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Before I start I will add that I understand the dangers of what I am doing and accept all responsibility should anything go wrong. Over the last little while I have been gathering a few things for the tank build (I'm building a 46% scale M4 Sherman tank incase I didn't mention this before) One of the things on the agenda this summer is the turret and cannon. I decided on an oxy acetylene cannon as I need a very small combustion chamber due to a lack of space. The material of choice is seamless DOM tubing and I already purchased the barrel which is 1 1/8"ID with a 1/8" wall which makes the barrel 1 3/8"OD I have a spacer that goes from 1 3/8" to 1 7/8" and so far it ends there.
The plans are to get another piece of 1 7/8 ID schedule 80 heavy tubing to fit over the spacer to make a 10" long combustion chamber. The barrel will go through the spacer and partly in to the combustion chamber (about 9" in) making the cannon a semi coaxial, this will allow me to breech load having the barrel set back that far but takes away from the total combustion chamber volume. This is where I need some help. The projectiles of choice will be formed from 1 1/8" wood dowel and will weigh about 1 to 1.5oz. The combustion chamber volume after subtracting the barrel volume is 14.25ci or 233.51cc
What I need to know is if there are any calculators out there that are specific to combustion spud cannons using oxy/acetylene because I need to determine if the mix in such a small chamber will produce enough energy to move the projectile all the way down the 1m long barrel. I'm not trying to break any records but I expect an effective range of about 100 yards so I wouldn't mind if some of you guys could chime in.
EDIT: Correction, I cannot get Acetyline in the small bottles so I will be using Oxy/Mapp gas or Oxy/Propane. Dont know how this changes things but I do have the ability to make a bigger combustion chamber just not longer. I'm not sure what kind of C:B is ideal at this point either.
Firstly, if you don't already have HGDT and GasEq, get them - they should be considered the bare minimum for designing any kind of combustion launcher, and they're both free and very easy to use.
Using oxygen/propylene (MAPP isn't actually made any more) will actually produce nearly identical pressure to oxygen/acetylene, with much less hazard involved. The theoretical maximum pressures (using adiabatic, equilibrium calculations in GasEq) are:
Oxygen/acetylene: -- 324psig
Oxygen/propylene: -- 325psig
Air/propane: ---------- 126psig
Air/propylene: -------- 130psig
These maximum pressures occur at 1:1 oxy/acetylene, 10:4.9 oxy/propylene, 100:5.1 air/propane, and 100:6.1 air/propylene. Note that all of these values are richer than the "stoichiometric" ratio, which is really only useful as a place to start, because it severely simplifies the combustion products (for example, the actual combustion products of the highest pressure oxy/propylene mix are mostly hydrogen and carbon monoxide, meaning that the combustion products are still very flammable).
Also note that the highest pressure is only one aspect to consider - the 10:4.9 oxy/propylene mix will produce a LOT of soot, a very bright muzzle blast, and internal ballistics that are difficult to model without non-equilibrium effects. Other factors of interest are the molecular weight of the propellant gas (should be low), sound speed of the propellant gas (should be high) and initial temperature of the propellant gas (should be low).
That being said, you can use HGDT to approximate the performance of your planned launcher by modelling an equivalent hybrid in HGDT - in this case one using a 2.5X mix with air/MAPP. This will be a rough estimate, especially since HGDT does support coaxial geometries (they're awful for heat loss, which is already a major loss mechanism in combustion guns).
The rough estimate yields a muzzle speed of about 450ft/s for a 1.5oz projectile. HGDT is usually accurate to within 40% on the muzzle energy, and it usually predicts high (sounds awful, but it's damned hard to model - even my quasi-1D equilibrium code can't do better than about 25% for hybrids). As such, I expect you'll be able to manage at least 380ft/s with a 1.5oz projectile, given an adequate ignition system and the correct fuel ratio.
As to the effective range: assuming you can achieve a drag coefficient of at least 0.7 with your (presumably Foster slug style) dowel rounds, you should be able to achieve a distance of 100 yards with an elevation of 3 degrees (aiming 16 feet above the target).
And about this "CB" garbage: you'd be surprised at just how large a chamber volume it takes to actually reach that "point of diminishing returns" (where the breech pressure is similar to its starting value throughout the launch). However, returns from increasing chamber volume (or barrel length, for that matter) are always "slow", compared to the returns from increasing the pressure. In your case, doubling the chamber volume from the one in your design would result in a muzzle energy increase of about 45%.
Last edited by DYI on Tue Feb 03, 2015 3:51 pm, edited 1 time in total.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
This is awesome and really good info to start with, thank you. I will keep this post updated as I go. As said before this wont happen for a while but I better start researching now. Going to download these two programs for now and play around.
