"Sloped Armour - Why?" Topic

OK so everyone knows that sloped armour is more effective for a given thickness than vertical armour. So for example a 50mm thick plate sloped back at 30 degrees from the horizontal gives the same line of sight thickness as a 100mm vertical plate.

While this is all fine and dandy, it doesn't save any weight – which is the main limiting factor for tank design. Our 50mm sloped plate now weighs just as much as the 100mm vertical plate as it must have twice the area.

Therefore why bother with sloped armour? Isn't it just as good and probably simpler to use a really thick vertical plate?

So for instance
Tiger I hull 100mm vertical = 100mm Line of sight (LOS)
Panther hull 80mm at 35 degrees = 140mm LOS
Churchill VII hull 152mm vertical = 152mm LOS

(Armour thickness calculator here link )

Would the Panther have been just as well off having a 140mm vertical plate? (Though of course it would have looked far less 'cool').

"Aha!" You exclaim – "What about shots bouncing off the angled armour then?"

Two things here –
Has anyone quantified how much of an effect this has?
If it's a major effect why do most wargames rules effectively give the Panther 140mm armour?

If it's not a major effect – then back to the slabby Panther question….

I'll admit my trig is rather rusty – so might well have screwed this up but would like to know if I'm making any sense or not!

Sloped vehicle armor looks better than slab-sided armor.

deflection of shot compared to a flat vertical slab.?

Actually, it does save weight. Take 45degree slope. Horizontal 1 unit, vertical 1 unit if they were the same thickness (I know,the horizontal wouldn't be) then 1 squared plus 1 squared = 2. Square root of 2 = about 1.42. Pythagoras theorem.

Actually, it does save weight. I think you're forgetting the horizontal component. Take 45degree slope. Horizontal 1 unit, vertical 1 unit if they were the same thickness (I know,the horizontal wouldn't be) then 1 squared plus 1 squared = 2. Square root of 2 = about 1.42. Pythagoras theorem.

GarrisonMiniatures, you're forgetting that the plate needs to be longer to stretch across that 45 degree angle than the straight up and down plate needs to be to reach the same height. In fact, the plate needs to be 2^0.5 times as long (1.42 times as long), which exactly offsets the reduced thickness that's possible.

Although the horizontal components are not always as thick as the vertical, GarrisonMiniatures has it.

A French tank like the FCM 36 wasn't all angled plate to take advantage of ballistic performance (though doubtless there was some) it cut all those corners to save weight (being built down to a weight specification as I understand it)..

That's right of course GarrisonMiniatures, but the 45 degree sloped armour doesn't have double the LOS thickness

eg 50mm plate at 45 degrees = 70.7mm LOS thickness ie 1.42 ish times as effective as the vertical plate for 1.42 times the weight.
(Edit as emckinney says)

Agreed on the horizontal component but this is either comparatively thin top and bottom armour or gives you the opportunity to make the tank shorter!

Edit – as Garrison and Phil say the horizontal is more of a consideration on the lighter, less well armoured tanks ie FCM, even MkVI etc. But presumably far less of penalty comparatively to the heavy tanks where the top and bottom armours are far thinner than the glacis.

Timbo,

The early single-turret Soviet T-26 is an extreme example of why sloped armor can sometimes be extremely efficient. The turret was completely cylindrical, which meant that there was a lot of wasted space at the top circumference. Consider what needs to fit inside the turret and where it needs to fit. Men are relatively broad in the shoulders and narrow at the head, so you need more space to accomodate them near the base of the turret than at the roof. So, going to a slightly conical design meant that the same thickness of armor plate gave more protection. (The surface area of the "side" of a truncated cone being less than the surface area of the "side" of a cylinder of the same height.)

I just realized what Garrison Miniatures was getting at; I was about to make the same point. By sloping the armor you're also reducing the size of the turret roof, so you're saving some weight there.

Now that does make sense emckinney.

However in the situation of a hull front glacis if you don't need to slope than you can make the hull a bit shorter surely?

Sloped armor is more aerodynamic.

To be serious, there has to be (one would think) some additional loss of penetrative power when striking an angled surface as opposed to a perpendicular one. The additional complexities of construction and internal layout with sloped armor would make thicker vertical plates the clear way to go if there were not some effect beyond the strictly geometric thickness.

