Northrop Grumman is great at building military airplanes. So why does it want to go back to doing something it's not as good at?Thirteen years ago, military aerospace and shipbuilding giant Northrop Grumman (NOC -0.10%) made the strategic decision to jettison its shipbuilding unit and focus its business on aerospace instead. At the time of the spinoff, the rejected unit, Huntington Ingalls (HII -0.22%), was valued at approximately $2.5 billion in market capitalization, leaving the rest of Northrop Grumman w ...