I don't think the camera angle can explain the difference. In the photograph I posted, the hollow space occupies about 30% of the total thickness along the line of sight... that is despite the fact that most of the armor is closer to the camera and hence perspective distortion enlarges it a bit. In your image, the hollow space accounts for 22% of the total line of sight.
Your protection "ideas" are way to optimistic and not based on any real life incidents. They read like the old and massively over-exaggerated protection estimates of the past.
Based on the Chinese data referenced earlier in this topic (which includes photos of the test set-up of side armor modules against ATGMs and RPGs), it seems likely that Merkava 3's side armor modules (that already featured integrated explosive layer(s)) could stop the PG-7V/VM warhead (capable of defeating 300 mm steel armor) only at 45° impact angle and the HOT-1 ATGM warhead (~700-750 mm penetration) at 30° impact angle. That is an level of armor protection comparable to the Leopard 2A4 from 1988 with Type C armor (stopping HOT-1 along a 60° frontal arc) and the M1 Abrams against the RPG (armor along the crew compartment designed to stop a 84 mm shaped charge warhead capable of defeating 380 mm steel armor at 45° impact angle). So it is on par with existing tanks, not higher - despite the use of explosive layers in the armor.
Armor technology probably has improved quite a bit between 1989 and 2003, but you expecting the side armor to stop METIS-M at 90° impact angle would not only require a doubling of effective protection - but also much improved performance, as Metis-M had a tandem warhead. NERA, NxRA, SLERA and ERA are more vulnerable to tandem warheads; how much depends on the exact armor array, but it is always worse than against a single stage warhead.
There are multiple reported incidents where Kornet and Metis-M managed to defeat the armor of the Merkava 4 and up-armored Merkava 3 tanks. According to the coverage of the 2006 Lebanon war by Israeli newspapers/websites, the administration of the Merkava tank program released figures in August of 2006, showing that 22 Merkava tanks were penetrated by ATGMs - 18 (!) of which were the Merkava 4 model. In the battle of Wadi Sulaki alone 11 Merkava 4 tanks were hit by ATGMs.
Russian sources put the figure of penetrated Merkava tanks a lot higher, claiming that Kornet alone was used to penetrated 24 Merkava tanks (not explicitly stated which variants, but they also claim that Hizbollah used Metis-M and Kornet to target the Merkava 4, and Konkurs and Fagot to target the Merkava 3).
Based on thickness and combat performance, it is a lot more reasonable to assume that Metis-M - and potentially also the RPG-29 - can defeat the turret side armor, unless hitting it at an angle. The side hull has been defeated by RPG-29. The frontal (hull) armor of a Merkava 4 being penetrated by a Kornet ATGM was the main drive for adopting Trophy.
It is still marketing talk. They basically improved the performance of their reactive armor and then claimed "its better than older NERA". That's what basically what everyone says. Åkers Krutbruk announced that MEXAS would be as good as ERA already in the 1990s (and this statement is just like any claims about the NxRA/SLERA performance massively exaggerated).
Aside of that, patents are neither verified on claims nor does the existence of a patent mean that this exact solution is used.
If you look at research papers for SLERA, tests with different configurations (GAP, Dottikon, RDX as explosives) to even simple ERA (4S20 from the 1980s...) is still massive - and that is in laboratory tests where only one layer is tested, so there are no issues with preventing sympathetic reactions ("chain reactions"). The more explosives are used (or the more powerful explosives are used), the better the performance - but also the armor becomes more vulnerable to tandem warheads/multiple hits and harder to integrate into complex armor arrays. This has to be counter-acted by using other design aspects (greater thickness, inter-layers acting as dampener) that tend to limit efficiency - space and/or weight efficiency.
The difference between SLERA and highly improved NERA/NxRA isn't really relevant, specifically if thickness is the limiting factor - because then optimal armor layouts and high explosive content cannot be used.
Marketing talk. The performance of NERA can vary drastically - mass efficiency ranges from just 1.3 to up to 10 depending on layout and material. ERA optimized for anti-KE roles doesn't reach much higher mass efficiency. Thickness efficiency of ERA cannot be reached at all.
Maybe I misread the article. For example you wrote on page 8: "A solution as such tunred out to be suprisingly effective, with the main armor surfaces being meticulously designed (with an assumption that primary protection would be provided at an angle of +/- 30° in relation to the longitudinal axis of the MBT)." My understanding of this sentence was that you said, that the Merkava was designed for "primary" protection along a 60° arc (+/- 30° from centerline). Is my understanding wrong?