One could, I suppose, break the vector of the incoming fire into components perpendicular to and along the impacted surface, with the supposition that the proportion of force applied along the surface is not as effective as that applied perpendicularly to it. This certainly seems reasonable, but it's undoubtedly a gross simplification, and I've no idea what the experimental results for such a scenario look like. The experiments HAVE been done, however, and people keep building things with sloped armor, so presumably there's an advantage.

Of course, people also keep building things with slab sides and nasty shot traps, so who's to say?

Doug

No doubt you're right Doug, but I'd like to at least start to understand why.

Or to put it another way, should the Panther glacis be the equivalent of 140mm perpendicular or more? If so how much?

I am wondering if its all due to needing somehwere for the driver to put his feet :-)

Surprisingly, wiki actually has some fairly accurate info on the subject:

link

As the article points out, the main defensive benefit comes from deflection effects, not weight savings – although they do mention the "fits the crew better" issue as well, which directly relates to weight in the end.

To quote the synopsis:

"The final effect is that of deflection, deforming and ricochet of a projectile. When it hits a plate under a steep angle, its path might be curved, causing it to move through more armour – or it might bounce off entirely. Also it can be bent, reducing its penetration. However, these effects are strongly dependent on the precise armour materials used and the qualities of the projectile hitting it: sloping might even lead to a better penetration. Shaped charge warheads may fail to penetrate and even detonate when striking armour at a highly oblique angle."

Long-rod penetrators were developed in part to help overcome radical sloping, which worked very well against the shorter penetrators of the WW2 era. I can remember several early boardgames with designer notes that talked about that.

Now that is a useful article Rich!

Deflection seems to be the key for WW2 era (and general shape issues affecting the weight).

Regarding Panthers, can anyone say if the testing of captured vehicles came out with a similar resistance to that expected (ie 140mm) or more?

It seems quite complicated and dependent on the nature both of the armour and the ammunition (short/long, capped/uncapped etc) as to how significant the deflection effect may be, and likely different for different diameter projectiles too…… Tricky!

Would be nice to have some 'rule of thumb' for WW2 armour at least.

Regarding Panthers, can anyone say if the testing of captured vehicles came out with a similar resistance to that expected (ie 140mm) or more?

I seem to recall a book that had a table of penetration from some British testing with captured German armor, but all of those books are in storage. Maybe Forty's "German Tanks of WWII"?

Doug

Shot traps are not a design feature, they are a mistake. People now are just as liable to make them as at any time in the past. We have a bridge a mile or so from me where they ordered the wrong size brackets( at a million a pop) so they cant finish it, and of course each set is unique to that project, so they cant return them. I dont believe they meant to do that.

From memory, Nathan Okuns site has a long series of articles on the subject. I read a lot of them like 10 years ago. I learned a lot, like new concepts such as shear forces and why armor petals when it breaks. Much of which i no longer remember.

I do however remember two good things. One, in most cases sloped is better. Two is that it is a very complex subject, with a lot of variables, some of which normaly defy measuremnet, and there is no moderate simple answer. Meaning simple is sloped is better. there is no good 5 minute answer after that, you need the hour long answer.

Not the answer you where looking for I am sure. It wasnt the one I was looking for when i started looking either. The answer is out there, it just takes a bit to dig it out. I could be wrong about the iste, but I know I looked up some stuff there,and perhaps it has links. I have not looked for a long time.

Owen

Sloped armor was somewhat more effective against old style kimetic energy armor piercing rounds (1940s-50s) than a non-sloped plate with the same LOS thickness. Perhaps more importantly, it is much easier to manufacture thinner armor plates. So even without an additional ricochet benefit, sloped armor is easier to manufacture. There is no weight savings, however.

Interestingly, modern APFSDS rounds are designed to turn into sloped armor on impact. This makes sloped armor plate less effective than non-sloped plate with the same LOS thickness. Of course, modern MBTs have composite armor packages, so this is not a big deal with them.

