The recent, too soon, death of Nathan Okun occasioned me to read four of the articles he published in Warship International in the 1970s and 1989. These are well written and referenced. [I found them in JSTOR, where you can view 100 articles free per month.] In the second of those pieces [1977, Vol. 14, No 2] Mr Okun gave a list of penetration formulas. One of these I had not seen before, the US Immunity Zone Slide Rule Mk Class A formula of the 1930s. Mr Okun at that time recommended it as a general class A formula. It was used by US experts in one of the USNTME reports in an analysis of German armor. The IZSR was notable in having no scaling [scaling exponent was zero], which was interpretated as Mr Okun explains, as shells breaking up or shattering on impact with the armor. Evidendently, if a shell breaks up into smaller pieces on the outside of the armor scaling is unlikely to be a feature in the results. Apart from that its dependences were quite close to those of the de Marre formula for nickel steel armor. Its exponents in the expression for T/D were 0.75 for M and 1.43 for v, compared with, respectively, 0.71 and 1.43 for de Marre. [Here I have rounded to two decimal places rather than five as in the historic practice.]
There also appears a graph taken from Rep. 380-45 in the USNTME series. [I have managed to find a list on the web of all these reports but not of this report or most of them.] This graph is of penetration in calibres against striking velocity for KC armor. Data points listed as belonging to shells in the L/4.4 series are plotted ranging from 15 to 40.6 cm calibre. Three lines are drawn through the data of which the one for Q = 0.9 was the best fit. This quality factor is a measure of armor quality relative to British KC of WW1 period. As a result of this the US experts concluded that German KC of 1934/35 had not changed since the original Krupp type of the 1890s. This graph was also used as a test of the universal Krupp formula and the IZSL with the former being wrong and the latter correct. Mr Okun says that the Krupp formula was correct for modern homogeneous armor but incorrect for KC type armors.
Under construction
There also appears a graph taken from Rep. 380-45 in the USNTME series. [I have managed to find a list on the web of all these reports but not of this report or most of them.] This graph is of penetration in calibres against striking velocity for KC armor. Data points listed as belonging to shells in the L/4.4 series are plotted ranging from 15 to 40.6 cm calibre. Three lines are drawn through the data of which the one for Q = 0.9 was the best fit. This quality factor is a measure of armor quality relative to British KC of WW1 period. As a result of this the US experts concluded that German KC of 1934/35 had not changed since the original Krupp type of the 1890s. This graph was also used as a test of the universal Krupp formula and the IZSL with the former being wrong and the latter correct. Mr Okun says that the Krupp formula was correct for modern homogeneous armor but incorrect for KC type armors.
Under construction
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