RE the burst disc idea, double bezel barrel has the same effect -it cuts the tater slightly bigger than the inner diameter of the spud. You can feel the difference when you push it back - a lot more friction. Have any of you timed hang time for spud if you shoot straight up? I think it's an easy way...

If you're using an inline design where you go from a large diameter chamber to a smaller diameter barrel..and fill from the back of the chamber...you could have the barrel extend into the chamber right? Then load the tater from the reservoir-side and press it onto the breech to the point it is part...

Question: does lubing the barrel help at all? It seems logical, but it might decrease force on potato in the end

now that is smart..i will definitely double bezel the barrel. We are metering the fuel, using syringes. My chamber is using 195ml of Propane, so that's basically 2 X 100ml syringe. I think the screw-on back was weakened by the holes we drilled for the fan. I hope this does not happen again, otherwis...

very interesting, thanks. It's always more complex that you'd first imagined - something I've learnt along the way. How about this explanation for the inertness of molecular nitrogen VS the unstable-ness of acetylene: the activation energy for the combustion of acetylene is very low (kinetically fav...

That's interesting... if reactant bonds need to be broken in order to start the reaction, then one would think that the activation energy is equal to the bond energy of the reactants. OK, I need to go read up and do a bit of research - any good links? So why is acetylene reactive, while molecular ni...

OK...the back screw-on end blew into bits today.. using LPG (70% propane, 30% butane), calculated that we needed 245cm3 of fuel for a 69cm long combustion chamber (11cm diameter)...yes, it's huge. Barrel = 2m. It was my fault..loaded the spud parallel to the barrel..note to self - don't do that agai...

Activation energy is equal to the bond energy of the products. Two ways to calculate heat of combustion of change in enthalpy. One is like you described, the other is by subtracting the energy released from the energy absorbed (where the energy released on the energy profile is from the highest peak...

Ok...this makes sense: double and triple bonds between carbon atoms are pi bonds, as opposed to sigma bonds (single bonds). There's stronger overlap with sigma bonds, compared to the weak overlap between p orbitals for additional bonds. So, alkene and alkynes are more reactive compared to alkanes......

120psi, that's more than 8 bar. The combustion chamber PVC is rated to 2 bar...not good. I did wrap a lot of duck tape around it, should it explode, hopefully that would mitigate the effects of flying shrapnel. Maybe then I'd rather stick with lower friction. How any of you experimented with lubing ...

Got the triple spark gap to work. Wow..it makes a difference. And, I'm using metered propane with a syringe. I calculated that I need about 187ml, taking into account the air that is pushed out when the propane goes it...man, does this work. Honestly, I had shots where the spud went out of sight. We...

Thinking of the syringe method. My chamber is 49cm long, and 11cm in diameter. Got a volume of 4656 cubic cm, or 4.656 liters. So, 21 percent oxygen = 978 cubic cm O2 in chamber. According to balanced equation for combustion of propane, I need 5 times more oxygen than propane, so around 190 cubic cm...

parallel!!! can't believe I missed this... going for triple spark again. Thanks for all the great info. Jes, of course, the bigger the molecule (hydrocarbon), the more oxygen is needed....so, though you get more energy per molecule, you need more oxygen. Bottom line: less butane (mol) can combust in...

This is one thing I don't get...heat of combustion, the bigger the hydrocarbon molecule, the more energy can be released during combustion, so butane has a higher heat of combustion compared to propane, and pentane would be higher than butane (albeit liquid at room temp). In theory, pure butane shou...

My chamber is roughly 284 cubic inches. How do I used a syringe to get my 200ml propane? Thanks shot off the old spud canon today. Let me just come out and say this: I hate multiple spark because it doesn't work! Current flows through the path of least resistance, and air had quite high resistance, ...