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Armor Scientific Forum HESH / HEP
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--Original Topic Post-- |
Author | Topic: HESH / HEP |
Ssnake Virtual Shiva Beast |
quote:From what I heard, that's right. |
Tyler Durden Mucho Scaree Cyclops |
Brought up issue of 120mm HESH against T-72’s in the old topic. So, can 120mm HESH stop a T-72 or not? I’ve always been under the impression that it couldn’t. |
Andrew Jaremkow Gunner |
Paul Lakowski wrote: "Bob Griffen told us that the training DS for the 120mm was steel , I wonder if the same is true for 105mm DS round?" It varies from model to model. Some are all steel, some are steel and aluminum. "I wonder what 25mm round couldn't penetrate the side armor when it was so thin?" If they were training rounds, as suggested, then you're probably looking at a hollow steel version of the HEI round. That should have rather limited performance. |
Paul Lakowski Gunner |
quote:
I wonder what 25mm round couldn't penetrate the side armor when it was so thin?On that note Sebastian Belos had a chance to measure the side armor on the tanks he was working on and he came up with different figures from the ones in the pic. As I recall they featured 20mm SHS outer plate 5-6cm airgap and 3 plates welded together for a total thickness of about 45mm . He was told these were VHS SHS & RHA in that order. I'm wondering how the same tank could have such different measurements.One of the LEO-2 tankers [Axl or Georg or Stephan] told us that the lower hull plates of there training tanks were only 20mm thick when the reported lower hull plate is 70mm thick. When pressed the answer he was given was that the plates had be worn away? Any comments would be welcomed. |
rixtrixau Crunchie |
Andrew + Paul The Leopard Turret display has now been removed.(I wonder why? . |
Paul Lakowski Gunner |
quote: No thats actually the way the pic came , and Richard Simmie reported to me that these were tests with 105 HESH rounds. |
Andrew Jaremkow Gunner |
Anyway, I think there are two hits here, one above and to the right of the Maple Leaf, and one to the right of the ammo door, right? What's surprising is that there doesn't seem to be any fragmentation scarring around the impacts, which I would have expected from a live round. Could these have been just training rounds, that penetrated by brute force? If there's only 47.5mm of steel total, in two thin plates, then an inert training round might be able to punch them without too much difficulty. |
Andrew Jaremkow Gunner |
HESH design considerations – 1964 HESH ammunition (or HEP in US Army terminology) has always been somewhat mysterious, compared to conventional armour piercing ammunition, both because its performance is hard to quantify, and because it seems to get little publicity in comparison to other ammunition types. Here are some HESH design considerations, further summarized from an American 1964 overview reflecting the state of the art of HESH engineering at that time. Please bear in mind that HESH shells were still considered relatively new at that point, and some development was still to follow. Function HESH rounds work by bringing a charge of high explosive into intimate contact with an armour plate and detonating it. Shock waves then travel through the plate at right angles to the surface, reflect off the back of the plate, reinforce each other, and cause the steel to rupture (spall) at the rear if the explosive charge was powerful enough. The spall detached by the HESH round is typically in the shape of a disc that is slightly wider than the explosive contact area, and travels between approximately 30 m/s and 240 m/s, depending on the quality and thickness of the armour, and the amount and type of explosive. Explosive The preferred explosives are those with high detonating velocities, and the best results are obtained when the explosive can squash into the shape of a flat cone. To do this the explosive cannot be crumbly, but must have a soft putty-like consistency. Composition A-3 (91RDX-9Wax) proved the best, with Composition C-4 (91RDX-9Polyisobutylene binder) coming second. Cast explosives such as TNT or Composition B (39TNT-60RDX-1Wax) have the wrong properties, and do not provide a HESH effect. Unfortunately, Composition A-3 and C-4 need to be press-loaded, which is more time-consuming and expensive than casting the explosive. Construction HESH rounds need a very thin soft nose and thin walls, to allow the explosive to deform and come into proper contact with the armour plate. An annealed steel nose was found to work best, better even than softer copper, which was a surprise. The preferred nose shape was an ogive, which provided a greater contact area on impact than shorter hemispherical noses. Thinner nose material was found superior to thicker material, but this was limited by the need to withstand the pressures during the explosive’s press-loading process. The thin walls are weak, and therefore difficult to launch at high velocity. They also cannot withstand the forces that are normally used to press on a driving band during manufacture, causing the designers to resort to welded overlay driving bands, which had previously been restricted to recoilless rifle rounds. The thin walls are also light, and thus have a low rotational moment of inertia, which makes the shell difficult to spin stabilize. This was overcome by the use of a blunt-nosed ogive shape, which drag stabilizes the projectile. However, the lightweight shell body and blunt high drag design mean the round has a poor ballistic coefficient, and slows down rapidly. Nonetheless, the shells were considered at least as accurate as HE shells of equivalent caliber. Initial British HESH shells were of two-piece construction (nose cap and body), and the Americans copied this design, until they developed a one-piece shell body. This lowered production costs, and allowed an increase in muzzle velocity (presumably due to the removal of the weak joint). Fuzing Choosing proper fuzing delay is important for HESH rounds, because the explosive must have time to deform and squash against the armour before it is detonated. Optimal delay time is less for oblique impacts than it is for vertical impacts, and will obviously vary with impact velocity. (This is presumably part of the reason for narrow range of effective velocities.) However, the engineers admit “Beyond that, not much is known about HEP fuzing requirements.” Performance HESH rounds will generally defeat steel tank armour of about 1.2 calibers in thickness, between the angles of 0 and 60 degrees. Since the shockwave from a detonating HESH round is transmitted approximately at right angles to the armour plate’s surface, the increased line-of-sight thickness presented by angled armour is not a factor, and spalling can be accomplished where the LOS thickness is greater than 1.2 calibers. However, the efficiency of a HESH round is greatest when it strikes at right angles to the armor plate, and in oblique impacts it is not quite as effective, so angle cannot be entirely ignored. The performance of HESH rounds depends on impact velocity, and there is a rather small range of velocities at which the rounds will function properly. HESH rounds need to impact between roughly 300 and 600 m/s to be effective. Rounds that are too fast will deflagrate prematurely (i.e., have a low order detonation) due to the force of the impact. (This seems to have been slightly mitigated by the use of an inert bitumen filler in the nose, which protected the explosive from the shock of impact. Thus the M393 HEP-T round has a muzzle velocity of 732 m/s.) Those that are too slow won’t come into proper contact with the armour. The minimum velocity requirement was seen as a serious handicap for lightweight low-velocity recoilless launchers. As of 1964, attempts were being made to widen the range of effective impact velocities by developing new fuzes. The British were reported to have done “a considerable amount of firing” against spaced targets, and had found that skirting plates could render HESH rounds ineffective, by preventing the shockwave from reaching the main plate. They also found that a spaced armour configuration of a layer of sponge rubber between the outer plate and the main armour could render HESH shells ineffective. (This seems to contradict the reported penetration of a spaced Leopard 1 turret by HESH. Paul, could you maybe send me a copy of that photo, so I can post it up?) One side effect of the extremely thin shell walls was that they were considered to provide a valuable secondary fragmentation effect against armoured vehicles, apparently because they would break up into many small high velocity fragments, unlike thicker walls, which would make fewer slower fragments. (I guess these would strip off external features, damage optics, etc., as well as being very effective against soft targets.) |
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