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How to Disprove/Deem Plausible Ridiculous Range Claims

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: D_Hall » Mon Mar 24, 2008 1:53 pm

Solar wrote:Can't forget to calculate what phase the moon is in or the latitude of the shot. :-p Sorry, couldn't resist.

LOL. While i've never done phase of the moon, I assure you that latitude isn't that horribly difficult.
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Unread postAuthor: koolaidman » Tue Mar 25, 2008 9:17 pm

Thats not too bad mark, those equations seem about right. I was producing a simular formula cause i dont have ggdt. Your's makes good sense, but im not sure you need to incorporate work, energy, or integration. I started off by saying that: range=v cos(angle) t. You can make v a function of the acceleration from the force of the average pressure in the barrel and T should be a the time the procetile stays in the air- t=-2v sin(angle)/g. I used mine more closely for a mortar that i made and the final equation had only pressure and the angle as variables. What i showed above was just a bit of what i actually did and i took more into account, i dont want to explain it all, im just trying to say what you did might have been overkill.
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Unread postAuthor: jimmy101 » Wed Mar 26, 2008 2:40 am

Markfh11q, it's a lot easier for the combustion gunners to do a similar analysis.

For a combustion gun you just need to calculate the energy in the chamber based on it's volume and the heat of combustion of the fuel.

Compare the result with the kinetic energy of the round. If the KE is more than ~10% of the chemical energy in the chamber then something is wrong. Combustion spudguns are rarely much more than 10% efficient. If the observed KE is 15% then it is remotely possible. If it is any more than 15% then something is wrong with the velocity or mass of the ammo (or volume of the chamber or ....)

If the claim is based on distance, instead of a measured muzzle velocity or the ole "it was really loud so the round must have been supersonic", then the external ballistics analysis is required like you suggested.

Some of the wilder claims for combustion guns would require gun efficiencies of well over 50%. That just ain't going to happen with these guns. I think it can be pretty safely stated that nothing you do with a combustion gun (including hybrids) will ever get the efficiency above about 30%. And, it is very likely that no combustion spud gun yet built has even gotten to 20%.
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Unread postAuthor: mark.f » Wed Mar 26, 2008 8:34 am

Yes, I remember using this approach when BigBang supposedly measured supersonic golfballs with a standard combustion.
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