BK90 compiled Bug report

So we got the nice BK90 update today and indeed the flight characteristics are much more accurate to the real thing from what I know which im happy about.

Of course I already ran some testing setups and this bug report will compile the issues I found with the current modelling.

There is a slight problem with low altitude releases but the main Problem is with the MJ2 performance.

The low altitude release problem is simply that even at the spec'd release altitude of 50m agl at mach 0.9 the bk90 keeps a negative pitch attitude for too long and thus runs into the ground.

A pitch up happens last moment before the missile hits the ground but its too late to achieve positive rate.

The most definitive reference that the pitch up to neutral vertical velocity happens way earlier is this footage from a DWS39 commercial showing a test launch with pretty immediate after seperation:

[42]

Now after that is out of the way this brings me to the main issue I found with the current implementation the MUSJAS2 submunition performance. I have already done calculations to determine its anti armor capability in in 2016 before the release, but I revamped all calculations to make sure its up to date with the soruces I found available and my memory doesnt fail me.

I will begin with listing the data available on the Submunition, then reverseengineer the missing values for the estimate of EFP performance and lastly present some comperative data and highlight the differences to the current situation in the sim.

The following list of datapoints can be compiled from the listed sources:

Diameter : 132mm for both [1]

Length : 600mm MUSJAS2 [1]

170mm MUSJAS1 [1]

Weight : 16,9kg MUSJAS2 [1]

3,7kg MUSJAS1 [1]

Explosive mass: 6kg MUSJAS2 [2]

1,4kg MUSJAS1 [2]

Parachute diameter: 2m MUSJAS2 [1]

0,8M MUSJAS1 [1]

Warhead length: 450mm MUSJAS2 [3]

Number of projectiles : 110 EFPs MUSJAS2 [3][4]

2000 steel fragments MUSJAS1 [2]

[1] The only direct primary source on the MUSJAS2 is the general description of the BK90 system in this document:

https://drive.google.com/file/d/0B8bCDRcq9BVeUGxlNW5CaFhFQ2c/view

The document lists the data on page 79 and provides a cut open sideview of both submunitions which led to the original discovery of its horizontal firing EFP nation.

[this document also lists controll forces and goes into some detail on the guidance as far as I can tell but my guessing at swedish isnt good enough to really dig into that too much in reasonable time.

[2]Also very helpfull in reverse engineering has been this page on the BK90 by the Arboga Robot Museum, since it lists the explosive mass of both munitions and the fragment count on the MUSJAS1.

It isnt http://robotmuseum.se/Mappar/Robothistorik/13_Bombk/ARM_bk.htm Even if this isnt a primary source I have good faith in it.

[3] https://www.unog.ch/80256EDD006B8954/(httpAssets)/73BAC38BBEA45AEFC1257B3B0039008D/$file/Sweden+Initial+Report+2013.pdf

This document lists the length of the warhead section of MUSJAS2 as 45cm with a conveniently placed centimeter scale next to it on page 24, despite the resolution,

the marks on the scale can be counted. This picture also lead to the following 3d model

[4]which was made by MYSE1234 to determine the amount of EFP forming indentations per Submunition.

https://forums.eagle.ru/showpost.php?p=2943944&postcount=3445. The total count of Indentations is 11 rings of 10 indentations putting the total count at 110.

A disclaimer before I switch to the reverse engineering part, I am not a professional in this field just a nerd that stuck his nose in some papers and took a "best guess" at things [https://www.youtube.com/watch?v=HBxcalnSzIk]. Overall I tried to guess conservative to prevent overshooting the realworld performance and rather undershoot.

The first missing data point which prevents an estimate at the kinematic energy of each EFP and thus its destructive potential is the weight of the overall liner material.

To determine this I will first try to estimate the liner material thickness by calulating the diameter the cylindrical explosive charge. This can be be done by determining the the height of the cylinder via the cutaway picture in [1] page 75 and combining this with the scaled picture of the munition in [3]. With the weight and an educated best guess at the explosives probable density the needed volume and thus diameter can be determined.

