[stephen f. polyak]

Further, the term "flak", although a German term, became, I believe, part of the lexicon of allied airmen in WW2, as they were subjected to the fury of various degrees of German anti-aircraft artillery.

High explosive projectiles/shells employ the effects of blast produced by detonation (resulting in shock wave and over-pressure loading) to inflict physical damage to aircraft structures; fragmentation adds to damage potential by perforating and weakening structures. Resistance to these effects may be gained by structural strengthening, load path redundancy, and incorporating damage tolerant design attributes. Thus, fuselage sections with two skins, while not weight friendly, would appear to be an attempt to otherwise reduce the lethality of HE munitions.