I realise that "bomb" can refer to all sorts explosives, but I am mainly interested in air-dropped bombs. Ever since reading about the USS Forrestal fire, I wondered about the safety of bombs when stored or transported in general- but in plain English: suppose I were able to hold a bomb in my hands- say, the B61 Nuclear bomb or the GBU-10 Paveway II- would it be okay to drop it on the ground? How often could I drop it before it damages the bomb? Is there a minimum speed/impact required that could be reached without using an aircraft?
In general a modern commercial/military explosive device will not explode if dropped. The most common commercial/military explosives such as C4, TNT, ANFO are catagorised either as Secondary or Tertiary explosives, which means they are fairly insensitive to shock/fire and require a severe shock (usually from a detonator) in order to explode. Tertiary explosives being the most stable. However, if the detonator is attached then they can be very sensitive. Which is why detonators are never attached until just before detonation and are always stored separately from the bulk of the explosive material. The shock required to set off an explosive is different for each explosive.
As an example, I was involved in a commercial explosive test involving 3 simultaneous detonations of C4. One of the detonators failed while the other two charges successfully detonated. This sent the third charge of C4 flying through the air due to the shock waves of the other two explosions (and could be seen on high-speed video) and it did not explode. Obviously a dangerous situation to locate and diffuse the unexploded charge, but everything was fine in the end.
In principal there is no limit to how often you can drop/reshape the explosive. However, common sense would obviously require explosives to be handled very carefully.
As mentioned already the bulk of the actual explosives used in munitions ect is pretty stable in itself. Clearly bombs need to be stored, handles and transported, often in difficult conditions so large quantities of sensitive explosives would be as much of a hazard to the user as the enemy.
The precise method for detonating a bomb will vary according to its intended use and may include impact fuses (sometimes with a time delay), timed fuses, altitude fuses, proximity detection (especially for air to air missiles) etc etc. For example bombs intended to create large craters may have an impact fuse with a short delay to enable them to penetrate the ground for some distance before detonating. Conversely anti-personnel weapons may be most effective detonated some distance above the ground.
Typically the detonator and associated mechanism won't be fitted untill it is required for use and there will be safety procedures to ensure that this is only done at the correct time, including visual indicators that a detonator is fitted.
In addition there may be other safeguards, for example a safety pin which prevents the detonator from actuating unless it is removed. For impact fuses this could be as simple as a cap over the fuse.
There may also be additional systems to ensure that a bomb cannot detonate untill it has been properly released from the aircraft. A simple way to do this is to have a small turbine at the nose of the bomb which is operated by the airflow over is as it falls and must turn a certain number of times before the bomb is armed.
This problem is somewhat more complex for artillery shells which need to be sensitive enough to detonate when required but not be set off by the forces involved in firing them.
So in summary
- A bomb will usually require a detonator to be fitted before it is capable of exploding at all.
- There will be a variety of additional safety features intended to ensure that the bomb isn't detonated untill the desired time, varying depending on the intended use and sophistication of the design.
- The type of fuse used will depend on where the bomb is intended to be detonated relative to its target.