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Help me figure out this hybrid

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Help me figure out this hybrid

Unread postAuthor: Marffy » Tue Dec 21, 2010 2:55 pm

I'm looking into making a portable hybrid. It will use a .5" union burst disk and a 30k igniter from Ulitimatespud. So far i have a 12" long by .75" diameter chamber made of seemed galv. Pipe. like This

I need help figuring out fueling. Does the same amount of air go in as propane? How do i find out how big the fuel meter should be? When i do make this, what kind of power will i be looking at? Could a 600psi spring loaded BV be more powerfull? And lastly what kinda "X" could this handle? 4x maybe?

Thanks in advanced.
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Unread postAuthor: Fnord » Tue Dec 21, 2010 3:05 pm

There's a topic for that.

http://www.spudfiles.com/forums/hybrid- ... 13602.html

The chamber you've picked is less than optimal because it may cause fuel mixing errors. I short chamber of 1.5" sch40 steel should be fine for 10x

I think you need to do more research on this personally. Haven't you made a combustion yet?
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Unread postAuthor: Technician1002 » Tue Dec 21, 2010 4:19 pm

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Unread postAuthor: MrCrowley » Tue Dec 21, 2010 4:40 pm

Does the same amount of air go in as propane?

Ahh no.

How do i find out how big the fuel meter should be?

Otherway round. You make the meter then calculate the fuel measurements for that meter. You will want a fuel meter (if you're doing volumetric metering, i.e #1 in the Hybrid 101 thread) that isn't too big or too small. For example a good meter, at say a 10x mix, will require you to inject propane up to a certain pressure which is below the room temperature pressure of propane (about 120PSI), this means you don't have to fuel and measure out to 'half' mixes and inject them separately.

Also, a good sized meter will also, say for a 4x mix, require you to inject propane at a pressure which isn't too low (say 15PSI) as it is harder to measure and requires a more precise gauge. Most gauges which go from 0-160PSI usually skip out the first 5, 10 or 15PSI markings on the gauge.

I'm a big fan of manometric metering but if you're only going up to a 4x mix, volumetric would be much better to use. I would suggest you buy a precise gauge with a degree of accuracy to about 1% or 2% F.S. (full scale). For example, a gauge which reads up to 160PSI and has a 1% degree of accuracy will fluctuate 1.6PSI. So while the gauge reads 50PSI, it could really be as high as 51.6PSI or low as 48.4PSI.

When I first read the Hybrid Fueling 101 thread, it took me a little while before I got my eureka moment and understood it all. Just keep at it and you'll figure it out no problem.

what kind of power will i be looking at?

No idea, you haven't really given us much information and I can't really put an exact figure on 'power'.

Could a 600psi spring loaded BV be more powerfull?

Yes. I'm sure even a slingshot could be more powerful if it were of mammoth size. You're dangerously close to asking about pieces of string.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 21, 2010 6:22 pm

You might want to consider a syringe meter to avoid clutter on the launcher.

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Unread postAuthor: Marffy » Thu Dec 23, 2010 12:28 am

K, thanks for the help. I've read over the hybrid fueling thread a couple times but i still haven't quite gotten it.

@Crowley I plan on using the max "X" measurement on steel fittings on a 1.5" short chamber. Is this enough info to tell me how much power i'll be looking at?
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Unread postAuthor: MrCrowley » Thu Dec 23, 2010 12:42 am

I plan on using the max "X" measurement on steel fittings on a 1.5" short chamber

Huh? Maybe it's just me but I don't get what you mean.

Is this enough info to tell me how much power i'll be looking at?

Ahhhh no. Think of it just like any other cannon.

Tell me, Marffy, how much power would I be looking at in a cannon using 1000PSI of compressed air? Yeah, you can't really answer it can you?

*cough* barrel diameter *cough* barrel length *cough* chamber length *cough* chamber diameter *cough* union diameter *cough* mix number *cough* projectile *cough* fuel *splutter*

Those would help. I understand you've listed some of them but it seems they may have changed since you posted the information.
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Unread postAuthor: Marffy » Thu Dec 23, 2010 12:58 am

Understood. I've been very vague. Things have changed since i was told 3/4 inch chambers wouldn't work. I'm not gonna bother trying to get an estimate on the power. I'll just start building and see what i come up with.

But before i start building, Whats the highest mix number a steel pipe can handle?
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Unread postAuthor: MrCrowley » Thu Dec 23, 2010 1:07 am

Why not just use HGDT to figure your estimates?

Whats the highest mix number a steel pipe can handle?

About as much as a piece of string :roll:

If you're talking about the usual hardware store galvanised iron pipe, it depends on the diameter but personally I wouldn't go above 10x handheld. DYI took his 2" diameter chamber up to a 20x mix I think, I probably wouldn't go above 15x mix with remote ignition.
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Unread postAuthor: Marffy » Thu Dec 23, 2010 2:31 am

The HGDT doesn't work on my computer for some reason. :(
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Unread postAuthor: Technician1002 » Thu Dec 23, 2010 9:31 am

Typical reasons for HGDT to not work are Mac, Linux, and Windows 7. There are fixes for a couple of those. It runs in WINE in Linux, and in Windows 7, there is some discussion, but the general fix is to run as Administrator for the proper privileges.
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Unread postAuthor: DYI » Thu Dec 23, 2010 3:50 pm

DYI took his 2" diameter chamber up to a 20x mix I think, I probably wouldn't go above 15x mix with remote ignition.


Remember, those 20X shots were with almost no buffer gas, so the pressure was significantly lower than it would have been in "normal" conditions. Also, I never used a disk which would fail higher than 1200psi or so. However, the 2" SCH 40 barrel on my 50mm gun did just fine at over 2500psi breech pressure. Still, considering the quality ( :roll: ) of hardware store fittings, I'd recommend a high safety factor on anything handheld.

But before i start building, Whats the highest mix number a steel pipe can handle?


The answer to your question lies in an equation derived by here by btrettel.
The strongest steel "pipe" possible with current technology would yield at around 350000 psi static pressure. However, this is not a case of static pressure, and thicker walls will still increase the time to rupture, if not the actual failure pressure. If one interprets "handle" as "survive indefinitely", then we must consider how compressibility of gases decreases at extremely high pressures. You'd have to find a suitable equation of state to know for sure, but the answer with this definition of "handle" would be somewhere between 1000X and 3000X. If indefinite survival is not a prerequisite, then there is no theoretical limit.

I find it intriguing that you're asking this question, considering that you likely don't have the manufacturing capabilities to even approach a quarter of the "highest mix number a steel pipe can handle".
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Last edited by DYI on Thu Dec 23, 2010 7:08 pm, edited 1 time in total.
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Unread postAuthor: Marffy » Thu Dec 23, 2010 4:00 pm

DYI wrote:
I find it intriguing that you're asking this question, considering that you likely don't have the manufacturing capabilities to even approach a quarter of the "highest mix number a steel pipe can handle".


Sorry, i was referring to The steel/iron pipe you buy at hardware stores.
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