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Advanced Combustion Ratio and Fueling questions

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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Advanced Combustion Ratio and Fueling questions

Unread postAuthor: cwazy1 » Sat Sep 29, 2007 11:38 pm

So, this is going to be my first propane powered combustion cannon in a linear design. i will be using a 100k stun gun with 3 or 4 gap jumps to ignite the fuel.

So my first question is that i plan on using a 5 foot long sdr21 1.5" in a sch80 2" sleeved (golfball) barrel. . . . .. i just dont know what size of a chamber i have to have. i want a 4" diameter one but when i do the c:b ratios, the barrel ends up to be 106 cubic inches and most people use a 1:1 ratio, that would mean that if i used a 4" thich chamber, it would only need to be like 10" long. personally, thats retarded. so i want a longer chamber, but dont want to increase barrel size or decrease the chamber thickness. so what is the maximum length of chamber @ 4" could i have without sacrificing power? or, should i just use 3" chamber instead and have a 18 inch long chamber. that ends up as 127 cubic inches which is close enough to 1:1?


I plan on making a propane metering system but i found nothing on how much psi should be in the main propane nipple? and also should that center nipple be 8" or 10" long?
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Unread postAuthor: hi » Sun Sep 30, 2007 12:39 am

you can make the chamber as big as you want it to be. if its a 5 foot sdr21 then i would recommend about a 18'' chamber 4'' in diameter. that should give you a fair amount of power

please note that i didnt do the math, but that will work.

as for the meter, it just trial and error. yes, there is math that you can use to calculate it, but i just use trial and error, it takes about 10 shot to get it right, after that it works every time.
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Unread postAuthor: SpudBlaster15 » Sun Sep 30, 2007 12:39 am

With 3 or 4 spark gaps in the chamber, it is unlikely that you would see a performance difference between the longer 3" chamber and the shorter 4" chamber.
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Unread postAuthor: cwazy1 » Sun Sep 30, 2007 2:10 am

ahh thanks guys!

so i guess 4" x 18" it is!

but i still would like for someone to use the burnt latke calc thing and calculate for me because i really dont understand that thing. . . 226 inches cub for chamber and i want 1/2 inch pipe. i need to know how long of a meter tube i should use and what psi. i hate trial and error.
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Unread postAuthor: Pete Zaria » Sun Sep 30, 2007 2:27 am

@Cwazy1...

I hate spoonfeeding, but here we go...

Your barrel, 5 ft of 1.5" SDR21 (which actually has an inside diameter of 1.70" +/- .010) is 136ci.

For a .8:1 ratio (most efficient, decent power) you want a 108ci chamber. You could do that with a 3" pipe x 15" long, or a 4" pipe 8.5" long.

For a 1:1 ratio (less efficient, a bit more powerful, a bit louder), you'd want a 136ci chamber (obviously), which could be accomplished with 19" of 3" pipe or 11" of 4" pipe.

For a 1.5:1 ratio (inefficient, powerful, and loud as a mofo) you'd want a 204ci chamber, 16" of 4" pipe.

For the .8:1 (108ci), use a 1/2" meter pipe 5" long at 40psi.
For the 1:1 (136ci) use a 1/2" meter pipe 6.5" long at 40psi.
For the 1.5:1, use a 1/2 meter pipe 6.5" long at 60psi.

You could use more pressure in a shorter pipe, or less pressure in a bigger pipe, for the same effect. The numbers I gave you are kind of "for example", but would work OK.

To calculate the volume of pipe: http://www.online-calculators.co.uk/vol ... volume.php

To calculate meter pipe length/pressure: http://www.burntlatke.com/ft_live.html

What kind of meter are you building? Are you going to use a regulator?

Personally, I use a simple meter consisting of two ball valves, two 3/8ths" nipples, a 3/8ths x 1/4" T, a pressure gauge, and some hose barbs. Cheap and effective; open tank-side valve, wait for 68psi (my correct mix pressure) on gauge, close tank-side valve, vent to chamber... A regulator takes the "wait for xx psi and close valve" out of the equation, but is not required.

You're welcome.

Peace,
Pete Zaria.
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Unread postAuthor: jimmy101 » Sun Sep 30, 2007 1:37 pm

Pete Zaria wrote:For a .8:1 ratio (most efficient, decent power) you want a ...


In a combustion gun maximum efficience = maximum power.

Actually, like a car engine, there is probably a very small shift between efficience and power. The difference is probably too small to (1) measure and (2) reproduce.
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Unread postAuthor: noname » Sun Sep 30, 2007 1:42 pm

I haven't read Latke's tests recently, but I thought the results were something like the most powerful and effecient CB ratio for potatoes was .6:1, and for projectiles with less friction (the gasket slugs), the best ratio was .8:1.
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Unread postAuthor: Pete Zaria » Sun Sep 30, 2007 2:07 pm

That doesn't make sense to me.