Figure 70.9 shows that the explosives cylinder reaches to the the first angle towards the first indentation on both ends. Combining this with [3] which shows those points at 5.5cm and 39.5cm the explosive cylinder is probably 34cm tall. The explosives used in EFPs are usually of very high detonation velocity in [5] section 2.1 a compound with a density with a density of 1.7g/cm^3 is used while the BLU108 submunition uses octol with a density of 1.81. So I am going to assume a density of 1.75g/cm^3 which requires a volume of 3428cm^3 for a charge of 6kg.

combining this with the estimated cylinder heigth of 34cm we get a cylinder diameter of 11.33cm which in combination with a calibre of 13.2cm gives us a liner thickness of 9.3mm.

We have to consider in this though that this simplification didnt account for the indentation depth and its volume which according to [4] is about 6.5mm. Liner thicknesses I have read of are usually

in the 3-4mm range, take [5] 2.1 as an example which's liner is 2.8mm copper. Usually EFP charges have a calibre of of 130mm or larger and a much flatter appearance. Since the indentations on the MUSJAS2 are just in the 3-4mm range a thicker liner is quite conceivable to reach reasonable projectile mass.

My best guess [42] is that the liner is 5-6mm. Which coincides with scaled measurement of the apparent thickness of the liner seen on the picture on page 75 in [1]. The resolution is quite low though, but considering that the former estimate without the indentation volume taken care off was 9.3mm it is quite believable when looked at in the context of 3-4mm liner for full calibre EFPs.

The liner material again is a best guess, usual materials for this are Copper, Steel or in rarer cases tantulum or uran for their density mostly.

In accordance to the colour in [1], blackish, and [3] shiny gold/orange I do believe it is some kind of copper alloy matching [3] quite nicely. [1] could indicate either tantulum or copper oxide so it is kinda ambigious, since [3] matches an copper alloy best though [i have no idea about metalurgy though so total laymans view] I am going to assume a density of 8.9g/cm^3 for the liner.

Using the warhead heigth of 45cm and an outer diameter of 132mm and an inner diamter of 121mm, corresponding to a liner thickness of 5.5mm, we get a tube with a volume of 984cm^3 which in copper would correspond to a weight of 8757gramm and leave us with 2.14kg for support structures, the radar altimeter fuzing and the parachute.

The ice is getting thinner the further we move out into the vastness of guestimates but we can try to compare our result to the non warhead weight of MUSJAS1.

MUSJAS1 has an non explosive mass of 2.3kg, subtracting the liner material for 2000 fragments yields about 1kg of mass for fuzing and parachute assembly, given the parachute of MUSJAS2 has 2.5 times the diameter a doubling in mass of the support infrastructure seems "about right" considering certain elements dont need to be enlarged proportionally.

To get the final mass of material available for EFP forming we have to subtract the 11cm of warhead height which dont have the corresponding indentations since from [1] page 75 it looks like the material continues past the explosive cylinder. We will assume a liner thickness of 3.5mm in this area since our cylindrical model doesnt account for the increased mass in the EFP section by means of the indentations. This gives us 1388gramms of liner material not available for projectile forming, putting us at a final 7368gramms of material for EFPs.

Main factors remaining to be determined are the mass recovery coefficient determining how much of the liner mass actually gets formed into the dominant projectile and the projectile initial velocity.

Removing both the best and worst result from the table on page index 564 in [7] and averaging the other results a recovery coefficient of 0.75 seems reasonable.

Given a total to be converted liner mass of 7368 gramms into 110 projectiles this leads to an EFP mass of 50.23 gramms. [This number feels so oddly round it has to be a design spec. :D]

For the initial speed of the EFP I am using stable flight velocity of [8] page 6 for copper at 1700m/s and [5] section 4.2 at 1809m/s as a reference for copper type EFPs.

The rate of speed decay in m/s per meter traveled also is a average of the quoted not closer specified 27.3m/s/m of generic and 15.6m/s/m worst case aerodynamic EFPs from section 4.2.