You later wrote on page in the article that the Merkava's armor protection on the sides is limited: "The side protection level, with angles of around 20 degress, was lower - relative 200 mm and less, but still decent, given the materials and technologies applied." That is good and I have to admitt, I didn't properly read it. I understood this as the armor being angled by 20° from the vertical plane rather than a protection up to an impact angle of 20°.
However in your table is still rather misleading, you do not show that other tank have a larger protected arc and the values are also very comparable. The Abrams for example is protected over a 50° arc (+/- 25°) against APFSDS and ATGMs and over a 90° (+/- 45° against RPGs). So the Abrams has a much higher protection level than the Merkava 1 despite being a lot lighter. Without reading another article about the T-64, T-72, M1 Abrams or Leopard 2, your article might create the illusion about the Merkava 1 (nearly) matching protection levels of other tanks just by being heavier and using simple spaced armor. But in reality the difference is much bigger.
You also use the RARDE value for minimum protection against a sheated/monobloc penetrated over a 60° frontal arc for the Leopard 2 tank. Meanwhile RARDE also stated that the Leopard 2 with original composite armor was believed to stop a 125 mm steel APFSDS at unspecified range (the round in question had an estimated penetration of up to 420 mm).
The CIA values for the Abrams turret are interesting, but unfortunately very, very unspecific. Is this protection achieved against monobloc penetrators, sheated APFSDS rounds like the M735, DM13 and BM-42 or against steel rounds with WC slug like Zakolka? Is this the protection level achieved over a 60° frontal arc or directly head-on? And why does the CIA document talk about "one version of the turret" achieving this? What is the other version, the prototype - or this talking about the short M1 and the long M1IP turrets?
British data from XM1 FSED phase (and the last FSED prototype was used for final ballistic acceptance tests in the USA) suggest protection was only 320-340 mm agianst sheated tungsten-cored APFSDS rounds over a 50° frontal arc (+/- 25°).
How does Israeli steel compare to Soviet, US or German steel? The early Leopard 2 prototype turret with spaced armor for example had similar frontal armor on the cheeks (in terms of total thickness 288.6 mm steel with a 300 mm air-gap when seen from the front) when compared to the Merkava 1, but was only designed for protection against 105 mm APDS rounds... and it used very high grade steel (with the outer plate made of HHS). So can the Merkava 1's turret armor actually stop a 105 mm APFSDS as the values suggest? Or is the cast steel on the 1960s US levels of quality?
Personally I dislike expressing protection as "RHA". It is a very approachable method, but also very flawed. For example the US stated that the NATO heavy triple target provided protection equivalent to 466 mm RHA against the old 105 mm M392 APDS round. Yet the British 120 mm L15 APDS could defeat the NATO heavy triple target at 500 metes - at this range, its penetration would be equvialent to just ~320 mm RHA. The XM735E2 APFSDS round (entered production as M735) could defeat the NATO heavy triple target at 2,000 meters. 1980's 105 mm APFSDS rounds such as M833, M426/DM63 and NP105A2 could defeat the same target at 6,000+ meters, where their theoretical armor penetration should be below M735 at 2,000 meters.
So one target has four different "RHA values".
Merkava 1 made compromises and protection was focused on stopping the ammunition available to enemies at the time. That would suggest that APDS and steel APFSDS with tungsten-slug were the main issues, while NATO was looking at more potent threats. M1 Abrams and Leopard 2 were tested against sheated steel APFSDS with (early) WHA core (XM578E4, 38mm KE for 105 mm SB, 120 mm DM13). Brits wanted protected against early monobloc APFSDS with WHA and DU core (requirement for MBT 80 and in reduced form for Challenger 1).
According to KPB Tula, Kornet-E was in development until 1998. Very unlikely that Kornet was available in the Middle East during 1995-1999.
Metis-M was in development until 1992. So it is possible, but not very likely to be a reference threat. In 2006 the IDF was surprised by the high amount of Metis-M and Kornet ATGMs in the hands of Hizbollah. Makes it appear very unlikely that they used these missiles as projected threats during the Merkava 4's development.
Spiraling flight pattern is common on all types of SACLOS missiles, even wire-guided systems like Milan and HOT. It really depends on the exact pattern, shape and size of missile to identify Kornet by the pattern alone.
Valryon got a reaction from David Moyes in Polish Armoured Vehicles
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