OK, must be getting old, all i could find on Nathan was Naval stuff. But there is a hell of a lot of it, much more than i could link to, just searcg his name you will get more than you want. I suspect much of the naval data is directly transferable to land, but certainly not all. I am also sure there are links to land based sirtes that will give you the indepth explination you will eventualy want. But, i mst go to work and dont have the time tonight to try to sort it all out.

i know it is there. I still remeber some ot the differences between shaped and kinetic penetraters. Just cant remember quite where it was.

owen

From my days at the US Army Armor Training Center, Ft. Knox we were taught that the main advantage to sloped armor was deflection of incoming rounds. This was true for HEAT and HEP. Less effective against APDS.
Of course, this didn't mean M60 series tanks didn't have plenty of shell traps.

Would be nice to have some 'rule of thumb' for WW2 armour at least.

For 20 degrees from vertical, multiply thickness by 1.1
For 25 degrees from vertical, multiply thickness by 1.2
For 30 degrees from vertical, multiply thickness by 1.3
For 35 degrees from vertical, multiply thickness by 1.4
For 40 degrees from vertical, multiply thickness by 1.6
For 45 degrees from vertical, multiply thickness by 1.8
For 50 degrees from vertical, multiply thickness by 2.1

The above were worked out using a spreadsheet, and rounded off.

It should be 1 over the Cos of the angle of slope for true LOS.

So a 60deg slope should double it.

Slope made a lot of sense in the first half of the 20th century. Of course the best option was thick plate, but the weight is a major problem. Slope was the most cost-effective way to increase protection without increasing weight or having to fiddle with expensive face-hardened plate.

New technology has reduced the importance of slope in recent decades.

R.M. Ogorciewicz goes into some detail on the effect of sloped armour on p.82-3 of 'Design and Development of Fighting Vehicles'. As mentioned above there is variation depending on armour quality and projectile quality and shape, but the rule of thumb is that effective armour thickness is not the geometrically apparent 1/cos (i.e. sec)of the angle from the vertical but for angles up to 55 degrees very close to sec squared.
Thus 55 deg gives 3x (not 1.74x), 45 deg 2x (not 1.41x)

the plate needs to be longer to stretch across that 45 degree angle than the straight up and down plate needs to be to reach the same height

Ah, but you are forgetting that the horizontal plate also needs a vertical plate to join to. i.e. along and up vs diagonal. The diagonal is shorter, in total.

Plus it prevents tea stains as your mug won't sit on it.

There are a number of factors that can have an effect on the slope multiplier. One is diameter of the projectile compared to the thickness of the armor. Another is shell nose shape. This table is from warship international and I believe the author is Nathan Okun, though all I have is the table. The data is for ogive shaped shells. I added a column E which I compiled a number of multipliers for Russian blunt or obtuse nosed shells. There are a large variety of different configurations resulting in a range of values.

(I would say this table over estimates the low angle multipliers as an example the US tank gun tables give only about 1.22 x for 30 degrees.)

The business of rounds turning and shape of them and whatnot.
It's mind numbingly complicated.
You're way better glossing over such stuff in rules design IMO.

Heavier tanks broke down more.
The Panther was originally designed to be lighter.
Someone forgot to fix some of the engine drive stuff when they upped the armour.
Result.
Big drain on them fix it guys.
Abandoned broken down super cats littering France.
Good guys have less panzers to fight.
Huzzah!

A big heavier front plate would have been a bad plan.
Something like a pz4 but angled might have been a better idea.

OK, so I think we're getting somewhere

- The LOS thickness is easy to find (see armour calculator in first post)
- Sloped armour can give extra protection compared with the same LOS thickness of perpendicular armour due to deflection effects
- Exactly how much extra protection is very very complicated to work out and is less relevant for modern projectiles and armours than for WW2 era

Thanks to Mobius, we can have a crack at the 'Panther question' just to get an idea while bearing in mind the above.

So the easy bit Panther front glacis = 80mm at 35 degrees from horizontal = 140mm LOS – this is 1.75x80mm

Looking at Mobius' chart (at 55 degrees from vertical) we find a number of figures depending on different ammo used and different armour used:

1.75 = 140mm – this just assumes LOS thickness with no deflection effect
1.9 = 152mm
2.05 = 164mm
2.1 = 168mm
2.2 = 176mm
2.35 = 188mm
So we could imagine that the slope has added somewhere between an extra 10% to 30% protection over and above that predicted by LOS thickness.

Though I'm sure that AONeill is right and this isn't so relevant in the big picture, but I was just curious ;-)

Couldn't the sloped armour also theoretically result in a slightly lower center of mass (for the same internal volume)?

