Building a Better (Combustion) Spudgun

[DYI]

To put it bluntly, the development of "advanced combustion" launchers peaked several years ago. In terms of ease of construction and ease of use, this class of launcher remains very attractive for entertainment purposes (totally useless otherwise, but there's a lot to be said for entertainment). I consider the lack of a large, high flow valve and capability to launch potatoes without destroying them in-barrel to be the main reasons for this.

I think a great deal more can be done to improve their performance without turning them into a TBMA-style launcher (which I would say is the natural progression if one starts with a spray'n'pray and moves on up).

Most of these ideas involve somewhat "exotic" (by SF standards, at least) fuels and oxidizers; nitromethane could be highly effective, and Myrol (methyl nitrate / methanol) would likely be even more so, if the handling difficulties could be overcome. Unfortunately, this solution doesn't achieve anything really "new", so much as it increases the pressure generation in the standard designs (possibly to levels which would destroy potatoes in-barrel). Even worse, nitromethane is becoming difficult to obtain, and Myrol would be illegal to even possess in most jurisdictions, it being a high explosive.

I recently got thinking along different lines. Take a standard barrel and chamber with a nozzle at the back and an ignition source directly in front of it. Plumb in one line carrying a liquid fuel (gasoline, for example) and another carrying high pressure air or oxygen (or even liquid nitrous oxide). With proper mixing, and the right pipe diameters and feed pressures, you could conceivably end up with what is essentially a giant torch firing into the chamber.

The advantage? Done properly, this could make it possible to maintain a more constant projectile base pressure by increasing fuel/oxidizer flow as the projectile moves farther down the barrel.

The disadvantage is that the pressure would need to be very high for the oxidizer line, barring use of a liquid oxidizer. High flow would be required as well, which could lead back to the problem which the builder would initially be trying to avoid. Still, if it turned out to be possible to use this design to create flatter base pressure profiles, it could still be worthwhile for the purpose of achieving greater performance from a low-pressure construction.


There is also, of course, the question of just how far the conventional designs could be pushed with judicious application of CFD. No one has really attempted to optimize performance in these launchers so far, that I'm aware of.

Feel free to discuss any novel improvements you can come up with, and/or tear into the ideas I discussed here. Please note that oxygen injection alongside fuel injection is not new - it's been around for years now (I believe Moonbogg built a very nice launcher using that method).

[samiam0295]

It is a very interesting topic, and I have though about it myself. However, if I were to use Gasoline injection, I would do it with a pressurized fuel line and an automotive fuel injector. It seems, to me, that this would be far more useful in a hybrid cannon, but could easily be incorporated into an "advanced combustion" cannon.

If I had to choose a method of improving combustion cannons, it would be ethanol. (I think) it would be possible to use the same methods as seen in "ethanol rocket" videos. It could be done with relative ease, and should produce good results.

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[wyz2285]

I have thought about use gasoline for fuel with pure oxygen mix, but I haven´t found a way to evaporate the gasoline first in able to get a good full ignition. Then I thought about having two separated chamber with a piston in between them. Both in the same mix, one side use air/propane, other side with gasoline/oxygen. First ignite the air/propane, the hot expanding gas pushs the piston, hopefully "diesel" the gasoline/oxygen

[natas]

atm i don think the problem is getting a big enough bang, or using exotic fuels, but getting a bang that dont destroy the potato.
Maybe looking at the construction, and the way we think when we think combustion spudgun must go back to the drawing board.

what about two bangs ?. one explotion to start the potatoe, and when the potato are at X speed, another explotion to accelerate it even more ?

two chambers going in to the same barrel, maybe with different fuels, and one with a burst disc ?


just brainstorming here

[Fnord]

Would this have any advantages over a large C:B ratio gun with an excessive number of ignition points, high gas turbulence and some type of heat-loss mitigation? That should give a more or less 'even' base pressure.


I imagine the math for an 'ideal' gun would not be too hard. Comparing that to a known launcher or hgdt simulation would be the first logical step.

I have thought about nitromethane in the past but have not found a lot of discussion here. Has anyone done even rudimentary testing with it?

[DYI]

what about two bangs ?. one explotion to start the potatoe, and when the potato are at X speed, another explotion to accelerate it even more ?

Multi-chamber gun systems are notoriously hard to implement, and there's been a lot of work put into them. They also immediately defeat any attempts at entertainment.

two chambers going in to the same barrel, maybe with different fuels, and one with a burst disc ?

Now that just won't work at all...

Would this have any advantages over a large C:B ratio gun with an excessive number of ignition points, high gas turbulence and some type of heat-loss mitigation? That should give a more or less 'even' base pressure.

