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Building a Better (Combustion) Spudgun

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Mon Jun 11, 2012 4:53 pm

DYI wrote:One can only increase chamber pressure up to the point where the projectile is destroyed, at which point it is worthwhile to strive for a flatter base pressure profile. As I mentioned above, even in the case of the combustion with a huge chamber and a very fast burn, base pressure will still decrease as the round moves farther from the breech. Rapid fuel/oxidizer injection during the acceleration process may be able to counter that problem to some extent.
As one of the few people on this board that actually measures things (on occasion) I have to ask ... why? Yes pressure drops during ammo movement but that is easily minimized to the point of no measurable affect with a large chamber. Heck even with CB of 2 the energy lost compared to a continuously injecting more fuel during the firing is trivially small. And, for spuds, probably lost in the normal shot to shot variation in performance.

Why make something as complex as a continuous injection system when you can just make the chamber a bit bigger? Or the barrel a bit longer? Is it really worth all that effort just to increase the muzzle velocity by a bit?
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Unread postAuthor: Fnord » Mon Jun 11, 2012 6:22 pm

In hindsight, starting this thread was probably futile. I don't think I've started a single non-showcase thread in the past three years that didn't turn into a great blob of half-baked "theories", "why do you want to do this" type responses, and total non-sequiturs.


^^ Warning, more of This ahead:


The practicality issue you mention is certainly the case. This would at least rival a modern piston hybrid in terms of cost and complexity. There is one distinct advantage over simply increasing chamber pressure though: it can be less heavily built, and fire potatoes without destroying them in-barrel. Which is pretty much the advantage I was looking for.


I would first tend to disregard potatoes, unless frozen. Used Golf balls can usually be had for cheap if you look around, possibly enough so to be considered 'disposable'.

I have thought about ways one could potentially get more power from something no more complex than a typical 'advanced' combustion, though nothing really stuck out as a practical solution. I often toyed with the idea of a, uhh... Heavy-Gas-Gun(?), where an initial combustion reaction compresses a second fuel/air mix enough to cause autoignition via the dieseling effect.

Unless it's cleverly built it also wouldn't be practical, and obviously pvc would not be an option. We might just have to accept this as a dead-end until some other technology which allows it to be improved is developed.
I wouldn't mind seeing some pulse-discharge-rated ultracaps applied to squeeze a few extra kilojoules into a chamber of fuel, but that's not gonna happen any time soon:)


As to your design, I'm not really sure what materials would be suitable. Your fuel/air injection could easily be tuned to safe power levels, but the potential for danger is there if, e.g. a regulator fails or other mechanical issues arise. A mis-fire of the spark gap could allow some amount of pressurized fuel/air to build in the chamber, before the projectile's static friction is overcome.

A few wraps of fiberglass might be in order if a plastic chamber is used.


Oh, and by the way, "high-level" math is definitely a relative term around here, as I'm sure you've noticed ;)
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Unread postAuthor: boyce123 » Mon Jun 11, 2012 11:10 pm

the diesel idea seems peculiar, and would generate alot of power, the issues would arise out of reinforcing and sealing the chamber. if you know much about diesels, it would become aparent of the compression required for ignition (generally around 700 psi to be on the safe side, which would require the power from a hybrid operating at high numbers of x's). But as the ignition cycle continues, the rididual heat (around 600 C in the chamber) aides further combustion. Diesels get around the inability of cold ignition with the introduction of a glow plug, which seems easy enough to impliment in a spud/golf ball cannon. It may be possible to get around the pressure barrier by using a more suitable fuel than diesel for the second stage. Overall, the idea of utilizing the diesel effect seems plausible, but would have to step beyond the average combustion cannon to a new form of hybrid cannon. If anyone is willing to pursue this, it may be interesing to design it around a coaxial hybrid design, with a segmented chamber (the top chamber being initial combustion, and the lower being the diesel chamber (it seems possible to either use a heavy burst disk or a high pressure piston system). Though the entire concept may be flawed if the secondary ignition ends up putting less energy out than was required to cumbust it. Even if it would be built out of futility (as far as efficiency is concerned), it would be an interesting conceptual idea that could be studied for further development.
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Unread postAuthor: DYI » Sun Jun 17, 2012 10:37 am

@Fnord: although I initially expected the result of discharging a few kilojoules worth of pulse capacitors into a combustion gun chamber would be the initiation of an overdriven detonation, it appears that this would not necessarily be the case for an air/propane mix or other relatively insensitive air/hydrocarbon mixes. I'd be comfortable doing a 5kJ exploding wire shot with my cap bank, but I don't currently have a chamber which would be suitable for it. Either way, using a 300lb capacitor bank as an ignition source for a spudgun isn't exactly the embodiment of practicality...

Assuming that a rupture diaphragm was not used, you might have trouble designing a "heavy gas gun" like you described. There'd certainly be a lot of dependency on the fuel mix used as to whether it was possible to gain anything from it. My LGG project was stalled by the problem of developing an adequate CFD program (without which an LGG is a total crapshoot... ), which I think I solved about two months ago. Finding the free time to actually write the program is another problem altogether :roll:
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Unread postAuthor: natas » Tue Jun 19, 2012 4:16 pm

hmmm...just some brainstorming here... dont cop my head of if im wrong... but how about a combustion/pneumatic hybrid spudgun ? :-)

Ignite the chamber to get up the pressure, then release the pressure with a valve, like a controlled burst disc :-)
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Unread postAuthor: wyz2285 » Tue Jun 19, 2012 5:03 pm

The pressure will be gone as soon as the gas cool down.
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Unread postAuthor: DYI » Tue Jun 19, 2012 10:01 pm

Ignite the chamber to get up the pressure, then release the pressure with a valve, like a controlled burst disc


Your idea is sound - no "cop"ing of heads here... That's actually how the very first hybrid ever posted on SGTC worked. :wink:
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Unread postAuthor: natas » Wed Jun 20, 2012 10:12 am

DYI wrote:Your idea is sound - no "cop"ing of heads here... That's actually how the very first hybrid ever posted on SGTC worked. :wink:


Really ?... must be a smart dude making that one.. hehe :-)

Guess its a nightmare to calculate pressure etc... lol... so guess its not really a improvement
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Unread postAuthor: jimmy101 » Wed Jun 20, 2012 1:12 pm

The problem is what wyz2285 said, the gases cool down and the pressure drops.

Note that the drop in temperature (hence pressure) is very fast. After 1 second the gases will be basically back to ambient temperature and the pressure will be back to about the starting pressure. Even with a fast opening, and properly timed, valve the passage of the hot gases through the restrictions in the valve will make the temperature drop even faster.
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