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HESH for Dummies

By edJanuary 25, 2016No Comments HESH FOR DUMMIES PART 1 – BASIC THEORY OF HESH BY VOLLKETTEN WITH SIGNIFICANT HELP Having spent a lot of time collating real life HESH and HEP data (probably far too long) from available public sources of primary evidence — such as real life firing trials and tests Vollketten was asked to spend some time on this article to try and explain what HESH/HEP is, how it works, why it works and why some countries are so attached to it. Part 1: Basic Theory of HESH Part 2: Testing HESH against armour Part 3: HESH vs Buildings Part 4: Modelling HESH (includes sources list) What is it? – Basic theory of HESH ‘HESH’ – High Explosive Squash Head and ‘HEP’ – High Explosive Plastic are essentially the same thing with different names. The shells are characterised by a thin walled shell containing soft explosive filler such as TNT, RDX, RDX/PWX, PE, RDX/TNT, PE3A, Composition A3, Composition A4, Composition B, or Octol. I’m sure more filler combinations have been tried but these are the ones listed from primary documents. (see part 4 for fillers explained) The explosive within the shell should be of a low sensitivity type so the shockwaves just from impacting the target won’t set it off. This is why something like PETN is not used as it is too sensitive and would cause the round to explode prematurely. Imagine a HESH shell like a large lump of soft clay thrown at a wall to get an image of how it will ‘splat’ on a surface. That brings us to two issues; padding and fuzing. To help protect against the severe shock of impacts HESH shells require cushioning around the nose which is usually TAR. This assists the explosive content in forming the ‘splat’ or “Pancake” on the armour. If the shell hits at too flat of an angle such as perpendicular to the target the forces may be too severe of the shell causing it to break up without even exploding or otherwise exploding without forming a sufficient sized splat first. In a nutshell HESH works at its worse flat onto a target. The fuze for the HESH shell is critical. US trials of HEP rounds revealed that too long of a delay and the force of the impact will spread the splat too thinly or have it peeling off before it explodes. Too sensitive and it will explode before the splat is properly formed. The shell also has to travel slowly enough so that the shell doesn’t just break up on impact. A maximum impact velocity was found by tests to be in the region of 800 meters per second. So now our shell has hit the target and splatted correctly, spreading out in a pancake in contact with the surface, the fuze causes the shell to explode. The next part is complicated, as the shock-waves from the shell expand just milliseconds after hitting they travel through the material until they reach the other side (usually the inside) of the armor plate. These are compressive waves and upon meeting the opposite side of the armour are partially reflected back as a tension wave. In the image below see the shockwaves travel through the plate and then reflect back. Shockwave propagation from HESH detonation on angled plate (Note: that this is different to the first graphic on the first page where the scab formed was in line with the shell through the line of sight thickness whereas it is in fact perpendicular to the outside armour face) As the shell hits the target one of several things can happen: 1) The shell strikes at such as extreme angle that it breaks up without detonating or so hard or so fast that the shock of impact break ups the shell (we will revisit angle of impact later on) 2) The armour (we’re only talking about solid steel just at the moment) is so thick that it absorbs all of the energy of the shell and the shockwaves run out of energy. (As you will see later on this will have to be very thick indeed compared to the caliber of HESH round but it is possible) 3 ) The armour absorbs nearly all of the energy but the compressive shockwave is insufficient to overcome the elasticity of the inner surface of the steel so the inside bulges inwards. (although it may actually be split internally such as shown in the image below) This is a nice cross section to show this bulging effect caused by a compressive shockwave (albeit not from HESH in this example) You can see the actual shape of the waveform causing that bulge which has separated inside the armour plate but not split or fractured. The armour is at this point significantly weakened at this location even though the scab has not come off. 4) The forces of the rebounding wave are too severe for the inner steel surface to cope with, but not quite enough to cause a complete scab separation to occur. This is the ‘cracked bulge’ point. Basically a boundary point where some small amount of spalling from this cracking could be expected but also where a spall liner would provide adequate protection. 