GURPS 3e/V2e question: Explosive Reactive Armor weight

[GURPS Fox]

So, I've been playing around with GURPS 3e and discovered that the explosive reactive armor is not the appropriate weight (note, I did some research, and ERA shouldn't be more than 3-5 metric tons, given the nation -USSR- having a 'must be less than 40-45 metric tons or ignored after WW2' rule for its tanks until the T-14).

So, have there been any fixes to this?

[Curmudgeon]

Not a fix, per se, but an update in GURPS Vehicle Expansion 1 [p. VXi27].

Under Extra Detail -Reactive Armor Plates (Late TL7), it is noted that instead of automatically destroying a shaped-charge warhead, the reactive armor plate provides DR 1,000 against it (due to powerful enough warheads actually penetrating RAP in real-life). Second, it introduces Heavy reactive Armor Plates which provide DR 2,000 (by being substantially thicker than the older plates). They weigh more than would be expected if they were just scaled up "old" reactive armor plates but multiplied by the surface area of the tank, shouldn't exceed the weight guidelines you've indicated.

One additional feature available (also p. VXi27) is Applique Armor which allows the RAPs to be bolted (or otherwise attached to the vehicle) giving 2 sets of DR and PD and two sets of weight and mass figures for the vehicle, one with the applique armor and one without. Performance statistics should have a second statistic if the vehicle's weight or mass was important to the calculation.

[GURPS Fox]

Originally Posted by Curmudgeon

Not a fix, per se, but an update in GURPS Vehicle Expansion 1 [p. VXi27].

Under Extra Detail -Reactive Armor Plates (Late TL7), it is noted that instead of automatically destroying a shaped-charge warhead, the reactive armor plate provides DR 1,000 against it (due to powerful enough warheads actually penetrating RAP in real-life). Second, it introduces Heavy reactive Armor Plates which provide DR 2,000 (by being substantially thicker than the older plates). They weigh more than would be expected if they were just scaled up "old" reactive armor plates but multiplied by the surface area of the tank, shouldn't exceed the weight guidelines you've indicated.

Here's the thing, it does and does by a lot. For an M1-style tank, I discovered that even if I halved the combined turret and hull surface area (as only front, left, and right is used), 'light' ERA only weighs about 6 metric tons and almost 50 metric tons if I use 'heavy' ERA.

Well out of line for ERA plates, as a 3rd gen 'heavy' ERA plate of Russian make only weighs 5.7kg/12.5663lbs per brick without attachment bolts, and a full set of 137 bricks weighs in at 2,700kg/5952.481lbs for both hull and turret front/left/right sides. The weight of the GURPS ERA bricks is well beyond that, as stated before.

[Curmudgeon]

I have no idea what you are using for a surface area value, however using fhe surface areas given for Main Battle Tank (TL7) (which is equated to an M1 Abrams) [p. VE141] and the Armor by Facing - Complex Option (p. VE23), I got the following values:

RAP: 400 (turret surface area) * 3/5 (facings) +500 (body surface area) * 3/6 (facings) = 240 sq. ft. (turret) + 250 sq. ft. (body) = 490 sq. ft. * 8 lbs./sq. ft. = 3920 lbs. = 1.78 metric tons.

HRAP: 490 sq. ft. * 90 lbs./sq. ft. = 44,100 lbs. = 20.05 metric tons.

The 90 lbs. for HRAP does seem a bit much. If it is roughly the same density as RAP, it has 11 times the volume of RAP and, if it was scaled up uniformly in all dimensions, would be 2.2 times as large in each dimension. If on the other hand, all that volume is extra thickness, 11 times the thickness probably should do more than just double the DR.

[GURPS Fox]

Originally Posted by Curmudgeon

I have no idea what you are using for a surface area value, however using fhe surface areas given for Main Battle Tank (TL7) (which is equated to an M1 Abrams) [p. VE141] and the Armor by Facing - Complex Option (p. VE23), I got the following values:

RAP: 400 (turret surface area) * 3/5 (facings) +500 (body surface area) * 3/6 (facings) = 240 sq. ft. (turret) + 250 sq. ft. (body) = 490 sq. ft. * 8 lbs./sq. ft. = 3920 lbs. = 1.78 metric tons.

HRAP: 490 sq. ft. * 90 lbs./sq. ft. = 44,100 lbs. = 20.05 metric tons.

The 90 lbs. for HRAP does seem a bit much. If it is roughly the same density as RAP, it has 11 times the volume of RAP and, if it was scaled up uniformly in all dimensions, would be 2.2 times as large in each dimension. If on the other hand, all that volume is extra thickness, 11 times the thickness probably should do more than just double the DR.

I must be using the wrong surface area, as I was using the SF on the bottom of a GVB sheet.

... and 90 pounds per square foot!?! That's insane.

[Rupert]

Originally Posted by GURPS Fox

I must be using the wrong surface area, as I was using the SF on the bottom of a GVB sheet.

... and 90 pounds per square foot!?! That's insane.

It seems rather heavy, but it's not insane - it's about 2-1/4 inches of steel per square foot. That seems far too much for even a heavy RAP, but it's not outright insane as armour weights go.

The normal RAP weight seems very light to me - 8 lbs per square foot for a metal box that encases another metal plate and a layer of explosive for the weight of 0.2" of steel seems questionable.

[GURPS Fox]

Originally Posted by Rupert

It seems rather heavy, but it's not insane - it's about 2-1/4 inches of steel per square foot. That seems far too much for even a heavy RAP, but it's not outright insane as armour weights go.

The normal RAP weight seems very light to me - 8 lbs per square foot for a metal box that encases another metal plate and a layer of explosive for the weight of 0.2" of steel seems questionable.

In terms of explosive reactive armor, yes it's insane. What little I could google indicates that even 'heavy' ERA is far lighter than what GURPS assumes.

[Fred Brackin]

Originally Posted by GURPS Fox

I must be using the wrong surface area, as I was using the SF on the bottom of a GVB sheet.

I haven't used GVB for a while but if you're talking about the number I think you are that's "total" surface area from all sources. You've got to use a limited armor profile to get your surface area or you're putting RAP on the belly of the tank, the treads and even the underside of the turret.

Also, using the MBT in VE2 as your base tank (yes. it's supposed to be a M1A3) will not give you good numbers for a Russian T-72. The M1 is 33% heavier than a T-72 and proportionately bulkier around the turret and perhaps generally.

Fortunately, RAP appears to have turned out to be a less revolutionary technology on actual battlefields than it was in the conventional wisdom of the mid-90s.

[GURPS Fox]

Originally Posted by Fred Brackin

I haven't used GVB for a while but if you're talking about the number I think you are that's "total" surface area from all sources. You've got to use a limited armor profile to get your surface area or you're putting RAP on the belly of the tank, the treads and even the underside of the turret.

Also, using the MBT in VE2 as your base tank (yes. it's supposed to be a M1A3) will not give you good numbers for a Russian T-72. The M1 is 33% heavier than a T-72 and proportionately bulkier around the turret and perhaps generally.

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

Fortunately, RAP appears to have turned out to be a less revolutionary technology on actual battlefields than it was in the conventional wisdom of the mid-90s.

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.

[Fred Brackin]

Originally Posted by GURPS Fox

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

n.

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