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small, high power hybrid

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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small, high power hybrid

Unread postAuthor: Raybateman » Sun May 06, 2012 1:10 pm

I was thinking of creating a small, all metal hybrid spud gun. Im no expert with hybrids and this would be my first one. The plans are hardly developed, but I plan on using a 4 ft 1/2 inch schedule 80 steel barrel. Is it feasible to fire it at 400 psi? If my calculations are right, that should be about a 28x mix. Does anybody have any tips or suggestions on the design?
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Unread postAuthor: jackssmirkingrevenge » Sun May 06, 2012 1:16 pm

Long thread but it traces my progress with small hybrids up to 28x: http://www.spudfiles.com/forums/mini-hy ... 21655.html
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Unread postAuthor: JDP12 » Sun May 06, 2012 5:22 pm

400 psi is NOT a 28x mix.... A "general" rule is that 1x generates 100psi, 2x, 200, et cetera. So a 28x mix produces a couple thousand psi of gas pressure, and its a pressure SPIKE as well. Make sure your material can handle it.
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Unread postAuthor: DYI » Sun May 06, 2012 6:49 pm

JDP12 - he was clearly talking about preignition pressure, and 400psi air/propane is roughly 28X.

I don't see any particular question here - Petitlu's "ThorJack" is probably the closest launcher on here to what you're thinking of, and it is quite recent. 1/2" SCH80 steel pipe will be more than sufficient for the barrel.

As JDP12 mentioned, design for about 3000psi firing pressure. High voltage ignition is still quite achievable at 400psi, but you'll need a very small gap - the rule of thumb here is an inverse relation between pressure and arc length for a given high voltage source, but it is currently unclear to us just what pressure range this is approximately true for.
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Unread postAuthor: Raybateman » Sun May 06, 2012 7:57 pm

Thanks for all the help.
JMR: its going to take me a while to read thru that thread but i will when i have the time.
How exactly do i use a high voltage ignition system when the gun is made out of metal? it will be difficult because the chamber should be very small.
I can't seem to find anything concrete about c:b ratios for hybrids. I hope to be able to fire the gun at a lot of different mixes up to about 28x. is there a certain ratio i should use?
How do you think the final product could compare to a black powder rifle?
Also, is there anything you can tell me about burst discs? I i don't know what kind of burst disc would do well at that pressure... what about the holder for the burst disc? does anybody know a good supplier?
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Unread postAuthor: DYI » Sun May 06, 2012 8:31 pm

You'll need to design an insulated pass-through for at least one electrode, just like everyone else here does. One of the joys of hybrid building :roll:

C:B ratios aren't something that's generally discussed when it comes to hybrids. Just play around with HGDT until you find what you consider to be acceptable performance.

Black powder rifles can, in general, have chamber pressures between 5000psi and 25000psi to the best of my knowledge (which is very little, because I have no interest whatsoever in BP). A 28X air/propane hybrid will generate somewhat over 3000psi in closed-chamber combustion (just use GasEq if you're interested in optimizing fuel mixes or seeing their pressure generation capabilities). So, the muzzle energy : internal volume ratio in a BP rifle will almost always exceed that of a 28X hybrid.

Well, you'll need to play with HGDT to determine which burst pressure you're aiming for. Do a basic shearing calculation for the disk to get a starting idea of the required thickness. From there, it's on to either FEM software like ANSYS (be aware that the dynamic nature of the process will throw off that result), or experimentation. Burst disk holders are typically either modified pipe unions, flanges, or machined specially for the purpose. Punching disks out of sheet metal is one way to go, although aluminum foil is a tried and true method as well. As regards the foil - I used 256 layers at 30X in a 2" union. In a 1/2" union at the same pressure, you'll want in the neighbourhood of 64 layers (exposed area increases quadratically with diameter, shearing surface increases linearly, which results in a linear thickness - rupture pressure relation.
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Unread postAuthor: pneumaticcannons » Sun May 06, 2012 9:29 pm

just like everyone else here does.

really?
I am in no way suggesting you make a plastic hybrid :D
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Unread postAuthor: jackssmirkingrevenge » Mon May 07, 2012 1:26 am

DYI wrote:You'll need to design an insulated pass-through for at least one electrode, just like everyone else here does. One of the joys of hybrid building :roll:


If you can find a way of threading the chamber for an off the shelf spark plug (something small is all you need, like a chainsaw or lawnmower model) then the work is done for you. Alternatively, epoxy is your friend ;)
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Unread postAuthor: Crna Legija » Mon May 07, 2012 1:46 am

jackssmirkingrevenge wrote:
DYI wrote:You'll need to design an insulated pass-through for at least one electrode, just like everyone else here does. One of the joys of hybrid building :roll:


If you can find a way of threading the chamber for an off the shelf spark plug (something small is all you need, like a chainsaw or lawnmower model) then the work is done for you. Alternatively, epoxy is your friend ;)


like this 10x1 mm pretty standard thread.
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Unread postAuthor: Raybateman » Tue May 08, 2012 3:19 pm

I don't have access to a windows computer so i can't use HGDT:(
after seeing what some other people have built, i lean more towards a 4ox mixture and 5 ft barrel.
I have also considered creating a a vacuum in my chamber then filling it with pure oxygen and propane. if i was to do a propane/oxygen mix at 40 bar but with more propane because of the greater oxygen levels, would it still be a 40x mix or something greater like a 200x mix? I'm also not sure how i can refill an oxygen tank.
Im not sure about the spark plug. i would really hate for it to blow out during combustion and i don't know how i would make threads in the chamber. also, would it still spark at 40 bar?
I would like to(if possible) have a simple ball valve on my chamber to connect everything needed to fuel the gun to. the only other opening i hope to have is for the ignition and of course the barrel.
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Unread postAuthor: DYI » Tue May 08, 2012 4:00 pm

I don't have access to a windows computer so i can't use HGDT:(


You may want to take a crack at running it with WINE then.