5 ) The shearing forces are too severe for the inner surface to cope with and a scab forms around the shockwave point of reflection. (Basically an angled strike should produce more of an ovaloid splat on the outside and a corresponding ovaloid scab from the interior.) Unable to cope with the forces the inner steel scabs off from the surface at very high speed. As an example of the damage you could expect here, picture how much space there is inside a tank, even a ‘roomy’ one. Compare this to the known performance of the 90mm T142E3 HEP shell containing 6.8kg of Composition A3. It hits the exterior of a target which is over 100mm thick at a 30 degree angle (from horizontal, therefore effective thickness of over 200mm for AP and HEAT rounds). Now spray the inside with some small supersonic steel shrapnel and a completely detached scab of steel measuring over 200mm (8 inches) in diameter weighing over 4kg (8.8lbs) rattling around as well. Even a good spall liner is only going to be partially effective in stopping smaller pieces, that huge fragment at supersonic speed is going to be devastating to the crew and modules. On the outside however all that may be visible is a dent along with shattered fittings. Dented exterior of M26 Pershing following test impact by HEP shell (white arrow) 1970 Test plate showing scabbing effect from an angled impact. HESH scab recovered from test range on 1cm2 paper. 6 ) One final possible outcome of a HESH shell strike is that the impact of the shell is sufficiently powerful to completely rupture the armour entirely and explode inside the crew compartment. Being thinly walled, HESH shells penetrate less than a regular thick walled HE shell but the effect of exploding inside a vehicle would be the same. Fatal. Turret side reinforced with applique armour As a final note regarding ‘penetration’ it is very hard to distinguish between penetration caused by the explosion bursting the plate and by the force of impact of the shell passing through the armour and still maintaining its structure. There is very little spaced armour testing data. That’s the basic theory and effects of HESH, the following parts will continue with showing you how well HESH performs against tanks and buildings and how HESH shell behavior may be predictable or calculated. HESH FOR DUMMIES – TESTING HESH AGAINST TANKS PART 2 Complete penetration of turret armour by 105mm HESH round Part 1: Basic Theory of HESH Part 2: Testing HESH against armour Part 3: HESH vs Buildings Part 4: Modelling HESH (includes sources list) So having looked at all 6 possibilities of what happens when your vehicle gets hit with HESH in the first part of this series we should cover the next commonly mentioned thing when people talk HESH; the effect of spaced armour. There is limited data available on the effect of spaced armour on the functioning of HESH shells with conflicting results. For example the T142E3 HEP shell (technically HEP-T) will completely penetrate a 12.7mm thick mild steel plate pass through 152mm of air gap and effectively spall a 100mm plate behind it at normal angle (a perpendicular impact), but will not spall exactly the same plate when angled 30 degrees to horizontal. (LoS thickness therefore 25mm). It appears in that case that the 25mm outer plate was sufficient to break up the incoming HEP round as LoS thickness does affect the physical shell as it has more material to pass through. Tests were carried out however against the skirting plates of the Chieftain tank using one of four types; 6mm steel, 6.35mm alloy, 9.52mm alloy, and 12.7mm alloy. These tests showed that versus a 120mm HESH shell that the 9.52mm alloy plate was marginal and the shell could still penetrate those skirts and impact the vehicle side hull although with reduced effectiveness. The 6mm steel and 12.7mm alloy plate were both successful in preventing the 120mm HESH shell from causing any damage to the 40mm thick side of the Chieftain tank. (Note here that for this test the suspension components were removed) It’s worth noting though that on one firing versus a 6.35mm alloy plate the HESH shell actually broke up and did not function at all even though it ripped a large hole in the plate. So spaced armour does have some effect of HESH shells, but how much is complex. Take for example this HESH firing trial versus the mantlet of an old Comet tank. The white cross denotes the target point for the shell and in the second image the mantlet has been struck dead on by the shell. On this occasion the shell has still managed to spall the turret on the inside as the force of impact has pushed the mantlet back onto the turret armour and allowed the shockwaves to travel through causing the spalling. For spaced armour to be effective versus HESH then it needs to be thick enough to cause the shell to break up or initiate prematurely which inevitably begs the question of stowage boxes. Here, a direct hit on the steel stowage