If you direct your attention toward the analytical solution of the 1-D semi-infinite ideal gas gun problem, you'll find that this is not the case. Take a look at "Fluid Mechanics" by Landau and Lifschitz.

I imagine the math for an 'ideal' gun would not be too hard. Comparing that to a known launcher or hgdt simulation would be the first logical step.

In that case, your math is well beyond that of anyone else here. Modeling gun performance is outside the capabilities of "normal" finite differencing CFD software. Modeling even a very simple pneumatic gun is difficult enough, most nicely done by a quasi 1-D Arbitrary Lagrangian Eulerian (ALE) approach. At this point, the math is already beyond what I'm "comfortable" with. Then you add in at least one more space dimension and a combustion process, and everything goes straight to hell. If modeling an ideal combustion gun is "not too hard" for you, I'd love to hear how you would do it.

I have thought about nitromethane in the past but have not found a lot of discussion here. Has anyone done even rudimentary testing with it?

SB15 was interested, but nitromethane dissolves ABS quite readily. I am interested, but I have no nitromethane (and, given my relationship with the authorities, should probably continue to have no nitromethane...). Apart from us, I've not heard anyone else mention it. GasEq certainly predicts great things from it.

[samiam0295]

SB15 was interested, but nitromethane dissolves ABS quite readily. I am interested, but I have no nitromethane (and, given my relationship with the authorities, should probably continue to have no nitromethane...). Apart from us, I've not heard anyone else mention it. GasEq certainly predicts great things from it.

Nitromethane is easily obtainable and legal. It is "top fuel" for drag cars, as well as "nitro fuel" for RC cars.

[Bowman]

What do you mean by ideal? If by an "ideal combustion cannon" you mean a perfect one....Well, let me give you the short answer.... There isn't one.

I like the idea of a gasoline combustion cannon. But I don't see how that would really be anything radically innovative....And it seems to me that you want something that just has insane performance but is also insanely simple (for entertainment purposes).

And if that's what you're looking for then maybe a gasoline hybrid IS the answer. But like I said before it's not really that innovative (well in my opinion it's not). Because in my opinion, building a gasoline combustion cannon is just another cannon that uses a "not so common fuel"....But hey, that's just my two cents...

Anyway, what I think would be innovative is instead of just another muzzle loading combustion with an "exotic fuel" how about we find a way to create a repeating combustion launcher. I mean it's been done before http://www.spudfiles.com/forums/gatling-1-t20903.html

But, other than that^^^ no one has really made a repeating combustion launcher. What I think would be innovative would be if we can take the design on the "combustion gatling cannon" and simplify it.

[Fnord]

Ok, the problem we have here is that I'm not communicating the boiled-down point effectively(as usual), and you're over-complicating it (as usual:P).

If modeling an ideal combustion gun is "not too hard" for you, I'd love to hear how you would do it.

I would not 'model' such a gun with any kind of complex math.
(Of course, this would not produce anything more accurate than sorta-kinda ballpark numbers for a 'standard' design, even less so in an exotic design such a yours).

Why? Well, I guess I should have just put it bluntly in the first place rather than playing along;
Gun simulation of this nature is currently outside the scope of this forum. I am aware some members are taking high-level math and physics and could contribute in some way to such a discussion, but they do not represent the majority. We are lucky to have D Hall's gun simulators, but as I understand it they were not originally created with spudfiles in mind.

I certainly would not discourage this type of advancement, however. I enjoy seeing stuff like this in the recent threads. Maybe someday I'll actually take the time to learn some semi-advanced math and physics, but, like most, it's not what I'm here for at the moment.


Here, exhibit A:

I think a great deal more can be done to improve their performance without turning them into a TBMA-style launcher (which I would say is the natural progression if one starts with a spray'n'pray and moves on up).

What you're shooting for is something between an advanced combustion and a low-mix hybrid, but in reality the design you've put forth would be less practical than either, as a result of the need for high pressure fuel/air injection. Scaling the design up could require calculations more complex than a simple propane meter, which would also be deterrent (though perhaps a positive one).

Which brings us to exhibit B:

...but there's a lot to be said for entertainment).

I'd reason that when factors of cost, build complexity, portability performance and required theoretical knowledge are all added up, you might be better off with a TBMA. I do like the semi-auto potential of your design, though that would add other practicality problems.


In conclusion, I would not mind seeing one of these guns being built as a testbed, but I wouldn't expect it to catch on like other advancements have. Theoretical discussion involving high-level math can't reasonably be expected to draw much interest here, either. I'm sure there are some other communities which would be able to appreciate it more than us.