Thus in the following analysis the initial speed is believed to be 1754m/s and the rate of speed decay 21,45m/s/m. I am going to reduce the rate of decay in 10m increments by inverse square law as the only slowing force in this case is aerodynamic drag. This is extremely rough around the edges but again should be worse then the actual performance so I am relatively confident the generated data can be taken as an performance indicator which is reasonable as an baseline expectation of destructive power.

[5]https://www.sciencedirect.com/science/article/pii/S2214914714000348

[6]http://armamentresearch.com/wp-content/uploads/2017/08/Skeet-HRW.jpg

[7]https://www.researchgate.net/publication/257726480_Explosively_Formed_Projectile_Soft-recovery_Force_Analysis

[8]https://www.researchgate.net/publication/257841624_Analytical_performance_study_of_explosively_formed_projectiles

EFP velocities/kinetic energy levels at given distances:

@0m 1754m/s 76,9kJ comparable to 20x139mm rh202 APDS 65,3kJ muzzle energy

rate of decay 0-10m 21.45m/s/m

@10m 1539.5m/s 59,2kJ comparable to 20x102mm m39 53,5kJ muzzle energy

rate of decay 10-20m 16.52m/s/m

@20m 1374.25m/s 47,2kJ

rate of decay 20-30m 13.17m/s/m

@30m 1242.58m/s 38.6kJ

rate of decay 30-40m 10.7m/s/m

@40m 1134.93m/s 32.2kJ same as 14.5mm AP muzzle energy

rate of decay 40-50m 8.98m/s/m

@50m 1045.12m/s 27.3kJ

rate of decay 50-60m 7.61m/s/m

@60m 968.96m/s 23.47kJ

rate of decay 60-70m 6.55m/s/m

@70m 903.49m/s 20.4kJ

rate of decay 70-80m 5.69m/s/m

@80m 846.59m/s 17.9kJ similar to 12.7mm AP muzzle energy at 16.8kJ

rate of decay 80-90m 4.99m/s/m

@90m 796.69m/s 15.9kJ

rate of decay 90-100m 4.42m/s/m

@100m 752.43m/s 14.15kJ

rate of decay 100-110m 3.95m/s/m

@110m 712.95m/s 12.7kJ

rate of decay 110-120m 3.54m/s/m

@120m 677.51m/s 11.4kJ

rate of decay 120-130m 3.2m/s/m

@130m 645.51m/s 10.4kJ

rate of decay 130-140m 2.9m/s/m

@140m 616.51m/s 9.5kJ

rate of decay 140-150m 2.6m/s/m

@150m 590m/s 8.7kJ

rate of decay 150-200m 2.47m/s/m

@200m 468.6m/s 5.5kJ

rate of decay 200-250m 1.53m/s/m

@250m 392m/s 3.8kJ

rate of decay 250-300m 1.1m/s/m

@300m 338.4m/s 2.8kJ

rate of decay 300-350m 0.8m/s/m

@350m 298.4m/s 2.2kJ comparable to ak47 7.62x39 2,1kJ muzzle energy

I have included the muzzle energies of relatively common weapon systems to make the kinetic energy levels relatable.

Now for the core part of this bug report, the damage effect in the simulator.

This screenshot is taken from maximum altitude at maximum zoom levels to ensure basically parallel perspective at the submunition parachute locations.

This is the directly resulting damage from the pictured 24 MJ2 submunitions.

Quite obviously the damage output is laughable considering the energy levels listed above and that 2640 projectiles of that kind have been fired in all directions combinging the 24 MJ2s.

Given that the projectiles have an kinetic energy 80m from the firing point, comparable to that of the .50cal AP projectile at the muzzle I did run a pretty simplistic but imho rather convincing emulation of the damage effect I would expect an MJ2 loaded BK90 to have. I did drive a Humvee M2 to each of the locations a MJ2 detonated and fired 100 rounds of m2 0.50cal AP ammo while rotating the turret at max rate, alternating the elavation and randomly choosing the start point of firing, within the 100 rounds the turret nearly does 2 full rotations so the distribution is _rather_ even.