box on the back of the Comet has had no effect in protecting the crew. The shell has penetrated the box, contacted the armour and exploded spalling the inside and blasting the back of the turret clean. A thin mild steel box would need to made thicker to have any benefit. Up to 14mm thick for mild steel per some later British studies on the subject. This would be consistent with the plate thickness found during those side skirt trials mentioned earlier. One further test of how well stowage boxes and external items this time done by the US in 1958 subjected an M48 tank to repeated hits from both 76mm and 120mm HEP rounds. The results found that overall the effectiveness of HESH/HEP rounds was reduced by about 20% by stowage and external items compared to decreasing the effect of HEAT, AP and HVAP by about 10%. As confusing or contradictory as that may seem German tests in the 1970’s showed that a 10mm thick plate 800mm off the target was sufficient to beat HESH/HEP shells. Striking this plate caused the fuze to trigger the shell prematurely as mentioned in part 1. Too insensitive of a fuze and the shell could simply penetrate a thin target and not explode or just break up instead. The UK created an experimental ‘Smart’ HESH round in the 1980’s which is capable of passing through spaced armour plates without being affected and thus negating those simple spaced armour arrays while still maintaining the effectiveness of HESH. The exact performance of the shell is still secret but suffice to say that this problem of spaced plates had been tackled to a degree. In summary then, HESH shells can penetrate some thin spaced armour and may be unaffected in the main by stowage and external fittings etc. If however the fuze is triggered without a splat having being formed on the main armour, the shell will not produce a scab and other fragments that will damages the inside of the tank. Although it will damage the local area. Spaced armour preventing HESH functioning on the main armour EFFECTS VERSUS COMPOSITE ARMOURS: So having seen the effects of HESH vs spaced armour the next question usually asked is what happens with regards to composite armours. Even just steel armours can be improved vs HESH by being backed with a softer ‘mild’ steel layer as it is more resistant to cracking and spalling than the harder outer layers but the effect is only going to protect against the small splinters which would otherwise be preventable with a spall liner anyway. The outer hardness of the armour tested seems to have no noticeable effect on HESH performance at all. Obviously multiple spaced layers will have a cumulative effect and where fuel tanks are used in the front of some tanks behind spaced armour it would be fair to assume that even should the outside spacing be penetrated that the armoured fuel tank and contents would protect against the main HESH-effect. It would not however prevent the exterior damage consistent with a High Explosive shell being detonated on the armour. Being thin walled HESH actually produces more small high velocity fragments than a standard HE shell and is more effective against infantry at when exploding in close proximity, it also has more effect on fittings and optical devices etc as more fragments can cause more crippling damage to exterior modules. In terms of contacting a laminated armour those results are unavailable to be shared publicly at this time but is very effective in dissipating the shockwaves and combined with modern spall liners the main effect of the HESH round will be negated. So if HESH even ‘Smart’ HESH is going to be rendered ineffective against modern tanks why is it still used? For tanks there’s the advantage of only having to carry two types of round with APFSDS for the antitank work and HESH for everything else, there’s the ability get a lot of firepower into a smaller gun providing fire support for troops with some anti-tank ability combined. Near miss outcome at front of light vehicle – result- Mobility kill. Direct hit on wheel – result – Mobility damaged Direct hit on track – result – Mobility kill Direct hit on lower track – result – Mobility kill That’s it for Part 2, the next part will continue the testing of HESH to include the effect against buildings. HESH FOR DUMMIES PART 3 – HESH VS BUILDINGS 120mm thick Armour test plate at the Royal Military College of Science at Shrivenham showing the large scabs detached by HESH strikes Part 1: Basic Theory of HESH Part 2: Testing HESH against armour Part 3: HESH vs Buildings Part 4: Modelling HESH (includes sources list) In part 2 we looked at some testing against spaced armour and the effects against tanks. Obviously main battle tanks are used for more than just tank vs tank action and are very important for supporting the infantry. As such, performance against buildings and fortifications has to be considered. HESH shells have thinner walls than regular HE shells and when exploding