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Hybrid test results.

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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Unread postAuthor: DYI » Sun Sep 16, 2007 3:40 pm

I just noticed something: your post says the chamber is 750 mL, which is equal to 750 cc, or about 116 ci, which is the volume of a 2" diameter chamber about 35" long. Are you sure that your measurement is correct?
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Unread postAuthor: spanerman » Sun Sep 16, 2007 3:42 pm

im pretty sure it is as his fueling would be waaay off ;)
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Unread postAuthor: noname » Sun Sep 16, 2007 3:43 pm

You stole my idea, except I'm using 1.5" cell core ABS!
Now people can believe me when I say 8x is fine for steel.
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Unread postAuthor: Fnord » Sun Sep 16, 2007 3:48 pm

google wrote:750 ml = 45.7678081 cubic inches



Also, in response to previous posts, I have no idea where I'm going to get a decent peak pressure gauge. I'm betting mcmaster has them, but I really don't want to spend more than $20 on one. I haven't found anything on ebay, and none of my hardware stores sell them
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Last edited by Fnord on Sun Sep 16, 2007 4:48 pm, edited 2 times in total.
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Unread postAuthor: DYI » Sun Sep 16, 2007 3:54 pm

1cc = 1mL
2.54cm x 2.54cm x 2.54cm = 16.387064cc = 1ci
750/16.387064 =45.767808071
Muhahaha! Google is wrong...
But so was I. I must have forgotten to do something the first time.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 16, 2007 4:54 pm

mopherman wrote:lol this is coming from the guy who took pvc to 400pi :lol:


and who also has all his fingers and a distinct lack of razor-sharp PVC fragments in his gut ;)
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Unread postAuthor: dongfang » Sun Sep 16, 2007 5:23 pm

Hi,

Great work! Always nice when somebody gets about to actually verify things, while the rest of just go on with assumptions and blah-blah ;)

_Fnord, in the Spudfiles threads as I see them, the more recent posts appear below the less recent ones... so I see your example as normal system behavior.

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Unread postAuthor: jackssmirkingrevenge » Sun Sep 16, 2007 5:26 pm

dongfang wrote:_Fnord, in the Spudfiles threads as I see them, the more recent posts appear below the less recent ones... so I see your example as normal system behavior.


That's what I was thinking, I don't see the anomaly.
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Unread postAuthor: Fnord » Sun Sep 16, 2007 5:49 pm

That's what I was thinking, I don't see the anomaly.


I just realised what was going on a little while ago. It was my mistake, partially due to small text/bad eyes :)
See original post.
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Unread postAuthor: clide » Sun Sep 16, 2007 6:01 pm

I'm not sure if you were trying to initiate DDT or not, but wouldn't having the spark at one side of the chamber be more prone to DDT than a spark in the middle? Thats how I understood it anyway.

Also did you vary the fuel mixture at all? I've also heard that lean mixtures are more likely to transition to detonation.

Pretty good test, but like others have said, a measure of peak pressure would have been nice
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Unread postAuthor: Fnord » Sun Sep 16, 2007 6:57 pm

I wasn't really trying to get ddt, as I don't think I can with my current setup. There just isn't enough run-up distance in the pipe.

The fuel mixture probably varied slightly, but if it did it was in error.

I don't know where to get a peak pressure gauge in my area. If you can find one relatively cheap online I might get it.
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Unread postAuthor: frankrede » Sun Sep 16, 2007 7:46 pm

pressure gauge+ 1/4" inline check valve.
The gauge may go bad after a while so a manifold of some type might be needed.

or perhaps this
mcmaster item #4031K44 or 4031K54
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Unread postAuthor: jimmy101 » Mon Sep 17, 2007 9:54 am

_Fnord wrote:I can't say for sure, as I don't have a peak pressure gauge that goes that high. 2" threaded sch40 steel has an estimated burst pressure of ~1300 psi.
Someone needs to do some gaseq number punching.

Nice work _Fnord!

For propane + air at 11x GasEq puts the pressure at 1274 PSIA.
Getting pretty close to the estimated burst pressure of ~1300 PSI.

I don't think you'll be able to get a closed pipe to DDT with propane + air unless you get the initial pressure and temperature up near autoignition conditions. According to GasEq, the speed of sound in your chamber at the end of combustion is ~900m/s (~Mach 3 compared to air at STP).
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Unread postAuthor: jimmy101 » Mon Sep 17, 2007 10:02 am

It's amazing, spudders go months (years) without any interesting new data then we get two posts, on two different boards, within 24 hours of each other on high pressure tests in hybrids.

Check out http://www.spudcom.com/forums/showthread.php?t=75. "Noname" is measuring peak pressures in a burst disk hybrid. At 6x he is getting ~650 PSI. Since Noname is using a burst disk gun the maximum pressure in the chamber is limited by the failure pressure of the burst disk.
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Unread postAuthor: noname » Mon Sep 17, 2007 11:20 pm

I may need to get a check valve to make operation easier and more accurate, but those were the general numbers I got out of it.
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