Make:

• Weekend Project: Blood Spurting Knife Wound (PDF)
• Weekend Project: Hot Glue Web Gun (PDF)
• Weekend Project: Ultimate LED Fan Sign (PDF)
• Weekend Project: Vortex Cannons (PDF)
• Weekend Project: Scanner Camera (PDF)
• Weekend Project: The Bullwhip (PDF)

This is the best weekend podcast yet!

Why?

Because it doesn't have the same tired format of Bre mugging for the camera in the pic.

:)

Putting the jar in the freezer to prevent cracking seems bass ackwards. If anything that will increase the ease of cracking because of the greater thermal gradient. Take a jar from the freezer and dump it in boiling water. Take a jar from an oven and dump it in boiling water. Which do you think will crack?

I think the reason the original article suggested putting the jar in the freezer was to make the jet easier to ignite. You want oxygen to mix with the fuel vapor in the jar. If the jar is too warm and the fuel vaporizes too easily then it will push air out of the ignition chamber. In other words, it gets flooded... Lowering the vapor point of the fuel make this less likely to happen.

Why am I unable to find stuff that can be made easily with matchboxes & rubberbands & things like that?

This was really awesome!
I tried this with a friend today and we had much fun!

For those of you who want to do the same:
The most important thing is the size of the diffusor inlet and outlet, the tubes we bought didn't work well, instead we used miniature flowerpots (of course with a selfmade hole in the bottom).
Since I haven't been able to buy methanol anywhere, ethanol had to do the job and it worked out. Noah is exactly right with his assumptions: ethanol isn't as critical with the temperature so the freezer wasn't really helpful, but if the glass was hot (making the ethanol vaporize) it didn't ignite (If the ignition doesn't succeed, don't even try to light it again, open the jar, blow fresh air into it and retry. If the jar is hot just wait...).

Furthermore the glass always made a clear crack at the bottom of the jar, in the area where the vaporizing alcohol cools the jar down (or at least limits the temperature) and the burning alcohol heats things up (high temperature gradient - high inner tensions...). Tomorrow we will try to make everything in a bucket of water, to keep everything at a similar temperature.
Instead of the ethanol we might try acetone as fuel, let's see what happens.

Nice experiments were other "jar" forms, we tried a very big bottle with a wide opening combined with a very long diffusor, which resulted in a very low frequency of explosions, making it sound like a very big diesel engine (quite impressive). A rather explosive fast burning model was a normal jar with the upper part of a plastic bottle as a diffusor (leaving approximately 5mm distance to the glas at each side), worked awesome for two runs, then the bottle had to be replaced (the small space between the jar and the diffusor "focussed" the explosion very nice probably the most efficient model we built).

Pics and videos might follow...

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