I was recently reading a What If articles on. One pointed me to you guys which is cool but more to the point a source used was from Cornell University. It is titled Studying the Internal Ballistics of a Combustion Driven Potato Cannon using High-speed Video and can be found here: https://arxiv.org/abs/1305.0966
The conclusion: Use acetylene.
In the ten minutes since I wrote this, I have found on this site an entire thread on the dis-merits of using acetylene. I suppose it's not for beginners.
Anyway, welcome me, thank you.
Re: Best fuel type - Cornell University
Posted: Wed Oct 25, 2017 7:44 am
by jackssmirkingrevenge
It's generally agreed by most practitioners of this hobby that if you want to dabble in higher pressures, so-called hybrid combustion cannons that use safer fuels like propane ignited in a mixture well above atmospheric pressure is the safest way to do so.
Also,
[youtube][/youtube]
Re: Best fuel type - Cornell University
Posted: Wed Oct 25, 2017 5:45 pm
by Zeus
Acetylene is not really the safest fuel for the newbie, despite being expensive, bulky and requiring regulation, it's also somewhat prone to detonation when severely mishandled. Those dangers are overplayed, but still, who wants to cart around a 30lb bottle to let out a few golf balls. If you really want a performance boost, oxygen enrichment from those wee little 40 gram canisters is a much better choice, or a hybrid. Not to mention you're limited to 15psi preignition pressures with acetylene (partial or relative, I don't know).
That said, if you want to build an acetylene powered spud gun, by all means do it! You get an impressive burn rate and secondary carbon burn, both of which are quite impressive.
There is something wrong with the report at https://arxiv.org/abs/1305.0966
Studying the Internal Ballistics of a Combustion Driven Potato Cannon using High-speed Video
E.D.S. COURTNEY AND M.W. COURTNEY
The numbers in Table I don't make sense. The "amounts between stoichiometric and the upper flammability limit" doesn't make sense. The massively better results with acetylene doesn't make sense.
208 cc of propane in a 1470 cc chamber is 14% (v/v)
160 cc of butane in a 1470 cc chamber is 11% (v/v)
Looks to me like their results are basically measures of how well the fuels perform if you use a grossly over-rich fuel mix.
Their muzzle velocities are terrible. Given the gun dimensions and ammo, propane and butane should be hitting about 100m/s instead of ~30m/s.
The air force academy really should be able to do better than that POS report.