NOSE-ONLY SHATTER
by Nathan Okun (3/24/2019)

In my FACEHARD naval face-hardened armor penetration program, I have a sub-category of projectile shatter (breakup of the projectile on the plate face prior to any significant penetration into the plate) called "Nose-Only" wherein only the upper end of the projectile shatters and the middle and lower body remain more-or-less intact, so that, unless the internal explosive-filled cavity of the shell extends into the broken region, the cavity would usually remain undamaged from this cause and regular fuze-initiated full-power explosion of the projectile could occur (within the reliability of the shell's components as with an unshattered projectile).  This only occurs for a specific set of circumstances, to be specified below.  Other damage may still happen in some cases, of course.

The source of my data are tests using the 1931-issue US Navy 260-pound 8" (203mm) Mark 17 (all MODs) "Special" Common projectile; that is, with a thin, mild-steel, sheet-metal Hood and long, pointed windscreen, the former soldered onto the nose, covering it almost completely with a skin-tight steel layer, to hold the threaded ring for screwing on the latter -- prior Common projectile designs with added windscreens, with the screw thread for the windscreen cut directly into the projectile nose, had suffered unacceptable damage on impact with thick homogeneous, ductile armor due to cracking and nose breakup at the threads.  These shells were not designed to be able to penetrate face-hardened armor of any significant thickness intact -- though, as with an armor-piercing (AP) shell with no AP cap fitted or which was lost due to a prior impact, hitting thin face-hardened plate and remaining undamaged (no shatter and minimal nose and body damage of other types) at near-right-angles impact obliquity was already known to be sometimes possible -- and were designed to penetrate the homogeneous, ductile armor used in most smaller warships, including up to heavy cruisers built under the Washington Naval Treaty of 1923.  Common shells in general had larger explosive payloads than thick-bodied AP shells and would therefore cause more damage inside the target, with these particular shells having about 4.4% Explosive "D" compared to circa-3.3% for WWI-era US Navy AP and 1.5% for WWII-era US Navy AP shells, which were on the low end for explosive weight of all AP shells used by other nations of these time-periods (maximum penetration ability overrode having the highest explosive power after penetration; even a dud penetration was considered better than bouncing off).

Explosive "D" was originally called Dunnite and was chemically ammonium picrate, a unique, medium-power, but extremely insensitive to impact shock, US Army and Navy explosive used in AP or US Navy Common (= US Army Semi-Armor-Piercing (SAP)) shells developed or retrofitted from circa 1900 for the Army and 1911 for the Navy where impact with any form of armor might occur and later, even with WWII Navy High Capacity (HC = HE) shells for use against thin steel plate when using their base fuze by replacing the nose fuze with a hardened steel nose plug.  The only exception to Explosive "D" in new Navy projectiles after 1911 was when the much more powerful Composition "A" was allowed in anti-aircraft projectiles to widen their danger zone when using VT (tiny radar proximity) fuzes when no direct target impact was usually expected -- they did have an impact portion of that fuze if in the unlikely event of a direct hit; they weren't stupid.

These 8" Special Common projectiles were fired at right-angles against 4.92" (125mm) German "Krupp Quality P255" KC n/A armor plates -- the first of the KC n/A type of Krupp post-WWI armors introduced in 1925 -- similar to those used for the main-armament turret faces of the then brand-new German "Pocket Battleships" (11.1" (283mm) guns on medium-sized heavy cruiser hulls) -- the plates were secretly bought and sent out of Krupp's armor plant via Sweden in 1931 and tested in 1933 at the US Naval Proving Ground, Dahlgren, Virginia.  Some US Class "A" face-hardened plates of similar thickness were also tested to get comparable results -- these were older plates made during and just after WWI, since no face-hardened armor was being made in the US in 1933.  The tests verified that the Krupp KC armor was, as expected, somewhat stronger than early-WWI-era KC a/A plates taken from the captured pre-WWI German battleship OSTFRIESLAND and tested about a decade previously using US Navy high-quality, soft-capped 8" and 12" Midvale Unbreakable AP shells at right-angles.

