When do bombs blow up?

[Thorlifter]

Sure.....when they hit the ground, but what if they are shot with MG's or cannons? You never hear of bombers being shot up and exploding due to the bombs being hit. So maybe that answers my question. Maybe the answer is only when the detonator is pressed.

 

[Shortround6]

It is possible to detonate a bomb with gun fire.

Please note the word possible.

Normally it takes a "high order" explosion to detonate military explosives. That means the detonator has to initiate a shock wave in the material that exceeds the speed the speed of sound in that material while getting getting it to ignite. Most bomb fuses used a train of 2-3 different explosives in ever larger quantities to go from the firing mechanism to the setting off the main bomb filling.
However High Explosives can be a bit fickle. Some bombs had up to a 20% dud rate in service, bad fuses or?
Even bombs dropped on concrete from thousands of feet would often split open and spill the HE in chunks across the surface without detonating.
on occasion some bombs did go off with much less provocation.
You can often shoot blocks of HE with rifle bullets without the HE detonating.

However in an aircraft bomb bay (or under wing) one MG bullet out of dozens may hit a fuse and the much more sensitive explosives inside. You could have a defective batch of HE or a defective bomb casing. Most were painted or varnished inside to keep certain explosives from reacting with the steel/iron and forming very sensitive "salts"from forming over time.

Hitting bombs with exploding ammunition, like 20mm shells gets a bit trickier and German 30mm shells is a whole different story. Not to say it is 100% certain but it sure shifts the probability on the scale of possible outcomes.

 

[pbehn]

Bomber command aircraft on night raids did sometimes explode with a direct hit. It could happen on the ground too as this picture shows.
434 Squadron - Airfield Activity - Lancaster explosion, KB-832, WL-F (1st of two images)

 

[stona]

Some Bomber Command crews liked to carry a load largely made up entirely of GP, MC, SAP or other hard case bombs rather than incendiary and HC bombs, referring to such loads as an 'armoured belly'.
This implies it was unlikely to explode except in exceptional circumstances, I'm sure a direct hit from a flak shell or heavy cannon might have done the trick.
Cheers
Steve

 

[tyrodtom]

Some unfused bombs detonated at Takhli , Thailand in 1967 or 8, a whole revetment of about 100 750lb bombs high ordered or low ordered, a forklift axle ended up over a mile away.
No one survived at the scene, but no bomb could have been dropped from much more than 6 feet, because the stacks were only that high.
When that Lot of bombs were examined at other places, they were found to have faulty linings, which allowed the explosives to get into the rough inner texture of the cast iron shell.
Mass produced bombs are not perfect.

The fuses probably cost more that the bomb, but their safety feathers can be overcome by brut force.

You can shoot a bomb with about any size weapon and nothing will happen, if it's built as designed, but don't expect that with a fuse. So don't be shooting at fuses.

I've read several accounts of a bombers entire load detonating when apparently struck by AA, or maybe a fighters gunfire.
Those would have been bombs with unarmed fuses. But since in every case no one on the aircraft survived , no one can say for certain what happened.

 

[bobbysocks]

just like the food you eat...explosives have either a chemical preservative or their purity in the manufacturing process guarantees a long shelf life. this is also the reason most homebrewers usually meet an unhappy ending...they cannot match the purity or do not have all the proper ingredients to ensure stability. Homebrews degrade quickly....certain ones become dangerously unstable within 72 hours. for commercial and military manufactured products how the explosive was stored...the temp, weather, handling, etc all contribute to help meet the shelf life or seriously shorten it. sometimes conditions in the field do not allow for proper handling and storage and some people do not understand how badly explosives can deteriorate. nothing worse than a pissed off piece of ordnance.. the less stable the explosive is the more it is prone to sympathetic detonation or detonation by minor shock. that is why EOD usually blows the stuff up in place when old stuff is dug up. lifting and the jostling of a truck ride may be enough to start the fireworks..

 

[msxyz]

Explosives used in ordnance are tested and ranked according to their 'sensitivity' to different kind of external forces. These are mainly external heat sources, direct flames, high velocity impact (ie. gunfire), shock (extreme G forces or hammer like impacts) and sympathetic explosions. To complicate matters further ageing and exposure to moisture or contact with certain metals may turn an otherwise stable explosive into a dangerous to handle substance.

Take for example TNT: it is hard to ignite (it's not even a good fuel for a fire), hot castable and insensitive to shocks (hence its use in shells, displacing ammonium picrate, which is equally insensitive to shocks but more prone to catch fire and deflagrate). Yet a TNT iron bomb directly hit by an explosive shell (i.e. a 30mm ammo) could probably burst as a consequence of the shell explosion working as initiator.

Speaking of initiators and fuzes, sometimes those are the weakest links. Unlike the main payload of a bomb, which is composed of relatively tame explosive compounds (and which are further phlegmatized by the addition of wax, vaseline, rubber and other moisture absorbing agents), fuzes contains explosive which are easier to ignite, that's why bombs are transported (and sometimes loaded onto an aircraft) unprimed. Many a times, a 'high energy event' that would not detonate the main explosive payload of a bomb, resulted nevertheless in a big boom because said bomb was primed and ready to go!

