Turret Armor

Discussions about the history of the ship, technical details, etc.

Moderator: Bill Jurens

marty1
Member
Posts: 219
Joined: Tue Nov 02, 2004 6:31 am
Location: Seattle

Post by marty1 »

An additional comment on spaced armor...

It wasnt so much to increase effective thickness or limit velocity as seen by an attacking projectile as much as a means of contain behind armor effects of penetrating shots. Spaced arrays were considered a means of containing blast, fragmentation, and high velocity spall -- similar to modern spall liners seen in tanks, IFV and APCs.
User avatar
Dave Saxton
Supporter
Posts: 3148
Joined: Sat Nov 27, 2004 9:02 pm
Location: Rocky Mountains USA

Post by Dave Saxton »

The Italian tests involved AP shells up to 381mm, and it showed effective thickness to be roughly equal to the sum thickness of the plates, plus a bit more. A 1:1 ratio does seem to be possible, even with large projectiles.

We do need to consider the material properties of plates used. Early 1920's tests were done with materials less advanced than the materials used by the Italians during the 30's. The British felt that one critical factor was shear strength of the resistance plate. Shear strength can be quantified as a percentage of the utimate tensile strength. By increasing the tensile strength of the resistance plate, the shear strength of the resistance plate is also increased. The system must be designed so that the thickness and strength (hardness) of the resistance plates and the pelimanary plates are such, that the critcal and limit velocities are manipulated favorably. Just using three decks 50mm thick will probably not give an effective thickness 150mm. Having a system, such as a deck 50mm over another one 100mm, is totally a different thing, and can give 150mm effective provided the material properties are condusive.

The Germans do metion that some of the reasons for scarps, and the location of the panzer deck was to deal with plate and projectile debris.

I'm willing to give Paul the benifit of a doubt. Perhaps I just don't understand.
marty1
Member
Posts: 219
Joined: Tue Nov 02, 2004 6:31 am
Location: Seattle

Post by marty1 »

Dave:

I am aware of the Italian approach -- although it is indirect knowledge through British attempts to imitate the Italian results. The British appear to have been somewhat under-whelmed when they conducted there own tests of this Italian philosophy using super caliber 14” APC.

But as I have already said – reports on the testing of spaced armor present a very mixed bag of results. Under some testing conditions a spaced array can show itself to be a bit more effective than a single plate of equivalent thickness. Under other conditions the spaced plates proved themselves to be far less effective than a single plate of same thickness.

As far as giving someone the benefit of the doubt -- I am all for it. But I see no reason why this sort of thing should not be the subjected to additional scrutiny. I personally am not inclined to take these sorts of things on blind faith, and he did choose to post. I have asked for clarification on the form of the equation being employed, as well as what the results of the equation are supposed to imply. If these are unreasonable requests, than I withdrawal my questions ;) Besides, without an understanding of the basic form of the equation, any further discussion of the equation is impossible.

Sincerely
Marty
User avatar
Dave Saxton
Supporter
Posts: 3148
Joined: Sat Nov 27, 2004 9:02 pm
Location: Rocky Mountains USA

Post by Dave Saxton »

A comment on spalling and spall control. I have enccountered some 1960's US Army research data and one that partially deals with this problem. The Army found that when two homogenous plates are layed over each other (laminated) the back plate will usually spall out reducing the effective thickness. They found that two plates touching each other must be strongly bonded, forming a composite plate. Even then, a defect in the bonding will produce spalling and reduced resistance. Furthermore, they determined that in a composite plate, the outer material should be harder than the inner material, and the inner material more tough. These are both homogenous materials, not a FH concept.

The army found that when two plates are spaced, there is a minimum spacing required to prevent spalling of the second plate. If the interspace distance exceeds the min requierment, then spalling of the second plate is unlikely. They also concluded that whith spaced plates, the inner plate should be harder and stronger than the outer plate.
marty1
Member
Posts: 219
Joined: Tue Nov 02, 2004 6:31 am
Location: Seattle

Post by marty1 »

Dave:

I have seen the same sort of conclusions that both pre-date and post date the US Army findings you refer to.