Another result of this test was a surprise to me when I studied it:  When the US 8" shells, which had no AP cap of any kind, merely that very thin mild-steel Hood layer covering the nose, failed to completely penetrate the German or US armor, they shattered into many pieces, as expected, but if the shells, at a higher striking velocity above the Shatter Navy Ballistic Limit (the minimum velocity for the intact shell body or, if broken up as with shatter here, 80% of its body weight pieces to go entirely through the plate and be found on the far side), did succeed in completely penetrating, in almost every case the tapered, ogival nose of the shell was destroyed by shatter, but the cylindrical body remained intact with a jagged, roughly-flattened front end and its filler cavity, though much larger than an AP shell, was not breached.  The shell remained "in effective bursting condition".  This was not expected by me at the time and I had to do some analysis to get some idea why this could happen.  This also did not occur using the new US Navy post-1935 Thick Chill Class "A" armor, with its superior toughness, when Hooded projectiles hit any plate of that kind (the thinnest ever used was 4.5" (114.3mm) at the tapered bottom edge of a cruiser waterline belt plate, with the usual minimum for a single-thickness plate being 5" (127mm) for cruiser barbettes and so forth), penetrating or not.

My idea is due to the time that the forces have to work on the projectile nose through the Hood.  While the Hood is very thin, it does cause some soft-cap-like protection to the extreme tip of the projectile nose, where it is relatively thick compared to the tip size.  This causes the initiation of shatter to be delayed slightly until the nose tip digs into the plate, still intact, and the rest of the nose immediately around that protected tip now slams into the plate as a ring surrounding that tip.  For that ring, the Hood is much too thin to offer any real protection and cracking due to impact shock, which will eventually cause shatter to the nose, begins.  By that time, however, the strong impact shockwave from the tip impact is radiating outward into the plate at the speed of sound in steel (circa 16,000 ft/sec (4877 m/sec), far, far faster than the projectile is moving).  In these older, brittle plates, when the shockwave hits the plate back surface and the impact velocity was high enough to punch a hole in that plate, even if the shell shattered, which is well above the penetration velocity for an intact-nosed, soft-capped shell to punch through at low obliquity (up to 20 degrees from normal), the shockwave reflects, bulging out the plate back before that plate snaps back and throws a reverse shockwave backward toward the face again.  At the rear surface, when the shockwave reflects, its force on the steel there is DOUBLED (action/reaction on top of the initial impact) and the rear of these brittle plates cracks even though they are of softer, more ductile steel than hardened face layer.  These cracks follow the reverse shockwave, at the same high speed, back to the face layer and cause the face layer to suddenly "have the rug pulled out from under it" from behind, with there suddenly being much less support due to the cracks.  The extremely hard and brittle face cannot take any such cracking at all and, combined with the enormous force of the projectile still ramming itself into the plate face, the face under the nose breaks up and is thrown back in pieces, tearing out a large, irregular shatter-hole plug of the face and back combined, on top of the finally breaking-up projectile nose.  However, the projectile nose in most somewhat superior Common and AP projectile types with hardness contours that allow some resilience at the joint of the hard nose and softer projectile middle and lower body (decremental or sheath hardening rather than the abrupt hard/soft joint of the older through hardening used in chilled cast iron and early chilled steel projectiles) can, if the resistance force is low enough and stops quickly enough, remain intact in the tougher middle and lower body even if the high-hardness nose is destroyed.  Such a condition is true for brittle plates, stronger shells, and Hoods to delay the onset of shatter just long enough to allow the plate face to begin breaking up before the shatter of the projectile nose spreads to the rest of the projectile.  Again, as noted, if the projectile fails to completely penetrate, then the time it is in contact with the plate is essentially infinite (as long as it takes the projectile pieces to bounce backward or jam themselves into the plate) and the shatter will spread to the entire shell, anyway.

This effect for the Hood does not work against the later, much tougher armors, with improved post-hardening tempering processes to minimize "temper embrittlement", used by most nations for new ships (post-1930) in WWII and in a couple of types during WWI.  The WWII exception was last Japanese face-hardened armor, their Vickers Hardened (VH) face-hardened armor used for the extremely thick plates of their YAMATO Class battleships, which retained most of the older pre-WWI British Vickers Company Cemented Armor (CA) hardening and tempering processes that they had adopted for their own Vickers Cemented (VC) battleship side armor used from KONGO through the NAGATO and MUTSU, as well as for armor upgrades to their older ships, even though VH used a new steel recipe and stopped using the very hard, but very thin, cemented surface layer used by most everyone else.  The two superior-toughness WWI face-hardened plate types were US Midvale Non-Cemented Class "A" (made from 1906-1912 or so) and Austro-Hungarian Witkowitz Company KC used in their several small, but rather advanced design, battleships.

