of results.

 

Another thing to remember is that penetration has some statistical dispersion and the penetration criteria is often 50% or some other % for some particular penetration configuration, which means that protection /= proof which often gets conflated. I think this last part answers your question; ie if the penetrator doesn't overmatch the target significantly it is possible for something rated at 1100 mm, to fail to penetrate a composite array rated to 1100, especially when you get into the details of penetrator-target interactions.

[TTK Ciar]

Agreeing with everything posted thusfar. RHA equivalence is handy and "good enough" for getting a ballpark result, but actual penetration performance is highly dependent on both armor and penetrator characteristics.

 

For instance, a 7.0kg rod moving at 1600 m/s is going to penetrate a marginally greater thickness of steel than a 5.8kg rod moving at 1700 m/s, but the lighter faster penetrator is less vulnerable to the dwell effect than the heavier slower one, so the 5.8kg rod would penetrate a greater RHA equivalent of a dwell-incorporating armor system than the 7.0kg rod. There are also different optimal matchups between the hardness of the armor material and the shape of the penetrator tip.

 

A good engineer will use parametrics first to figure out a good system based on RHAe, then book time on big iron for hydrocode simulations and use the results to justify the expense of an empirical test, then build a real system and test it under laboratory conditions to find out what really happens (which often deviates significantly from the hydrocode simulation). If all goes well, the system finds its way into the field, where trials under less-controlled conditions might reveal hidden complications impacting actual real-life performance. If these complications aren't too bad, the system gets acquired and dumped through the logistics meatgrinder (shipped across the sea, bounced around in the back of a truck, left crated in a desert or swamp for a few months, then finally carried on a vehicle or infantryman's back through rain and snow and sun and mud and sand, perhaps get shot or used as a prytool to get a stubborn MRE to open) and used under all manner of wacky combat conditions, by people who may or may not be well-trained in its use.

 

It's guesswork all the way down, but RHAe estimation is an important guess in the guessing process.

[Guest Jason L]

You can do a lot of hydrocode stuff on a state of the art personal computer now because the geometry is so simple and so you get convergence with relatively few nodes in 2D.

[Ssnake]

RHAe figures are usually just meant for a quick orientation. Even if broken down into smaller segments (e.g. turret front, side, top, rear, and the same for the hull) it doesn't say anything about ballistic windows (particularly weak or strong spots). Tank XYZ having "1100mm RHAe" protection frontally is per se a meaningless buzz factoid. It would probably be the best value in a particularly strong spot, assumes a frontal and near-horizontal angle impact case, and is probably valid only against a specific type of ammunition.

Just to give a very simplistic example: Every MBT has a large caliber main gun. Obviously the gun barrel itself is an opening in the frontal silhouette, therefore the protection level in this particular spot is limited to the thickness of the breech block (if closed), no matter what other figure may be floating around.

Of course it is very, very unlikely to receive a hit in this particular spot - but that's quite the point: The RHA figures are usually given for the best case, or the most likely one, which leaves ample room for ... deviations ... in other areas.

 

RHA figures are popular because

a) they are popular, and

they allow for quick orientation without having to look very long into the details

 

Anything that is really meaningful is classified anyway. One can make certain more or less educated guesses, but the error margins usually are substantial. Take them always with a big grain of salt ... unless someone can document how he arrived at a specific figure, and under which circumstances the figure is valid.

[istvan47]

What are the tecnical caracteristics of RHA (WWII-'60s-modern)?

 

How costly is the testing vs such amount of RHA? I read here that many tests were made vs carrier's deck plates (after demolition), surely the RHA was not the best available today.

[TTK Ciar]

What are the tecnical caracteristics of RHA (WWII-'60s-modern)?

In an American context, RHA is a catch-all term for any steel formulation complying with specification MIL-A-12560. RHA is also used very broadly to refer to steel armor with similar characteristics (Brinell Hardness Number between 330 and 450, and moderately high ductility).

 

Armox 370T is one example of a product which complies with MIL-A-12560. Here is a brochure describing its characteristics. In laboratories, AISI-4340 steel is often used as an approximation for RHA, because they are very similar (and commodity AISI-4340 is relatively cheap), but not all AISI-4340 products are strictly MIL-A-12560 compliant.

