rab@ginger.Berkeley.EDU (Robert A. Bruce) (08/30/90)
From: rab@ginger.Berkeley.EDU (Robert A. Bruce) In article <1990Aug28.042707.29047@cbnews.att.com> you write: >They were originally designed with an >immune zone of 18,000-30,000 yards against the 16"/45 (on the North Carolina >and South Dakota classes) and 21,700-32,100 yards against the 16"/50 they >carried themselves, and these are respectable immune zones. > Could you explain what an immune zone is? I checked a couple of my reference books, and they don't explain it. Thanks. -bob rab@ginger.Berkeley.EDU [mod.note: A shell fired from a battleship's gun, like all projectiles, follows a ballistic path. At short ranges, the path is rather flat; at longer ranges, the shell arcs higher through the air. At any range, the angle of descent (AOD) of the shell can be calculated. The shell's penetration of armor depends upon this angle. The ship can be hit on either its vertical (side) armor, or its horizontal (deck) armor. The higher the angle of incidence against the armor, the less likely the shell is to penetrate. Best penetration, of course, occurs when the shell hits perpendicular to the armor. At short ranges, the AOD is small, so that the shell will be more likely to strike the side armor, and will do so at nearly a perpendicular angle; penetration is optimum. As the range increases, so does the shell's AOD; incidence is therefore higher against the side armor, and penetration decreases. Further, it becomes more likely that the falling shell will strike the deck armor, and as the AOD increases, the deck will be struck closer to the perpendicular, and penetration will improve. Penetration is also a function of the velocity remaining with the shell; at longer ranges, penetration decreases. A battleship's armor is compared to the penetration of the gun in question (typically, for design, this is the same gun carried by that ship). All modern battleships were capable of penetrating their own armor at point blank range; but as the range increases, the point will eventually be reached at which the side armor cannot be penetrated; the ship is "immune" to side hits at all ranges beyond this. Similarly, even if a deck hit were obtained at point blank, the angle of incidence would be so high that the shell would glance off. But as range increases, the point is eventually reached where falling shells can penetrate the deck armor. The ship is "immune" to deck hits at all ranges below this. The range between the inner (side immune) and outer (deck immune) points is called "the immune zone." A ship within this zone is immune to serious hits (i.e., against magazines and engineering spaces) from the gun in question. In some cases, the immune zone can be negative; the deck can be penetrated at ranges shorter than that at which the side is immune. This is a bad thing. Immune zones were a major consideration in WWII US warship design, and ship's captains were provided with charts showing their immune zones against the estimated performance of various enemy guns. - Bill ]