One theory I have reviewed suggests that part of the reduction in limit velocity seen in spaced array tests is a result of fragmentation and high velocity spalling from the first plate impacting the second plate. It is suggested that these fragments and spall act like a series of precursor projectiles striking the second plate before the actual AP projectile impacts the second plate. High velocity fragmentation from the first plate will spread out in a cone shaped pattern behind the plate. The closer the second plate is to the first plate the more focused the fragmentation impact pattern, and the more focused will be the energy imparted to the second plate by fragmentation and spall from the first plate. If you pull the plates progressively further apart, the area of the second plate impacted by secondary fragmentation becomes larger and larger, and the spall energy imparted to the second plate becomes progressively smaller and smaller per unit area.

The thicker the first plate the greater number of fragments developed and the greater number of precursor fragments striking the second plate. I think the interest in spaced arrays in naval architecture is for decapping APC projectiles before they strike cemented armor. The thinnest possible initial plate that is deemed sufficient for decapping action is desirable in order to limit fragmentation and spall damage associated with the projectile perforating the decapping plate. But this approach is well documented and does not really require a new equation, or new model. This is simplified into an AP projectile vs. Cemented Armor.

The old standard for determining limit velocity of conventional large bore steel projectiles against multiple armor steel plates used to be

VLa = SQRT(VL1^2 + VL2^2 ... + VLn^2)

Where VLa = Limit Velocity of the Spaced Array as a whole
VL1 = limit velocity of the first plate by itself
VL2 = limit velocity of second plate by itself

I have an ADM report laying about here somewhere that details the efficacy of this particular approach.

A solved example – lest there be any lingering doubts as to how to apply the above equation ;)

Projectile is say 3” APC, weight = 15-lbs
The First Target consists of a spaced plate array – 1” plate followed by a 3” plate both at 0-degrees slope.

The second target consists of a single 4” thick plate.

Using the 1931 USN Universal Equation we get the following for the First Target – the spaced array:

VL1 for the 1” plate ~719-fps
VL2 for the 3” plate ~1504-fps

VLa = SQRT(719^2 + 1504^2) = 1667-fps

For the Second Target a single plate that is 4” thick
the limit velocity is ~1886-fps

The difference works out to be a +13% advantage for the single 4-inch thick plate.

marty
User avatar
Dave Saxton
Supporter
Posts: 3148
Joined: Sat Nov 27, 2004 9:02 pm
Location: Rocky Mountains USA

Post by Dave Saxton »

Hi Marty,

It would be nice if we could simply work a solution for each plate in turn using a simple equation, but this may not give us an accurate result. Most formulas consider striking velocity and striking angle, but the characteristics of the shell, and also the resistance properties of the plate are equally important, and only very advanced calculations take these accurately into account. In a properly designed spaced array, the characteristics of the projectile have been significantly altered. In an improper design, the degree of change in this crucial factor may be insignificant Nonetheless, in a good arrangement, the shell is now de-capped, and if it did not strike the pelimanary plate at a right angle, it will be yawed. Both these factors reduce the shell's penetration capability. It no longer has the cap to act as a sledge hammer on the next plate, and yawed impacts always require more velocity to obtain penetration. (here's one place where any difference in effective thickness may be made up) American firing range data on yawed impacts from 1941 indicate, that a yawed impact requires an increase in velocity. Both these circumstances shift the nessasary velocity for penetration upward. The shell needs more energy, but has much less usable energy, because it has lost speed, and it now has less mass. Moreover, the shell is no longer protected by the cap, and additionally, a yawed impact subjects the shell to greater desructive forces. This shifts the critcal velocity for shell breakup downward. These are what a designer must accomplish. It seems to work out to about a 1:1 ratio of effective thickness overall, PROVIDED THE SYSTEM IS DESIGNED PROPERLY ( and if the materials possess greater shear strength). In a right angle impact, the inducement of post penetration yaw may not be that significant, therefore less than overwhelming results may be possible. Fortunately, almost any impact, on armour of a ship, will be oblique to some degree.

It can get very complicated in a hurry, but the Germans took a slightly different approach than using very complicated equations at each little step along the way, and simplfied matters somewhat. The Germans knew the the amount of energy (from actual firing range trials) a given plate type, at a given thickness, would consume, vs a given shell type and caliber, considering the plates relative angular positions, plus an arbitrary surplus. They then designed their systems accordingly. They also factored in the calculations for critcal velocities of a degraded, post initial penetration shells. Thus all four important factors in armour pentration were accounted for, and quantified.
Post Reply