 

How costly is the testing vs such amount of RHA? I read here that many tests were made vs carrier's deck plates (after demolition), surely the RHA was not the best available today.

Well, there's not much wiggle-room for "better" RHA, since steels with much "better" characteristics will fall outside of the range specified by MIL-A-12560. There are better steels for armor which are not MIL-A-12560 compliant, but compliant steels are often preferred so that their behavior, capabilities, and failure modes are predictable. Typical is to make a vehicle's chassis from RHA, and add armor to this which incorporates non-RHA steel (or aluminum, or titanium, etc) components.

[DavidDCM]

Thank you all for your input. Basically your answers confirmed what I already thought about RHA equivalent, but you guys put it into much better words than I would have been able to.

[istvan47]

In an American context, RHA is a catch-all term for any steel formulation complying with specification MIL-A-12560. RHA is also used very broadly to refer to steel armor with similar characteristics (Brinell Hardness Number between 330 and 450, and moderately high ductility).

 

Armox 370T is one example of a product which complies with MIL-A-12560. Here is a brochure describing its characteristics. In laboratories, AISI-4340 steel is often used as an approximation for RHA, because they are very similar (and commodity AISI-4340 is relatively cheap), but not all AISI-4340 products are strictly MIL-A-12560 compliant.

 

 

Well, there's not much wiggle-room for "better" RHA, since steels with much "better" characteristics will fall outside of the range specified by MIL-A-12560. There are better steels for armor which are not MIL-A-12560 compliant, but compliant steels are often preferred so that their behavior, capabilities, and failure modes are predictable. Typical is to make a vehicle's chassis from RHA, and add armor to this which incorporates non-RHA steel (or aluminum, or titanium, etc) components.

 

A- but so, the average RHA would have been around? 200-250 BHN? What kind of difference is expected if the RHA is, let's say, 280 BHA (example, the T-54 vs M-47)?

 

B - when it is claimed '1000 RHA penetration', then they really tested the shells/missiles vs a 1 m RHA? Or it was usually a calculation? I saw some 'sketchs' with AT missiles and the section of armour they penetrated with the 'tunnel' deep in the steel (a tunnel as long as the missiles displayed near there).

[TTK Ciar]

A- but so, the average RHA would have been around? 200-250 BHN? What kind of difference is expected if the RHA is, let's say, 280 BHA (example, the T-54 vs M-47)?

"The average RHA" is a bit vague to have a good answer. It depends on the time period and differs from country to country. Mild steel is only about 170 BHN, so that's about the worst to be expected. Above about 500 BHN most formulations get pretty brittle, and acceptably ductile-yet-hard formulations start to get expensive to manufacture and process. Harder steels than this are used in armor, but usually limited to a component of a greater system with a special purpose, to produce a specific effect.

 

The rule of thumb for eRHA of steel of a given hardness within a range of about 170-500 BHN is the ratio of the square roots of the hardnesses of the steel and RHA. Thus if RHA is assumed to have a hardness of 350BHN, then a 280 BHN steel would have a thickness efficiency of sqrt(280)/sqrt(350) = 0.9 or so.

 

B - when it is claimed '1000 RHA penetration', then they really tested the shells/missiles vs a 1 m RHA? Or it was usually a calculation? I saw some 'sketchs' with AT missiles and the section of armour they penetrated with the 'tunnel' deep in the steel (a tunnel as long as the missiles displayed near there).

It depends on the vendor, and how honest (or desperate) they are. Showing a tunnel like that is primarily a marketing gimmick, so maybe they tested it against a 1m slab of MIL-A-12560 compliant armor steel, or maybe they tested it against a 1m slab of AISI-4340, or maybe they tested it against a 1m slab of mild steel.

 

I really don't know how many base their marketing numbers on empirical tests vs hydrocode vs parametric. For most sources it's impossible to tell. The rules for estimating RHA penetration from shaped charge cone diameter and standoff distance are pretty well-known by now. I've been guilty myself of slapping a ruler against the screen on a zoomed-in image, making some assumptions, and writing down a DOP.

 

-- TTK

[EchoFiveMike]

A ruler? For shame, where's your dial calipers? S/F....Ken M

[TTK Ciar]

A ruler? For shame, where's your dial calipers? S/F....Ken M

They're never far :-) for when there's something to physically clamp them against