 

[soulezoo]

April 1973. Roseville Railyard incident.

I was about ten miles away... bad juju.

 

[tyrodtom]

Read up on the USS Forrestal fire of 1967.
Use of probably improperly stored AN-M65 ( Korean era munitions ) escalated what would have just been a fire on deck into a major tragedy that took over 130 lives.

 

[Shortround6]

About the only thing you can say with 100% certainty about high explosives is that there is nothing 100% certain about high explosives.

 

[bobbysocks]

yeah I completely overlooked the initiators/detonators they are usually way more sensitive than the main charge. if they deteriorate, setting them off would be almost a matter of looking at them cross eyed. back when I was in elementary school in the early 60s there were signs hanging in each classroom warning us not to pick up or play with the detonators. they had 2 dummy ones fixed to the sign....one with wires and the other with cannon fuse. there were all kinds of mines...coal...clay..etc around and I did stumble across more than a few of them in my youth.

 

[parsifal]

normal procedures for aircraft carrying live ammunition aboard that is not expended over the target is to discharge or expend that ammunition in full before returning to the carrier. The risk of cook offs is considered too great and the spread of fire too great to risk a live ammo being carried back on board.

It is usually also preferred that the aircraft land with tanks as near to empty as possible. Maybe its different in wartime, but an aircraft landing in military conditions with full tanks is not something you want as a deck officer.

 

[vikingBerserker]

China's 30th Squadron lost 5 of their Martin 139 bombers when during a Japanese raid one of the fully loaded bombers was hit by shrapnel which started a fire, which reached the bombs and exploded, destroying the other 4.

 

[msxyz]

Read up on the USS Forrestal fire of 1967.
Use of probably improperly stored AN-M65 ( Korean era munitions ) escalated what would have just been a fire on deck into a major tragedy that took over 130 lives.

I've read that those old bombs contained compound B explosive, which becomes more unstable with age, moisture and contact to metal.

I'm not saying that if they where loaded with composition H6 or any modern 'safe' explosive, the incident wouldn't have happened, but probably the fire fighting crews would have had more time to dump them overboard before the heat from the fuel fires detonated them.

 

[Milosh]

The bombs should have been able to stand 10 minutes of fire but went off in less than 2 minutes.

 

[tyrodtom]

The modern ( for the Vietnam era ) Mk84, , Mk83 and other bombs of that type had been tested for resistance to fire and had taken about 10 minutes to cook off.
But there was no similar test done on a new AN-M65, and these bombs were not new.
Everyone who saw these bombs knew they had some ill stored bombs on their hands, and wanted to roll them overboard into the ocean.
But they were over ruled by mission requirements, and those higher in the chain of command

 

[Fighterguy]

Bombs have a fuse that is pinned, similar to a hand grenade. The fuse is armed by a little turbine, either at the front or rear of the bomb, that the previously mentioned pin prevents from spinning during loading and flight. The pin is attached to the bomb rack with a lanyard. When the bomb is dropped, the lanyard pulls the pin, allowing the arming turbine to spin from airflow. The turbine has to make a pre-determined number of rotations before the bomb fuse arms. Artillery shells have a spring tensioned centrifugal switch, the shell has to spin at a certain RPM in order for it to arm. Some bombs were equipped with a proximity fuse (like the one used for ant-aircraft munitions and also used on artillery shells) so the bomb would detonate at a pre-determined altitude for greater explosive circular error probable (CEP) effect.
I've seen missile warheads and motors survive significant crashes, with accompanied fire, without exploding or cooking off. During weapons loading integrated combat turns, we had a chart with the particular munitions explosive CEP and probable cook of time when engulfed in fire so we knew how far to run in the event of a mishap.

 

[stona]

A lot of bombs were dropped 'safe' in WW2.
The British system, on bombers, had the safety wires (lanyards as you call them) wound on drums. In 'safe' mode the drum rotated freely, so any ordnance dropped took the wire with it and could not arm itself. When switched to 'live' the drums were locked by solenoids so that they retained the wires, which were pulled from the departing ordnance, allowing arming vanes to spin and the bombs to arm. A load dropped 'safe' was obvious when an aircraft returned because the safety wires had gone with the bombs.
I don't know how the US system worked, but the Americans managed to make estimates of the percentage of loads dropped safe too.
Cheers
Steve

 

[Fighterguy]

Excellent point Steve. Emphasizes the fact that it takes more than just sheer impact to cause detonation. Some bombs are purposely set to have a delayed, or when jostled, fuse system as a denial measure. Much like the BBC television series "Danger UXB." When touring the Normandy Invasion sight at Pointe du Hoc, there were numerous signs warning people to stay on the marked path because of this hazard. We continuously performed unexploded ordinance sweeps during post attack exercises while in the Air Force because of this.

 

[fastmongrel]

The explosion at RAF Fauld in 1944 was one of the biggest explosions ever. RAF Fauld explosion - Wikipedia

Even such a big explosion didnt set off all the bombs in the mine only approx 1/3 went off.