If your chamber is too big for your barrel, at least out to a certian extent, there shouldn't be any loss of velocity... Basically, the chamber would be putting out more power than the barrel can harness (because the barrel is too short to capture all of the hot expanding gases before the projectile has exited the muzzle), so the excess power is wasted as muzzle flash and report. Unless you used a crazy-huge chamber (think waterheater tank) I'd seriously doubt that having an overly large c:b ratio would hurt performance. It seems to me like it would just increase muzzle flash and report.

.8:1 is the ratio at which the barrel is able to take advantage of all the power being put out by the chamber, and very little energy is wasted. I don't think it's the most powerful, but I guess we'd need to do some pretty extensive testing on that to be sure.

Latke's tests were important and informative but hardly scientific - I'd like to see similar tests repeated with more standardized conditions (and non-organic projectiles.... lol, superbouncy balls or golfballs would be much better). Maybe this forum could get involved with sponsoring such a test.....

Just my two cents.

Peace,
Pete Zaria.
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Unread postAuthor: MikeNice » Sun Sep 30, 2007 2:58 pm

I've been using a 1.5:1 ratio for a long time now (big over under) specifically mine is 18"x4" chamber and a 48"x2" barrel. This is a WAY powerful gun. With Static Guard it will easily put a spud WAY out of sight. I'm building a 1:1
gun now, doing everything the same as my big boy, just to see which c:b ratio I can get more range / hang-time out of. I have 2 rigs, 1 holds the gun at vertical for hang-time tests and the other holds the gun @ 45 degrees for distance test. My new 1:1 is going to be 12"x4" chamber with a 48"x2" barrel. Later I'll build a 72" barrel for my 18"x4" chamber and run the tests again. I'll be amazed if I get more distance or hang - time out of the 1:1, in the past longer barrels haven't gained me much although I've never used anything above a 62"x2" with that chamber. Almost 2 years ago I tested 50", 52", 54", 56", and 62" ,all x 2" sched 40 pvc BTW, and got the best results with the 48 so it's what I stick with. I've lost my excel spreadsheet my results were on so I can't quote my hang-times and distances anymore. I'll poast my new results when I get them done.
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Unread postAuthor: DYI » Sun Sep 30, 2007 3:12 pm

The larger the chamber is, the slower the fuel burns, so, at some point, increasing chamber size would decrease performance.
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Unread postAuthor: psycix » Mon Oct 01, 2007 5:29 am

With 3 or 4 spark gaps you can have one hell of a huge chamberwithout performancedrop.
ALson then it will make a big bang :D
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Unread postAuthor: jimmy101 » Mon Oct 01, 2007 4:17 pm

Pete Zaria wrote:That doesn't make sense to me.

If your chamber is too big for your barrel, at least out to a certian extent, there shouldn't be any loss of velocity... Basically, the chamber would be putting out more power than the barrel can harness (because the barrel is too short to capture all of the hot expanding gases before the projectile has exited the muzzle), so the excess power is wasted as muzzle flash and report. Unless you used a crazy-huge chamber (think waterheater tank) I'd seriously doubt that having an overly large c:b ratio would hurt performance. It seems to me like it would just increase muzzle flash and report.

.8:1 is the ratio at which the barrel is able to take advantage of all the power being put out by the chamber, and very little energy is wasted. I don't think it's the most powerful, but I guess we'd need to do some pretty extensive testing on that to be sure.

Latke's tests were important and informative but hardly scientific - I'd like to see similar tests repeated with more standardized conditions (and non-organic projectiles.... lol, superbouncy balls or golfballs would be much better). Maybe this forum could get involved with sponsoring such a test.....

Just my two cents.

Peace,
Pete Zaria.

Latke's studies are scientific, damn good science at that. Most of the studies have ten shots per barrel length. Most of the studies were done with the same slug, a wood dowel with a rubber gasket. Only one of the studies, out of three IIRC, was done with spuds. The fuel was metered and fan mixed.

I really can't think of any way to do the studies more accurately than what Latke did. There may be ways that would require a bit less work, but they wouldn't be any more accurate.


Combustion guns behave completely differently then do compressed air guns. If the chamber is too big the performance drops way off. Presumably because a large chamber burns too slowly and sufficient pressure to launch the spud is created long before maximum pressure is obtained. This is sometimes called the "mine sized chamber affect".

Looking at Latke's data, a well built gun (except for the CB) would only have about 1/3 the kinetic energy (and ~40% the muzle velocity) at a C:B of 1.5 as it would at a CB of ~0.8. (Using the 1.5" spud data.)

There are a lot of questions that Latke's studies didn't address. For example, what is the best CB for various numbers of sparks? Or, for hybrids? Or for low versus high friction projectiles?
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