I assume that Nose-Only Shatter would also happen to soft- or tough-capped AP shells against the toughest armor types when their AP caps fail, but the caps would have worked against the weaker, more brittle older face-hardened armors mentioned in the last paragraph -- that is, a thick soft AP cap does the same thing against armors that the cap always fails against, whenever it would succeed with the older armors, as a Hood does against those older armors themselves.  Whenever the soft/tough cap or Hood does not work against the older armors (for example, over 20 degrees obliquity for soft caps or when the soft cap fails in the 15.1-20-degree Zone of Mixed Results) or any AP cap or Hood would fail against any face-hardened armor, then complete shatter will occur, as is usually the case.

Note also that if the cavity is so large that it extends into the nose region (a few base-fuzed Common and HE shells that have Hoods are of this type), then even if the "Nose-Only" conditions are satisfied, the upper end of the cavity is going to be broken open anyway and the results are not going to be much different from Complete Shatter occurring.  About the only thing that might be true in this last case is that the average size of the filler explosion due to the impact is larger, due to the fuze working and setting off the lower part of the explosive charge that is sometimes still intact and not already undergoing a low-order deflagration or explosion due to impact compression and/or friction.

As a rough estimate, if the cavity size is 5% or more with a soft/tough-capped or Hooded projectile, then the cavity upper end is always broken open even with Nose-Only Shatter.  If the filler weight is 4.5-4.99% or more, the cavity upper end is destroyed 50% of the time when Nose-Only Shatter occurs.  If in the 4-4.49% range (the 8" tests above), then I would guess that the cavity remains intact 75% of the time.  There were not a lot of complete penetrations in these tests and I am sure that more tests would have found more failures even if completely penetrating, from my previous experience.  Only under 4%-filler-weight cavity size, Nose-Only Shatter, if it occurs, never damages the cavity, though other damage can still do so.

[NOTE:  These are cavity sizes for cavities only if completely filled by an explosive filler.  Some foreign nations used more impact-sensitive explosives and had inert cushioning material wrapped around the actual explosive, so they may have had smaller explosive charges, but their actual internal cavity sizes were noticeably larger and, thus, more susceptible to impact damage.  The actual percentage cavity size must always be used here.

For post-WWI "AP"-type shells (as designated by the nation using them):

Japanese cruiser-sized shells always used higher-power, but extremely sensitive, filler types (Shimose (= British WWI Lyddite) for AP and Common shells up to 1931 and Type 91 Explosive (TNA, only slightly weaker and slightly less sensitive than Lyddite, but it could barely be cushioned successfully for inert heavy armor penetration if the shell remained intact) thereafter for new designs.  Their 1928-issue Mark 6/Type 88 AP shells (7.9" (200mm), 14" (356mm), and 16.1" (410mm)), the cavity size was 2.5% (same as British APC shells in the 1920s), even though the filler was smaller, due to thick cushioning.  In their improved-ballistic-streamlining, 1931-issue Type 91 AP shells, the 14", 16.1", and 18.1" (460mm) AP-capped battleship sizes had cavities of 2.5%, as before, but TNA fillers of only 1.5-1.6%, the rest being extensive cushioning material, while the 6.1" (155mm) and 8" cruiser and battleship secondary gun AP shells had 4% Common shell cavities with only 2.5% filler, again with the rest being cushion.

20th Century German AP and Common shells used block TNT fillers form-fitted to the cavity and with felt and paper liners between and around each block.  They also had thin wooden cushion plugs filling the tip of their cavities.  This means that they had about 0.5%-explosive-equivalent larger cavities than given by their actual filler weights.

British shells using TNT fillers after WWI, mostly cruiser SAP-type shells, capped or not, had solid cast TNT fillers with beeswax mixed in and a thin fiber cushion at the tip of the cavity, meaning that their cavities were about 0.2% larger than their nominal explosive percent weight.  Most battleship-sized shells used Lyddite for APC or black powder for CPC up through 1917-or-so and the rather insensitive filler Shellite (Lyddite mixed with a much less sensitive and much less powerful derated form of Lyddite in a 70/30 ratio) after 1917 and had no cushions.  The only exception was the 16" (406mm) Mark IB APC shell for the NELSON and RODNEY, which also used a TNT/beeswax filler, I believe.

Any nations (other than Japan, which obviously went its own way) using British- or German-type ammunition had the same filler adjustments needed.  French post-WWI AP/SAP shells used their own version of Shellite, M.f.Dn, (a different low-power explosive (Dn) mixed with Melanite (their name for Lyddite), but otherwise more-or-less identical) and used no cushions, to my knowledge.]

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