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Armor Strength of BMP-1 and BMP-2

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I just tested this with 25mm AP M791 on 34 BMP-1s from ranges 290m ... 2500m. In the engine area it protects its crew reasonably well (the engine still dies, and there is a chance of fire coming from it, but that's not the usual way how a BMP-1 dies if shot up from the front).

From the sides it's often a matter of one or two hits. From the front, if hit on the left side, usually with the first two rounds the driver and commander get killed (no big surprise there). Hitting the turret usually kills the gunner, or damages the weapon or turret. So for all practical manners the BMP can quickly be immobilized and will suffer terrible damages to equipment and crew. What it won't do is to immediately, always burst into flames.

I don't see what is "fatally flawed" here. We're still going to address the one or other minor issue, but all in all it seems to be a tempest in a teapot to me.

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Well I'm sorry I didn't take a video of the mission I was in when I noticed this while crewing ASLAV-25. I practically emptied an entire AP supply in the ready compartment just trying to kill a BMP-1 at 400-450 meters. I usually play as TC and noticed my AI gunner kept firing, and firing and firing...the shots seemed to hit square in the middle of the BMP, below the turret...I then took over the gunnery and continued emptying the AP rounds into its rear and open doors.

Thank you gentlemen for looking into this.

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Well I'm sorry I didn't take a video of the mission I was in when I noticed this while crewing ASLAV-25. I practically emptied an entire AP supply in the ready compartment just trying to kill a BMP-1 at 400-450 meters. I usually play as TC and noticed my AI gunner kept firing, and firing and firing...the shots seemed to hit square in the middle of the BMP, below the turret...I then took over the gunnery and continued emptying the AP rounds into its rear and open doors.

Thank you gentlemen for looking into this.

Ah yes, well actually I think I realize the cause of some confusion: the BMPs have two fuel tanks (three if you count both rear doors separately). I thought I explained that but perhaps I didn't. It seems that (or at least I guess now) the previous posts may have been referring to the rear door fuel tanks whereas we were investigating and mentioning the large primary fuel tank in the rear of the troop compartment. A hit on the inner troop compartment primary fuel tank must pass several kill checks before the round even makes it to the tank itself, so everything seemed fine in that regard. However, once some more specific information was provided hours ago, we looked at the rear door fuel tanks and found something that can possibly be improved a bit. This is probably a cause of what you are referring to (firing at the rear doors with KE and seemingly nothing happening in return), although I think that Sky Hunter was referring to killing damages in general, which is a topic that often can never be rectified with what some believe it should be.

All I can say is, this is a perfect example that if there is a complaint about something, or a possible bug, then everyone needs to be as specific as possible. The more specific the information that can be provided, then the better we can investigate, otherwise general complaints will result in us doing the best guess investigation that we can, and then explanations when nothing is found. Specific information also avoids seeming like a "drive by complaint" which are often, out of necessity and the need for efficiency, not always thoroughly investigated. Of course the other problem is that I am human and may have caused the confusion myself; someone may have mentioned the rear door fuel tanks specifically before and I missed it. Apologies all around if that was the case.

Other than that, we have to accept that at the basic level everything abides by a series of die rolls (basically) and probabilities; that is the nature of all computer based simulation that does not rely on the hit point system. You can rest assured that all vehicles follow the same methods, techniques, and protection rules, so there really should never be a case of one vehicle feeling "just right" and another vehicle being "fatally flawed". Essentially if one is fatally flawed, then all of them are fatally flawed because they are all directly related to one another. In other words, you can expect that one vehicle is perfectly comparable to another. That doesn't mean the system is perfect of course, but we do strive for the highest realism. Unfortunately however, what is "realistic" doesn't always jive with what others believe is the same, especially when we update a very old vehicle model that many have gotten accustomed to (we heard this again and again when the M1A1(HA) and Leo2A4 were updated for example). Some may feel that six M791 rounds should kill a BMP, while others might think it should be two, or twenty. All we can do is take the best data that is available to us and implement it on the damage model, then let SB do the rest.

Well anyway, the the good news is that in the coming patch we will likely have an improvement on both BMPs in the engine and rear door areas. In the meant time it shouldn't be too disastrous to continue using them for the time being. ;)

Edited by Volcano

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...while crewing ASLAV-25. I practically emptied an entire AP supply in the ready compartment just trying to kill a BMP-1 at 400-450 meters.

Well, it's the nature of a stochastic damage model that there may be cases where you simply have no luck; this may be especially true if many of the rounds fired by a computer-controlled gunner hit the one area where the BMP actually offers comparatively good protection, the glacis area, when firing from the front. The impact angle of typically around 9° results in more than a six-fold increase of the actual armor strength; going with the 41mm RHAe strength suggested on the previous page that's a whopping 260mm LOS thickness, which means immunity against pretty much ANY medium caliber APFSDS round in SB Pro.

On the other hand, the bow at about 60° impact angle and 35mm base thickness has a mere 40mm resistance against incoming KE projectiles.

So, as you can see from this very simple example, it matters A LOT where you actually hit the vehicle.

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Glacis is nearer 7mm@80 and nose 19mm@57 degrees - both roughly 30mm LOS.

they are rolled plates, not cast steel armour, so closer to 40mm is a better estimate.

the engine and gearbox is much thicker however, and can soak up around 150-200mm KE rounds.

