Friday, April 9, 2010

The day math nearly killed me – a cautionary tale about checking your calculations

I was once a soldier, what seems like an eon ago now. As a newly trained junior officer I was given my first command, a troop (platoon) of about 25 soldiers. A week after I took command, I was deployed along with my troop for an exercise on the demolition range. I had run demolition ranges in training, but always someone was there to watch me and catch my mistakes. This day I was in charge, I knew what to do and I was naive.

We were cutting metal with C4 explosives – it was a bit more complex than that, but cutting metal was essentially what we were doing. It was early spring, not yet warm enough to want to spend the day dilly-dallying in the sun, so I wanted to get what I had to be done completed without unnecessary delay. As soon as we arrived, I briefed my soldiers on how the day would go and assigned tasks. I put Sergeant 1 in charge of cutting the time fuse, while Sergeant 2 and myself supervised the soldiers laying out the C4 to cut the metal.

Time fuse is a tricky thing, because a roll of time fuse can't be counted on to burn at the same rate as any other role. So, for every roll someone needs to time how long it takes to burn over a known distance, let's say 1 meter. Then that someone needs to figure out how long it takes to walk from where the charge was to be detonated to the safe area. On this day we had a lovely concrete bunker to hide in for safety, about 200 meters from our detonation spot.

It's important to get this right because shrapnel from cutting metal with C4 can travel up to a kilometer.

Sergeant 1 carefully measured the time it took the burn 1 meter of time fuse. He then took out a stopwatch and walked at a brisk but not hurried pace from where we were working on the charges to the bunker, adding about 30 seconds to his result as an extra safety measure. We wanted the explosives to go off shortly after we got into the bunker. In addition to wanting a nice count down for dramatic effect, it is important that we know precisely when the detonation was to occur because a detonation that doesn't occur when it is supposed to is the kind of thing that can wreck a day.

The next step is to figure out how much time fuse is needed to set off the charge at the appropriate time. So this is what we have:

time taken to burn one meter of time fuse = time taken to walk to bunker / length of time fuse

Which can be rearranged to give:

length of time fuse = time taken to walk to bunker / time taken to burn one meter of time fuse

Sergeant 1 figured out how much time fuse was needed, cut and delivered it to where we were setting up the charges. I should have checked his math – math with times can be tricky.

Sergeant 2 and I waited until everyone was safely in the bunker before we lit the time fuse. We walked at a brisk but not hurried pace towards the safety of the bunker. The bunker door was situated so it was facing away from the explosions so we would have to walk around the building to get inside. When we were about 5 m from the bunker door at the edge of the bunker, the explosives detonated.

We ran into the bunker and slammed the door shut. We could have been riddled with little slivers of flying metal, but neither of us were. And those injuries which didn't happen, they would have been fully my responsibility. Sergeant 1 was very apologetic, I think he expected me to punish him profusely – but I didn't. My squadron (company) commander and the squadron sergeant major had shown up just before we detonated, so I had a little chat with them. Since I accepted responsibility and would never ever make that mistake again, I was allowed to continue. From that day on, I always checked the time fuse calculations. If I did them myself, I had someone check my work. I never had a mistimed explosion again – instead I had to deal with a whole whack of new problems.

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