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combustion help please

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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combustion help please

Unread postAuthor: beastmode986 » Fri May 11, 2012 4:15 pm

well i have a regular old combustion and i use hairspray as a propellant but i want to make it nicer. now i cant spend alot of money because im ordering some stuff but any suggestions are appreciated. i want to adda propane meter but i dont know how much propane to use also can the meter be made using pvc pipe? thanks!
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Unread postAuthor: jsefcik » Fri May 11, 2012 4:41 pm

Research my friend research !!

Your best bet for propane metering is steel pipe my friend

Add a fan nd switch to starting fluid or carb cleaner!!
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Unread postAuthor: Bowman » Fri May 11, 2012 5:29 pm

I would not reccomend using a PVC chamber for propane at all. If you want propane you will pretty much have to build a whole new gun.
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Unread postAuthor: jsefcik » Fri May 11, 2012 5:33 pm

Bowman wrote:I would not reccomend using a PVC chamber for propane at all. If you want propane you will pretty much have to build a whole new gun.


dude there are alot of guns made of pvc using propane,


do some research

watch this video, is it steel no! you do not have to switch to steel

http://www.youtube.com/watch?v=HvFiPJ1zzcM
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Unread postAuthor: Bowman » Fri May 11, 2012 5:52 pm

jsefcik wrote:
Bowman wrote:I would not reccomend using a PVC chamber for propane at all. If you want propane you will pretty much have to build a whole new gun.


dude there are alot of guns made of pvc using propane,


do some research

watch this video, is it steel no! you do not have to switch to steel

http://www.youtube.com/watch?v=HvFiPJ1zzcM


That's not really what I meant. Sorry I should've said this in the last post. as a general rule of thumb 1x mix will create around 100psi. So that's about as far as he can go he can maybe go a little higher but PVC really limits performance.

Just take a look at this:

http://www.engineeringtoolbox.com/pvc-c ... d_796.html

Look at the chart. If he wanted let's say a 3" chamber (pretty basic size) he can only safely operate at 158psi with schedule 40 and 255psi with schedule 80. I do plenty of research just because I didn't lay out the obvious doesn't mean I don't research. Also here's a quote from our very OWN spud wiki on THIS site: "A hybrid is one of the more dangerous types of launchers to build"

If he uses propane it's a hybrid.

Just because some guy on Youtube used PVC doesn't mean anything. Sure he can use PVC. My point was that it limits performance and is more likely to be a safety hazard.

MY reccomendation would be that if he doesn't want to spend a whole lot on a hybrid then there is really no point to even attempting it. But hey it's not my decision so if he wants to use PVC I really don't care but it's his safety.
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Unread postAuthor: DYI » Fri May 11, 2012 6:07 pm

PVC is not recommended for compressed gases as it is rather brittle - steel fittings are preferable.

Also, Bowman needs to read a bit more - a hybrid uses a propellant mix which is pressurized above ambient, and propane/air at atmospheric pressure won't exceed 120psig or so on ignition.

@Bowman: he's asking about using PVC for a meter pipe. Not a hybrid chamber.
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Unread postAuthor: Bowman » Fri May 11, 2012 6:19 pm

DYI wrote:PVC is not recommended for compressed gases as it is rather brittle - steel fittings are preferable.

Also, Bowman needs to read a bit more - a hybrid uses a propellant mix which is pressurized above ambient, and propane/air at atmospheric pressure won't exceed 120psig or so on ignition.

@Bowman: he's asking about using PVC for a meter pipe. Not a hybrid chamber.


