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Comment by jules | Hacker News Reader

I'm calling bullshit. I did that many times and never blew any significant hole in the ground (though because of impatience I started using 220V -- not a great idea). Yes it makes a loud boom, but a 2 foot hole no way.

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The joy of being 12/13 is that you're still in that cusp between child and semi-adult. You've got enough motor skills and knowledge to be dangerous, but still have the childlike sense of awe that can make things look significantly larger than they are!

> still have the childlike sense of awe that can make things look significantly larger than they are!

Considering some of the fishermen[0] I know, that one never goes away.

[0] and guys I subsequently shared a locker room with

1) Your 220 made mostly steam and little browns gas.

2) We had partially buried the bottle before setting it off.

Certainly, that experiment didn't last more than a tenth of a second. Lighting a 3L bottle produces about 20kJ if my calculation is correct, so if you bury the bottle in sand or other loose soil I can see how it can move that, but well then most of the hole is because you dug it. If you have a bottle full you can safely light that with the cap off while holding the bottle in your hand, though I wouldn't advise doing the same with the cap on (you certainly won't lose your hand or anything, but it may be painful).

For the people wondering, here is a video:

http://www.youtube.com/watch?v=HmcaC0vxaSo

Interesting. Considering we never took the cap off and lit it off with the spark, wouldn't the amount of energy in the bottle go up with the pressure? Some people were saying here that a 3 liter might take as much as 250psi. We waited until the bottle was visibly distended. How would the calculation vary with pressure? I'd be interested to know the best(worst?) case scenario!

The calculation is already best/worst case. I simply calculated the enthalpy of 3 liters of the mixture. The energy due to the pressure buildup is negligible. If you didn't make sure that there is no air in the bottle, then the energy would in fact be considerably lower simply because then the bottle is mostly air (the way to make sure is by submerging and filling the bottle with water and wait until the bottle is completely filled with gas, and then closing the cap under water).

BTW, speaking of pressure it's unfortunate that these days the soda bottles are so thin & weak, some can't even stand 5 bar anymore, they used to be a lot thicker and stronger.

> Considering we never took the cap off and lit it off with the spark, wouldn't the amount of energy in the bottle go up with the pressure?

I have been playing a bit with a potato/hairspray cannon. If you don't have a good seal (that's why you use a potato), you get no boom. I tried to shoot snow, but it would just go hsssst.

I'm thinking the initial combustion raises the pressure enough for the fuel mixture to detonate, and if the seal is't good enough, the fuel will just burn gradually.

I agree. Though only if he used a plastic bottle. My father and I did this when I was in middle school. He (stupidly) held it in his hand. All it did was make a very loud boom, and not a two foot hole in his arm.

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