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another DDT question.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: SpudFarm » Thu May 08, 2008 8:58 am

i am going to make a camera and a ignition coil setup, but if i talk to Larda he may digg up a flyback or something for me.

or i run like 10camera caps in one ignition coil
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Unread postAuthor: psycix » Thu May 08, 2008 4:45 pm

MrCrowley wrote:And I can't remember, but does DDT have a higher chance of occurring at a lower mix then higher?


IIRC, in the rule you can divide the DDT runup distance on atmospheric pressure by the amount X mix.
If a certain fuel has a runup distance of 100" at atmospheric pressure, then a 2x mix would only need 50", a 3x mix 33, 4x mix 25" and so on.

Im not sure about what I stated above.
Its just a vague memory.
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Unread postAuthor: DYI » Thu May 08, 2008 4:59 pm

Im not sure about what I stated above.
Its just a vague memory.


A vague memory indeed, from way back when people actually thought that 4x was approaching DDT.

In reality, the Nx has very little effect on the DDT runup distance of propane/air mixes. The increased pressure decreases burn rate, and the increased temperature of higher mixes just barely overcomes that effect. A 20x mix might burn a few m/s faster, but not enough to be really worrisome.
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