I said, "general rule of thumb" didn't I? Of course it's not exact. My point was that it's not really that safe to use PVC for a chamber. Yes, he can have a PVC meter. That was his question, but he said that he wanted to just convert it from his PVC spud gun. Meaning he would use PVC as a chamber...No? I know a hybrid uses a propellant mix. If he just puts propane in the chamber it won't ignite because he needs air to MIX with the propane to be able to ignite..... If you read my post you would know that a 3" schedule 40 chamber has a max operating pressure of 158psi. You said a propane at atmospheric pressure won't exceed 120.... I would say that's pretty close to 158. So here is my point:

he can use propane for a propellant but it will limit performance

It's as simple as that. That's all I'm trying to get across :D
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Unread postAuthor: mark.f » Fri May 11, 2012 7:27 pm

What on earth are you talking about Bowman? OP wants to build a 1x combustion so whether or not he uses PVC as a chamber won't "limit performance" or safety (in fact, one could make an argument towards less heat loss to a plastic chamber as opposed to a metal one and its effect on performance, but this would be pretty insignificant).

As far as the meter pipe goes, most of your expensive parts are the ball valves, regulator/needle valve, gauge, and tank connection. The piece of pipe between these doesn't effect cost that much so it's probably a better idea to go with steel, but anything will work, just bear in mind that PVC gets brittle at lower temperatures (which can happen even in summer by "burping" your fuel meter or using it frequently).
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Unread postAuthor: Fnord » Fri May 11, 2012 8:19 pm

I'd go with steel or copper for the meter pipe. I've gotten liquid propane in mine by accident -on more than one occasion- and it does get quite cold. It's better to use a material that isn't brittle at -40 degrees ;)

*Also has no idea what bowman is talking about *
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Unread postAuthor: DYI » Sat May 12, 2012 9:27 am

Alright Bowman, time for a more thorough response.


I said, "general rule of thumb" didn't I? Of course it's not exact. My point was that it's not really that safe to use PVC for a chamber.


I was not contesting that 100psi/X was a good general rule of thumb - in a gun, as opposed to a closed chamber combustion, it is very good approximation. Considering the typical rating of PW PVC pipe in the sizes commonly employed for handheld combustion guns, and the fact that the original poster (OP) will only be using 1X, I must contest your point on safety - so long as excessive loading is avoided, a PVC chamber with an adequate pressure rating is quite sufficient for a 1X combustion gun.


I know a hybrid uses a propellant mix. If he just puts propane in the chamber it won't ignite because he needs air to MIX with the propane to be able to ignite.....


Perhaps you should reread my previous post, as it currently seems you left off immediately after reading "propellant mix", and before you got to the bit about "pressurized above ambient".

The air, in this case, is the air already present in the chamber when the propane is injected. The propane will displace air out of the chamber, leaving it at atmospheric pressure while containing an ignitable mix.

A point on common Spudfiles terminology: when we say "combustion cannon" (as opposed to "hybrid cannon"), we imply that the propellant mix will NOT be above ambient pressure before ignition. Being aware of the subtle difference will help you avoid embarrassing mistakes like this one in the future.


If you read my post you would know that a 3" schedule 40 chamber has a max operating pressure of 158psi. You said a propane at atmospheric pressure won't exceed 120.... I would say that's pretty close to 158.


Don't worry, Bowman, I read your post very thoroughly :wink:
The maximum rated operating pressure incorporates what the relevant standards association considers to be an adequate safety factor. If the pipe is being used below its maximum operating pressure, there is no cause for concern.

So here is my point: ...


Your point is incorrect. The pressure generation in what we call a "combustion cannon" is not variable. It either performs, or it explodes. There is no "reduced performance". He may not be able to use oxygen/hydrazine in his PVC chamber, but that's not what he was asking for.
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Unread postAuthor: beastmode986 » Sat May 12, 2012 6:09 pm

ok, so i should add a fan propane meter and im considering an electric igniter if im going to do one of these ''upgrades' which one will give me the highest increase in performance? or should i just build a new gun if that would just give me better peformance at the same price as all these upgrades would???
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Unread postAuthor: MrCrowley » Sat May 12, 2012 11:26 pm

Bowman wrote:[If he uses propane it's a hybrid.

Almost sig worthy :lol:

ok, so i should add a fan propane meter and im considering an electric igniter if im going to do one of these ''upgrades' which one will give me the highest increase in performance? or should i just build a new gun if that would just give me better peformance at the same price as all these upgrades would??
Propane meter would be my first upgrade and then the fan. The ignition shouldn't matter too much as long as it's sparking fine.

