Does a nuclear bomb blow up on contact with the ground?

Answer 1

A nuclear bomb blows up when critical mass is reached. Critical mass is some amount (depending on the fuel--uranium, plutonium--and how pure the material is. A nuclear bomb typically has two pieces of the fuel, one in the head of the bomb and one near the tail. when the bomb hits the ground, the piece from the tail slides forward to combine with the piece at the head.

First, a nuclear bomb designed as above could not work, all it would do is heat the two pieces of fissile material to melting point before they touched causing a "fizzle" yield explosion accomplishing nothing but scattering molten fissile material around the area producing significant alpha emitter radiological contamination. Any nuclear bomb requires explosives to "assemble" the piece(s) fast enough to supercritical condition and prevent a "fizzle" yield.

A nuclear bomb (as any other bomb) blows up when triggered by its fuse. There are many types of fuses:

• altitude - this fuse is for airbursts, when the fuse determines the bomb has fallen to the set altitude the fuse blows up the bomb
• contact - this fuse is for surface bursts, typically when an accelerometer in the fuse detects sudden deceleration the fuse blows up the bomb
• depth - this fuse is for subsurface bursts (underground or underwater), when the fuse determines the bomb reaches the set depth the fuse blows up the bomb (underground bursts are either for cratering or destroying hardened installations like missile silos, underwater bursts are usually for destroying submarines)
• lay down - this fuse is for low altitude precision delivery by airplane to let the airplane escape the blast, the bomb is dropped by parachute when an accelerometer in the fuse detects sudden deceleration the fuse starts a short delay timer, the parachute gently lays the bomb down on its side on the ground, when the delay timer runs out the fuse blows up the bomb
• time delay - this fuse is designed to let the bomb bury itself underground where it "hides", on impact the fuse starts the timer which may be set for hours to months or even longer, when the timer runs out the fuse blows up the bomb
• vibration - this fuse is designed not to blow up the bomb unless it is disturbed, then an accelerometer or "tilt switch" mechanism triggers the fuse to blow up the bomb
• booby trap - this fuse is designed only to blow up the bomb if someone is actively attempting to disarm the bomb
• etc.

Answer 2

No, a nuclear bomb does not automatically detonate upon contact with the ground. It requires a specific sequence of events to trigger the explosion, typically initiated by the activation of a detonation mechanism controlled by the bomb's operator.