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Well I'm sorry I didn't take a video of the mission I was in when I noticed this while crewing ASLAV-25. I practically emptied an entire AP supply in the ready compartment just trying to kill a BMP-1 at 400-450 meters. I usually play as TC and noticed my AI gunner kept firing, and firing and firing...the shots seemed to hit square in the middle of the BMP, below the turret...I then took over the gunnery and continued emptying the AP rounds into its rear and open doors.

Thank you gentlemen for looking into this.

this should definitely kill the BMP-1 because of the exposed ammunition.

it could be however, that you're hitting the fuel tank between the soldier seats.

from the rear, it's thick enough to completely stop a 25mm round.

diesel is usually safe in liquid form. it's when it's vaporized that it becomes dangerous and flammable. so a nearly empty tank of diesel is actually more dangerous than a full one.

in SB, the vehicles are simulated with all of the tanks filled to the top.

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Glacis is nearer 7mm@80 and nose 19mm@57 degrees - both roughly 30mm LOS.

I was going with the figures from page 2, not the actual ones used in our model, to illustrate the effect of the low impact angle on the glacis plate in contrast to the bow. Indeed we go with 7mm RHAe on the glacis (which ends up being in the 40...45mm LOS area) and 19mm for the bow (which results in a 35...40mm protection range). The trim vane may add a further 3mm resistance on the glacis, and each rib adds a further 7mm (but without LOS increase from the impact angle as they are near perpendicular to the trajectory).

Therefore the glacis can have up to 64mm resistance in the trim vane area, about 50mm above that, and 35...40mm in the bow area.

The upper third of the engine box is filled filled with low resistance material like air filters (due to the multitude of many objects there we set it to 2x8mm resistance), the lower two thirds are filled by the engine block (up to 95mm RHAe against KE rounds passing fully through), and the lowest third has the gearbox in addition to that (up to 85mm).

Finally comes the engine separation wall of 15mm strength.

So, for hits on the trim vane or upper glacis, a round will usually lose 52 + 16 + 15 = 83mm penetration power.

Upper bow is 35 + 95 + 15 = 145mm RHAe before a round enters the crew compartment. Lowest bow third is 230mm (add the gearbox) in resistance.

From these examples it should be clear that the exact impact location and impact angle play a major role in the overall vulnerability of the BMP. Hit the BMP from the side or from the rear, and things look A LOT different.

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Related question: why would an APFSDS, esp. DU penetrators, (from a tank gun) be less effective in killing BMPs than HEAT rounds?

Don't sabots have more post-penetration damage potential than HEATs?

My untrained brain was imagining things like 'catastrophic penetration overmatch and armor failure' but it seems this isn't the case, right?

Admittedly though, endurance of BMPs in current version more 'feels right' than before.

Before, I think 2.5x onward, BMPs occasionally acted as 'pin cushions' for tank sabot rounds, withstanding several hits and often retaining mobility/combat power before finally silenced by one more sabot hit or a single HEAT hit.

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Related question: why would an APFSDS, esp. DU penetrators, (from a tank gun) be less effective in killing BMPs than HEAT rounds?

The armor strength is part of the defeat mechanism. A KE round will form a crater during the perforation process of about 110% the diameter of the projectile multiplied by the length of the perforation tunnel, and a certain proportional amount of that volume will be injected into the vehicle to form the "behind armor debris" (BAD) cloud.

Obviously, with all other factors being the same, a 7mm skin of the BMP will create only about one percent of the BAD volume than the 60...70cm of an MBT turret front.

A HEAT round has in principle the same problem. But aside from the debris HEAT also creates a much bigger overpressure shockwave and has a higher incendiary effect, therefore the lack of debris formed from armor material has less of an overall effect than in the case of a kinetic energy projectile.

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The armor strength is part of the defeat mechanism. A KE round will form a crater during the perforation process of about 110% the diameter of the projectile multiplied by the length of the perforation tunnel, and a certain proportional amount of that volume will be injected into the vehicle to form the "behind armor debris" (BAD) cloud.

Obviously, with all other factors being the same, a 7mm skin of the BMP will create only about one percent of the BAD volume than the 60...70cm of an MBT turret front.

A HEAT round has in principle the same problem. But aside from the debris HEAT also creates a much bigger overpressure shockwave and has a higher incendiary effect, therefore the lack of debris formed from armor material has less of an overall effect than in the case of a kinetic energy projectile.

But what about the engine, gearbox, etc that you just mentioned? How do those behave differently from armor plates in making BAD clouds?

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In principle you are correct. However, there are two factors to consider. In real life, the engine compartment is not one homogeneous material. There are lots of air gaps that allow the engine material to displace to the sides of the impact canal, which isn't possible in the case of one highly dense armor plate. This dramatically reduces the amount of debris that will actually reach the crew compartment. Most of the fragments will be caught and neutralized by the engine separation wall.

The other factor is a limitation of our damage model. We do not track the entire penetration canal through a vehicle to determine how much debris might be generated. We simplify things, sure, but IMO this is a justified simplification due to the factors in real life that I described in the beginning. A full analysis of the impact canal would be in no proportion to the small gains in fidelity, both from the computational time aspect (you want to resolve the shots at near-real time, right?) and the amount of programming effort that would need to be poured into it.

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This is great information. I was prepared to write up a big complaint about the BMP's invulnerability to 25mm fire. With this diagram I can consistantly put rounds with the M791 APDS-T (older round) onto the BMP-2 and kill it, frontally. Simply aim for the BMP's left front hull 1/3 and either side of the turret mantle.

Thanks

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