throw out a large amount of small high velocity fragments which makes them actually better against unprotected infantry than standard HE. It is also very useful for making holes in buildings, too, big enough for troops to enter. Reinforced concrete wall hit with 105mm M39 Based on a 50 foot wide street (tank in blue) the angle and range for being able to create entry holes using HEP-T shell This hole breaching is effective even at extremely sharp angles up to 80 degrees allowing for a tank in a street to breach a building on the roadside in front of it. Even at these sharp angles (that particular test found the limit to be 85 degrees where the shell would ‘bounce’ off the wall without functioning) These images are from a US test from 2002 and demonstrate perfectly the best use for HESH. It’s at least as good as HE shells versus soft targets, vehicles and troops as HE but also has this high angle ability. This is not obviously limited to buildings but is of particular value against vehicles too. Take the below example of a steep angle strike on the turret of a tank. An APDS or even APFSDS round may ricochet at an acute angle where the HESH round will still function. HESH flies more slowly and therefore has a greater ballistic arc meaning it is more likely to strike higher up on a tank above the composite armour and where most of the sensitive items like optical equipment is fitted. In a nutshell it’s better than HE against tanks, it’s as useful as HE against troops and light targets and it’s very very useful for cutting mouseholes in buildings. As a low velocity shell it can also be fired from relatively small vehicle like the Scorpion CVRT (76mm gun) giving it a very powerful potential to do potentially crippling damage to even more modern contemporaries and able to actually knock out older generation Soviet tanks. Acute angle hit on curved surface – note that the wave propagation is perpendicular to the face of the armour and not in the direction of the shell as seen in the very first graphic. So these tests are all well and good for the HESH shells most people may have heard of. The 76mm HESH used in vehicles like the British Scorpion, 105mm or 120mm but what else is there? Well as a result of research I have actually found HESH/HEP rounds in various forms in calibres as small as 40mm and as large as the 233.6mm (9.2”) British (the largest US HEP round I can find is a mere 175mm calibre) HESH demolition gun. The smaller 165mm HESH round for demolition work is well known and this requirement dates all the way back to the need to have a gun which could knock out enemy bunkers. The old design requirement being based on destruction of a 7’ thick reinforced concrete bunker. There is of course the rather terrifying 183mm gun from the FV4005 which as a tank destroyer is a truly scary thing as even if the shell didn’t cause scabbing of the armour inside, the blast would wreck the whole area around the impact and damage or dislodge interior items or fittings. For reference the 165mm HESH round weighs in the region of 18kg, the 183mm HESH round about 20kg and the 9.2” round over 43kg….. If people ever ask what the west could have developed to take on the last Soviet heavy tanks like the IS-7 – HESH firing vehicles like the FV4005 were it but are in truth overkill, a hit anywhere across the tank is at a minimum seriously damaging. A shame therefore that it was never developed further but the 120mm HESH round would have sufficed so such large HESH guns were really only needed for demolition work. Anyway, that’s it for Part 3, Part 4 will concluded this short series with information for the modelling of HESH and HEP shells. HESH FOR DUMMIES PART 4: MODELLING HESH PERFORMANCE 76mm HESH tested against an old T-34 tank (the yellow rectangle is a matchbox for scale) Part 1: Basic Theory of HESH Part 2: Testing HESH against armour Part 3: HESH vs Buildings Part 4: Modelling HESH (includes sources list) So having looked at all this HESH stuff you may be interested in how the performance of HESH can be modelled. To do this I took every scrap and piece of firing trials data and charted it all. Once this was charted I made a 3 dimensional graph of the data points plotting shell calibre against angle and the depth of the effect the shell would cause. Assumptions used in modelling HESH performance: No account was taken on fuze type – insufficient dataNo account was taken on filler type – insufficient data although some information relating to fillers and their contribution was found. (a list of fillers can be found with the source material to use as reference)Any result ending with a bulge or crack would be classed as the limit of effect.Any result of a separated bulge where the crew would be affected would be classed as successfully ‘effect’.Where a shell breaks up or doesn’t function no effect or data point was plotted.Spaced armour results were not plotted First data plot