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Fracture of U.S. 90mm Solid Shot AP


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Based on penetration data for U.S. 90mm AP shot available during WW II, M77 AP and T33 APBC, penetration of the Panther glacis should have been a regular occurrence at short and medium range.

At ranges of 1250 yards and less, 90mm T33 APBC penetrates 3.15" (80mm) at 55 degrees impact (source is TM9-1907). 90mm M77 AP penetrates the Panther glacis at about 400 yards. Both ranges assume Panther hull is facing gun and ground is level.

Despite the apparent ability of 90mm solid shot to defeat the Panther glacis at many combat ranges, the U.S. reports on 90mm gun effectiveness usually sound disappointing. This raises the possibility of other factors turning a round that should be effective into something much less.

Research into the issue has turned up many references that point to a shatter tendency on the part of solid shot 90mm rounds.

SPACED ARMOR, by A. Hurlich (WAL 710/930-1, November 1950), points out that when 57mm M70 and 90mm T33 solid AP was fired at a 1/2" plate at 30 to 60 degrees, 26 of the 57mm rounds went through the armor without shatter but 26 of 28 90mm AP shattered against armor that presented insignificant resistance. The 90mm shatter was attributed by Mr. Hurlich to longitudinal wave phenomena particular to 90mm AP, since the 57mm round was relatively long and thin and should have shattered before 90mm AP.

The page from spaced armor that discusses 90mm T33 AP shatter against a thin plate has been loaded into the Files section at Yahoo! Tankers.

Further evidence is provided in the penetration data for 90mm T33 and M77 solid shot. TM9-1907 shows T33 shot shattering against armor at 30 degrees that should be easily penetrated (penetrates 4.85" on half the hits at 2800 fps but should do about 6.5"), and the penetration range curves in THE VULNERABILITY OF ARMORED VEHICLES TO BALLISTIC ATTACK show M77 with a 1.80 slope effect at 30 degrees impact (about 1.25 to 1.30 is expected).

During the December 1944 tests against two Panthers (subject of recent discussion about picture submitted by Packfanone on Yahoo! Tankers site), 90mm M77 AP lodges in the Panther glacis at 50 degree impact even through complete penetration probability is almost 99% probability, and only one of two hits against 55 degree glacis armor penetrate despite a very high success probability per hit.

It would seem that something other than penetration-vs-armor resistance ratio's significantly impacted U.S. 90mm solid shot, and seriously detracted from the projectile effectiveness. 90mm T33 APBC is the same as M77 AP except for a ballistic cap (windscreen) and heat treatment to a higher hardness.

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Recently surfaced data strongly suggests that solid shot AP without an armor piercing cap has significantly lower slope effects than APCBC-HE at angles from 45 through 65 degrees. This result is rather surprising since common wisdom is that sharpy pointed AP outpenetrates APCBC at small angles but is inferior at steep slopes since the sharp nose is a handicap (tends to bend, which absorbs energy and decrease sloped penetration).

Review of newly found data for 6 pdr, 17 pdr, 90mm T33 and 90mm M77 solid AP without armor piercing caps results in similar slope effects at similar T/D ratio's. Following compares solid AP slope effects vs APCBC-HE at similar T/D ratio:

55 degree impact, T/D=1.00

Solid AP, 2.17

APCBC-HE, 2.55

45 degree impact, T/D=1.00

Solid AP, 1.79

APCBC-HE, 1.82

65 degree impact, T/D=1.00

Solid AP, 2.91

APCBC-HE, 4.00

The British report on German 75mm and 88mm Ammunition at Oblique Angle states that solid projectiles should have superior slope effects, since the absence of an HE burster cavity increases the bending resistance.

For angles from 45 to 60 degree, the following equation describes the slope effect multipliers for solid AP without armor piercing cap (^ indicates that previous term is raised to following factor):

slope effect = A x (T/D)^B,

where A = 0.7152 x 1.02047^(angle)

B = 0.07791 x 1.00949^(angle)

At 65 degrees impact, the slope effect equation is:

2.908 x (T/D)^0.1864

When 90mm M77 or T33 strikes the Panther glacis at 55 degrees impact, the slope multiplier will be 2.15 and the effective vertical resistance will be 172mm at 0 degrees.

When 90mm M82 APCBC-HE hits 80mm armor at 55 degrees the vertical resistance is about 200mm at 0 degrees.

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