SpudFiles

How powerful (in a very general sense) would a mixture of pure oxygen and propane under pressure (60 psi prior to ignition) in a 200 cubic inch combustion chamber? Would this mixture require a chamber fan to adequately mix the gas? or would simply adding the gases sequentially and allowing their differing densities to mix them be adequate? any answers to these questions would be greatly appreciated

I don't think a fan would survive an oxy-propane mixture even at atmospheric pressure... bit of a crazy idea

A pressurised mix generally doesn't need a fan anyway as the more energetic gasses mix better naturally.

How powerful?
You have around 329,000 joules reaction energy avalible in the chamber.
An efficient hybrid can utilize 20-25% of this.
Other than that I'd be more worried about detonation due to lack of an inert buffer gas like nitrogen. Detonation will fatigue your chamber and eventually lead to a failure.

First, download GasEq so you don't have to ask these questions again.

That mix would result in a peak adiabatic combustion pressure of ~98 atmospheres at 4000K, with the resulting gases having a sound speed of just over 1300m/s, a respectably high value. Chamber energy would be 220kJ or so, and you'd be looking at muzzle energies, for a well built gun, possibly as high as 40kJ (realistically, more like 20-30kJ). Compared to most launchers here, that's quite powerful. Put simply, you'd be able to launch large, blunt projectiles through 3/8" steel plate with it.

There is, however, a problem: without any sort of buffer gas, there may be, depending on chamber geometry, issues with detonation. These high pressure transient events don't contribute greatly to propulsion and tend to cause cracking and localized plastic deformations that ruin chambers. Other than for very special cases that don't apply to you, detonations are useless and dangerous, and thus should be avoided. At a 20X mix like you're using, you'll want at least one atmosphere of buffer gas - something as simple as leaving the air (or burned gases) in the chamber and increasing fueling pressure to 75psi will mitigate the problem, for chambers of "typical" dimensions (all bets are off if you make it excessively long and thin).

the chamber is 1026 steel (5.5" outside diameter, 4.5" inside diameter, .5" wall, 8"-12" in length) with a 4.5" brass burst disk on the side facing away from me in the event that the pressure was going to exceed the rating of my chamber and other components the combustion would be safely released from a rather large opening and be rendered safe. Also the fan could survive it would be hand built out of stainless steel and it would not be electrically driven it would be driven like a turbocharger is driven with the ability for the blades to rotate within place to minimize the surface air when combustion takes place. So far the simulation results of the gun on my computer have been promising.


If there is a concern about my chamber rupturing I can increase the wall thickness and decrease the volume as well as lowering the pre ignition pressure.

One last question could that size chamber contain detonation?

The oxygen is coming out of the cylinder from a (heavy duty 2 stage regulator) and being input into a metering cylinder at 395 psi. and the propane will hopefully be entering another metering cylinder at 90 psi.
both those have triple safeties as well double burst disks should anything happen.

as long as the pressure can be kept under 20,000 psi the steel should not experince determental fatigue but just in case the 4.5" burst disk as well as many other safety overrides. yes i understand it will be heavy, the chamber alone would be 25-30 lbs

I have no plans of constructing this until I can get the proper licensing Class 2 NFA. Which will be sometime in the future.


Edited by jrrdw, triple post. Use the edit button!

the chamber is 1026 steel (5.5" outside diameter, 4.5" inside diameter, .5" wall, 8"-12" in length) with a 4.5" brass burst disk on the side facing away from me in the event that the pressure was going to exceed the rating of my chamber and other components the combustion would be safely released from a rather large opening and be rendered safe.

Yes, except for the part where the blast overpressure pops a lung or two. If the chamber failing is a possibility, you shouldn't be near it. That's all there is to it. Now in your case, the wall thickness is ridiculous overkill, and the thing's going to be about as safe as riding an elevator (safety factor of ~10 on YIELDING). A burst disk venting to the atmosphere would be a pointless addition which would probably serve to make the apparatus MORE dangerous (i.e. detonation which wouldn't rupture the chamber pipe due to inertial confinement and/or insufficient pressure might rupture the disk, leading to a significant unconfined explosion).

Now you don't seem to be listening too well about this detonation business - if you build the chamber to contain a detonation of the fuel mix, it'll end up being ten times as heavy as it should be. What you want to do is prevent detonation from occurring. Would it really kill you to add a bit of buffer gas? A little air is all it takes, and with the dimensions you list, you'd have a hard time detonating the mix without aid of high explosives unless you also happened to be using acetylene for the fuel.

If you consent to add ONE atmosphere of air or other buffer gas: I have used the exact same mix (20X oxy/propane + 1atm air), several times, in a chamber with a geometry much more prone to detonation than yours is, made of standard SCH40 pipe. That pipe was more than sufficient for the task. What you describe would be more suited to a 50X or 60X mix.

Your chamber would likely contain one detonation like that, but it shouldn't have to. If you obtain detonation in a chamber like that with any reasonable fuel mix, then you should stop using blasting caps as ignitors

The oxygen is coming out of the cylinder from a (heavy duty 2 stage regulator) and being input into a metering cylinder at 395 psi. and the propane will hopefully be entering another metering cylinder at 90 psi.
both those have triple safeties as well double burst disks should anything happen.

Metering oxygen at that pressure is something of a concern - proper cleaning procedures would need to be employed on all the exposed parts. If you simply did without the meter pipe for the oxygen and injected it directly from tank to chamber, monitoring chamber pressure, you'd save yourself an awful lot of trouble and retain the same functionality. I've run a similar mix up to 30X that way without undue hassle.

