"Sabot and HESH." Topic

I assume the modern fin stabilised sabot rounds have solved the problem with accuracy of the spin stabilised 6 and 17pdr as APFSDS rounds are now the norm for tank on tank engagements. What was the problem with the early rounds?
With respect to HESH I understand the reason the UK stuck to rifled barrels rather then the more fashionable smooth bore was to enable the use of HESH, but why can't you fire HESH from a smooth bore?

For HESH, rifling is simply for accuracy – HESH warheads perform poorly when fired at very high velocities, so have a fairly modest propellant / low velocity, which if coupled with a smoothbore weapon compromises accuracy to an unacceptable degree.

I'm not certain on the APDS accuracy issue, but I'd imagine it was due to imperfect sabot separation – as the sabot leaves drop away from the penetrator, any contact between the two is going to affect the penetrator's trajectory, sometimes severely. With modern metallurgy and aerodynamics you can probably be sure of a near-perfect separation every time, but in 1944….

Can't you fin stabilise a HESH round with pop out fins then?

No idea on that one.

IIRC, the US calls HESH … HEP = High Explosive Plastic …

Yes, you could use fins to stabilize a HESH round, but you would not gain any benefit. HESH rounds are much heavier than a sabot or HEAT round, so have a far lower muzzle velocity. For the British L30 120 mm cannon, the APFSDS has a muzzle velocity of 1,534 meters per second, compared to the HESH round with a muzzle velocity of only 670 mps. To get the same range with HESH, the gun tube must be elevated significantly higher. The rifling provides excellent spin and accuracy, especially at long range. Technology has improved to the point where a HEAT round will detonate as a shape charge against armor, but as a fragmentation round against a soft target, so most countries abandoned the HESH round as redundant.

I assume the modern fin stabilised sabot rounds have solved the problem with accuracy of the spin stabilised 6 and 17pdr as APFSDS rounds are now the norm for tank on tank engagements. What was the problem with the early rounds?

Technical term is "sabot tipoff", where the sabot doesn't separate cleanly.

With respect to HESH I understand the reason the UK stuck to rifled barrels rather then the more fashionable smooth bore was to enable the use of HESH, but why can't you fire HESH from a smooth bore?

IIRC, HEAT rounds don't like being spun (didn't GIAT make a 105mm HEAT round that has a ball-bearing driving band?), as it somewhat de-focuses the shaped charge. So the HEAT rounds are better from smoothbores. And that doesn't count the improved fuses which have largely left HESH redundant as a main-gun round.

Note that the 165mm demolition gun is the only HESH thrower still in service, everyone else uses HEAT.

The British still have a HESH and WP for their 120 mm rifle.

I assume the modern fin stabilised sabot rounds have solved the problem with accuracy of the spin stabilised 6 and 17pdr as APFSDS rounds are now the norm for tank on tank engagements

The question jumps too far in one step for a clear answer.

Modern fin stabilization is not related to the accuracy problems of 6 and 17pdrs. The accuracy problems of the early British APDS rounds were resolved by later APDS rounds. The British 20pdr was known to be a VERY accurate gun firing APDS. So also the L7 105mm gun (M68 in US Army parlance) was a very accurate gun firing APDS. These were rifled guns firing spin-stabilized APDS rounds. They did just fine.

There was more than one contributing factor to early accuracy problems with APDS. 1st was "seperatation", as others have mentioned. But … the first rounds were a "pot" sabot design rather than the "petals" sabot. The pots never managed to separate cleanly and consistently. Petals worked better, but for consistent performance the manufacturing tolerances must be very tight. Also the 6pdr had a flash suppressor that interfered with clean separation, and the 17 pdr had a muzzle brake which was even worse. Later guns (like those mentioned above) did away with variances in muzzle shape, and performed better.

Also, early rifled guns had a spin-rate optimized for stabilizing full bore projectiles. Sub-caliber sabotted penetrations need to be spun at a different rate for optimal stability. It took time for militaries to recognize that the accuracy of their sabotted rounds was the guns' first priority, with minor variances in performance of HE or full-caliber AP rounds being an acceptable result of getting a really good APDS round.