I have also considered creating a a vacuum in my chamber then filling it with pure oxygen and propane.


Just remember that, without the nitrogen as a buffer gas, DDT runup distance will decrease significantly. It's possible to use either fuel or oxygen as your "buffer" too, but excess fuel appears to have some theoretical advantages.

if i was to do a propane/oxygen mix at 40 bar but with more propane because of the greater oxygen levels, would it still be a 40x mix or something greater like a 200x mix?


Get GasEq. Don't get caught up on "X" numbers. Propane/oxygen mixes have a pressure multiplication ratio of between 18 and 22 (compare to 8-9 for propane/air mixes), depending on fuel ratio. For a given quantity of fuel, pure propane/oxygen mixes produce lower final pressure, if that's what you're getting at.

I'm also not sure how i can refill an oxygen tank.


The typical method is to fractionally distill liquefied air to obtain liquid oxygen, then allow expansion in a closed vessel to generate high pressure oxygen gas (actually a supercritical fluid). The smaller tanks like the one you probably have would be filled from that high pressure supply.

If you don't feel like getting into cryogenics or chemical oxygen generators, you'll need to pay to have the tank filled by a company like Praxair or Air Liquide, or whoever your local equivalent happens to be. You'll need to make an account with them, which will require a credit check, but it's really quite easy if my experience is anything to go by.


Im not sure about the spark plug...


At 800 bar or so (roughly what you'd get from igniting oxygen/propane starting at 40 bar), some spark plugs will experience a catastrophic shear failure which fires out the core of the plug at considerable speed (I had several fail at 200 bar, but those were non-standard).

The threads would be made with a tap, which is readily available in any sparkplug thread size (you'd need to order it though, you'll not find them at hardware stores unless you live in a metric country).

As a rule of thumb, estimate that the dielectric strength of the gas is proportional to the pressure. You will need a very short gap, but it is certainly possible.


I would like to(if possible) have a simple ball valve on my chamber to connect everything needed to fuel the gun to. the only other opening i hope to have is for the ignition and of course the barrel.


I suggest a needle valve with steel sealing faces. Ball valve sealing faces are large and precise, and fare poorly against the hot gases. I've had a 2000psi 1000F rated ball valve fail after exposure to three 30X shots, and Larda had several valves rated for 1000 bar fail as a result of shots between 100X and 200X.

P.S.: I'm not sure I've ever seen a single post with more questions than that one had...
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Unread postAuthor: Raybateman » Sat May 19, 2012 11:19 am

P.S.: I'm not sure I've ever seen a single post with more questions than that one had...

lol well be prepared because i have many more :lol:
You may want to take a crack at running it with WINE then

I can't seem to understand how to do that... I'm not the most computer savvy person and everything on the website is like chinese to me. lol
Just remember that, without the nitrogen as a buffer gas, DDT runup distance will decrease significantly. It's possible to use either fuel or oxygen as your "buffer" too, but excess fuel appears to have some theoretical advantages.

What exactly is ddt runup distance? What does a buffer do? and what are these "theoretical advantages?"
Get GasEq

again, that is a windows application. until i figure out WINE or get access to a windows computer, I'm stuck.
At 800 bar or so (roughly what you'd get from igniting oxygen/propane starting at 40 bar), some spark plugs will experience a catastrophic shear failure which fires out the core of the plug at considerable speed (I had several fail at 200 bar, but those were non-standard).

Would you trust it on a handheld hybrid such as mine? i would like to be as safe as possible.
sorry this took so long. I have been very busy.
Thanks for all your help!
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Unread postAuthor: DYI » Sat May 19, 2012 12:29 pm

What exactly is ddt runup distance?


That would just be the distance between the ignition point, and where the flamefront transitions from deflagration to detonation. Obviously this isn't really a quantitative value because it is highly dependent on geometry.


What does a buffer do?


A buffer dilutes the energetic mixture with an relatively inert substance, decreasing burn temperature and flamefront propagation speed. This is obviously useful in avoiding detonations and reducing barrel erosion, but it has direct performance advantages as well, assuming the correct gas is chosen. A buffer gas can increase the sound speed in the combustion products as well as allowing higher final pressure for a given oxidizer pressure. This has obvious advantages for gun design, seeing as how much of a handling concern high pressure oxygen presents.


...and what are these "theoretical advantages?"


Fuel rich oxygen/fuel mixes will tend to produce carbon monoxide preferentially over carbon dioxide, as well as producing a good deal of hydrogen. These being lighter gases than carbon dioxide and water, the sound speed increases significantly. Highly fuel rich oxygen/fuel mixes are also less likely to cause barrel erosion owing to the deficiency of oxygen in the propellant gas mix.


Would you trust it on a handheld hybrid such as mine? i would like to be as safe as possible.


Are you asking me if I'd trust a component I knew was likely to fail on a handheld hybrid which I intended to run at 800 bar? :lol:

If so, yes. I would trust it to fail and cause me some sort of grievous injury. You'll need to design your own insulated pass-through for the ignition electrode.
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Unread postAuthor: Raybateman » Wed Jun 13, 2012 4:38 pm

I am at my friends house using his windows computer for hgdt. Is there a way i can simulate a propane/oxygen mixture?
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Unread postAuthor: MrCrowley » Wed Jun 13, 2012 7:04 pm

Not sure about that but if you ever need a HGDT simulation and you don't have access to it, you can request one here:
http://www.spudfiles.com/forums/ggdt-hg ... 23256.html
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