What triggers a bomb to explode?

I was wondering if bombs (conventional and nuclear) explode on impact, or is there a detenator? In general, what is the mechanism that triggers the bomb to explode? Please mention if you are explaining a conventional or nuclear bomb.

Also, with nuclear, how is the nuclear reaction prevented from going off until it is desired to start? I mean, a nuclear reactor has control rods, so what controls a nuclear bomb prior to reaching its target? …Or, am I totally misunderstanding? - Jinx



Bombs can have any number of detonation methods, including on-impact, time-after-impact (for tunneling bombs), or altitude (for airburst bombs) among others. And this is only for bombs dropped from aircraft or lauched with a missile.

An atomic bomb is detonated when the fissionable material is compressed to a critical mass using conventional high-explosives. before then, it is sub-critical and wil not explode.

Early bombs exploded on impact.

Later ones had propeller fuses on threaded shafts. As they dropped, the propeller would turn and screw in the shaft. At a predetermined depth, the shaft would set off the bomb. This allowed airbursts.

Now it’s mostly all electronic sensors. For example, from howstuffworks, you find out that the sidewinder missile (AIM-9X) has an infrared device that tells the electronic fuse when it is next to the target, and then it explodes.

Is that what you were asking?

The trigger mechanism.

Generally, primary charges blow base charges. A fuse will have a small explosive that can be detonated by electricity, or a chemical reaction, or percussion, and that small explosive triggers the larger base charge.

Same way rifle cartridges work, for that matter.

Well, not in WWII. The fuze propeller was a safety feature which prevented the firing pin from initiating the explosion. A wire which was connected to the bomb shackle ran through a small hole in the propeller so as to prevent its turning. When the bomb was dropped the wire stayed behind and the propeller was free to turn. It then unscrewed and that released the firing pin so that on inpact it could initiate the charge. Any delay was built into the fuze by adjusting the length of the powder train between the initiator and the detonator. If the bombs had to be salvoed, shackle and all was released from the rack, the wire then stayed in place so the fuze remained in “safe” and the bomb wouldn’t detonate.

Artillery shell fuzes had the same feature. The firing pin was prevented from initiating the charge by fingers that engaged detents in the pin. The fingers disengaged by centrifugal force from the rotation of the shell, like the weights of a fly-ball governer and that armed the shell.

As far as I know there were no WWII aerial bombs that provided an air burst. Late in the war, radio-actuated proximity fuzes that initiated air bursts were developed for artillery shells.

The trigger for a nuclear bomb is an extremely complex system. Without going into, um, too much detail, I used to work at a Department of Energy facility (in Miamisburg, OH) that built the triggers for our nuclear bombs. It’s no cakewalk; there were literally dozens of buildings and hundreds of scientists & engineers working on the systems. We’re talking about exacting systems with sub-nanosecond timing, micron mechanical tolerances, and absolute reliability…

And yea, I know how they work. But I won’t discuss it. Yes, some of it was classified and/or secret, but we were taught to not even discuss the unclassified stuff. It’s the ‘ol “need to know” principle…

I don’t think that is quite right, the purpose of the “propeller” was as a safety feature. When the bomb was dropped from an aircraft a release wire is pulled out allowing the propeller turn, as the the bomb falls the propeller winds in some sort of screw thread that completes the fusing mechanism ready for whatever it is that actually detonates the bomb. This allows the bombs to be handled safely - they cannot go off while being loaded into the aircraft even if you drop it.

Ahh, now I remember, on the V1, there was a little “propeller” and its purpose was to cut off the fuel to the engine after a certain distance flown. Perhaps that is what Joe_Cool had in mind.

Of course, it is not a propeller as such, “turbine” or “vane” would be a better expression.

And, in fact, it is quite analogous to the way you build a campfire. You first light off some easily ignited kindling which then ignites slightly heavier pieces and things build up gradually until the logs are burning. Of course, in an explosion it all happens in a millisecond or so.

The first nuclear bombs were very simple, they had a big sphere of fissionable material (uranium or plutonium) with a plug cut out, and a sort of gun barrel to fire the plug into the sphere. The sphere-with-a-hole was subcritical, but adding the plug would take it over critical mass and start a chain reaction, which leads to a nuclear explosion. The trigger doesn’t have to be real exact on one of these, just something that slams the chunk into the sphere. They weighed in at something like 10 tons apiece, so weren’t exactly suitcase-nukes.

Modern nuclear weapons take advantage of the fact that ‘critical mass’ is more accurately ‘critical density’, and use a carefully constructed pattern of conventional explosives to crush a much smaller chunk of fissionable material into a dense enough ball for a chain reaction. This requires a lot less fissile material, but is also a lot harder to do - the conventional explosives need to be set off in a very exact pattern, and finding exactly how to set up and trigger the explosives is complicated work (and extremely secret). They can be much smaller, though ‘suticase nukes’ are still pretty hard to make.

http://www.fas.org/ has a lot of information on nuclear weapons, treaties related to them, and who probably has them.

Riboflavin, you are confusing the two entirely different designs of the first atom bombs.

One used the “gun barrel” design to fire a cylindrical plug of U[sub]235[/sub] into a hollow cylinder of U[sub]235[/sub].

The other used conventional explosives to collapse a hollow sphere of Pu[sub]239[/sub].