The move to smooth-bore designs was done for other reasons. It is possible to make a sabotted round with significantly better penetration using a smooth bore design. There are 2 reasons. First, it is possible to get better velocityin a smooth bore. Rifling induces a great deal of drag in the barrel. A lot of the propellant gasses' forces are used to impart spin, rather than to accelerate the projectile. A smooth bore just lets things come out the muzzle at a higher speed.

The problem with smooth bores is accuracy. If that can be resolved by fin-stabilization, well then you get a faster round WITH accuracy. What's not to like?

The second issue driving the move to fin-stabilization is the recognition that building a longer penetrator leads to better penetration. With rifling you can't effectively stabilize a projectile that is longer than about 6 times it's own diameter. Modern APFSDS rounds are closer to 20 times as long as their own diameters. This L/D ration (length to diameter) means you get significantly better penetration from a round of the same caliber at the same velocity. But you can't fire "long rod penetrators" accurately with spin stabilization. So you are forced to use fins. This is now done even with rifled guns (firing fin-stabilized APFSDS rounds) for their superior penetration. But if you're going to fire a fin-stabilized round anyways, why use a rifled gun???

So … there's a lot that has transpired between a 6pdr's APDS and a modern APFSDS round. Not just one simple cause-and-effect.

-Mark
(aka: Mk 1)

Interesting. So what do the UK get from HESH that they feel they can't get from HEAT or plain HE?

Fred,

I guess HESH works just fine fired from a rifled gun, whereas HEAT does not – and both HEAT and HESH are dual purpose and much more capable than plain HE. I think the rifled versus smoothbore debate began back in the '60s when the British stuck to sabot whereas the Soviets and French went smoothbore to enable HEAT rounds. Sabot was much more effective at range against armour and so became round of choice for NATO. It was only when long penetrators came into use and required fin stabilisation that smoothbores regained an advantage, by which time the UK had invested in a 120mm rifled ammo supply chain (most everyone else in NATO having stuck to the British developed 105mm rifled design). With hindsight the UK should have collaborated with Germany and installed the 120mm smoothbore design into a redesigned C2 turret, but no-one was predicting the end of the cold war and the reduction in demand for MBTs when those decisions were made.

Interestingly, the capability sustainment programme for Challenger 2 was originally going to fit a Rhienmetal 120mm smoothbore, mostly due to the fact that L30 HESH ammunition was no longer in production and UK needed to drop depleted uranium APFSDS rounds to meet international treaty commitments. However this idea has now been shelved. Although the gun dropped sweetly into the C2 mantle, ammo storage was another issue. 120mm smooth-bore is a one-piece munition stored in the turret bustle, L30 ammo a two-piece split between the turret and hull. The upshot was that C2 would need a complete new turret to upgrade, costing way more than any savings on sharing NATO ammo supply chains. I think new HESH and Tungsten APFSDS (FIN) ammo is now being manufactured for the L30 in Belgium.

Two rounds are reported on from the 20-pdr; APDS and HESH (HESH is the British name for HEP) round.
First round hit chance of a 7.5 foot square target at 1000 yd.
71.8% APDS 25% HESH

Cold Steel wrote:

most countries abandoned the HESH round as redundant.

Most countries never really adopted HESH in the first place. It has always been something of a British peculiarity, and even though everyone who used the L7 105mm gun had the possibility of using it, I do not believe any other nation ever made it one of their main rounds for MBTs.

The accuracy problems with the early 17-pdr sabot were I believe due to poor shot seating. Both the 6-pdr and 17-pdr were relatively inaccurate guns anyway because of their high muzzle pressures; more conservative designs would have been more accurate, but far bulkier. The high muzzle velocities associated with high muzzle pressure meant flat trajectories, which kept up a fairly high P(hit) at likely engagement ranges regardless of being less accurate than, say, the US 3-in.

Lion in the Stars wrote:

IIRC, HEAT rounds don't like being spun (didn't GIAT make a 105mm HEAT round that has a ball-bearing driving band?), as it somewhat de-focuses the shaped charge.