The changing elevations have been included to simulate the EFPs not beeing fired perfectly at waterline.

This test assumes that the rare impacts at full rotational speed of the turret beyond 80m, at which the muzzle energy of the 0.50cal projectile should be very similar to that of the EFPs, are more then compensated by the much higher destructive power impacts within 40m should have.

The resulting damage after all 2400 rounds had been fired looked like this.

The massive difference is obvious, as to be expected the BMP2s have been barely scratched, lightly armored or unarmored units especially close to firing points have been riddled down by a lot of single impacts but even low health units arent automatically dead because the damage type is KE.

Now this is one example and it took me long enough to assemble it. Also the usage of the m2 round is obviously flawed since at ranges beyond 90m the EFP kinetic energy is a lot lower then that of an m2 round, but at 0-40m it is higher even more so. So in the end due to the large distance between rounds beyond 90m I believe that this example of an damage pattern is still conservative.

In the end this all is an fair amount of guesswork and even if the units that are destroyed in my example in an future version of the bk90 would be down to below 60% health, where radars stop working on AI units I would deem the result fair. But the current state that 2640 projectiles with energy levels between 20mm apds and 12.7mm AP minimally damage an unarmored radar and command post and and only inflict note worthy damage on an direct hit on an armored supply truck is simply unbelievably bad given the former analysis.

I know that simulation of 2640 projectiles per weapon inside DCS is let alone due to the synchronus game loop probably not really an option due to performance concerns but the Area of effect type damage probably can be adjusted to believably reflect the to be expected damage.

Amazing post! Really well laid out, thanks!

I also noticed the abysmal performance of the MJ2's during some extensive testing yesterday.

The 20 mm RH202 round has an armor penetration of 55mm according to the Steel Beasts Wiki (http://www.steelbeasts.com/sbwiki/index.php/Ammunition_Data#Autocannon).

I have no idea how APDS penetration given a certain kinetic energy translates into EFP penetration at the same level of kinetic energy, but even if EFPs have only 50% pen given the same kinetic energy, it should still be enough to penetrate APCs and the side armor of many IFVs at 20-30 meters?

Of course, penetrating something doesn't mean destroying it - but I think DCS should at least register damage / hits at much greater ranges from each MJ2 impact point than what seems to happen now.

My best guess is that DCS models each MJ2 as a really small bomb doing blast damage in a similarly small radius of effect.

Excellent research !

Amazing post! Really well laid out, thanks!

 

I also noticed the abysmal performance of the MJ2's during some extensive testing yesterday.

 

The 20 mm RH202 round has an armor penetration of 55mm according to the Steel Beasts Wiki (http://www.steelbeasts.com/sbwiki/index.php/Ammunition_Data#Autocannon).

 

I have no idea how APDS penetration given a certain kinetic energy translates into EFP penetration at the same level of kinetic energy, but even if EFPs have only 50% pen given the same kinetic energy, it should still be enough to penetrate APCs and the side armor of many IFVs at 20-30 meters?

 

Of course, penetrating something doesn't mean destroying it - but I think DCS should at least register damage / hits at much greater ranges from each MJ2 impact point than what seems to happen now.

 

My best guess is that DCS models each MJ2 as a really small bomb doing blast damage in a similarly small radius of effect.

Yeah I am not sure either how much the penetration power is reduced vs "proper" armor piercing projectiles, but i feel similar that at 10-20m I would expect it to at least pen stuff like BMPs and other aluminium tubs. :D

Excellent research !

thanks for the kudos ! :)

Amazing post! Really well laid out, thanks!

 

I also noticed the abysmal performance of the MJ2's during some extensive testing yesterday.

 

The 20 mm RH202 round has an armor penetration of 55mm according to the Steel Beasts Wiki (http://www.steelbeasts.com/sbwiki/index.php/Ammunition_Data#Autocannon).

 

I have no idea how APDS penetration given a certain kinetic energy translates into EFP penetration at the same level of kinetic energy, but even if EFPs have only 50% pen given the same kinetic energy, it should still be enough to penetrate APCs and the side armor of many IFVs at 20-30 meters?

