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Help needed with O2 and C3H8

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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Help needed with O2 and C3H8

Unread postAuthor: Philbob1 » Thu May 29, 2008 3:47 pm

This is my first post, so don't be mean :(

I am fairly new to all this cannon thing, and decided a few weeks/months ago with a few friends to build a basic propane powered cannon.

After a few tests and seeing what were the major problems, I did some research and saw that a low amount of propane was needed.

And I came with those questions that I would like to ask.

With more oxygen, would the cannon shoot even more?
Is it possible (technically) to get pure oxygen in it without it blowing up?
If yeah, is steel a necessity?
And last question, would a few starters be greater?

Thank you for your time.
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Unread postAuthor: SpudFarm » Thu May 29, 2008 3:54 pm

in a atmospheric mix you need 4.2% propane.

steel is needed to use oxygen.

if you use oxygen you have made a hybrid

welcome to the forum and i hope you make a nice hybrid out of STEEL :D
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Unread postAuthor: Philbob1 » Thu May 29, 2008 3:56 pm

O thanks, I'll check out that section.

Thanks for the quick answer
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Unread postAuthor: BC Pneumatics » Thu May 29, 2008 5:14 pm

Also make sure to measure your 4.2% propane to 95.8% air ratio in moles or volume, not mass. In order to use volume, you must be at equal pressures. (Or account for the differences with P1V1=P2V2)
O2 would warrant a steel or copper chamber, and would require a different ratio. (1:5 C3H8:O2)
Multiple ignition points are theoretically helpful, but it does not make a huge deference.
If you are more comfortable starting out with a kit that includes what you need to make a basic launcher, along with instructions, they are available from BCARMS in the link below.
I suggest you get your feet wet by building a couple basic launchers before trying to use O2, you can always move up to it later. This will also ensure that any mistakes you might make are likely to be both cheaper and less dangerous.
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Unread postAuthor: DYI » Fri May 30, 2008 5:08 pm

I've used O<sub>2</sub>/C<sub>3</sub>H<sub>8</sub> mixes with great success, at preignition pressures up to 70 psi in SCH 40 welded steel pipe. While this obviously isn't recommended, it shows that even relatively weak materials (SCH 40 2" steel fails at ~2300 psi constant pressure) can hold oxy/fuel mixes successfullly. It's hydrogen and acetylene that you should probably steer clear of.
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Unread postAuthor: jimmy101 » Sat May 31, 2008 10:51 am

spudfarm wrote:in a atmospheric mix you need 4.2% propane.

BC Pneumatics wrote:Also make sure to measure your 4.2% propane to 95.8%

Actually, the 4.2% propane goes with 100% air. 4.03% propane goes with 95.8% air.

4.20% if your fuel is added to the chamber and does not displace any air, such as when you use a meter.

4.03% if your fuel displaces some of the air in the chamber, such as when you fuel with a syringe or by spray-n-pray.

Of course, it probably doesn't matter a heck of a lot which way you calculate and measure the fuel. I doubt you can tell the difference, even with a chrony, between a 4% and 4.2% final mix. Rough estimate would be that if the fuel should be 4.2% and it is actually 4%, then the muzzle velocity would drop by only about 2.5%, which would be an unmeasurable difference.
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Unread postAuthor: BC Pneumatics » Sun Jun 01, 2008 2:48 am

Thanks for the correction Jimmy. I really didn't give the numbers much thought, I was more just making a point not to fuel by mass. (With those numbers at least.)
Also, a chrony could easily measure a 2.5% change in the ~400fps most cannons shoot.
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Unread postAuthor: SpudFarm » Sun Jun 01, 2008 8:38 am

i hate when jimmy post almost just after me, then i know i did something wrong :D
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Unread postAuthor: Philbob1 » Sun Jun 01, 2008 10:04 am

Thanks for the answers guys, I have decided to make it in steel or good aluminium, depends what I can get cheaper here in Canada.
I have a few questions tho.

Would 7972K43 or 7972K123 be better?

And does anyone knows a good shop like McMaster in canada? transport cost are going to double the price of the order.
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Unread postAuthor: SpudFarm » Sun Jun 01, 2008 10:15 am

why don't just use ordinary galvanized steel pipe and fittings?

if you are going with O2 you don't need much more that a 8" by 2" chamber. then you can put a marble barrel on.
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Unread postAuthor: Philbob1 » Sun Jun 01, 2008 12:37 pm

Heh yeah well the thing is I want to shoot potato with it, so I'm thinkin 'bout sticking with a 3 1/2'' chamber w/ a 1 1/2'' copper barrel (keeping the 0.8:1 or 1.5:1 C:B ratio, haven't decided wich yet)
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Unread postAuthor: BC Pneumatics » Sun Jun 01, 2008 12:52 pm

Potatoes make a poor ammo in anything with real power. They just blow apart in the barrel and exit as a cloud of mist. Are you sure you are confident enough in your building skills for this project?
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Unread postAuthor: Philbob1 » Sun Jun 01, 2008 5:02 pm

Thanks for being worried about not havin' a newspaper frontpage, but I am pretty sure I will be OK. :D

I just chose to start pretty hard, it's how I like it :P I'll start w/o oxy hybrid, and if it goes well I'll add oxy.

The general idea is there in my head, I just need to put all the plans that I have made togheter and make it come to reality. I have all the summer for that last thing.
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Unread postAuthor: SpudFarm » Sun Jun 01, 2008 5:08 pm

don't be near it when it goes off with oxy.
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Unread postAuthor: Philbob1 » Sun Jun 01, 2008 5:14 pm

Yeah I know that,

I have a R/C system with some cervo (not sure how to spell that in english) able to trigger it from a long range
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