These mixes don't just go off by themselves. They have low ignition energies, but clearly aren't prone to many of the initiation modes which cause accidents with solid primary explosives. Your concern for safety is admirable, but this proposed build isn't half the deathtrap you seem to be convinced it is

(If you ignore everything above, just remember that the chamber-venting burst disk is a terrible idea - if there are people proximate, there is NO safe way to vent the chamber rapidly.)

Why not just use compressed air and propane though, the use of pure oxygen implies greater safety hazards and the need for specialist materials. Also for an equivalent mix you have a lower pre-ignition pressure.

At atmospheric pressure, pure oxygen:propane is a 5x mix, meaning at 60 psi you have a 20x mix. With compressed air, this would imply a chamber pressure of just under 300 psi, nothing out of this world if you're working with cylinders.

The meters are nessecary for semi auto firing, the meters are all aluminum and therefore non ferrous and won't spark it can contain the gases. I understand the concern of a burst disk but considering it is scored it is designed to peel. In addition I have no plans of standing near it when firing and the burst disk is facing 2 ft in the total opposite direction. I will take that consideration to heart and I can boost the mix I just need to buy a more expensive oxygen regulator. I have to wait til I'm 21 to get a permit so I'm not in trouble. I'll post cad models of the whole system so you can better critique it. In addition I can't go too crazy with pressure or my parents will stop me from even designing it. Thanks for the advice I'll bear it in mind. Pure oxygen is highly explosive....under high pressure you obvious never had high pressure oxygen or you would know just how volatile it can be and it maybe 300 psi for compressed air but that's 21% oxygen not 80% oxygen.

The concern for the burst disc is not shrapnel, I believe it is the fact the such high pressures can do damage on their own without a projectile of any kind involved. In essence, the pressure IS the projectile. Removing the burst disc is certainly a good idea.

Semi auto...

You obviously have a very ambitious project in mind but I think you should start small. Something like this can be put together for very little financial outlay and will give you an idea of the basics and the power potential.

I do not recommend the use of Aluminum near high pressure Oxygen. The oxide coating on the Aluminum can be compromised by a high speed dust particle. Use Brass. Oxygen regulator fires are no fun and can be started simply by opening the cylinder valve. There have been several casualties from Aluminum regulators and Oxygen.

After studying this, I refuse to use an Aluminum oxygen tank or an Aluminum regulator near Oxygen.


See the video on the bottom of the page.
http://www.therebreathersite.nl/01_Info ... nfire.html

http://www.floteco2.com/htm/Press%20Rel ... eport.html

The meters are nessecary for semi auto firing...

Ah. This explains the necessity (or at least usefulness) of the license. I'd been wondering about that.

I understand the concern of a burst disk but considering it is scored it is designed to peel. In addition I have no plans of standing near it when firing and the burst disk is facing 2 ft in the total opposite direction.

You've missed the point. I already assumed the use of a scored disk - the danger is the blast overpressure. If you're nowhere near it, this isn't dangerous. It's just damned pointless for a chamber which won't even reach a tenth of its yield pressure, much less its rupture pressure.

Pure oxygen is highly explosive....under high pressure you obvious never had high pressure oxygen or you would know just how volatile it can be and it maybe 300 psi for compressed air but that's 21% oxygen not 80% oxygen.

Pure oxygen is not explosive in the slightest. It is actually very, very stable. And just how "volatile" it can be? Considering that volatility describes how readily a substance evaporates, applying the term to something which is already a gas doesn't do anything for the argument that you somehow know what you're talking about better than we do. The very nature of our hobby - applying products to purposes for which they were not designed - necessitates that we learn how things actually work, rather than memorizing effective-yet-inaccurate safety lessons. Also, just FYI, you appear to be tilting at the windmills here - if your reread everything a bit more carefully, you'll realize that no one actually made the statements you appear to be responding to.

Now one thing that pure, high pressure oxygen IS, is an awful good oxidizer... Which makes me wonder about the choice of a highly flammable metal for your "safety" meter.

EDIT: Aw, ninja'd by Tech...

Regulators are all brass don't worry they are heavy duty welding regulators 153x-250 gentec I believe should do the job it's rated for it. Also potatos were not really my main projectile in mind I was thinking 20-30mm bearings, DYI hence the need for licensing, and I know it sounds crazy but given a simple foolproof design can you restrain the projectile to seal the chamber instead of having burst discs contain the mixture, it e
Would be a tight fit but would allow much faster reload times... Just an idea.

Thank you for clarifying DYI, yes oxygen is stable but I personally DON'T want to be creating a spark in a 60 cubic inch aluminum or other soft metal pipe with 400 psi oxygen, because if piston seals fail and the reaction from the combustion chamber reach the meter pipe it will explode. I also plan on increasing the pressure to utilize the massive yield strength and potienally try something a little more crazy with H.... So the chamber will not be "over engineered" I just like to stay in one piece and out of jail if I play with highly explosive mixtures of gas

Crazy Ideas wrote:I know it sounds crazy but given a simple foolproof design can you restrain the projectile to seal the chamber instead of having burst discs contain the mixture

No it doesn't

Crazy Ideas wrote: I personally DON'T want to be creating a spark in a 60 cubic inch aluminum or other soft metal pipe with 400 psi oxygen, because if piston seals fail and the reaction from the combustion chamber reach the meter pipe it will explode.

Wow. You're assuming a hell of a lot here. Firstly you assume that the seals do fail (which is possible, given SB15's experiences), that there is somehow a magic spark across the inside of the pressurized oxygen, which has an incredible amount of resistance compared to the aluminium casing, and also that the meter pipe and the oxygen cylinder may fail. Do you have yield calculations for those to show that they may yield under post-ignition pressure? This will get you started.

Crazy Ideas wrote:try something a little more crazy with H

That subject has been beaten to death on this forum. The general consensus is that there is no reason to use hydrogen over an alkane.