The meter and fan electronics shouldn't cost you more than $20USD; cheaper than building a new cannon. Unless you've drilled some 1/2" holes through a single layer of chamber PVC, I'd stick with the cannon you have now and just do the upgrades. When you add the fuel meter, make sure to thread it through two layers of PVC where a fitting overlaps the chamber pipe.
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Unread postAuthor: battlelava » Sun May 13, 2012 7:47 am

MrCrowley wrote:
Bowman wrote:[If he uses propane it's a hybrid.

Almost sig worthy :lol:

seems worthy to me
Anyway I just put fuel metering and a fan in my spray and pray combustion cannon, it will make it perform like a new cannon
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Unread postAuthor: jimmy101 » Sun May 13, 2012 10:39 am

Dang, BOWMAN is clueless. And JSEFCIK isn't making sense (starter fluid with a meter WTF?)

The simplest things you can do to get 100% of the performance that a particular gun configuration is capable of are:

1. Meter the fuel. A syringe + needle + short length small ID tube (e.g., wire insulation) + $0.95 disposable butane lighter. (Or a Bernzomatic propane torch, but if you don't have one already that adds $20 or more.) Search the wiki for "syringe". The above includes everything you need, including fuel, and should cost about $5. (see also http://www.inpharmix.com/jps/Fuel%20Meter%20From%20A%20Disposable%20Butane%20Lighter.html) This works just as well as a meter pipe, and is a bit cheaper, but doesn't have nearly the Bling factor.

2. Add a chamber fan made from a computer CPU fan with a 9V battery. If you (or a friend) has an old PC collecting dust in a closet just swipe the CPU fan (not the case fan, which is way too big). Add a 9V battery with battery clip. A switch would be nice but isn't required. The only thing even remotely difficult with adding a chamber fan is figuring out how to mount it in the chamber. One option is to use two small "L" brackets bolted to the fan and with bolts through the chamber wall. You can use the bolts through the chamber wall to carry the current from the battery into the chamber and to the fan.

A chamber fan is really optional. If you omit it you just have to be patient and let the gun sit for a minute or two between fueling and firing to let the fuel fully mix with the air in the chamber.

Edit: In practice most 1X propane (or butane) fueled spudguns generate about 70 PSIG maximum pressure, and that requires very tight fitting ammo with a fairly high static friction (say 50 pounds of force just to get the ammo moving). Theoretical pressure is about 135 PSIG and that is the true absolute maximum that a 1X will generate in an ideal chamber. In practice, even a closed chamber won't get to the theoretical maximum because there is too much energy lost as heat transfer from the gases to the chamber wall. Pressure rated 3" and 4" PVC pipe, even as a closed chamber, has no problem with the maximum theoretical pressure.

Schedule 40 4" PVC is typically rated to 220 PSIG, 3" to 260 PSIG, so your pressure safety margin is ~2X the pressure rating.

BUT, and this is the BUT people generally forget, the pressure rating of a typical combustion spud gun is not really controlled by the pipe or most of the fittings. It is controlled by the pressure rating of the clean out fitting. A clean out is, by definition, a DWV part and is not pressure rated. If anything on the gun is going to fail (assuming everything was properly glued) it is the clean out cap. That is why we always recommend that nothing is mounted on the clean out cap. Mounting a flint sparker on the clean out cap is a particularly bad idea.
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Unread postAuthor: natas » Tue May 15, 2012 6:03 am

I'd say the easyest way to have metered propane, is to build a external meeter device. A pipe with a ball valve for the metered gas, and just have a female hansen coupling on the gun.

Connect the pipe to the gun, and release the gas to the chamber, disconnect the pipe, and fire.
There is many examples inside this forum on how to do it :idea:

And as mentioned before here, a fan is never fail :-)

I have a goldfish memory when it comes to remembering what cannon, and where it is on the forum. So little help from me on that part. But there is a video here with a golfball combustion, showing the metering system i mean really good :-)


But for me personally, the most fun, is to have a onboard fuel system, lots of bells and whistles :-)
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