Right -- Obus G (for Geissner, the inventor). A brilliant idea that, at the time, combined long-range accuracy with a powerful tank-killing punch, as well as being useable as a general purpose shell (though there was a plain HE round). With the stereo-coincidence rangefinder mounted on AMX-30, the French planned to engage enemy armour at greater ranges than anyone else in the world.

Another way of compensating for the defocusing effect of spin on HEAT rounds, which I only duscovered last week, is to design a fluted liner, so that the formation of the penetrator occurs with a slight spin to compensate for the rotation of the shell. I doubt you could have designed such a thing before the days of hydrocode modelling.

Fred Cartwright asked:

So what do the UK get from HESH that they feel they can't get from HEAT or plain HE?

Sir Dennis Burney called his original invention "Wallbuster", and HESH is better than anything for demolishing concrete. I believe that there was an attack on a bunch of antenna masts in Basra with concrete bases that was undertaken by CR2s because no other weapon in anyone's inventory would have done the job of demolishing them without getting out and planting demolition charges.

Back in the old days, the question of behind-armour lethality was another consideration that favoured HESH over HEAT. At one point the UK was considering a 183mm HESH-firing gun with an autoloader as its main tank-killing weapon. Trials against an up-armoured Conqueror showed that any hit by HESH in that calibre was a sure kill. The trouble was, the same was also true of 165mm HESH and 120mm HESH -- only at 105mm did the target begin to show any chance of surviving a single hit.

All the best,

John.

Thanks John. The UK have never followed fashion. Sticking with Horstmann suspension when everyone else went with torsion bars, ranging MG when the fashion was coincidence range finders and big guns/heavy armour when everyone else went with lighter, more nimble designs.

Most countries never really adopted HESH in the first place.

Actually not correct. As pointed out, the US called it HEP. It was part of our basic load on the M48/60 series and I have seen it used by ROKs and the FRG. I was told by Israeli and Japanese officers they carried it. There are also references to Canada, India and China using it, plus possibly on the Stryker with the 105 mm.

According to war diaries the problem with APDS and accuracy was the unfamiliarity of the round. When troops first got them the accuracy was poor as they were used to firing APCBC.
The APDs has a flatter trajectory than the APCBC round and hence when troops were compensating for the above the rounds tending to go high.
Strangely the CRA 1st airborne on his after action report wrote "due to short ranges encountered suggest increasing amount of APCBC issued" (paraphrasing). So it seems the APCBC round was either more superior at shorter ranges or something else?

Cold Steel wrote:

As pointed out, the US called it HEP. It was part of our basic load on the M48/60 series and I have seen it used by ROKs and the FRG. I was told by Israeli and Japanese officers they carried it. There are also references to Canada, India and China using it, plus possibly on the Stryker with the 105 mm.

Yes, anyone who ever used the L7 (which must have been pretty much everyone in the free world except the French) would have had the ability to use HESH. I believe the US even developed their own HEP rounds for the 76mm, 90mm, 106 RCL and 152mm (which also fired Shillellagh). But I do not believe that anyone other than the British -- and those with strongly British military traditions, so perhaps the Canadians, where's Ditto the Abdominal alternate callsign? -- ran MBTs with a basic load consisting of mostly HESH and the rest sabot. Everybody else's second choice of tank-killing round -- if it wasn't their first, with sabot second -- would have been HEAT, and the US eventually decided on HEAT for the 152mm. Apart from the 165mm dem gun -- obviously specialised for concrete-busting -- I do not believe anyone else used any weapons intended to fire HESH only, as the British series of 120mm recoilless weapons intended for infantry battalion A/Tk platoons (BAT, Mobat, Wombat, Conbat).

I'd be interested to hear if any non-British tanks have ever fired a HESH round in anger. Iranian Chieftains in the first Gulf War, I expect, and perhaps the odd Israeli, but I would doubt that anyone else did, especially against armour.

All the best,

John.

Due to the lower muzzle velocity, HESH gave guns less barrel wear. I'm not sure that APFSDS also had a slip band, so it didn't engage the rifling and badly degrade it, amongst other things.

It's been a long time, so I'm probably remembering wrong.