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Hybrid Design and Questions

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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Hybrid Design and Questions

Unread postAuthor: tyrant_bb » Sat Mar 15, 2008 10:48 pm

Alright... so I made a basic combustion spud gun and it works great. I realize I should have more experience with spud guns than 3 shots with 1 cannon...haha. I've built 2 pneumatic cannons before, not for spud gunning though so I think I know the basics about combustion and pneumatic cannons... yay.

Well onto the hybrids. I've done as much searching as I could but I still have questions.

This is my design:

Image

The chamber will be 2" x 12" of galvanized steel. I can only find nipples though so I'm going to have to use a coupler to join them together. If I can't find a coupler, then I'll just use 2" x 6" and make it fire something smaller. If it's safe, I'll make it 2" x 18". After the ball valve behind the chamber, I'll be using SCH 40 PVC since that shouldn't have any combustion going on.

Alright so that's my design, now onto the questions:
1. Will this design effectively fire potatoes? (And go around 1000 feet...)
2. Do PVC male threaded connections fit with galvanized steel female threads?
3. Are the multiple spark gaps necessary? Is it bad to put the sparks there, even though it's over couplers?
4... Propane meters I'm having some trouble with. This is what I've come up with, but I want to make sure before I go and spend $30 on a meter. I'll use my combustion gun as an example because I know it's volume.
The chamber is 50.868 cubic inches. At atmospheric pressure, I should use 2.136456 cubic inches of propane. I will adjust the propane regulator to 2 BAR (easier to work with than PSI)... which means I should adjust the regulator to 1 BAR since there is atmospheric pressure? Then my meter volume should be 1.068288 cubic inches, right? If that's correct, and I were to use 1/2" PVC which has a ID of .602, then I should use slightly less than 1 inch of it? (Numbers are getting too precise to use now...)

Hmm well I think that's it for now... thanks for the help.
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Unread postAuthor: bigbob12345 » Sat Mar 15, 2008 10:56 pm

1.If your barrel is long enough
2.Yes
3. they are not nessesary but do help. dont get the coupler question
4. 1/2in pvc has an ID of.608 not .602. Other than that Im confused with your question
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Unread postAuthor: tyrant_bb » Sat Mar 15, 2008 11:02 pm

Alright cool. About number 3, I meant that since couplers would be there the metal would be twice as thick. Therefore drilling a screw there shouldn't affect the strength as bad as if I were to drill it directly on the chamber.
About number 4... um well I just used the wiki to try and figure it out.

Number 1 reminds me, what is the best C:B ratio for a hybrid?
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Unread postAuthor: bigbob12345 » Sat Mar 15, 2008 11:35 pm

It depends on the mix. The more pressure the smaller the CB can be and still gain power.
so pretty much for this just get the longest barrel you can transport and store.
And #3 yes you are right it will not affect the strength of it as much But you cant just screw a screw in the electricity will travel all ove rthe chamber. You need some sort of spark plug
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Unread postAuthor: tyrant_bb » Sun Mar 16, 2008 1:24 am

How do you drill holes in steel?

Also, is it safe to use SCH80 PVC for a hybrid? I read on the archive of Spudtech forums that a lot of people are using that, but that might be old now. Is it like certain it's going to blow up after a while? Cause that would be so much easier to work with...
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Unread postAuthor: SpudFarm » Sun Mar 16, 2008 7:05 am

no way i would have used PVC in a hybrid if it is not SCH 340 or something.
just use steel and the performance will not get any better or bader with just one spark since you don't need rapid combustion in a hybrid. it dosen't go until the disk does.

just tap threads for a sparkplug in there.

question: how big are the barrel going to be?
how high are you going with it?
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Unread postAuthor: tyrant_bb » Sun Mar 16, 2008 10:37 am

Well since I want to fire potatoes, it's going to be 1.5" x 60" most likely (If that will still be an OK ratio). At the most I'll take it up to 6X because I will be remotely firing it from far away.

And when you put a sparkplug in there, can you just use a simple BBQ ignitor to create a spark?
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Unread postAuthor: jimmy101 » Sun Mar 16, 2008 2:56 pm

tyrant_bb wrote:And when you put a sparkplug in there, can you just use a simple BBQ ignitor to create a spark?

yes

PVC is OK for the parts of the meter but everything in the combustion envelope needs to be metal. Most people make their meters out of metal though it isn't absolutely necessary.
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Unread postAuthor: SpudFarm » Sun Mar 16, 2008 5:03 pm

with that barrel you will get some decent power :)
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Unread postAuthor: tyrant_bb » Sat Mar 22, 2008 9:33 pm

OK I know most of this now, except for the propane meter. I'm using Fuel Tool to find this out now...

When it asks for atmospheric pressure, is that talking about atmospheric pressure in our area (If we're not at sea level)?

Then it's asking for supply tank pressure... is that what you set the propane regulator at? Or do propane tanks just supply different pressures?

And finally it asks for fuel mixture by volume, by default it's at "0.042%", I'm guessing that's not a typo and I should just leave it... instead of making it 4.2% right?
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