 

Of course, penetrating something doesn't mean destroying it - but I think DCS should at least register damage / hits at much greater ranges from each MJ2 impact point than what seems to happen now.

 

My best guess is that DCS models each MJ2 as a really small bomb doing blast damage in a similarly small radius of effect.

I'm not an expert but the EFP projectile is formed out of copper I would imagine ? APDS use tungsten rods; depleted uranium rods for more modern shells. Which are much denser and stronger.

Also maybe EFP projectile have a worse shape for penetration, by that I mean that the energy of the projectile is spread over a larger surface area on impact, they are engineered to be as effective as possible so the exact amount of explosives required, the shape of the copper before the weapon is released,... were calculated to get the most effective shape possible.

I would expect the EFP projectile to have alot less penetration then a 20mm APDS round. But a BMP-1/-2, the -3 is probably not much better but I have not found any data, with it's tinfoil grade armor would almost surely be penetrated.

EDIT : Have found some data, http://www.steelbeasts.com/sbwiki/index.php?title=BMP-3 Since the BMP-3 in steel beasts doesn't have a complete armor model detailed on the wiki these values are probably estimated. It says it's protected against 30mm AP from the front, now we don't know what kind of AP, I guess that would be at any range, but it's definitely better then the BMP-2/BMP-1, anywhere else, sides, rear, top is probably just as bad though. To be taken with a grain of salt.

This should really be looked at

From todays changelog:

 

• Tweaked BK90 warheads.
  

I like the fact that the BK90s are no longer one big bomb and have a fantastic visual effect in MP now. But whilst I can't comment on the very detailed report made to initiate this thread, the damage output of the BK90s seems very much "less than expected".

Utilizing the nice quick training mission pack by cor.vinus:

https://www.digitalcombatsimulator.com/en/files/3302241/

I had been trying to use a mix of MJ1 and MJ2 to get good coverage, but barely made a scratch on artillery units.

So I then switched to a pure MJ2 composition, and I only managed to kill 3 x T55s, despite fairly good coverage (see screenshots). I can't imagine this being viable for more modern tanks at all.

So I then switched to a pure MJ2 composition, and I only managed to kill 3 x T55s, despite fairly good coverage (see screenshots). I can't imagine this being viable for more modern tanks at all.

From what I know about MJ2 it wouldn't scratch a T55 anywhere except the rear maybe. But even the rear armour is 45mm RHe inclined at 17 deg. So, yeah it isn't viable against modern tanks, it shouldn't be viable against t55, but should shred lightly armoured tin cans like BMPs BTRs Bradleys and most artillery pieces.

I wasn't able to retest after the change yet how it goes against light armour atm.

MJ1 wont do anything against any armour, we are talking about 0.6-1 gram or lighter fragments.

Any armour is going to stop them basically.

Yesterday I used the BK90 extensively for the first time after it recieved the overhaul, because I'm very limited on time to play DCS as much as I would like lately. I can't say anything about the damage output, because the low altitude release problem gave me a huge headache:

So we got the nice BK90 update today and indeed the flight characteristics are much more accurate to the real thing from what I know which im happy about.

 

Of course I already ran some testing setups and this bug report will compile the issues I found with the current modelling.

There is a slight problem with low altitude releases but the main Problem is with the MJ2 performance.

 

The low altitude release problem is simply that even at the spec'd release altitude of 50m agl at mach 0.9 the bk90 keeps a negative pitch attitude for too long and thus runs into the ground.

A pitch up happens last moment before the missile hits the ground but its too late to achieve positive rate.

The most definitive reference that the pitch up to neutral vertical velocity happens way earlier is this footage from a DWS39 commercial showing a test launch with pretty immediate after seperation:

[42]

I played a mission where external views were prohibited and wondered why I was not able to get any BK90 on target within several tries. It was untill after the mission when I discovered in TacView that the BK90s just fell into the ground upon release every time, even though I released them above 50m.

So yeah, it would be nice if this could get fixed :)