Myth Buster explosive question

[davidmash]

I was just watching the green hornet episode (orig air date 12/15/2010).

At about 57 min into the show they blow up a bulldozer. They use some amphor (sp) with what I think were blasting caps. They show the scene in slow motion and you can see a light travel up the lines to the caps, you then see an explosion (an here is the thing I don't get) then you see light traveling down three lines back to the detonator? Is it a closed loop of some sort?

The light I am referring to is not an actual light (I just don't know how else to explain it). It kind of looked like that cord you light with a match and it burns really quick (it was lighting fast on the show).

Anyone know what the deal is?

[Aquaticedge]

Probably PrimaCord or other explosive cording

[davidmash]

But why the loop? I get a cord to the explosives but why have a cord going away from it after the explosion already happened?

[Aquaticedge]

no idea

[t walgamuth]

An electrical circuit must be a loop, maybe that primer cord is similar.

[Eskimo]

Originally Posted by davidmash

But why the loop? I get a cord to the explosives but why have a cord going away from it after the explosion already happened?

Redundancy?

[C Sean Watts]

It's called 'detonation cord' or 'primer cord.' I believe there is a brand called PrimaCord but I'm not sure if the military still buys it. I'm sure it's still around for mining, etc. Cord is more reliable than old timey fuse.

I didn't normally deal with 'bang stuff' every day but sometimes I did. The way the engineers explained it, it carries a good but small shock wave or 'over pressure' all the way almost to the end. The wave ends just before the cut end. The wave actually pushes out a little of the packed powdery/crystal explosive when it gets there, and dies off. You want the wave to go all the way through and actually surround the cap or charge you want to set off. You use more cord than the actual length to get a good shock wave. If you don't plan on any more shots before the roll expires, you normally just set it off rather than let it go bad and have to dispose of any potential hazmat.

THE GOOD part of the story...in '94 I was on a testing range standing by with a couple of walkie talkies and an ambulance (sadly, Amber Lamps wasn't there or it would not have been such a boring week.) At the end of the week, the OIC (officer in charge) asked me to stay an extra half hour on that Friday and promised I would have fun. We had to get rid of some bang stuff expiring that month. IIRC, it was 4 lbs of TNT, two half rolls of det cord (something like 100 meters) and over 30 lbs of C-4 He let me set it all off. We used wire > electric cap > good det cord > crimp cap on the end of three winds of the other end of det cord > one 1/4 lb block of fresh TNT in the center of the pile of expired stuff. It was all in about a 6 ft. deep pit, all sitting on top of a 5 ft piece of I beam. The beam looked like some kind of twisted modern art after. To this day I have a reverent respect for explosives.

[layback40]

amfo is the correct term. Short for ammonium nitrate / fuel oil .
It is actually quite hard to set off. It needs a very good kick to get it going, hence the loop to insure it gets a kick in several places. I have seen soft drink bottles of it set off with a cap in the top & only 1/2 goes bang. The other 1/2 is still sitting there unreacted.

[Walrus]

layback40 is partially correct on the term (ANFO for Ammonium Nitrate and Fuel Oil). The material is not actually classed as an explosive, due to its desensitized nature (is classed as a blasting agent). He is also correct in that it does take quite a kick to ignite, and must be dry (does not tolerate water).

Ammonium Nitrate is not the same grade as fertilizer, and fuel amount (diesel)is critical.

Burns at a rate of approximately 13,000ft/sec (slow), but generates enormous gas pressure. Used extensively in the mining and quarrying industry. Can be ignited with SOB (slip on boosters) of essentially C-4, or other cast boosters (high energy explosive "primers"). Electric, NONEL (non-electric), and electronic caps are typical means of detonating the booster, but "fuse" still works and fuse caps are still available.

a 50# bag can be set on fire and it will not detonate, but just smolder/burn...

Det cord (PrimaCord, etc), shock tube, and percussive caps are usual means of setting off a charge. In quarrying, a typical "round" would consist of ANFO, 1# cast Booster, cap, det cord, shock tube, shotgun primer, and stomper (literally stomp on the device to fire the shotgun primer, which sets of the powder in the shock tube, which sets off the powder in the cap line, which detonates the PETN in the cap, which detonates the cast booster, which sets off the ANFO.

Did not watch the Green Hornet episode all the way thru, so I will not comment on the lines of "fire" seen, other than to say, it is not too uncommon to use redundant lines, one to fire, and a second if the first does not set off the charge. It is therefore possible that in the case you mention, two lines were run, and the det cord burned from the initiator to the charge, and back along the second line. If the first line had not fired, the second would have been fired with another initiator, thereby allowing for a second attempt without someone having to return to the charge to "re-wire"...

Incidentally, IIRC, detcord burns at about 18,000ft/sec. It can be wrapped around a tree several times, and cut the tree down... Lain out on the ground, it will produce quite a "bang" itself, which is why shock tube is used to run from the initiator to the round (too help control noise), rather than all det cord, as was done in the Mythbusters episode... Of course, cost is also a big factor!

All in all... Kinda cool!