Well I've been running some numbers and according to HGDT the performance I've been looking for will not happen from a regular combustion cannon. It looks as though I will have to go hybrid on this one which brings fourth another set of obstacles I have to factor in. First being a method to quickly change burst discs at the breach after loading and second is getting the mixtures right.
Go for a piston valved hybrid, no need for burst disc. It can be done in coaxial cannon too.
Definitely considering it but it is very hard to do right using all DOM construction without a lathe.
I just realized something. Since this was supposed to be Oxy/Propane or Oxy/Mapp do I need to go with a hybrid setup? The reason I thought I did was because HGDT does not account for Oxygen in the mix. Just propane or mapp so the performance numbers were really low. So now I have to figure out what amount of Oxygen will produce the equivalent to a 7x - 10x mix. Having this setup helps me greatly as I don't need a small air compressor inside the turret anymore. Which would be one more device that would constantly draw power.
Re: Oxy/Acetylene mini cannon questions.
The maximum you can have with oxygen at atmospheric pressure is 5x, and HGDT assumes that you have a pre-ignition pressure which it adds to the mix.
Ok good, so that gets me closer to my goals. Now is it safe to say that when I take the 5X mix and pressurize to 2X atmospheric (which would be much easier than pressurizing to 10X with just air) that I will get the same effect as 10X? This would mean that I will still need a compressor and burst disc. I have been reading through the FAQ and have determined that 4% of total combustion chamber volume is the ideal amount of fuel? I've been having a hard time with that GasEq program but from what was written above I need a 10:4.9 oxy to propane mix? So pretty much 2:1 Oxy/Propane with only the propane counting as 4% and the oxygen is extra?
Not exactly, because for a 10x mix, you've pressurized to 130 psi, while with a pure oxygen mix you've only pressurized to 15 psi, the precompression adds a significant boost to the firing pressure.
If you run pure oxygen through a normal compressor, it will burst into flames.
Time to wheel out the usual photos of airguns that have been filled with oxygen instead of compressed air:
If you're going to bother compressing, use air, not pure oxygen.
Propane is ideally 4% concentrated in air, which has 20% oxygen. If you have pure oxygen, the ratio by volume is now 1:5, so 20% propane in 80% oxygen. Might I suggest the glory of syringe metering?
Thanks for the reply, I did not mean running oxygen through a compressor. I meant that I would add my propane/oxygen mix to the combustion chamber and theb pressurize the mix with air,to whatever pressure necessary to obtain a 10X mix, but trying to avoid having to use a compressor period so I may just stick with Oxy/Propane or Oxy/mapp at atmospheric pressure and see what performance numbers I get. It won't be that big of a deal to add a compressor afterwards. I'm not worried about the cannon exploding because it will be welded from Sched 80 DOM tubing. Combustion chamber will be tiny @ 4"x6"
Unfortunately syringe fueling won't cut it. This is a cannon that is going inside of an army tank. Eventually (once my arduino skills get up to par) I plan to have an automated logic system that will turn a valve to allow gas flow in to a metering chamber and then in to the combustion chamber. The only human involvement will be loading the gun then hitting the ready button then the fire button, boom, repeat process.
Have you thought of having preloaded cartridges? Purge with oxygen, syringe squirt of propane, seal with duct tape and you're golden. No need to fuss about metering inside the tank, and more realistic too.
That's not really how it works, at least not linearly - adding compressed air only adds 20% more oxygen per atmosphere of pressure, adding compressed oxygen adds 100% more oxygen to the mix.
I have considered preloaded cartridges but then each one would have to be the size of my combustion chamber would it not? It would get pretty expensive to make that many 4"x6" DOM cartridges. And I dont think I'd have that much room inside the tank. I could always make them longer but smaller diameter. But then it would be a different burn. Right now I'm going by what the program is telling me is ideal.
Not necessarily, you could have cartridges that are cardboard tubes, loaded into a strong chamber. If it just has to contain gas at atmospheric pressure it doesn't have to be pressure resistant material.
It would be nice but is just not practical in my application. My biggest thing now is figuring out fueling. I have already determined the ideal combustion chamber size using HGDT and from what I'm understanding it looks like I need fuel at 4% chamber volume give or take a bit. Plus a 1:4 ratio of fuel and oxygen, someone mentioned 1:5 but 20% and 80% is 1:4 is it not? So (and correct me if I'm wrong) For every 1cc of fuel I need 4cc of oxygen.
My new combustion chamber volume (without going coaxial like the original plan) is 1235cc so 4% of that is 49.42cc so roughly 50cc. This means that at a ratio of 1:4 I need 200cc of oxygen?
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