Normal approach course?

[razark]

I'm just starting to read the old instructions, and trying to get a better grasp on how this was done during the war. I've got a question, and hopefully someone can give me some insight.

From http://www.ibiblio.org/hyperwar/USN/...SS-Doct-4.html:

Normal approach course -- The course which, at right angles to the bearing of target, closes the range.

Also mentioned in http://www.hnsa.org/doc/attack/index.htm#platei

Can someone explain why? Is there something in the geometry of the approach that makes the normal approach course better than another method? Why 90 degrees, instead of 80, or 70, or any other value?

Also, in what situation is the NAC used? Would the NAC be constantly recalculated to keep the target on the 90 degree bearing, or would it be calculated once?