Not sure if this change would be large enough to have any practical effect, though.

The height of the center of mass of the sloped plate is exactly the same as for an unsloped plate of the same height.

You get a slight advantage from having a smaller roof area and you may get a really slight reduction from having a truncated conical or pyramidal turret. However, that assumes that you're willing to tolerate a reduction of internal volume, as the Soviets were happy to do with the T-26.

A friend of mine who is a Viet Nam vet told me that an RPG-1 round hitting the slightly sloped frontal armor of a M-113 ACAV almost never went off because the fuse failed to contact the plate sufficiently. The same could happen with any HEAT weapon, given sufficient "off-angle".

BTW, RPG-7 didn't have the problem, to the disgust of the armored cav.

Bazookas were particularly prone to this effect.
There was a unit did field tests comparing their bazooka to a panzershreck.
The panzershreck coped with acute angle contacts way better.
Obviously better when shooting tanks but very important when shooting at people in a field. Pretty flat, most fields.

There's some ad hoc field tests carried out shooting panther glacis from 100 yards with 17pdr.
Some just bounced.
Some cracked the glacis.
The hardness and hence brittleness of armour used to make tanks varied.
These panthers were of the previously used variety ( KO'd ) and the "tests" don't seem to have been terribly scientific. It's unclear just how previously damaged the targets were.

This wasn't just a problem for the jerries.
There's an account of one us unit discovering that 50 cal drilled into the armour on their shermans.
Not all the way through but far enough to think a proper AT round would go through easily.

Apologies for the length of this post; I don't have current links for the following reports but they all were (and presumably still are) available on-line. I will just quote enough of each report to give the scope of each of the tests. All contain firing tests specific to the Panther.

Firing Tests Conducted 12-30 July 1944 by 1st U.S. Army in Normandy

Purpose: To conduct tests to determine the effectiveness of tank and anti-tank weapons in First U.S. Army, against the German Mk V "Panther" and Mk VI "Tiger" tanks.

1a. Firing was conducted on terrain permitting 1500 yards maximum range with zero angle of site. All guns and types of ammunition, suitable for anti-tank purposes, available to First U.S. Army were defeated on targets whose armour plate was slightly burned. Upon determination of critical ranges , all penetrations were proven against the armor plate of a German Mk V "Panther" Tank with armor undamaged and in excellent condition. All firing was conducted normal to the target. No firing was conducted against the German Mk VI "Tiger" Tank as there were none available.

1b. The following normal types of tank and anti-tank weapons and ammunition were tested;
Launcher, Rocket, AT, 2.36" Rocket, AT, 2.36", M6A1 Launcher, Grenade, M8 Grenade, AT, M9A1 37mm Gun, M6, Mounted on Light Tank, M5A1 APC M51 40mm Gun, M1, AA AP M58 57mm Gun, M1 APC M86 Sabot 75mm Gun, M3, mounted on Medium Tank, M4 APC M61 HEAT M66 (Special) 3-inch Gun, M5, mounted on Motor Carriage, M10 APC M62, w/BDF M66A1 AP M79 90mm Gun, M1A1, AA AP M7 105mm Howitzer, M4, mounted on Medium Tank, M4 HEAT M67
…

U.S. Army Firing Tests Conducted August 1944 by 12th U.S. Army Group at Isigny, France

Final report of board of officers appointed to determine comparative effectiveness of ammunition of 76mm gun and 17pdr gun.

1. The board convened pursuant to the attached order at the firing range established by First U.S. Army near Isigny, France at 1030 hours, 19 August 1944 and conducted firing tests against the front plate of German Panther Tanks. The firing was continued, as the weather and the availability of target tanks permitted, on 20 and 21 August 1944. Because of the urgency of the test, a preliminary report, dated 21 August 1944, was submitted on 22 August 1944.
…
3. Nature of Test
a. The above ammunitions were fired at the front plate of three Panther tanks.
…

HEADQUARTERS
5th TANK DESTROYER GROUP
APO 758, U.S. ARMY
Effects of Bazooka Fire on the German Mk. V Panther Tank

1. To ascertain from first hand observation the effect of bazooka fire on the Mk. V Panther tank, sixteen rounds were fired at a knocked out 130 Pz Lehr Mk. V. which had been immobilized by the 776 TD Bn's 90mm fire.
…