What was the difference in function and purpose between the impact fuse on the MK-III atomic bomb and the contact fuse on the MK-5 atomic bomb?

Answer 1

The Mk-III had radar and barometric fuzing, as it was intended to be detonated as an air burst. However, since nuclear weapons were so secret, the Mk-III (Fat Man) atomic bomb had four AN-219 piezoelectric fuzes installed in its nose, which would destroy the weapon, and prevent any recognizable fragments of the weapon, and elements of its design from being salvaged by the enemy, that might give them an idea as to how it worked.

The contact fuze on the Mk-5 nuclear bomb was used as a "salvage fuse," in other words, in case the radar fuses failed and the bomb impacted the ground, the contact fuses were meant to set off the firing circuits to try and ensure a nuclear detonation. If the bomb hit the ground, it would probably explode, but the explosion would not create the symmetrical implosion which was necessary to produce a nuclear yield.