GURPS 3e/V2e question: Explosive Reactive Armor weight

[GURPS Fox]

Originally Posted by Fred Brackin

If you think the ERA has _truly_ full coverage then yes. I'd be expect some gaps.

Well, I was thinking that, well, I use that number with the equation that Curmudgeon provided and get the surface area for individual facings from that...

[Rupert]

Originally Posted by GURPS Fox

Thanks for the heads up. So just look up the armor's surface area and use that?

Largely because of tandem-charge warheads quickly coming onto their own, though the current situation in Ukraine indicates that some serious corruption gutted the ERA on the tanks.

Still, it shouldn't be almost 18 metric tons for 'heavy' ERA on just the turret of the tank if what I currently checking with is any indication.

Even at the rather high 90 pounds per square foot of VX1, that's 400 square feet of coverage. As the tank's total deck area plus frontal area is less than that I very much doubt this.

A quick eye-balling of a T-72 with ERA and doing some maths suggest to me no more than ~160 square feet of ERA, for 160 x 90 = 14,400 pounds, or 7.2 tons.

[dcarson]

Originally Posted by Rupert

Even at the rather high 90 pounds per square foot of VX1, that's 400 square feet of coverage. As the tank's total deck area plus frontal area is less than that I very much doubt this.

A quick eye-balling of a T-72 with ERA and doing some maths suggest to me no more than ~160 square feet of ERA, for 160 x 90 = 14,400 pounds, or 7.2 tons.

From a PDF I fond on msm.sk

The total weight of all blocks of ERA is 2700 kg per one tank whilst the weight of blocks there on
turret is 1 170 kg. Increased weight effects have been tested in the course of qualification and user tests.

[Curmudgeon]

Originally Posted by dcarson

From a PDF I fond on msm.sk

Using the figures and coverage given in the PDF, we arrive at the following formulae and results, which suggest that you are actually looking at HRAP from VXi, with a weight errata being necessary:

Surface Area (using Complex Option for Armor by Location): 400 sq. ft. (turret) x 4/5 (facings, top of turret is indicated as being included in the ERA facings, which wasn't the case in the previous calculation) + 500 sq. ft. (body) x 4/6 (facings, top of hull also counts as an ERA facing which wasn't indicated earlier) = 320 sq. ft. (turret) + 333 1/3 sq. ft. (body) = 653 1/3 sq. ft. (total).

Total weight of ERA = 2700 kg. = 2.7 metric tons = 5,940 lbs.

Weight of ERA/sq. ft. = 5.940 lbs./653.33 sq. ft. = 9.09 lbs./sq. ft.

This is very close to being 9 lbs./sq. ft. As said, 90 lbs./sq. ft. seemed excessive, although as Rupert pointed out, not insanely so. It's possible that NEXREARM is, despite its marketing, effectively just RAP with a weight of 9 lbs./sq. ft. instead of 8 lbs./sq. ft., and thus just a difference between models that can be statted using the real world weight. On the other hand, it's also possible that it really is HRAP, which may indicate that the 90 lbs./sq. ft. was subject to a misplaced decimal point and should be the subject of an errata as 9.0 lbs./sq. ft.

Given that errata for Classic GURPS isn't the highest priority, I wouldn't necessarily expect it to be reported, assuming that an errata is needed.

[GURPS Fox]

Originally Posted by Curmudgeon

Using the figures and coverage given in the PDF, we arrive at the following formulae and results, which suggest that you are actually looking at HRAP from VXi, with a weight errata being necessary:

Surface Area (using Complex Option for Armor by Location): 400 sq. ft. (turret) x 4/5 (facings, top of turret is indicated as being included in the ERA facings, which wasn't the case in the previous calculation) + 500 sq. ft. (body) x 4/6 (facings, top of hull also counts as an ERA facing which wasn't indicated earlier) = 320 sq. ft. (turret) + 333 1/3 sq. ft. (body) = 653 1/3 sq. ft. (total).

Total weight of ERA = 2700 kg. = 2.7 metric tons = 5,940 lbs.

Weight of ERA/sq. ft. = 5.940 lbs./653.33 sq. ft. = 9.09 lbs./sq. ft.

This is very close to being 9 lbs./sq. ft. As said, 90 lbs./sq. ft. seemed excessive, although as Rupert pointed out, not insanely so. It's possible that NEXREARM is, despite its marketing, effectively just RAP with a weight of 9 lbs./sq. ft. instead of 8 lbs./sq. ft., and thus just a difference between models that can be statted using the real world weight. On the other hand, it's also possible that it really is HRAP, which may indicate that the 90 lbs./sq. ft. was subject to a misplaced decimal point and should be the subject of an errata as 9.0 lbs./sq. ft.

Given that errata for Classic GURPS isn't the highest priority, I wouldn't necessarily expect it to be reported, assuming that an errata is needed.

Given that 2700kg weight is based on Russian Kontakt-5 'heavy' ERA (given that it works against APFSDS rounds, although not as extensively as shaped charge warheads), yeah, someone, somewhere, screwed up and misplaced a decimal.

[The_Ryujin]

Ok, while hard data is hard to come by, based on figures I've been able to dig through in published papers, Jane's and so on; a block of Russian Kontakt-5 weighs ~23 lbs, covers ~0.26 square feet, and ~2.8 thick and gives an additional ~600mm extra RHA equivalent vs shaped charges and ~300mm vs APFSDS.

So around DR 830 vs kinetic and 1,630 vs HEAT. Though I think these are probably best case so I'd probably drop it to closer to maybe DR 600 v Kinetic, 1,200 v HEAT in practical cases. Then again these numbers might be outdated, since the pen figures are from the late 90's so modern models might be improved.