for this information was then grouped as angles at 90 degree (from horizontal, i.e. vertical) and 30 degrees (from horizontal, i.e. very steeply sloped) as calibre of HESH round against effective depth. This is NOT the NATO standard way of explaining the angles just to be clear. Note that the effective depth here is to equate it to AP and HEAT performance. HESH waves travel perpendicular to the exterior angle of the armour and NOT line of sight. So this is equivalent effect. This graph shows in red the predicted complete penetration a HESH or HEP shell will achieve in an armored plate or spaced armour. The orange part is the effect against a vertical slope and is noticeably worse than the effect against a steeply inclined target. That’s right- HESH works significantly better at steeper angles than against a vertical plate. You’ll also notice a disproportionately effective area at about 100mm calibre where this effect is most prominent. I don’t have a good explanation for why that may be except that it is exactly what the available data shows. This graph is at first glance more confusing but shows the relationship between the calibre of the shell (x – axis) and the calibre to effect ratio (y-axis). The calibre to effect ratio is simply explained as a 50mm HESH round can produce the desired HESH effect at depths of up to 1.0 x its calibre at 90 degrees; in other words 50mm, whereas a 100mm calibre shell shows a maximum effect at 90 degrees of about 1.5 x the calibre which is about 150mm. In fact the relationship between calibre and the depth of effect those HESH/HEP shells can produce is roughly linear across all calibres at 90 degrees of impact. The most interesting thing though is that effect at 30 degrees; the really steep angled impacts. There’s a huge spike at around the 100mm calibre mark where some HESH shells have achieved effects up to 3.5 x their calibre. My only explanation for this would be the huge amount of design effort put into 105mm shells in the 1950’s which seem to have reached a pinnacle in this type of shell. The problem though is that the graph is too simple. For the number of shells achieving those 3 to 3.5x calibre effect depths there were a large number which would consistently fail at just 2x their depth and the results in that whole region were very inconsistent. As a consequence I re-plotted these results to reflect consistent and inconsistent results and likewise I added those results for the 90 degree impacts where the shells failed to function as well (usually because they were impacting too fast) and what we get is this: Again, note that this is the effective depth to equate it to AP and HEAT performance. HESH waves travel perpendicular to the exterior angle of the armour and NOT line of sight. So this is equivalent effect. The one on the right is the re-graphed calibre to effect-ratio and the linear relationship previously seen for 90 degree impacts has ended and the highly angles impacts are now divided into consistent and inconsistent effects. So if you are looking at for example a 140mm HESH shell and want to predict what you should be able to expect performance wise from a 90 degree impact of about 2.5x the calibre which is about 210mm. For highly angled impacts based on the 30 degree results you have an estimate range of about 2.5 to 3.5 for consistent effects and up to 4x calibre for inconsistent effects. Potentially then a 140mm HESH shell per this model could scab a solid steel plate up to 560mm thick. Now look at the left hand graph. Here I have plotted all of the data points. The lowest part is the complete penetration of the shell through a plate so a 140mm HESH shell should be modelled as being able to penetrate up to about 75mm thick, consistently spall a steel plate at 90 degrees up to about 375mm thick LoS and scab plates at 30 degrees up to about 625mm thick LoS above which there is just no data on which to plot information. Normalisation: Obviously my graphs only go to 30 degree impacts as there was insufficient data points to make any conclusions from the data I had access to. From other reports however a few quotes of relevance on the subject: …”in the region of of slope of 10 degrees or less the round tends to separate or splatter upon impact” [degrees from horizontal] …”The impact angle of 20° to 90° had no influence on the spalling effect” [degrees from horizontal] …”HEP warheads are designed to defeat standard tank armour 1.2 calibers in thickness at angles of obliquity of 0 to 60 deg.” …”the area of displaced spall is usually slightly greater than the area of the charge in contact with the plate” …”Present HEP shells are shown to be capable of defeating about 1.3 calibers of plate regardless of obliquity…” So in summary then remembering that HESH shells DO NOT follow line of sight thickness here, those graphs show the equivalent effect to compare to AP and HEAT rounds. At the point of contact with the armour