(Oh, and yes I did download Fluid mechanics along with the other 9 volumes in the collection, and did have a look through the relevant sections. However, I am going to hold off for now and refer back to them at a time which is not 1:00 in the morning ).

[Crna Legija]

Anyway, what I think would be innovative is instead of just another muzzle loading combustion with an "exotic fuel" how about we find a way to create a repeating combustion launcher. I mean it's been done before http://www.spudfiles.com/forums/gatling-1-t20903.html

[Bowman]

Anyway, what I think would be innovative is instead of just another muzzle loading combustion with an "exotic fuel" how about we find a way to create a repeating combustion launcher. I mean it's been done before http://www.spudfiles.com/forums/gatling-1-t20903.html

Not exactly practical.....

Also, maybe you didn't read my entire post but I said

simplify it.

I didn't insinuate that my example of a repeating combustion cannon was the only one in existence..... I think you missed the point. No one has made a "practical" single chamber repeating combustion cannon.

[jimmy101]

The math behind a combustion gun really isn't all that complicated. I've done it, Dave's done it (probably better) and others have done it. The math doesn't have to be perfect (indeed combustion modeling, such as for guns, ICE, rocket engines etc., even with an army of engineers isn't perfect) since it can be, fairly easily, calibrated with real world data.

Large chamber, multiple ignition points, fans, high static friction (roughly the same as a burst disk), fast burning fuel, and you are pretty much at the situation where the chamber pressure is nearly constant though out the firing cycle. Efficiency would suck but nobody cares about that anyway.

I really don't see anything fundamentally wrong with the a typical advanced spud gun. The efficiency is a fair bit below a modern rifle but what do you expect from something that costs 1% of a decent rifle? The thought that you could engineer up a spud gun to match a real rifle is OK but ultimately you'll just reproduce a modern rifle with all the associated costs.

[Bowman]

I really don't see anything fundamentally wrong with the a typical advanced spud gun. The efficiency is a fair bit below a modern rifle but what do you expect from something that costs 1% of a decent rifle? The thought that you could engineer up a spud gun to match a real rifle is OK but ultimately you'll just reproduce a modern rifle with all the associated costs

It's not that there is something wrong with the "typical advanced spud gun". Basically what DYI is saying is that there has been no real progression or innovation with the advanced combustion cannons.

[boyce123]

to go off innovation, there isn't much to do with an advanced combustion besides changing fuel composition/mixing, or pressurizing the chamber (then putting it into hybrid territory). Beyond exotic fuels, the only thing really to be done is to radically change the design of the cannon, such as with the tac6 on ultimate spun gun. Yet as depicted in test videos of the tac6 firing, it is pitifully weak, so it could be possible to redesign the currently accepted pump action spud gun to implement faster reloads and potentially more power. Another concept would be cartridges (though borderlining hybrid) to simply combine fuel/oxidizer (pre-charged),ammunition, and reusable container. This concept would require copious machining and engineering, yet is feasible to someone willing to pursue it (either with a revolver or a magazine design). With any of these concepts, it could be taken further by implementing a gas return system to utilize expanding gasses at the end of the barrel for a semi-auto system (as in an AK-47). With all this said, the engineering is, in most cases beyond what the average person is willing to do for weekend entertainment, but once completed it would be a highly viable and even more effective form of entertainment over the accepted advanced combustion.

@DYI i'm not sure why you would have such difficulty in aquiring nitromethane, though its possible to find higher concentrations, this was the first result on an ebay page http://www.ebay.com/itm/Torco-RC-Fuel-2 ... 2c58c27081 the issues would then become finding a chamber that would be chemically resistant to nitromethane. And as far as the fuel dispersion is concerned, the most utilitarian combustion entity of all time has already been created... the internal combustion engine, so why not use its proven method of fuel dispersion, use a fuel injector. i haven't done any research yet, but it might be possible to play around with vapor pressures and make a reverse hybrid (vacuum in the chamber, probably requiring a larger chamber) in order to make light hydrocarbon fuels, such as ethanol, into their vapor form for easier combustion.

[DYI]

Firstly, to everyone who thinks my suggestion regarding gasoline was simply using it as a fuel along with a convoluted injection system: please, stop talking.

What you're shooting for is something between an advanced combustion and a low-mix hybrid, but in reality the design you've put forth would be less practical than either, as a result of the need for high pressure fuel/air injection.

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.


Just to clarify my first, potentially unclear post, this was supposed to be for firing potatoes. 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.

Theoretical discussion involving high-level math can't reasonably be expected to draw much interest here, either.

Well, I don't know the first thing about high-level math, but the basic applied math I know doesn't draw much interest around here either...

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.