British Tank Development.

If we're looking at the 1.25" armour on the front of the Crusader III turret it's completely vertical though, isn't it? So Peasant's calculation would seem to hold true.

 

Thanks for interesting data. Honestly, after pondering a while on this, I'm not sure how to interpret these statements.

Thanks author. Why was this considered worthy of note? The very next paragraph states that:

Obviously then it is not immune at shorter distances.

And at the end the author contradicts his previous statement:

A much lower figure. Maybe he mixed up the figures for the basic (20mm/30°) nose plate and the uparmored (20+14mm/30°) one?

Also, how would they examine the rear of the nose plate for signs of a bulge with an open crack to determine the Ballistic Limit if it's riveted to a half in. solid steel plate? Is this then supposed to be the Ballistic Limit for the entire assembly (20+14mm of RHA + 12.7mm of mild steel frame)? That's 46.7mm in total and are we supposed to believe that it's less resistant than a single 34mm thick RHA plate? (1750yrd vs 1530yd)?

Some photos I found that show the internal structure of this tank:







For reference, the minimum immunity spec for a vertical 40mm I.T.80c armor plate is 1375f.s:



Since this report is from Aug. 1942 the specs would've changed in the I.T.80D iteration but I doubt the figures would've been drastically different.


Edit: Interesting thread: Armor on all Crusader tanks still wrong

 

The backing plate in that instance is 0.75 in. of IT 110, which was defined as carbon manganese steel backing plate, and commercially known as D1 steel. This was a slightly harder type of mild steel, and was the same material that prototype tanks were made of. I haven't got any armour trials against the Crusader III front turret plate, although I don't have every Crusader armour trial, so it might have been tested. It would have been less effective than 50mm of IT 80 RHA, though.

I should also say that the Crusader II and III had a 50mm armour basis (not applied everywhere), and it was only the Crusader I that had a 40mm basis, so Peasant might need to readjust his calculation to take this into account.

 

The 1300 yard figure came from the Middle East, which I suspect was from an impromptu test. ME probably didn't test for a further distance away, which this trial did.

This plate was set at 30 deg. from vertical, which is the 1750 yd immunity (i.e. directly ahead). The 2200 yd figure is probably from normal, although this isn't explicitly stated.

There's no mild steel frame there. There is just 20mm of IT 80 RHA plus 12.7mm of IT110 carbon manganese steel backing plate (i.e. D1 mild steel). So really the quoted 34mm should be 32.7mm.

I will upload the full report for you so you can ponder it yourself. So keep an eye on your PM's.

 

Don Juan Alright, I've looked through that report and here's what I got to say:

What initially threw me off track was the fact that the the armour schematic I posted shows the I.T.110 plate behind the nose to be exactly 0.5in. thick, while on the actual vehicle it was 0.562in. (~14mm), which confused me, as the total thickness reported (20+14mm=34mm) was the same as that of base I.T.80 plate and the 14mm add-on armour plate that was supposed to be welded on to up-armour Crusader tanks.

The nose armour plate (20mm) plus the 14mm thick rear plate (called "skin" in the report) at 30° obliquity have indeed been perforated by the 2pdr AP projectile at 1320fps, which, I concur with the writer, is equivalent to the performance of a single, solid 28mm I.T.80 RHA plate set at 30°.

Therefore the 14mm thick rear plate of I.T.110 added 8mm of effective thickness to the ballistic resistance of the 20mm frontal plate, which is only 57% of it's geometric thickness.

Extrapolating this result to the turret face of the tank, (assuming the schematic is accurate this time) the 0.75in. thick rear plate of I.T.110 would in turn add ~11mm of effective thickness to the turret front, which would result in 43mm of effective thickness.

Using the NPL equation and my own external ballistics calculator, I estimate that the german 37mm Pak 36 gun firing full caliber shells would be able to perforate the turret front of these early Crusader II tanks at up to 350m distance and the nose plate up to 600m.

 

This is at normal, yes?

 

Yes, the worst case scenario, no side angle.

I have collected the data I have on the ballistic performance of mild steel against AP projectiles, here is some data:

This is from this document here.

I have applied this formula to the scenario above, 125 BHN mild steel vs 255 BHN RHA and the results match the experimental data exceedingly well:

 

Found among my documents partial specs for the british I.T.80 revision E armour. Unfortunately only for these three thicknesses, the rest had to be extrapolated.

T(mm): Min. BL spec I.T.80E (fps):
40 1320
45 1445
50 1570
55 1695
60 1820
65 1945
70 2070
75 2195
80 2320

values extrapolated by 25fps per mm of thickness, as instructed in I.T.80C spec document.

 

I was working on something and thought you might be interested to see this graph:



As you can see, extra ~0.2calibers of armor thickness is usually enough to stop high velocity rounds at normal.

 

What does calibre mean in this context? Is it the calibre of the 2 pounder (i.e. ~40mm)?

 

Yes.