of the tank assuming the shell functions properly then multiply the calibre of the shell by 1.3 to see whether the shell will cause a behind armour effect or not. In the simplest possible terms then at the point of impact of the shell ignores the line of sight thickness so in this illustration where an APDS may bounce off the steeply sloping side armour of the turret HESH simply laughs at the slopes, splats and then draw a line from the centre of the splat directly through the armour. CONCLUSION: So there you go, that’s the sum total of over a year’s work finding, collating and plotting all available HESH/HEP data to try and find some kind of predictive model for how HESH works and how to model it properly. I know it looked like a lot of maths but really it was just laborious to plot these things. Of the big tank related games out there I can tell you that this information has already been made available to them to try and improve the modelling of HESH/HEP in game although any changes are yet to be forthcoming. What I have learned is a newfound respect for frankly what I considered to be a rather anachronistic British love of HESH and discovered that really it’s far better than expected. In a nutshell and to oversimplify things a little I would consider HESH shells as being able to consistently scab plates up to 1.3x their thickness (measured perpendicular to the point of impact NOT LoS) regardless of what angle they hit at up to 80 degrees and to give them about 10% less blast radius and penetration than the same calibre HE round. Finally a summary from a US document from 1951 on the subject. Transactions of Symposium on Shaped Charges Held at the Ballistic Research Laboratories, Aberdeen Proving Ground, Maryland 1951 FILLERS: KNOWN FILLERS USED IN HESH AND HEP SHELL TRIALS FOR REFERENCE PURPOSES: RDX – also known as Cyclonite, Hexogen (German/Russian), T4 (Italian) and RDX4 and was one of the first plastic explosives, widely used in WW2 and may have up to about 9% added beeswax as a plasticizer. By weight RDX is about 1.5x more powerful than TNT.Composition A3 – 91% RDX, 9% wax and as such can be used as the same name for the RDX filler as ‘RDX’ is.Composition A4 – 97% RDX, 3% waxComposition B – 60% RDX, 39% TNTOctol – 75% HMX, 25% TNT (HMX is High Melt Explosive ‘cyclotetramethylenetetranitranine’ also known as ‘Octogen’)PE – 88.3% RDX, 11.7% PE OilPE3A – a complex mix of 87.7% RDX with oils and other ingredients.RDX/TNT – mixture of RDX and TNT is a ratio from 60/40 to 40/60 respectivelyRDX/PWX is a 86/14 mix of RDX and Paraffin wax respectively although the ratio is different in some of the test data as 86/14 and as 88/12- SOURCES: FVRDE Report TR.7 Firing Trials with the 120mm Tank Gun in Conqueror FV214 using APDS/T and HES Shell Explosive Codes and Abbreviations Photographic Record of 76mm HESH Test Firing on T-34/85 Static Target ~1985 HESH Firing Targets Otterburn Moor, Bovington Tank Museum, and Fort Nelson, UK 105mm M393A2 Terminal Ballistic Performance Against Concrete Wall, April 2002 USAREUR Pamphlet ‘Military Operations – Tips for Tankers’, 525-1, 30th September 1970, HQ, US Army Europe & 7th Army The Impact of Armor on the design, utilization and survivability of ground vehicles, 2006 Gunfire Qualification test of model HRS-2 and -3 Helicopter self sealing fuel cell installation DEFE 15-326 Technical Report 8/52 – 1951 Engineering Design Handbook Recoilless Rifle Weapon Systems HEP Design 1961 Determination of the Time Interval Between Impact and Deflagration of 75MM T165E11 Composition A-3 HEP-T Shell Engineering Design Handbook: Ammunition Series, Section 6, Manufacture of Metallic Components of Artillery Ammunition ‘Materiali in allestimento presso il 12 Autogruppamento’ RARDE Memo. B 66/63 76mm HESH vs APC’s Janes Armour and Artillery 1985 Distribution of Armor of the M48 Medium Tank Test of shell HEP, T170E3 for 76mm gun T91 DEFE 15-326 Technical Report 8/52 – 1951 Effects of size and shape of inert pads on HEP shell performance Development of 90-mm Gun Tank, T49 Test of shell HEP T294E3 for 90mm rifle, T149 Bundeswehr firing trials result of HEP versus target plates Transactions of Symposium on Shaped Charges Held at the Ballistic Research Laboratories, Aberdeen Proving Ground, Maryland, 1951 Development of 105mm High Explosive Plastic shell M327, T81E28, 1957 Jump in Tank Guns Report 72015 Firing trial WT57/65(1) Appendix A Methodology for Dynamic Characterization of Fragmenting Warheads ADE Tech Report 8/47 726349 Recoilless Rifle Ammunition, 1971 OB Proc. Q 7987 OB Proc. Q 8218 WO194/463 WO 291-1416 Development of 120-mm Gun Tank, T57 Development of 155-mm Gun Tank, T58 Register 63/PP/40 The effect of System design characteristics on first round hitting probability of tank fired projectiles, 1959 Research Wing Report No.94 [Resurrection of "HESH for Dummies" post, part 1, text]

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